Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2024 Mar 12;63(12):5701–5708. doi: 10.1021/acs.inorgchem.4c00176

Crystal-Like Atomic Arrangement and Optical Properties of 25La2O3–75MoO3 Binary Glasses Composed of Isolated MoO42–

Atsunobu Masuno †,‡,§,*, Sae Munakata ‡, Yoshihiro Okamoto ∥, Toyonari Yaji ⊥, Yoshihisa Kosugi #, Yuichi Shimakawa #
PMCID: PMC10966733  PMID: 38471976

Abstract

graphic file with name ic4c00176_0008.jpg

Transparent and brown La2O3–MoO3 binary glasses were prepared in bulk form using a levitation technique. The glass-forming range was limited, with the primary composition being approximately 25 mol % La2O3. The 25La2O3–75MoO3 glass exhibited a clear crystallization at 546 °C, while determining its glass transition temperature was difficult. Notably, despite its amorphous nature, the glass possessed a density and packing density comparable to those of crystalline La2Mo3O12. X-ray absorption fine structure and Raman scattering analyses revealed that the glass structure closely resembles La2Mo3O12 due to the presence of isolated MoO42– units, whereas disordered atomic arrangement around La atoms was confirmed. The glass demonstrated transparency ranging from 378 to 5500 nm, and the refractive index at 1.0 μm was estimated to be 2.0. The optical bandgap energy was 3.46 eV, which was slightly smaller than that of La2Mo3O12. Additionally, the glass displayed a transparent region ranging from 6.5 to 8.0 μm. This occurrence results from the decreased diversity of MoOn units and connectivity of Mo–O–Mo, which resulted in the reduced overlap of multiphonon absorption. This glass formation, with its departure from conventional glass-forming rules, resulted in distinctive glasses with crystal-like atomic arrangements.

Short abstract

Transparent and brown La2O3−MoO3 binary glasses were prepared in bulk form using a levitation technique. Mo L3-edge and K-edge X-ray absorption fine structure and Raman scattering spectra revealed that the 25La2O3−75MoO3 glass structure highly resembles crystalline La2Mo3O12, which consists of completely isolated MoO42−. The structural similarity with the crystalline phase and the slight disorder caused the additional transparency in the infrared region.

Introduction

Oxide glasses are constructed on a three-dimensional network of MOn polyhedra where element M and oxygen (O) are bonded covalently. The constraints in glass formation are that the oxygen coordination number, n, should be as small as three or four, and that the MOn polyhedra are connected to each other not by edge- or face-sharing but by corner-sharing. SiO2, P2O5, etc., can form corner-sharing MO4 tetrahedral networks, and they are thus referred to as network former oxides. Modifier oxides, such as alkali metal oxides, alkaline earth metal oxides, and rare earth oxides, typically break the network and introduce nonbridging oxygens. Al2O3, TiO2, Nb2O5, and other intermediate oxides act as either a network former or modifier, depending on their composition in a glass.1,2 In addition, among the intermediate oxides that cannot vitrify without additives, Al2O3, Ga2O3, MoO3, and WO3 can vitrify relatively easily, even at a binary composition, in combination with a specific modifier or intermediate. They are especially called conditional glass formers.3 The glass-forming region of the binary system that includes conditional glass formers is generally narrow. Nevertheless, an additional small amount of network former oxide added to a binary system drastically improves the glass-forming ability.

MoO3 is a conditional glass former, and glass formation in some of its binary systems has been reported. However, compared to Al2O3- and Ga2O3-based binary glasses, the number of reports is considerably small. Nassau et al. prepared Li2O–MoO3 binary glasses during a search for glasses with high ionic conductivity in the 1980s.4 However, glass formation required the use of the twin-roller quenching method with a very high cooling rate (104–106 K/s). Ag2O–MoO3 binary glasses with a thickness of approximately 1 mm were obtained by a conventional melt-quench method to investigate their ionic conductivity.5 Recent structural analysis combined with X-ray and neutron diffraction revealed that the coordination number of Mo decreased from 5.4 to 4.2 with increasing Ag2O content and a variety of MoOn polyhedra (n = 4, 5, and 6) connected to each other to develop the network structure.6 Dimitriev et al. have fabricated a variety of binary and ternary glasses that included MoO3 as a main component since the 1990s and summarized a large amount of research on nontraditional molybdate glasses.7−9 They succeeded in the glass formation of the 10La2O3–10Nd2O3–80MoO3 and 10R2O3–90MoO3 systems (R = La or Nd) using slow cooling rates (102 K/s). It was reported that the 10La2O3–90MoO3 glass had the glass transition temperature at around 325 °C and crystallization temperature at 410 °C. From structural analysis using Fourier transform infrared (FT-IR) and X-ray absorption fine structure (XAFS) spectroscopies, it was suggested that the glass network was build up with corner-sharing MoO6 or MoO5 without short isolated Mo=O bonds.9

A levitation technique is an effective method to vitrify a composition without a sufficient network former oxide in bulk form despite its low glass-forming ability. Various unconventional oxide glasses have been fabricated recently using a levitation technique in the last 20 years.10−13 New glass compositions and the expansion of the glass-forming region were reported in systems with conditional glass formers as the main component. R2O3–Al2O3 glasses have very high elastic moduli,14,15 and R2O3–Ga2O3 and R2O3–WO3 glasses exhibit good optical properties such as infrared transparency, strong luminescence, and high refractive index with low wavelength dispersion.16−19 In this study, La2O3–MoO3 binary glasses were successfully obtained by a levitation technique in the bulk form. Physical properties of the binary glasses, including thermal and optical properties, were measured. Structural analysis using Mo L3-edge, K-edge, and La L3-edge XAFS and Raman scattering spectroscopies were performed to investigate the atomic arrangement of the glass by comparing it with the crystalline phase structure with the same composition as the glass.

