Synthesis and Characterization of Two New Second Harmonic Generation Active Iodates: K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl

Abstract

Transparent single crystals of two new iodates K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl have been synthesized hydrothermally. Single-crystal X-ray diffraction was used to determine their crystal structures. Both compounds crystallize in non-centrosymmetric space groups. The compound K₃Sc(IO₃)₆ crystallizes in the orthorhombic space group Fdd2. The crystal structure is made up of [ScO₆] octahedra, [IO₃] trigonal pyramids, and [KO₈] distorted cubes. The compound KSc(IO₃)₃Cl crystallizes in the trigonal space group R3. The building blocks are [ScO₆] octahedra, [KO₁₂] polyhedra, and [IO₃] trigonal pyramids. The Cl^– ions act as counter ions and reside in tunnels in the crystal structure. The second harmonic generation (SHG) measurements at room temperature, using 1064 nm radiation, on polycrystalline samples show that the SHG intensities of K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl are around 2.8 and 2.5 times that of KH₂PO₄ (KDP), respectively. In addition, K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl are phase-matchable at the fundamental wavelength of 1064 nm. The large anharmonicity in the optical response of both compounds is further supported by an anomalous temperature dependence of optical phonon frequencies as well as their enlarged intensities in Raman scattering. The latter corresponds to a very large electronic polarizability.

Introduction

New strategies for designing non-centrosymmetric (NCS) materials are attracting interest because of their potential technological applications. Some properties of NCS materials, for example, ferroelectricity, piezoelectricity, and optical activity like second-harmonic generation (SHG), are often linked to the manufacturing of data storage devices, sensors, and laser devices.¹⁻³

A synthetic strategy that has proven successful to increase the probability to form NCS compounds is to include acentric building units in the crystal structure.⁴⁻⁶ Lone electron pair cations of p-elements such as I⁵⁺, Te⁴⁺, or Se⁴⁺ comprise an active electron pair that occupies space and has a volume that can be compared with the volume of an O^2– anion. Those cations tend to adopt asymmetric one-sided coordination geometries such as trigonal pyramidal or see-saw coordination. The heaviest cations of this kind are Bi³⁺ and Pb²⁺, and because of their stronger attracting nucleus, they are equipped with a less stereochemically active lone pair occupying a relatively small volume. These ions often have a high coordination number, and they may form asymmetric building units such as distorted cubes.⁷

There are several optically active compounds reported that comprise p-element lone-pair cations. One such compound is BiSeO₃F, which involves the two lone pair cations Bi³⁺ and Se⁴⁺ and in addition a halide ion as well.⁸ There are also NCS iodates reported in the literature that belong to the L–I⁵⁺–O system, with L = Bi³⁺, and show optical activity, e.g. BiO(IO₃)⁹ and Bi₂(IO₄)(IO₃)₃.¹⁰ Additionally, the system A–L–I⁵⁺–O also exists with relevant reported compounds belonging to A₂BiI₅O₁₅, where A = K⁺ or Rb⁺ and L = Bi³⁺.¹¹

Another family of iodates is A–M–I–O that combines I⁵⁺ with highly coordinated A ions, Na⁺, K⁺, Rb⁺, or Cs⁺, with a transition metal that does not give colored compounds, for example, Zn²⁺, Mn²⁺, or Mo⁶⁺. Examples of reported NCS compounds belonging to this system are K₂Mn(IO₃)₄(H₂O)₂ and K₂Zn(IO₃)₄(H₂O)₂.¹²

In this study, the K⁺–Sc³⁺–I⁵⁺–O and K⁺–Sc³⁺–I⁵⁺–Cl systems are explored, and two new NCS compounds are reported, K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl. Both are SHG active with intensities that are approximately 2.8 and 2.5 times that of KDP for K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl, respectively.

Results and Discussion

Crystal Structure of K₃Sc(IO₃)₆

The new iodate K₃Sc(IO₃)₆ crystallizes in the orthorhombic NCS space group Fdd2 with unit cell parameters a = 39.507(3) Å, b = 8.2507(6) Å, and c = 11.3283(7) Å. Crystal structure refinement data are summarized in Table 1. There is one crystallographically independent Sc³⁺, two K⁺, three I⁵⁺, and nine O^2– ions in the crystal structure. The oxidation states of the cations are confirmed by bond valence sum (BVS) calculations using the theoretical bond distance values from Brown and Altermatt.¹⁹ There are three building units in the crystal structure, [KO₈] distorted cubes, [ScO₆] octahedra, and [IO₃] trigonal pyramids. An overview of the structure is shown in Figure 1a.

