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. 2016 Nov 18;1(5):963–970. doi: 10.1021/acsomega.6b00274

Crystal Structure, Infrared Spectra, and Microwave Dielectric Properties of Temperature-Stable Zircon-Type (Y,Bi)VO4 Solid-Solution Ceramics

Di Zhou †,‡,*, Jing Li †, Li-Xia Pang ‡,§, Guo-Hua Chen ∥, Ze-Ming Qi ⊥, Da-Wei Wang ‡, Ian M Reaney ‡,*
PMCID: PMC6640743  PMID: 31457176

Abstract

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A series of (Bi1–xYx)VO4 (0.4 ≤ x ≤ 1.0) ceramics were synthesized using the traditional solid-state reaction method. In the composition range of 0.4 ≤ x ≤ 1.0, a zircon-type solid solution was formed between 900 and 1550 °C. Combined with our previous work (scheelite monoclinic and zircon-type phases coexist in the range of x < 0.40), a pseudobinary phase diagram of BiVO4–YVO4 is presented. As x decreased from 1.0 to 0.40, the microwave permittivity (εr) of (Bi1–xYx)VO4 ceramics increased linearly from 11.03 to 30.9, coincident with an increase in the temperature coefficient of resonant frequency (TCF) from −61.3 to +103 ppm/°C. Excellent microwave dielectric properties were obtained for (Bi0.3Y0.7)VO4 sintered at 1025 °C and (Bi0.2Y0.8)VO4 sintered at 1075 °C with εr ∼ 19.35, microwave quality factor (Qf) ∼ 25 760 GHz, and TCF ∼ +17.8 ppm/°C and εr ∼ 16.3, Qf ∼ 31 100 GHz, and TCF ∼ −11.9 ppm/°C, respectively. Raman spectra, Shannon’s additive rule, a classical oscillator model, and far-infrared spectra were employed to study the structure–property relations in detail. All evidence supported the premise that Bi-based vibrations dominate the dielectric permittivity in the microwave region.

Introduction

Microwave dielectric ceramics have played an important role in modern wireless communication systems such as dielectric resonators, filters, capacitors, and duplexers. To meet the requirement of miniaturization and reliability of microwave devices, large dielectric permittivity (εr), large microwave quality factor (Qf), and near-zero temperature coefficient of resonant frequency (TCF) must be achieved.1−4

Many classic microwave dielectric ceramics based on ABO3 perovskite structure have been explored, and the structure–property relation has been extensively studied.5,6 Recently, many complex oxides with a classic ABO4-type structure, such as scheelite (CaMoO4, BiVO4),7,8 Fergusonite (LaNbO4, NdNbO4),9 stibiotantalite (BiNbO4, BiTaO4),10,11 wolframite (ZnWO4, MgWO4),12 zircon (CeVO4),13 and rutile ((Zn,Nb)TiO4, (Cu,Nb)TiO4)14,15 structures, have been reported to have good microwave dielectric properties. Zircon-type (I41/amd) vanadates (AVO4, A = Sc3+, Ln3+, Bi3+) are a subset of ABO4-type compounds and were widely investigated due to their potential optical, magnetic, elastic, and electric properties.4,13 The zircon structure is composed of alternating [VO4] tetrahedra and edge-sharing [AO8] dodecahedra, forming chains parallel to the c axis. Two [AO8] dodecahedra are arranged along the a, b, and c axes, which results in a linear increase in cell volumes with A-site ionic radius.4,16 As reported by Watanabe and Zuo,13,17 LaVO4 prefers to crystallize in a monoclinic monazite-type structure with a space group P21/n. The microwave dielectric properties of zircon-type CeVO4 ceramics were first reported by Wang et al.13 with a εr ∼ 12.3, Qf ∼ 41 460 GHz, and TCF ∼ −34.4 at a sintering temperature of ∼950 °C. It can be deduced that other rare-earth orthovanadates, such as YVO4, SmVO4, and NdVO4, may also possess similar microwave dielectric properties. However, their large negative TCF values must be modified to near zero. There are two traditional methods to modify the TCF value of microwave dielectric ceramics to near zero, which are broadly separated into a composite and a solid-solution approach.18 In our previous work, a temperature-stable microwave dielectric composite ceramic, which contains both scheelite BiVO4 and zircon-type (Bi0.6Y0.4)VO4 phases with a εr = 45, Qf ∼ 14 000 GHz, and TCF = +10 ppm/°C, was achieved.4 The end member, (Bi0.6Y0.4)VO4, takes on the zircon structure with a positive TCF (+32 ppm/°C). This result inspired the design of a zircon-type solid-solution ceramic in the (Bi,Y)VO4 system with a near-zero TCF. Undoped BiVO4 ceramic prepared via the solid-state reaction method crystallizes in a monoclinic scheelite structure, not a zircon structure. As reported by Watanabe,17 the solid solubility of the Bi3+ ion in zircon-structured (BixCe1–x)VO4 ceramics is ≥60%, as confirmed in our previous work,19 which is attributed to the presence of the zircon-type BiVO4 synthesized by a precipitation method.20,21 Undoped zircon-type BiVO4 is not stable and irreversibly transforms into a monoclinic scheelite structure at >300 °C,21 and it is plausible that the substitution of Bi for Y in YVO4 stabilizes the zircon-type lattice. In the present work, a comprehensive study on the phase assemblage and evolution in the (Bi,Y)VO4 system was performed on the zircon-type solid-solution region and its associated sintering behavior, microstructure, and microwave dielectric properties were studied in detail.