Experimental Procedure

Glass syntheses of the (100–x)La2O3–xMoO3 binary system were examined by a levitation technique. High purity La2O3 and MoO3 were stoichiometrically mixed and pressed into pellets. The pellets were sintered at 600 °C for 12 h in air and then were crushed to target pieces for the aerodynamic levitation (ADL) furnace. A piece of the target was placed on the nozzle of the ADL furnace and levitated by an O2 gas flow. A CO2 laser was used to melt the levitated sample for several seconds. The melt was rapidly cooled to room temperature by turning off the laser power, and then it solidified. The cooling rate was estimated to be approximately 500 °C/sec. Glass formation was confirmed by Cu Kα X-ray diffraction measurements (XRD: Rigaku, MiniFlex600). Although the size of the fabricated glasses was approximately 2 mm, the thermal, optical, and structural properties could be sufficiently investigated. Crystalline La2Mo3O12 as a reference was prepared by conventional solid-state reaction. High purity La2O3 and MoO3 were stoichiometrically mixed and pressed into a pellet, and it was sintered in air at 800 °C for 24 h several times with intermediate pulverization.

The glass transition temperature (Tg) and crystallization temperature (Tx) for the glasses were investigated by using differential scanning calorimetry (DSC: NETZSCH, DSC 404 F1 Pegasus). The temperature was raised from room temperature to 700 °C with a heating rate of 10 °C/min in a N2 gas flow. Prior to measurement of the physical properties, the glasses were annealed at 475 °C for 5 min to remove any internal stresses. Furthermore, post annealing of the glass in an oxidized atmosphere was conducted to diminish Mo5+ and improve transparency at the 400–600 nm region. The glasses were annealed in an O2 gas flow at 400 °C for 24 h. The density of the glasses was measured using a He-gas pycnometer (micromeritics, AccupycII 1340). More than a dozen glass beads were placed in a 0.1 cc sample cell to reduce experimental errors.

Mo L-edge XAFS measurements were performed at the soft X-ray beamline, BL-10, in the SR Center of Ritsumeikan University.20 The incident X-ray was monochromated with a double crystal using Ge(111) planes. Powdered samples of glasses and reference crystals were put on the carbon seal on the sample holder in a high vacuum chamber. The X-ray absorption spectra were recorded with the fluorescence yield method. Photon energy was calibrated at the energy of a peak (2481.69 eV) of white line of S K-edge XANES spectra of K2SO4 standard samples.

Mo K-edge and La L3-edge XAFS spectra were recorded in transmission mode using two ionization chambers at the BL27B and BL9A beamlines in KEK-PF, respectively.21−23 The incident X-ray was monochromated with a double crystal by using Si(111) planes. The samples were ground into powders and mixed with a high purity hexagonal boron nitride powder to form pellet specimens. Energy calibration for Mo K-edge spectra was performed by setting the inflection point of the spectra of a metallic Mo foil as E = 20003.9 eV and that for the La L3-edge was done by setting the energy of the pre-edge peak of the Ti K-edge of a metallic Ti foil as E = 4966.0 eV. XAFS data analysis was conducted using the Athena and Artemis programs in the Demeter software package.24 The k3-weighting EXAFS spectra were Fourier-transformed (FT) to real space. Structural parameters regarding the first peak in the FT magnitude, such as the oxygen coordination number of Mo and La, the cation–oxygen distance, and the Debye–Waller (DW) factor, including both structural and thermal disorder, were evaluated by curve fitting analysis using phases and amplitude functions calculated using the FEFF8 code.

Unpolarized Raman scattering spectra of the glasses and crystalline La2Mo3O12 were obtained in a 180°-scattering geometry by a confocal Raman microscope (JASCO, NRS-4500). The incident source was a 532 nm semiconductor laser.

For transmittance measurements, both sides of the glass samples were optically polished to a thickness of approximately 500 μm. Transmittance spectra were obtained using an ultraviolet–visible (UV–vis) spectrometer (Shimadzu, UV3600 plus) in the range of 250–2000 nm and an FTIR spectrometer (Shimadzu, IRAffinity-1S) in the range of 2000–10000 nm. The diffuse reflectance spectra of powder samples were also obtained using an integrating sphere installed in a UV3600 plus. BaSO4 was used as a reference.

Results and Discussion

The glass synthesis results are shown on the La2O3–MoO3 binary phase diagram25 in Figure 1. Bulk glasses were obtained only at x = 75 and 80 in the composition of the (100–x)La2O3–xMoO3. Note that the x = 75 glass has the same composition as that of crystalline La2Mo3O12. The glasses were homogeneously transparent and brownish colored, as shown in the inset. The glass-forming region of the La2O3–MoO3 binary system is similar to that of the La2O3–WO3 binary system.19 The brown color of the La2O3–MoO3 glasses is a unique characteristic compared to that of colorless La2O3–WO3 glasses. The diameter of the spheric glasses was approximately 2 mm. At x = 50, 70, 85, 90, and 95, violent evaporation occurred during melting, and the targets were not vitrified. The glass formation in this study is clearly different from the previous report that showed glass formation with x = 90 using the conventional melt-quench method.7,8,9 Glass formation at x = 90 rarely succeeded in our levitation experiments, but only where the melting time was long and the melting temperature was high. This may be due to composition deviation, resulting in an increase of La2O3 content because of MoO3 volatilization.

Figure 1.

Figure 1

Open in a new tab

Glass-forming range indicated on the (100–x)La2O3–xMoO3 phase diagram. Circles and crosses indicate glass formation and crystallization, respectively. The inset shows a picture of the 25La2O3–MoO3 glass annealed in an O2 gas flow at 400 °C in 24 h.