Table 1. Crystal Data and Refinement Parameters for K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl.

empirical formula	K3Sc(IO3)6	KSc(IO3)3Cl
formula weight	1211.66	644(1)
temperature (K)	294(2)	294(2)
wavelength (Å)	0.71073	0.71073
crystal system	orthorhombic	trigonal
space group	Fdd2	R3
a (Å)	39.507(3)	8.087(1)
b (Å)	8.251(1)	8.087(1)
c (Å)	11.328(1)	27.577(1)
volume (Å3)	3692.5(4)	1562.11(13)
Z	8	6
density calc. (g cm–3)	4.359	4.109
F(000)	4320.0	1728
crystal color	colorless	colorless
crystal habit	plate	orthorhombic
crystal size	0.05 × 0.05 × 0.01	0.1 × 0.08 × 0.05
θ range for data collection (deg)	3.098–26.361	2.216–46.484
index ranges	–49 ≤ h ≤ 47	–16 ≤ h ≤ 16
	–7 ≤ k ≤ 10	–16 ≤ k ≤ 16
	–14 ≤ l ≤ 14	–54 ≤ l ≤ 44
reflections collected	6040	20,073
independent reflections	1378 [R(int) = 0.039]	4576 [R(int) = 0.0316]
Flack parameter	0.11(4)	0.47(3)
data/restraints/parameters	1703/1/83	5220/1/91
refinement method	full-matrix least squares refinement on F2	full-matrix least squares refinement on F2
goodness-of-fit on F2	1.133	1.105
final R indices	R1 = 0.0401	R1 = 0.0223
[I > 2σ(I)]α	wR2 = 0.0562	wR2 = 0.0443
R indices all data	R1 = 0.0625	R1 = 0.0300
	wR2 = 0.0603	wR2 = 0.046

Figure 1.

(a) Overview of the K₃Sc(IO₃)₆ unit cell along [010]. I atoms: yellow and O atoms: red. (b) [K₃O₁₈]_∞ zig-zag trimers along [100]. (c) Alternating [IO₃] and [I₂O₅] groups due to splitting of I2 and I3.

According to Brown’s operational definition of bond lengths, a ligand should contribute with at least 4% to the valence of the central cation in order to be considered to belong to the primary coordination sphere; this definition sets the maximum distance for the K–O bonds to be 3.32 Å.²⁰ There are eight K–O distances that fulfill this condition, and they are in the range 2.644(13)–3.168(15) Å. The distorted cubes [K1O₈] and [K2O₈] are connected to each other via edge-sharing to form [K₃O₁₈] trimers along [100]; see Figure 1b.

All the three crystallographically independent iodine atoms have a trigonal pyramidal [IO₃] coordination. The one-sided coordination is due to the presence of the stereochemically active lone-pair on I⁵⁺. The I–O bond distances are in the range 1.748(10)–2.100(17) Å that is close to the I–O bond distance range, 1.77(3)–1.95(3) Å, reported for I₂O₅.²¹ The [I1O₃] trigonal pyramids are isolated from each other, and are connected to one [ScO₆] octahedron via corner-sharing and to two [K1O₈] distorted cubes via corner- and edge-sharing, respectively. The [I2O₃] and [I3O₃] trigonal pyramids alternate and almost form [I₂O₆]_∞ chains along [010]. However, the O7 and O8 atoms compete for both I2 and I3, resulting in that they split into two positions, each with 0.5 probability. In practice, this means that there are no chains, but rather have alternating [I₂O₅] groups and [IO₃] groups (see Figure 1c).

The [ScO₆] octahedra connect to a [IO₃] trigonal pyramid at each corner (two of each of I1, I2, and I3) and two [K2O₈] distorted cubes via edge-sharing. The Sc–O bond distances are in the range 2.035(7)–2.137(14) Å, and are close to the Sc–O bond distance range 2.08–2.16 Å, as reported for Sc₂O₃.²²

A family of polar compounds is reported in the literature, A₂Pt(IO₃)₆ (A = H₃O, Na, K, Rb, and Cs). The iodine atoms form [IO₃] trigonal pyramids with I–O bonds from 1.782(9) to 1.905(10) Å. In this group of compounds, there are highly coordinated A ions, where, for example, Na is nine coordinated with Na–O distances in the range 2.570(11)–2.997(19) Å. Finally, the platinum atoms form regular isolated [PtO₆] octahedra with Pt–O bond distances between 1.994(8) and 1.998(6) Å.²³ This family has similar building groups, with bond length values comparable to K₃Sc(IO₃)₆ iodate. An isostructural compound KH(IO₃)₂ is previously described that crystallizes in the same space group Fdd2 having similar unit cell parameters.²⁴