Results and Discussion

The X-ray diffraction (XRD) patterns of the (Bi1–xYx)VO4 ceramics calcined and sintered at their optimal temperatures are presented in Figure 1a. All peaks were indexed as a zircon-type phase for x ≥ 0.4. As Bi concentration increased (decrease of x), all diffraction peaks shifted to the lower 2θ, commensurate with an increase in the cell volume, which is attributed to the larger ionic radius of Bi3+ (1.17 Å) compared with that of Y3+ (1.019 Å). The cell parameters of (Bi1–xYx)VO4 (x ≥ 0.4) ceramics as a function of x value are presented in Figure 1b. a and c parameters increased from 7.116 to 7.225 Å and 6.287 to 7.394 Å, respectively, as x decreased from 1.0 to 0.4, commensurate with an increase in the cell volume from 318.35 to 333.79 Å3. As shown in Figure 1c, the refined values of lattice parameters for the (Bi0.3Y0.7)VO4 sample are a = b = 7.163(7) Å and c = 6.336(2) Å with a space group I41/amd (141). All of the atomic fractional coordinates are listed in Table 1. The goodness of fit of refinement, which is defined as the ratio of Rwp to Rexp, is ∼3.15. The V–O distance in the zircon-type (Bi0.3Y0.7)VO4 sample is 1.7005 Å, similar to that in YVO4 (1.7058 Å). The long and short (Bi,Y)–O bond distances are 2.3301 and 2.4520 Å, respectively, which are larger than those in YVO4 (2.2986 and 2.4434 Å, respectively),22 consistent with the larger ionic radius of Bi3+ than that of Y3+. Combined with our previous work,4 the schematic phase diagram of the (1 – x)BiVO4–xYVO4 binary system is presented in Figure 1d. In the region x ≥ 0.4, a zircon-type solid-solution phase field was formed. In the region x < 0.4, both monoclinic BiVO4 and zircon (Bi0.6Y0.4)VO4 coexisted with stable cell parameters.

Figure 1.

Figure 1

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XRD patterns of (Bi1–xYx)VO4 ceramics calcined and sintered at different temperatures (a), cell parameters as a function of the x value (b), and experimental (circles) and calculated (line) X-ray powder diffraction profiles of the (Bi0.3Y0.7)VO4 composition sintered at 960 °C for 2 h at room temperature (c) (Rp = 12.7%, Rwp = 12.9%, Rexp = 7.27%). The short vertical lines below the patterns mark the positions of Bragg reflections. The bottom continuous line is the difference between the observed and the calculated intensity and the schematic phase diagram of the BiVO4–YVO4 system (d).