Figure 2 shows the DSC curve of the x = 75 glass. The crystallization temperature, TX, was found at 546 °C, and the crystallization peak was remarkably sharp. TX of the x = 75 glass was lower than those of 20La2O3–80WO3 glass, which was at 594 °C. Determining the glass transition temperature was challenging due to minimal change in heat flow. Previous reports based on differential thermal analysis (DTA) data of the x = 90 glass indicated a crystallization peak temperature at 410 °C, and a subtle change, assumed to be Tg, was observed at approximately 325 °C.7,9 However, no significant signal was observed in the temperature range from 300 °C to TX in this study. Two possible reasons explain why the glass transition temperature was not detectable. One possibility is that Tg might be so close to TX that Tg is obscured by the onset of the crystallization peak. Another possibility is that the difference in specific heat between the glassy state and supercooled liquid might be too small to be detected.

Figure 2.

Figure 2

Open in a new tab

DSC curve of 25La2O3–75MoO3 glass.

Figure 3 shows XRD profiles of the as-melted sample of x = 75, the crystallized sample after DSC measurement, and a calculated profile using the crystal structure of La2Mo3O12.26 The profile of the as-melted sample does not have any peaks, indicative of an amorphous nature. It is clearly shown that the profile of the crystallized sample is almost identical to the calculated profile of La2Mo3O12. This implies that the atomic arrangement of x = 75 glass and crystalline La2Mo3O12 may resemble each other. The crystal structure of La2Mo3O12 is shown in the inset of Figure 3. There are five Mo sites and three La sites in the La2Mo3O12 unit cell. The Mo atom forms highly isolated MoO4, without any Mo–O–Mo connections, while the La atom is coordinated by eight oxygens. It can be seen that the MoO4 tetrahedra connect at all four oxygen with LaO8 by corner-sharing. Each of the six oxygens in LaO8 is shared with MoO4 at the corner, and each of the remaining two oxygens is shared by MoO4 and two edge-shared LaO8. Alternatively, Mo6+ forms isolated MoO42–, all of whose oxygen atoms are nonbridging, and La3+ coordinates it as a charge compensator.

Figure 3.

Figure 3

Open in a new tab

XRD profiles of the as-melted 25La2O3–75MoO3 sample and the crystallized sample after DSC measurement. The calculated profile of La2Mo3O12 is shown at the bottom. The inset shows the crystal structure of La2Mo3O12 drawn using VESTA software.27 Purple and green polyhedra in the inset represent MoO4 and LaO8, respectively.

The density of the x = 75 glass was measured to be 4.67 g/cm3, indicating a 4.1% deviation from the calculated value of 4.87 g/cm3 of La2Mo3O12. The packing density (PD) was calculated using the equation PD = VP/Vm, where VP is the sum of the volume of component ions and Vm is the molar volume of the glass. Assuming that ions in the glass are spherical, the volume of the ith ion is expressed as 4πri3/3, where ri represents the ionic radius of the ith ion. Shannon’s ionic radii were utilized for volume calculations: 1.16 Å for La3+(VIII), 0.41 Å for Mo6+(IV), and 1.35 Å for O2–(II).28 The PD of the 25La2O3–75MoO3 glass was determined to be 0.510, closely resembling 0.532 of the La2Mo3O12 crystal. Such similarity in PD between glass and crystalline phase, often observed in glasses prepared by a levitation technique,29 suggests that the glass structure is highly packed, akin to the crystalline phase.

X-ray absorption near edge spectroscopy (XANES) spectra mainly originate from a superposition of electronic transitions and provide information on valence and chemical states of the elements to be measured. The relative height and position of the pre-edge peak in XANES spectra are often helpful to estimate the coordination number and distortion of the polyhedron.30−32 XANES analyses for Mo were reported for silicate glasses and melts by comparing a variety of reference crystals.33 XANES spectral shape and the pre-edge peak of the reference crystals strongly depend on the atomic arrangements and the valence state of Mo. Linear correlation was clearly shown between the average valence state of Mo and the Mo K-edge position at a normalized absorbance equal to one. The XANES spectra of silicate glasses also showed this variation; however, the degree of difference is much smaller than the case of crystalline phases. It was concluded that the valence state of Mo was six in silicate glasses prepared in ambient conditions, and the reduction of Mo requires a strong reduced condition. The coordination number of Mo estimated from the pre-edge peaks is almost four; however, there are some glasses with higher coordination numbers, ranging from 4 to 6 even in silicate glasses.33

Mo K-edge XAFS spectra of the La2O3–MoO3 binary system were clearly different from silicate glasses, and they provide clear evidence of the similarity of local structure around Mo atoms between glassy and crystalline phases. Figure 4a shows Mo K-edge XANES spectra of the 25La2O3–75MoO3 glass and La2Mo3O12 with reference MoO3. Mo in MoO3 has a valence state of 6+ and is coordinated by six oxygen atoms. MoO6 polyhedra are connected to each other by sharing corners and edges. The spectrum of the glass is clearly different from that of MoO3, especially at the pre-edge region; however, it is almost the same as that of La2Mo3O12. Compared to MoO3, the pre-edge peak of the glass is higher, and the position shifted to the lower energy side, indicating that Mo forms MoO4 tetrahedra as well as La2Mo3O12. From the absorption edge energy, Mo in the glass is estimated to be 6+, as well as that in the two reference crystals.

Figure 4.

Figure 4

Open in a new tab

Mo K-edge XAFS spectra of 25La2O3–75MoO3 glass and crystalline La2Mo3O12. (a) XANES spectra, (b) k3-weighted EXAFS spectra, and (c) the FT magnitude of EXAFS oscillation. Inset of (a) shows Mo L3-edge XANES spectra. Inset of (c) shows the fitting result of the glass.