Crystal Structure of KSc(IO₃)₃Cl

The new compound KSc(IO₃)₃Cl crystallizes in the trigonal space group R3 with unit cell parameters a = 8.0875(3) Å and c = 27.5774(12) Å. The crystal structure was solved from single-crystal X-ray data, and the refinement data are presented in Table 1. There are two crystallographically independent atoms of the ingoing cations K⁺, I⁵⁺, and Sc³⁺, respectively, and six independent O^2– and two Cl^– anions. The oxidation state of the ions was confirmed by BVS calculations.¹⁹

An overview of the crystal structure is shown in Figure 2a. The structure consists of two types of [KO₁₂] distorted polyhedra, [IO₃] trigonal pyramids, and [ScO₆] octahedra, respectively. The [KO₁₂] and [ScO₆] building blocks form a framework with tunnels along both [100] and [110], where the Cl^– ions reside as counter ions. The Cl1 atom is surrounded by six iodine atoms, three I1 at a distance ranging from 3.297(2) to 3.298(2) Å, and three I2 atoms with distances that average at 3.182(3) Å. The Cl2 atom is also surrounded by three I1 and three I2 with distances that average at 3.224(2) atoms and 3.265(2) Å, respectively. Those long distances to the closest cations make both the Cl^– anions under-bonded with calculated BVS values of 0.57 and 0.58 for Cl1 and Cl2, respectively; see Figure 2b. The long distances make it likely possible to replace Cl^– with e.g. Br^– or I^– in the crystal structure.

Figure 2.

(a) Overview of KSc(IO₃)₃Cl with tunnels along [100] where the Cl atoms reside. (b) Cl1 and Cl2 in the channels of KSc(IO₃)₃Cl with surrounding I1 and I2 atoms.

The I⁵⁺ ions adopt one-sided coordination to form [IO₃] trigonal pyramids that do not polymerize. The I–O bond lengths are in the range 1.730(6)–1.848(6) Å that is in agreement with the values of I₂O₅, 1.77(3)–1.95(3) Å.²¹ The K–O bond distances are in the range 2.888(7)–3.315(7) Å in the [KO₁₂] distorted polyhedra. The longest K–O bond length is on the wedge of the aforementioned acceptable upper limit of 3.32 Å.²⁷ The Sc–O distances are in the range 2.054(6)–2.088(7) Å, and they are in agreement with the Sc–O bond distance range 2.08–2.16 Å reported for Sc₂O₃.²² There are two crystallographically unique Sc atoms in the structure, Sc1 and Sc2. The [Sc1O₆] octahedron connects to one [K1O₁₂] and one [K2O₁₂] polyhedron via edge-sharing as well as to three [I1O₃] and three [I2O₃] trigonal pyramids via corner-sharing, see Figure 3a. The [Sc2O₆] octahedra link with three [K1O₁₂] and three [K2O₁₂] polyhedra. The latter form a ring around [Sc2O₆] of edge-sharing alternating polyhedra. In addition, three [I1O₃] and three [I2O₃] trigonal pyramids connect to [Sc2O₆] via corner-sharing; see Figure 3b.

Figure 3.

Two different Sc atoms in KSc(IO₃)₃Cl: (a) Sc1 and (b) Sc2.

Several iodates that also contain chlorine are reported previously. In Pb₃[IO₃]₂Cl₄, the I⁵⁺ ions form [IO₃] groups with I–O bond distances in the range 1.808–1.812 Å. The Pb atoms are six- and eight-coordinated, forming [PbO₄Cl₂] and [PbO₄Cl₄] groups, respectively, with a Pb–Cl bond range of 2.868(2)–3.042(2) Å and a Pb–O bond distance range of 2.506(7)–2.762(7) Å.²⁶ K₂Bi(IO₃)₄Cl contains [IO₃] and [KO₇Cl₂] groups with an I–O bond distance from 1.780(5) to 1.865(5) Å and a K–O bond distance range 2.705(6)–3.339(5) Å. The I–Cl distances are between 3.088 and 3.219 Å.²⁷ Another example of iodate chloride is Cd(IO₃)Cl, which is a chain compound where the Cl atoms bond to Cd to form [CdO₄Cl₂] octahedra with a Cd–Cl bond distance of 2.566(2) Å and Cd–O bond distances that are in the range of 2.308(3)–2.314(2) Å. The iodine atoms form trigonal pyramids that connect to the chains with I–O bond lengths of 1.813(2)–1.833(3) Å.²⁸ The distance values in the aforementioned examples are close to the corresponding ones in KSc(IO₃)₃Cl.