Table 1. Refined Atomic Fractional Coordinates from XRD Data for the (Bi0.3Y0.7)VO4 Samplea.

atom site occ. x y z biso.
Y 4a 0.08750 0.00000 0.75000 0.12500 0.27426
Bi 4a 0.03750 0.00000 0.75000 0.12500 0.27426
V 4b 0.12500 0.00000 0.25000 0.37500 0.14327
O 16h 0.50000 0.00000 0.06795 0.20273 0.12035
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a

The lattice parameters at room temperature are a = b = 7.163(7) Å and c = 6.336(2) Å with space group I41/amd (141).

Undoped YVO4 belongs to the D4h19 crystal structure (I41/amd), and the theoretical group analysis gives an irreducible representation of the vibrational modes after Miller et al.23 and Vali24

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The 2A1g, B1g, B2g, and 5Eg modes are the Raman-active optical modes, whereas the 3A2u and 4Eu modes are infrared-active. In addition, there are two further inactive translational modes A2u + Eu. As seen from Figure 2a, the main Raman modes created by stretching and bending of [VO4] tetrahedra at ∼890, 378, 815, and 488 cm–1 are assigned to A1g(ν1), A1g(ν2), B2g(ν3), and B2g(ν4), respectively. The weak mode at 837 cm–1, which is close to the B2g(ν3) mode, is assigned to Eg. The peaks at 260 cm–1 and overlapping at about 160 cm–1 are external modes caused by translation. As seen from Figure 2b, the main Raman modes shift to the lower wavenumber as x decreases, along with broadening and a decrease in peak intensity, due to the A-site disorder in the solid solution. According to Hardcastle and Wachs’ study on the relation (ν = 21349 e(−1.9176×R),25 in which ν is the Raman shift and R is the bond length of V–O) between the Raman shift and the V–O bond length, the decrease in the A1g(ν1) Raman mode (stretching mode of [VO4], Figure 2b) is caused by the increase in the V–O bond length.

Figure 2.

Figure 2

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Raman spectra of (Bi1–xYx)VO4 (x ≥ 0.4) ceramics (a) and as a function of the x value (b).

Scanning electron microscopy (SEM) images of the (Bi1–xYx)VO4 ceramics are shown in Figure 3. For pure YVO4 (x = 1.0), the densification temperature is above 1550 °C, and many pores are observed in ceramics sintered at 1550 °C for 2 h. As seen from Figure 3a–c, substitution of Bi for Y effectively lowered the sintering temperature and induced a homogeneous dense microstructure in (Bi1–xYx)VO4 ceramics with x = 0.7, 0.8, and 0.9. As x decreased from 1.0 to 0.7, the sintering temperatures of (Bi1–xYx)VO4 ceramics lowered from 1550 to 1025 °C along with a decrease in the grain size from ∼5 to ∼1 μm.

Figure 3.

Figure 3

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SEM images of (Bi1–xYx)VO4 ceramics sintered at different temperatures: x = 0.70 at 1025 °C (a), 0.80 at 1075 °C (b), 0.90 at 1225 °C (c), and 1.00 at 1550 °C (d).

The microwave εr and Qf of the (Bi1–xYx)VO4 ceramics as a function of sintering temperature are shown in Figure 4. The εr of (Bi1–xYx)VO4 ceramics increased with sintering temperature due to the elimination of pores before reaching a stable value. The Qf increased with sintering temperature, reaching a maximum value before decreasing sharply for all samples except for x = 1.0. The Qf is composed of intrinsic and extrinsic contributions and affected by grain size, grain boundaries, and pores. For (Bi1–xYx)VO4 ceramics, high Qf values were attained only in a relatively narrow sintering temperature range. Optimal microwave properties of (Bi1–xYx)VO4 ceramics are listed in Table 2, in which εr decreases linearly from 30.9 at x = 0.4 to 11.1 at x = 1.0 in accordance with Shannon’s additive rule. Shannon26 suggested that polarizabilities of oxides may be estimated by summing the polarizabilities of constituent ions. The polarizabilities, αx, of (Bi1–xYx)VO4 may be calculated as follows

graphic file with name ao-2016-00274w_m002.jpg 2

where αBi3+, αY3+, αV5+, and αO2– are the polarizabilities of Bi3+, Y3+, V5+, and O2–, respectively. Because of smaller polarizabilities of Y3+ relative to those of Bi3+ (αBi3+ = 6.123 Å > αY3+ = 3.81 Å3),26 there should be a linear decrease in (Bi1–xYx)VO4 as x increases. Using the Clausius–Mossotti relation, which relates εx, αx, and molar cell volume Vx, εr may be calculated