Mo L3-edge XANES spectrum, in which the peaks are mainly ascribed to the transition from 2p to 4d states, is helpful to predict the oxygen coordination number of Mo as well as K-edge XANES spectrum.34 Mo L3-edge XANES spectra of the glass and reference crystals are shown in the inset of Figure 4a. Two peaks were clearly observed in the spectra of MoO3. The peak at the higher energy side was less intense than that on the lower energy side, implying that they are associated with the eg and t2g empty states of octahedral MoO6. The spectra of the glass and crystalline La2Mo3O12 were almost the same shape, which is evidence of the local structural similarity between them. The main peak consists of two peaks with an energy difference smaller than that of the MoO3 spectrum. The intensity ratio of the peak at the higher energy side and lower side was the opposite of that of MoO3. These results suggest that the two peaks are attributed to the electron transition in the tetrahedral crystal field splitting.

Figure 4b shows the k3-weighted EXAFS spectra, k3χ(k) of the 25La2O3–75MoO3 glass and crystalline La2Mo3O12. The oscillation was observed clearly in a high k range. The glass and crystal are almost the same in shape and intensity. This means the homogeneity and uniformity of the local structure around Mo in the glass. Figure 4c shows the FT amplitude of the k3-weighted EXAFS spectra with kmax = 14.0 Å–1. They are almost the same; however, a relatively smaller peak height was observed for the glass, indicating a slightly smaller coordination number or disorder in atomic arrangement around Mo. Furthermore, there is no correlation at 3.8–4.0 Å for the second nearest shell in the glass, while a small but clear correlation is observed in the crystal, showing disorder in the glass. A correlation smaller than expected from the FEFF calculation is often observed in molybdate crystalline phases, such as Na2MoO4 and Ag2MoO4.33,35

In order to more quantitatively investigate the local structure around the Mo, fitting of the K-edge EXAFS spectra was performed. Prior to the fitting of EXAFS spectra for the glass, it is necessary to determine the inelastic multielectronic losses, S02, of the EXAFS formula in this experiment from the fitting result of a reference crystal. The fitting for La2Mo3O12 was performed in the r range from 1 to 2 Å with a fixed value of 4.0 for the coordination number, N. The value of S02 was 0.885. The Mo–O distance, R, and DW factor, σ2, are 1.778 ± 0.002 and 0.0022 ± 0.0002 Å2, respectively, as shown in Table 1. The fitting of the glass was performed by using the S02 value. The coordination numbers of Mo, R, and σ2 were 3.9 ± 0.1, 1.778 ± 0.002 Å, and 0.0022 ± 0.0002 Å2, which are almost the same as that of La2Mo3O12. The coordination number of the glass certainly smaller than that of the crystal but they are almost the same within an error. It should be noted that there is almost no difference in the structural parameters N, R, and σ2 between the glass and crystal, which strongly indicates the similarity in the local structure of the Mo atom in the first coordination shell. Therefore, it is highly estimated that Mo in the glass forms regular tetrahedra with four nonbridging oxygens as in the case of La2Mo3O12.

Table 1. Parameters Used to Fit the Mo K-edge EXAFS Spectra of 25La2O3–75MoO3 Glass and Crystalline La2Mo3O12.

  N R (Å) σ2 ΔE (eV)
25La2O3–MoO3 glass 3.9(1) 1.778(2) 0.0022(2) 2.1(7)
crystalline La2Mo3O12 4.0a 1.778(2) 0.0022(2) 2.4(7)
Open in a new tab
a

Fixed value, S02 = 0.885.

Figure 5 shows La L3-edge XAFS spectra of 25La2O3–75MoO3 glass and crystalline La2Mo3O12. In contrast to Mo K-edge XAFS, there is a clear difference between the glass and the crystal. The peak height of the white line in the glass XANES spectrum is clearly smaller than that of the crystal, and the oscillation above 5.50 eV for the glass is not obvious compared to the crystal (Figure 5a). This is indicative of the disordered structure of the glass. Figure 5b shows EXAFS spectra of the glass and the crystal. The intensity of the glass is much smaller than the crystal; however, the oscillation periods are similar to each other, indicative of local environment similarity around the La atoms. Because the La L2-edge absorption is nearly located, there is a limited k range available for FT of the EXAFS spectra. Figure 5c shows the FT magnitudes of the glass and the crystal. Because of the limited range of k (3 Å–1 ≤ k ≤ 8.4 Å–1), it is difficult to estimate structural parameters by fitting. From the simple fitting of crystalline spectra with a fixed coordination number value of eight, the inelastic multielectronic loss, S02, is estimated to be 0.95, and the La–O distance R and DW factor σ2 are 2.48 ± 0.02 Å and 0.007 ± 0.001 Å2, respectively. By using the S02 value, the coordination number of La, R, and σ2 for the glass were 6.4 ± 0.1, 2.46 ± 0.01 Å, and 0.009 ± 0.002 Å2, respectively. The La–O bond length of the glass is slightly shorter than that of the crystal. The apparent small coordination number of glass might be due to disorder in the atomic arrangement and does not indicate that the number of oxygen atoms around La in the glass is smaller than that of the crystal. The disorder sometimes diminishes the longer side peak by averaging the distribution in amorphous materials. Accordingly, XAFS analysis using Mo K-edge and La L3-edge absorptions indicates structural similarity around Mo and La between the glass and crystal, although disorder is obvious, especially around La in the glass.

Figure 5.

Figure 5

Open in a new tab

La L3-edge XAFS spectra of 25La2O3–75MoO3 glass and crystalline La2Mo3O12. (a) XANES spectra, (b) k3-weighted EXAFS spectra, and (c) the FT magnitude of the EXAFS oscillation.