SHG Properties of K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl

SHG intensity versus particle size revealed that K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl have SHG intensities that are approximately 2.8 and 2.5 times that of KDP, respectively, see Figure 4. The SHG intensities of both samples are comparable to similar materials.^11,12 The measurements also indicate that both K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl are phase-matchable, and falls in the class A category of SHG materials.^29,30

Figure 4.

Polycrystalline SHG results of K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl in comparison to KDP.

Optical Spectroscopy of K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl

Raman scattering in the temperature range 4–300 K was carried out to study if there were signs of phase transitions.

The symmetry analysis of K₃Sc(IO₃)₆ with Fdd2 (43) leads to 44 A₁ + 44 A₂ + 46 B₁ + 46 B₂ modes with a total sum of 180 modes that could be observable in Raman scattering or IR spectroscopy. To these modes also, the I02A and I02B iodine split positions contribute with reduced intensity due to their partial occupation. The experimental Raman spectra show approximately 50 modes in the frequency range 35–860 cm^–1. The difference between the expected and observed modes can be explained by a smaller intensity or near degeneracy of some modes.

The experimental spectrum of modes can be divided into three frequency ranges with the typical frequency ranges 35–190 and 205–420 cm^–1, and the high frequency range 700–860 cm^–1; see Figure 5; the latter range is attributed to IO₃ modes.

Figure 5.

Raman scattering spectra of K₃Sc(IO₃)₆ at four different temperatures. The insets show the frequency, linewidth, and intensity of selected phonons at 47, 260, 720, and 860 cm^–1, respectively.

The modes at 740 cm^–1 and 780 cm^–1 have extremely high scattering intensity. With 200 and 90 ct/s peak intensities, they are, respectively, factors 20 and 9 more intense than the optical phonon of Si under similar experimental conditions. Both modes show a softening of the peak frequency with decreasing temperatures, different from the anharmonicity of a normal optical phonon; see the inset of Figure 5.

The symmetry analysis of KSc(IO₃)₃Cl with R3 (146) leads to 30A + 30 ¹E + 30 ²E modes. This corresponds to a sum of 90 optically active modes. In the Raman spectra, approximately 23 modes are observed in the frequency range 0–950 cm^–1, see Figure 6.

Figure 6.

Raman scattering spectra of KSc(IO₃)₃Cl at four temperatures. The insets show the frequency, linewidth, and intensity of selected phonons at 40, 418, 790, and 780 cm^–1, respectively.

The spectrum of KSc(IO₃)₃Cl can also be divided into three frequency ranges. However, because of the smaller number of modes, it is easier to analyze. There exists an equidistant ladder-like spectrum of modes at 30–160 cm^–1 that resembles rotational excitations of molecules. Furthermore, there exist modes with different frequency separations in an intermediate frequency range 190–420 cm^–1 and in a high frequency range 730–920 cm^–1.

In the latter range, the mode at 790 cm^–1 has the highest scattering intensity. With 140 ct/s peak intensity, it is a factor 14 more intense than the optical phonon of Si.

This mode also shows an anomalous softening with decreasing temperatures, while most of the phonons harden with decreasing temperatures, see insets of Figure 6. Nevertheless, further phonons with anomalous frequency evolution exist at 36 and 420 cm^–1.

As mentioned above, the phonon spectra of K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl show rather distinct groups of modes with different intensities and frequency separations. This grouping is rationalized within the composition and functional groups of K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl. Regarding their frequency, K and Sc displacements contribute mainly to lower frequencies. The iodate group mainly contributes to high-frequency local vibrations. For the latter, stretching vibrations in the frequency range of 750–800 cm^–1 have indeed been reported in the literature. However, bending modes are also expected in the range 300–450 cm^–1.^31,32 The rather different effect of temperature on the modes is due to the different mechanisms of anharmonicity that collective phonon vibrations and local modes are experiencing. A collective optical vibration can decay into pairs of acoustic modes based on the anharmonicity of the lattice potential. Such a decay process is strongly reduced for a local mode enhancing their thermal occupation. Therefore, the electronic polarizability is enhanced leading to a large scattering intensity in Raman scattering experiments. The observed scattering intensity with a ratio of 1.4 for the largest intensity modes compares well with the ratio of SHG enhancements of K₃Sc(IO₃)₆ compared to KSc(IO₃)₃Cl.