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The values of molecular αx calculated from eqs 2 and 3 along with the macroscopic microwave dielectric properties are listed in Table 2. As x decreases, the relative deviation between αcal. and αmeas. increases. Employing eq 3, it can be obtained that αBi3+ – αY3+ = (αx2 – αx1)/(x1 – x2) ≈ 5.8 Å3. This result is greater than that reported by Shannon (2.31 Å3), which suggests that within its coordination in the zircon-type structure, Bi3+ contributes much more than Y3+ to the dielectric polarizability, compared with that in higher-symmetry systems.

Figure 4.

Figure 4

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Microwave dielectric permittivity (a) and Qf values (b) of (Bi1–xYx)VO4 ceramics as a function of sintering temperature.

Table 2. Densification Temperature, Microwave Dielectric Properties, and Molecular Polarizability of the (Bi1–xYx)VO4 Ceramics.

x S.T. (°C) εr Qf (GH) TCF (ppm/°C) V (Å3) αcal. (Å3) αmeas. (Å3)
0.4 975 30.9 15 570 +103 333.79 16.16 18.11
0.65 995 20.5 16 380 +32 325.43 15.58 16.83
0.7 1025 19.35 25 760 +17.8 324.55 15.46 16.65
0.8 1075 16.3 30 100 –11.9 322.76 15.23 16.11
0.9 1225 13.49 40 720 –31.2 319.96 15.00 15.40
1.0 1550 11.03 28 600 –61.3 318.35 14.77 14.63
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The highest Qf (∼40 700 GHz) was obtained for (Bi0.1Y0.9)VO4, but as x decreased to 0.65 and 0.40, the Qf decreased linearly to 16 000 GHz. The intrinsic dielectric loss determines the upper limit of Qf and is quantitatively described by the classical one-phonon damped oscillator model as follows27

graphic file with name ao-2016-00274w_m004.jpg 4

where ε*(ω) is the complex permittivity, ε(∞) is the electronic part of the static permittivity, εT is the transverse frequency of the polar phonon mode, γ is the damping parameter, z is the equivalent electric charge number, e is the electric charge of an electron, m is the equivalent atom weight, and V is the unit volume. In the microwave region, considering ω2 ≪ ωT2, the relationship between the Qf and εr is obtained as follows

graphic file with name ao-2016-00274w_m005.jpg 5

The above relation was successfully used to explain the reciprocal relationship between the εr and Qf in scheelite solid solutions.28 The same qualitative relationship holds for zircon-type solid solutions in (Bi1–xYx)VO4 ceramics. The decrease in Qf for undoped YVO4 is attributed to the absence of a sintering temperature, at which the density of the end member compound may be optimized. The TCF of undoped YVO4 is ∼−61.3 ppm/°C. As x decreased to 0.40, the TCF linearly increased to +103 ppm/°C. In contrast, +17.8 and −11.9 ppm/°C were obtained in the (Bi0.3Y0.7)VO4 and (Bi0.2Y0.8)VO4 ceramics, respectively, suggesting that zero TCF may be achieved for 0.7 < x < 0.8. The TCF is usually defined as follows

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where αl is the thermal expansion coefficient and τε is the temperature coefficient of dielectric constant. Usually αl of microwave dielectric is <+20 ppm/°C and independent of x. Hence, the TCF value mainly depends on τε. Using the Clausius–Mosotti relation, Bosman and Havinga29 derived an expression for τε at constant pressure as follows