Further evidence of the structural similarity between the glass and the crystal is clearly shown in the vibration spectra. Figure 6 shows Raman scattering spectra of the glass and the crystal. It should be noted that the spectrum of glass clearly resembles the crystalline spectrum in peak position and peak height. It seems that broadening the spectrum of the crystal matches well with that of the glass. The agreement of the glass and the crystal strongly suggests that the local environment around the Mo atoms, such as the bonding distance to oxygen, the oxygen coordination number, and the oxygen–cation–oxygen bond angles, is almost the same. Detailed analyses of Raman bands of crystals including Mo have been reported in the literature.36−38 The bands at 880–970 cm–1 and 750–880 cm–1 are attributed to the Mo–O symmetric stretching mode and antisymmetric stretching modes in MoO4 tetrahedra, respectively. The band at 320 cm–1 is attributed to the O–Mo–O bending mode in MoO4 tetrahedra.

Figure 6.

Figure 6

Open in a new tab

Raman scattering spectra of 25La2O3–75MoO3 glass and crystalline La2Mo3O12.

Finally, the optical properties of the glass are shown in Figure 7. Figure 7a shows the transmittance spectra of the x = 75 glasses in the UV–vis region. Compared with the spectrum of the as-melted glass, the transparency of the glass annealed in the O2 gas flow was improved in the visible range. The UV absorption edge is approximately 378 nm, from which the optical bandgap energy is estimated to be 3.40 eV. The small absorption at 400–700 nm is obvious, which might be due to the d–d transition in Mo5+. The amount of Mo5+ should be very small because it was difficult to find evidence of a lower valence state of Mo in the Mo K-edge XANES spectra. The maximum transmittance was 80% at 1000 nm, which can be attributed to reflection on both sides of the surface during the measurement. Assuming that there is only surface reflection and no light scattering inside the glass, the refractive index, n, of the glass is estimated to be 2.0 using the equation Tmax = 1 – [2R′/(1 + R′)], where Tmax is the maximum transmittance and R′= [(n – 1)/(n + 1)]2. The estimated refractive index at 1000 nm is slightly larger than that of 20La2O3–80WO3 glass.19 Since the optical bandgap energy of the x = 75 glass is not so large, the wavelength dispersion of the refractive index will be large, which is not suitable for lenses in the visible region. Nevertheless, the La2O3–MoO3 glasses are possible candidates for optical applications using high refractive index in the near-infrared region.

Figure 7.

Figure 7

Open in a new tab

(A) UV–vis transmittance spectra of 25La2O3–75MoO3 glasses. Red line represents the glass annealed in O2 gas flow, while gray line represents the as-melted glass. (Inset) Tauc plot using the Kubelka–Munk function, F(R), obtained from diffuse scattering spectra of 25La2O3–75MoO3 glass and crystalline La2Mo3O12. The factor m is 1/2. (b) IR transmittance spectra of a 25La2O3–75MoO3 glass.

The optical absorption edge of the glass can be compared to that of the powdered crystalline phase using diffuse reflectance spectroscopy. The Kubelka–Munk function, F(R), is obtained from the reflectance spectra. The Tauc method equation is transformed to eq 1 by substituting F(R) for the absorption coefficient, α:

graphic file with name ic4c00176_m001.jpg 1

where h is the Planck constant, ν is the photon frequency, Eg is the optical band gap energy, and B is a constant. The factor m depends on the nature of the electron transition and is equal to 1/2 or 2 for direct or indirect transition band gaps, respectively. The inset of Figure 7a shows the Tauc plot with m = 1/2 by using the Kubelka–Munk function. The Tauc plot curve of the glass clearly shifts to the lower energy side compared to that of the crystalline sample. The x-axis intersection point of the linear fit of the Tauc plot gives an optical band gap energy. The estimated Eg of the glass is 3.46 eV, which is in good agreement with the value obtained from transmittance spectra. The Eg of the crystalline sample is 3.60 eV.39 The bandgap of the glass is slightly smaller than that of the crystal, which is reasonable considering the disorder in atomic arrangements characteristic to amorphous materials.

Figure 7b shows the infrared transmittance spectra of the glass. The glass was transparent to 5.5 μm, and the absorption at 2.9 μm is attributed to the presence of the O–H vibration. Compared with conventional oxide glasses, the O–H absorption of the glass is considerably small, indicating a scarcity of OH groups. Furthermore, there is the additional transmittance window in the range 6.5–8.0 μm. A transparent region at the wavelength side longer than the infrared absorption edge was also found in R-rich R2O3–B2O3 binary glasses (R is a rare earth element or Y).40−42 The additional transmittance in the R-rich borate glasses was explained by considering the local structure around the B atoms. B atoms in the glass form isolated BO3 units without any three-dimensional network structure. The simple environment around the B atoms resulted in the suppression of the overlap of the absorption bands of the B–O multiple vibrations and their overtones observed in conventional borate glasses. As a result, the absorption bands discretize, and then an additional transmittance window emerges. The local structure around the Mo atom in the 25La2O3–75MoO3 glass is confirmed to be the isolated MoO42– from XAFS and Raman scattering spectra, which is seen in crystalline La2Mo3O12. Therefore, the additional transmittance window at 6.5–8.0 μm can be explained by suppression of the overlap of the absorption bands of Mo–O multiple vibrations and their overtones because of the simple environment around Mo as in the case of crystalline La2Mo3O12.