Conclusions

Two new NCS iodate compounds are synthesized hydrothermally, K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl. The crystal structures are determined from single-crystal X-ray diffraction data. The compound K₃Sc(IO₃)₆ iodate crystallizes in the orthorhombic space group Fdd2 with unit cell parameters a = 39.507(3) Å, b = 8.2507(6) Å, and c = 11.3283(7) Å, and the compound KSc(IO₃)₃Cl crystallizes in the trigonal space group R3 with unit cell parameters a = 8.0875(3) Å and c = 27.5774(12) Å. SHG measurements at 1064 nm indicate that both K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl are attractive candidates for nonlinear optical applications. The related large SHG intensities that are around 2.8 and 2.5, respectively, compared to that of KH₂PO₄ (KDP), are attributable to a very large electronic polarizability that also shows up in the enhanced Raman scattering intensity of stretching modes of the iodate groups.

Experimental Section

Transparent single crystals of plate-like K₃Sc(IO₃)₆ and block-like KSc(IO₃)₃Cl were synthesized via hydrothermal synthesis. The compound K₃Sc(IO₃)₆ was prepared from the starting materials KIO₃ (Alfa Aesar, 98%), I₂O₅ (Sigma, 98%), and Sc₂O₃ (Koch-Light Laboratories, 99%) mixed in a molar ratio of 3:1.5:0.5. KSc(IO₃)₃Cl was prepared from KIO₃ (Alfa Aesar, 98%), I₂O₅ (Sigma, 98%), and ScCl₃ (Strem. Chemicals, 99%) mixed in a molar ratio of 1:1:1. The starting materials were each mixed with 3 ml of deionized water in a 23 ml Teflon-lined autoclave. The reaction conditions were 230 °C, a temperature plateau of 72 h, and a cooling rate of ca 21 °C/h for both compounds. The product was purified by rinsing in hot water to dissolve the unreacted starting materials. The compounds were characterized by scanning electron microscopy (SEM, HITACHI-TM3000), and their composition was analyzed by energy dispersive spectroscopy (EDS). Structural characterization was done by single-crystal X-ray diffraction data obtained from a Bruker D8 Venture diffractometer with Mo Kα radiation; λ = 0.710730 Å. Data integration was done with software SAINT¹³ and the absorption correction with software SADABS¹⁴ provided by the manufacturer. The structures were solved by dual space methods using software SHELXT-2014/7¹⁵ and refined by the full matrix on F² using the program SHELXL-2014/7.¹⁶ All atomic positions were refined with anisotropic displacement parameters for both compounds. Structural drawings were made with software DIAMOND.¹⁷ Powder X-ray diffraction was employed for the confirmation of the sample purity using a PANalytical X’Pert PRO diffractometer and comparing the experimental patterns with the relevant theoretical patterns. Data collection was done in a scanning mode between 5° and 60° and with a θ-step of 0.013.

Raman scattering experiments have been performed using a microspectrometer (Dilor LabRAM), using 532 nm-laser radiation of 32.7 μW, and a microscope optics with 50×. The spectra were recorded without a polarization analyzer, as the shape of the single crystals leads to an internal light polarization scrambling in the transparent sample. The temperature dependence has been recorded using a microcryostat in the range 4–300 K. The phonon intensity has been compared with scattering on the 520 cm^–1 phonon of silicon under similar experimental conditions. Such an intensity is proportional to the electronic polarizability at the energy of incident laser radiation with respect to the investigated phonon displacement.

For second-harmonic generation (SHG) measurements, K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl polycrystals were ground and sieved into distinct particle size ranges (<20, 20–45, 45–63, 63–75, 75–90, and 90–125 μm). Relevant comparisons with known SHG materials were made by grinding and sieving crystalline KDP into the same particle size range. A 1064-nm pulsed Nd:YAG laser (Quantel Laser, Ultra 50) generated the fundamental light, and the SHG intensity was recorded at room temperature on an oscilloscope (Tektronix, TDS3032) regarding different particle size ranges. No index matching fluid was used in any of the experiments. Please refer to Ok et al. for a detailed description.¹⁸

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b04288.

• Tables of atomic coordinates, bond lengths, and BVS calculations. Further details on crystal structural investigations can be obtained from the joint CCDC/FIZ Karlsruhe deposition service, deposit numbers CSD 1959925 and CSD 1959926 for KSc(IO₃)₃Cl and K₃Sc(IO₃)₆, respectively (PDF)

Author Present Address

^∥ Physics Faculty, Lomonosov Moscow State University, 119991 Moscow, Russia.

The authors declare no competing financial interest.