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The sum of the A and B terms is approximately 6 ppm/°C. Term C usually lies between −1 and ∼−10 ppm/°C and represents the direct dependence of the polarizability on temperature. Figure 5a shows a plot of the temperature dependence of permittivity (αl was assumed to be +20 ppm/°C) as a function of (ε – 1)(ε + 2)/ε for all of the (Bi1–xYx)VO4 ceramics to extrapolate the (A + B + C) value ∼ −16.1 ppm/°C, which is within the acceptable range. Using this relation, any TCF value can be achieved by precisely adjusting the composition. Furthermore, it is similar to the empirical logarithmic rule (ln ε = y1 ln ε1 + y2 ln ε2/TCF = y1 ln τf1 + y2 ln τf2)30 suitable for composites. The temperature dependence of the εr and Qf of the (Bi0.3Y0.7)VO4 and (Bi0.2Y0.8)VO4 ceramics in the temperature range 25–130 °C is shown in Figure 5b. εr is stable at ∼19 and 16 for the (Bi0.3Y0.7)VO4 and (Bi0.2Y0.8)VO4 ceramics, respectively. Qf decreased linearly with temperature from 28 000 to ∼20 000 GHz, confirming that the dielectric loss increased with temperature. Taking into account the above arguments, temperature-stable microwave dielectric ceramics may be obtained for 0.7 < x < 0.8 with 16 ≤ εr ≤ 19 and Qf > 20 000 GHz between 25 and 130 °C. The sintering temperature of this system (below 1050 °C) is much lower than that of SrLnAlO4 (Ln = Nd and Sm) microwave dielectric ceramics31 (1475–1500 °C) with similar properties (permittivity 17.8–18.8, Qf values 25 700–54 880 GHz, and TCF values −9 ∼ +2 ppm/°C), and this can save a lot of energy during processing.

Figure 5.

Figure 5

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τε (temperature dependence of permittivity) as a function of (ε – 1)(ε + 2)/ε (a) and permittivity and Qf as a function of temperature (b) for the (Bi0.3Y0.7)VO4 and (Bi0.2Y0.8)VO4 ceramics.

Far-infrared reflectivity is a useful tool to investigate intrinsic dielectric properties of microwave ceramics. As discussed above, there are seven infrared-active modes including three A2u and four Eu modes. In Vali et al.’s work,24 a perpendicular electric field of the incident light to the c axis of YVO4 single crystal was employed to obtain the infrared reflectivity and only four Eu modes, at 195, 263, 309, and 780 cm–1, were observed. As shown in Figure 6, besides the four Eu modes, two other modes at 439 and 916 cm–1, assigned to A2u modes, were also observed to contribute to the infrared reflectivity spectra of the YVO4 ceramic sample. The high wavenumber peak at 916 cm–1 belongs to the stretching mode of [VO4], which is similar to that at 977 cm–1 caused by the Si–O stretching in ZrSiO4 material.31,32 As x decreased to 0.8, four new modes are observed, at 69, 136, 233, and 749 cm–1. These new modes are also assigned to Eu modes caused by the partial occupation by Bi on the A site of the zircon structure. The external Eu modes below 450 cm–1 are conventionally caused by translation and rotation between A-site ions and [VO4] tetrahedra, as shown in Figure 6b. Because of the different relative masses of Bi and Y atoms, the translation and rotation strengths are different and lead to different Raman shifts; consequently, the strength of the Eu modes increased with Bi concentration.

Figure 6.

Figure 6

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Far-infrared reflectivity of (Bi1–xYx)VO4 ceramics (a) and schematic of external Eu modes caused by translation and rotation (b).

Using eq 4, the relation between complex reflectivity R(ω) and permittivity can be obtained as follows

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The fitting of infrared reflectivity was employed to study the intrinsic dielectric contribution from each vibrational mode. Infrared reflectivity spectra of (Bi0.3Y0.7)VO4 were fitted using eight modes as shown in Figure 7a, and the related parameters are listed in Table 3. Among the eight modes, Nos. 1, 2, 4, and 6 belong to Bi-based vibrations, and their total contribution to the microwave permittivity was ∼8, whereas all others were attributed to the Y-based and [VO4] tetrahedral vibrations (value about ∼7.2). The optical permittivity is ∼4.41. This result indicates that Bi-based vibrations mainly contribute to permittivity in the microwave region, in accordance with the analysis using Shannon’s additive rule. The measured and fitted real and imaginary parts of permittivity are plotted in Figure 7b. The measured part corresponded well with the fitted values, confirming that in the microwave region the dielectric polarization is mainly caused by phonon absorption in the infrared region.

Figure 7.

Figure 7

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Measured and calculated infrared reflectivity spectra (a) (solid line for fitting values and circle for measured values) and fitted complex dielectric spectra (b) of the (Bi0.3Y0.7)VO4 ceramic (the circles represent the experimental values in the microwave region).