Crystallization of the glass, XAFS and Raman scattering spectra, and the additional infrared transmittance as mentioned above strongly indicate that the atomic arrangement of the 25La2O3–75MoO3 glass is almost the same as that of the crystalline La2Mo3O12, even though structural disorder in the glass was confirmed in the La L3-edge XAFS and the absorption edge in the UV–vis region. MoO3 is classified as a conditional network former, and it has been reported that MoOn (n = 4, 5, or 6) formed a network in MoO3-based glasses.5,6 Contrary to previously reported glasses, there is no network in the La2O3–MoO3 glasses and Mo forms completely isolated MoO42–. Furthermore, the La2O3–MoO3 glasses are highly packed and comparable to those of the crystalline phase. The highly dense packed structure and the structural similarity of the crystal are unique and distictive in glass science; however, similar results have been reported recently in glasses prepared by a levitation technique.12,40 As suggested in the previous study, the structural similarity of the glass and the crystalline phases might enhance the glass-forming ability of the densely packed glass system. Assuming that R3+ and O2– are arranged in the nearly closest packed structure and small cations, such as Mo6+ ions, are inserted into interstitial sites (tetrahedral sites in the case of La2O3–MoO3 binary system) in the glass, the amorphous nature is produced by the slight displacement from the atomic arrangement of the crystalline phase. The small displacement enables a fall in metabasins in the free energy landscape43,44 and thus loses long-range order in the case of densely packed glass systems.45 In this case, glass formation becomes possible even when the three-dimensional regular tetrahedral networks are not developed.

Summary

Bulk glass formation of the La2O3–MoO3 binary system was achieved using the levitation technique. The glass-forming region was narrow at the vicinity of 25-mol % La2O3. The fabricated glasses were brownish but transparent in the visible region. The physical and structural properties of the 25La2O3–75MoO3 glass were investigated because competing crystalline phase of La2Mo3O12 exists at the same composition. The glass transition temperature was not detected due to minimal change in heat flow, while the crystallization temperature was 546 °C. After the sharp crystallization peak, a single phase of La2Mo3O12 crystallized. Mo L3-edge and K-edge XAFS and Raman scattering spectra revealed structural similarity between the glass and the crystal. The glass is formed by isolated MoO42– tetrahedra without any network formation. La L3-edge XAFS and optical bandgap in the UV–vis range indicate the disorder in the atomic arrangement of the glass. An additional infrared transparent window at 6.5–8.0 μm was explained by the simple environment of the local structure around Mo, which has been commonly observed in highly packed glasses prepared by the levitation technique. The estimated refractive index of 2.0 at 1.0 um and broad infrared transparency open up the possibility of optical applications in the infrared range. Furthermore, it was suggested that the glass formation of the La2O3–MoO3 system was caused by slight movement from the ordered La2Mo3O12 structure and, thus, it does not require a three-dimensional tetrahedral network.

Acknowledgments

The synchrotron radiation experiments were performed at KEK-PF (Proposal Nos. 2019G133, 2021G091, and 2023G040), and at SR Center of Ritsumeikan University (Proposal No. S22010). This study was supported in part by JSPS KAKENHI (Grant Nos. JP18K18928, JP19H05163, JP20H00397, JP20H02429, JP20H05880, JP20K20547, JP21K18800, and 23H05457), JST-AdCORP (No. JPMJKB2304), and the International Collaborative Research Program of Institute for Chemical Research, Kyoto University (Grant Nos. 2021-103 and 2022-110).

The authors declare no competing financial interest.

References

  1. Zachariasen W. H. The Atomic Arrangement in Glass. J. Am. Chem. Soc. 1932, 54 (10), 3841–3851. 10.1021/ja01349a006. [DOI] [Google Scholar]
  2. Sun K.-H. Fundamental Condition of Glass Formation. J. Am. Ceram. Soc. 1947, 30 (9), 277–281. 10.1111/j.1151-2916.1947.tb19654.x. [DOI] [Google Scholar]
  3. Rawson H.Inorganic Glass-Forming Systems; Academic Press, 1967. [Google Scholar]
  4. Nassau K.; Glass A. M.; Grasso M.; Olson D. H. Rapidly Quenched Tungstate and Molybdate Composition Containing Lithium: Glass Formation and Ionic Conductivity. J. Electrochem. Soc. 1980, 127 (12), 2743–2747. 10.1149/1.2129583. [DOI] [Google Scholar]
  5. Bhattacharya S.; Ghosh A. Electrical Properties of Ion Conducting Molybdate Glasses. J. Appl. Phys. 2006, 100 (11), 114119. 10.1063/1.2400116. [DOI] [Google Scholar]
  6. Hoppe U.; Schöps A.; Hannon A. C.; Ghosh A. Structure of Silver Molybdate Glasses by X-Ray and Neutron Diffraction. J. Non-Cryst. Solids 2021, 573, 121143. 10.1016/j.jnoncrysol.2021.121143. [DOI] [Google Scholar]
  7. Dimitriev Y.; Iordanova R. Non-Traditional Molybdate Glasses. Phys. Chem. Glasses 2009, 50 (2), 123–132. [Google Scholar]
  8. Dimitriev Y.; Iordanova R.; Aleksandrov L.; Kostov K. L. Boromolybdate Glasses Containing Rare Earth Oxides. Phys. Chem. Glasses 2008, 50 (3), 212–218. [Google Scholar]
  9. Aleksandrov L.; Iordanova R.; Dimitriev Y. B.; Handa K.; Ide J.; Milanova M. Glass Formation in the MoO3-La2O3-Nd2O3 System. Adv. Mater. Res. 2008, 39–40, 37–40. 10.4028/www.scientific.net/AMR.39-40.37. [DOI] [Google Scholar]
  10. Weber J. K. R. The Containerless Synthesis of Glass. Int. J. Appl. Glass Sci. 2010, 1 (3), 248–256. 10.1111/j.2041-1294.2010.00026.x. [DOI] [Google Scholar]
  11. Benmore C. J.; Weber J. K. R. Aerodynamic Levitation Supercooled Liquids and Glass Formation. Adv. Phys.: X 2017, 2 (3), 717–736. 10.1080/23746149.2017.1357498. [DOI] [Google Scholar]
  12. Masuno A. Structure of Densely Packed Oxide Glasses Prepared Using a Levitation Technique. J. Phys. Soc. Jpn. 2022, 91 (9), 091003. 10.7566/JPSJ.91.091003. [DOI] [Google Scholar]
  13. Masuno A. Functionalities in Unconventional Oxide Glasses Prepared Using a Levitation Technique. J. Ceram. Soc. Jpn. 2022, 130 (8), 563–574. 10.2109/jcersj2.22073. [DOI] [Google Scholar]
  14. Watanabe Y.; Masuno A.; Inoue H. Glass Formation of Rare Earth Aluminates by Containerless Processing. J. Non-Cryst. Solids 2012, 358 (24), 3563–3566. 10.1016/j.jnoncrysol.2012.02.001. [DOI] [Google Scholar]
  15. Rosales-Sosa G. A.; Masuno A.; Higo Y.; Watanabe Y.; Inoue H. Effect of Rare-earth Ion Size on Elasticity and Crack Initiation in Rare-earth Aluminate Glasses. J. Am. Ceram. Soc. 2018, 101 (11), 5030–5036. 10.1111/jace.15760. [DOI] [Google Scholar]
  16. Yoshimoto K.; Masuno A.; Ueda M.; Inoue H.; Yamamoto H.; Kawashima T. Low Phonon Energies and Wideband Optical Windows of La2O3-Ga2O3 Glasses Prepared Using an Aerodynamic Levitation Technique. Sci. Rep. 2017, 7 (1), 45600. 10.1038/srep45600. [DOI] [PMC free article] [PubMed] [Google Scholar]
  17. Yoshimoto K.; Ezura Y.; Ueda M.; Masuno A.; Inoue H. 2.7 Μm Mid-Infrared Emission in Highly Erbium-Doped Lanthanum Gallate Glasses Prepared Via an Aerodynamic Levitation Technique. Adv. Opt. Mater. 2018, 6 (8), 1701283. 10.1002/adom.201701283. [DOI] [Google Scholar]
  18. Yoshimoto K.; Ezura Y.; Ueda M.; Masuno A.; Inoue H.; Mizuguchi M. Fluorescence Characterization of Heavily Eu 3+ -Doped Lanthanum Gallate Glass Spheres with High Quenching Concentration. Opt. Lett. 2019, 44 (4), 875–878. 10.1364/OL.44.000875. [DOI] [PubMed] [Google Scholar]
  19. Yoshimoto K.; Masuno A.; Inoue H.; Watanabe Y. Transparent and High Refractive Index La2O3–WO3 Glass Prepared Using Containerless Processing. J. Am. Ceram. Soc. 2012, 95 (11), 3501–3504. 10.1111/j.1551-2916.2012.05439.x. [DOI] [Google Scholar]
  20. Nakanishi K.; Yagi S.; Ohta T. XAFS Measurements under Atmospheric Pressure in the Soft X-ray Region. AIP Conf. Proc. 2010, 1234, 931. 10.1063/1.3463369. [DOI] [Google Scholar]
  21. Nomura M.; Koyama A. Design of an XAFS Beamline at the Photon Factory: Possibilities of Bent Conical Mirrors. J. Synchrotron Rad. 1999, 6 (PT 3), 182–184. 10.1107/S0909049598016823. [DOI] [PubMed] [Google Scholar]
  22. Nomura M.; Koyama A. Performance of a Beamline with a Pair of Bent Conical Mirrors. Nucl. Instr. Methods Phys. Res. A 2001, 467–468, 733–736. 10.1016/S0168-9002(01)00482-X. [DOI] [Google Scholar]
  23. Konishi H.; Yokoya A.; Shiwaku H.; Motohashi H.; Makita T.; Kashihara Y.; Hashimoto S.; Harami T.; Sasaki T. A.; Maeta H.; Ohno H.; Maezawa H.; Asaoka S.; Kanaya N.; Ito K.; Usami N; Kobayashi K. Synchrotron Radiation Beamline to Study Radioactive Materials at the Photon Factory. Nucl. Instr. Methods Phys. Res. A 1996, 372 (1–2), 322–332. 10.1016/0168-9002(95)01241-9. [DOI] [Google Scholar]
  24. Ravel B.; Newville M. ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy Using IFEFFIT. J. Synchrotron Rad. 2005, 12 (4), 537–541. 10.1107/S0909049505012719. [DOI] [PubMed] [Google Scholar]
  25. Rode E. Y.; Lysanova G. V.; Gokhman L. Z. Phase Diagrams of the Systems of Rare Earths Oxides with Molybdenum Trioxide. Izv. Akad. Nauk SSSR, Neorg. Mater. 1971, 7, 2101–2103. [Google Scholar]