Table 3. Phonon Parameters Obtained from the Fitting of the Infrared Reflectivity Spectra of the (Bi0.3Y0.7)VO4 Ceramica.

mode ωoj ωpj γj Δεj
1 86.97 170.99 32.31 3.77
2 142.85 245.14 45.68 2.74
3 187.97 444.15 26.78 5.38
4 229.35 266.24 39.57 1.25
5 256.07 168.85 15.32 0.44
6 750.00 365.67 54.21 0.24
7 803.83 967.63 93.02 1.35
8 871.76 154.23 32.06 0.03
  ε∞ = 4.41 ε0 = 19.61
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a

Bold type represents the modes coming from Bi-based vibrations.

Conclusions

A zircon-type solid solution is formed in (Bi1–xYx)VO4 (0.4 ≤ x ≤ 1.0) ceramics. Substitution of Bi effectively lowered sintering temperatures from above 1550 °C for undoped YVO4 to ∼900 °C for (Bi0.6Y0.4)VO4. As x decreased from 1.0 to 0.4, cell parameters increased linearly, along with the increase of εr from 11.03 to 30.9 accompanied by a shift of TCF value from −61.3 to +103 ppm/°C. Excellent microwave dielectric properties with a εr ∼ 19.35, Qf ∼ 25 760 GHz, and TCF ∼ +17.8 ppm/°C were obtained in (Bi0.3Y0.7)VO4 ceramic sintered at 1025 °C. For (Bi0.2Y0.8)VO4 ceramic sintered at 1075 °C, εr ∼ 16.3, Qf ∼ 31 100 GHz, and TCF ∼ −11.9 ppm/°C were obtained. It is demonstrated that temperature-stable microwave dielectric ceramics can be obtained for compositions with 0.7 < x < 0.8. Therefore, we conclude, with reference to our previous work,4 that there are two methods to design temperature-stable microwave dielectric ceramics in the (Bi1–xYx)VO4 system for gigahertz frequency applications: (1) the fabrication of composites of scheelite and zircon phases and (2) the formation of zircon-structured solid solution with 0.7 < x < 0.8.

Experimental Section

The (Bi1–xYx)VO4 (x = 0.4, 0.65, 0.7, 0.8, 0.9, and 1.0) (abbreviated BYVx) samples were prepared via the solid-state reaction method as described in our previous work.2,3 Samples were sintered at temperatures from 850 to 1550 °C for 2 h.

XRD was performed with Cu Kα radiation (Rigaku D/MAX-2400 X-ray diffractometry, Tokyo, Japan) using a powder sample. The diffraction pattern was collected over 5–65° (2θ) at a step size of 0.02°. The Rietveld profile refinement method was employed to analyze the data using the FULLPROF program. As-fired surfaces were observed by SEM (FEI; Quanta 250 F). Raman spectra were performed with a Raman spectrometer (inVia; Renishaw, England), excited by an Ar+ laser (514.5 nm). Infrared reflectivity spectra were measured using a Bruker IFS 66v FT-IR spectrometer on the infrared beamline station (U4) at the National Synchrotron Radiation Lab. (NSRL), China. Microwave dielectric properties were measured using the TE01δ method with a network analyzer (HP 8720 Network Analyzer; Hewlett-Packard) and a temperature chamber (Delta 9023; Delta Design, Poway, CA). The temperature coefficient of resonant frequency TCF (τf) was calculated with the following formula

graphic file with name ao-2016-00274w_m010.jpg 9

where fT and fT0 are the TE01δ resonant frequencies at temperatures T and T0, respectively.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (U1632146), the Young Star Project of Science and Technology of Shaanxi Province (2016YFXX0029), the Fundamental Research Funds for the Central University, Guangxi Key Laboratory of Information Materials (Guilin University of Electronic Technology), P.R. China (Project No. 161004-K), and the 111 Project of China (B14040). The authors would like to thank the administrators of the IR beamline workstation of the National Synchrotron Radiation Laboratory (NSRL) for their help. The SEM work was done at the International Center for Dielectric Research (ICDR), Xi’an Jiaotong University, Xi’an, China, and the authors thank Yan-Zhu Dai for her help in using SEM.

The authors declare no competing financial interest.

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