  26. Jeitschko W. Crystal Structure of La2(MoO4)3, a New Ordered Defect Scheelite Type. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1973, 29 (10), 2074–2081. 10.1107/S0567740873006138. [DOI] [Google Scholar]
  27. Momma K.; Izumi F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44 (6), 1272–1276. 10.1107/S0021889811038970. [DOI] [Google Scholar]
  28. Shannon R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr. A 1976, 32 (5), 751–767. 10.1107/S0567739476001551. [DOI] [Google Scholar]
  29. Masuno A.; Inoue H.; Yoshimoto K.; Watanabe Y. Thermal and Optical Properties of La2O3-Nb2O5 High Refractive Index Glasses. Opt. Mater. Express 2014, 4 (4), 710–718. 10.1364/OME.4.000710. [DOI] [Google Scholar]
  30. Farges F.; Brown G. E.; Petit P.-E.; Munoz M. Transition Elements in Water-Bearing Silicate Glasses/Melts. Part I. a High-Resolution and Anharmonic Analysis of Ni Coordination Environments in Crystals, Glasses, and Melts. Geochim. Cosmochim. Acta 2001, 65 (10), 1665–1678. 10.1016/S0016-7037(00)00625-6. [DOI] [Google Scholar]
  31. Farges F. Ab Initio and Experimental Pre-Edge Investigations of the Mn K -Edge XANES in Oxide-Type Materials. Phys. Rev. B 2005, 71 (15), 155109. 10.1103/PhysRevB.71.155109. [DOI] [Google Scholar]
  32. Chalmin E.; Farges F.; Brown G. E. A Pre-Edge Analysis of Mn K-Edge XANES Spectra to Help Determine the Speciation of Manganese in Minerals and Glasses. Contrib. Mineral. Petrol. 2009, 157 (1), 111–126. 10.1007/s00410-008-0323-z. [DOI] [Google Scholar]
  33. Farges F.; Siewert R.; Brown G. E. Jr.; Guesdon A.; Morin G. Structural Environments Around Molybdenum in Silicate Glasses and Melts. I. Influence of Composition and Oxygen Fugacity on the Local Structure of Molybdenum. Can. Mineral. 2006, 44 (3), 731–753. 10.2113/gscanmin.44.3.731. [DOI] [Google Scholar]
  34. Aritani H.; Tanaka T.; Funabiki T.; Yoshida S.; Eda K.; Sotani N.; Kudo M.; Hasegawa S. Study of the Local Structure of Molybdenum–Magnesium Binary Oxides by Means of Mo L3-Edge XANES and UV–Vis Spectroscopy. J. Phys. Chem. 1996, 100 (50), 19495–19501. 10.1021/jp9615464. [DOI] [Google Scholar]
  35. Rocca F.; Kuzmin A.; Mustarelli P.; Tomasi C.; Magistris A. XANES and EXAFS at Mo K-Edge in (AgI)1–x(Ag2MoO4)x Glasses and Crystals. Solid State Ionics 1999, 121 (1–4), 189–192. 10.1016/S0167-2738(98)00546-3. [DOI] [Google Scholar]
  36. Ait Ahsaine H.; Zbair M.; Ezahri M.; Benlhachemi A.; Bakiz B.; Guinneton F.; Gavarri J.-R. Structural and Temperature-Dependent Vibrational Analyses of the Non-Centrosymmetric ZnMoO4 Molybdate. J. Mater. Environ. Sci. 2016, 7 (9), 3076–3083. [Google Scholar]
  37. McKeown D. A.; Gan H.; Pegg I. L. X-Ray Absorption and Raman Spectroscopy Studies of Molybdenum Environments in Borosilicate Waste Glasses. J. Nucl. Mater. 2017, 488, 143–149. 10.1016/j.jnucmat.2017.03.012. [DOI] [Google Scholar]
  38. Stenzel D.; Issac I.; Wang K.; Azmi R.; Singh R.; Jeong J.; Najib S.; Bhattacharya S. S.; Hahn H.; Brezesinski T.; Schweidler S.; Breitung B. High Entropy and Low Symmetry: Triclinic High-Entropy Molybdates. Inorg. Chem. 2021, 60 (1), 115–123. 10.1021/acs.inorgchem.0c02501. [DOI] [PubMed] [Google Scholar]
  39. Wang Q.; Liao M.; Lin Q.; Xiong M.; Zhang X.; Dong H.; Lin Z.; Wen M.; Zhu D.; Mu Z.; Wu F. The Design of Dual-Switch Fluorescence Intensity Ratio Thermometry with High Sensitivity and Thermochromism Based on a Combination Strategy of Intervalence Charge Transfer and up-Conversion Fluorescence Thermal Enhancement. Dalton Trans. 2021, 50 (26), 9298–9309. 10.1039/D1DT00882J. [DOI] [PubMed] [Google Scholar]
  40. Masuno A.; Iwata T.; Yanaba Y.; Sasaki S.; Inoue H.; Watanabe Y. High Refractive Index La-Rich Lanthanum Borate Glasses Composed of Isolated BO3 Units. Dalton Trans. 2019, 48 (29), 10804–10811. 10.1039/C9DT01715A. [DOI] [PubMed] [Google Scholar]
  41. Sasaki S.; Masuno A.; Ohara K.; Yanaba Y.; Inoue H.; Watanabe Y.; Kohara S. Structural Origin of Additional Infrared Transparency and Enhanced Glass-Forming Ability in Rare-Earth-Rich Borate Glasses without B–O Networks. Inorg. Chem. 2020, 59 (19), 13942–13951. 10.1021/acs.inorgchem.0c01567. [DOI] [PubMed] [Google Scholar]
  42. Sasaki S.; Masuno A. Composition dependence of the local structure and transparency of Gd2O3–B2O3 binary glasses prepared via aerodynamic levitation. J. Ceram. Soc. Jpn. 2022, 130 (1), 60–64. 10.2109/jcersj2.21124. [DOI] [Google Scholar]
  43. Greaves G. N.; Sen S. Inorganic Glasses, Glass-Forming Liquids and Amorphizing Solids. Adv. Phys. 2007, 56 (1), 1–166. 10.1080/00018730601147426. [DOI] [Google Scholar]
  44. Pérez-Castañeda T.; Jiménez-Riobóo R. J.; Ramos M. A. Two-Level Systems and Boson Peak Remain Stable in 110-Million-Year-Old Amber Glass. Phys. Rev. Lett. 2014, 112 (16), 165901. 10.1103/PhysRevLett.112.165901. [DOI] [PubMed] [Google Scholar]
  45. Eguchi T.; Inoue H.; Masuno A.; Kita K.; Utsuno F. Oxygen Close-Packed Structure in Amorphous Indium Zinc Oxide Thin Films. Inorg. Chem. 2010, 49 (18), 8298–8304. 10.1021/ic1006617. [DOI] [PubMed] [Google Scholar]

Articles from Inorganic Chemistry are provided here courtesy of American Chemical Society

RESOURCES