Shedding Light on the Intrinsic Characteristics of 3D Distorted Fluorite

Apr 19, 2019 - Transition-metal tellurites have motivated growing research interest in both fundamental and applied chemistry, and the corresponding s...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Shedding Light on the Intrinsic Characteristics of 3D Distorted Fluorite-Type Zirconium Tellurite Single Crystals Weiqun Lu, Zeliang Gao,* Xiaoli Du, Xiangxin Tian, Qian Wu, Conggang Li, Youxuan Sun, Yang Liu, and Xutang Tao* State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, People’s Republic of China

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S Supporting Information *

ABSTRACT: Transition-metal tellurites have motivated growing research interest in both fundamental and applied chemistry, and the corresponding single crystals could serve as rich and fascinating platforms to regulate, explore, and elucidate the intrinsic characteristics of different structures from 0D to 3D architectures. In this context, a zirconium tellurite (namely, ZrTe3O8) single crystal featuring a 3D distorted fluorite-type structure with a size of 35 × 32 × 21 mm3 was successfully harvested by the top-seeded solution growth (TSSG) technique. The X-ray diffraction rocking curve reflects that the crystallinity of the as-grown ZrTe3O8 crystal is quite perfect with a small full-width at half-maximum (fwhm) value (∼39 arcsec). The temperature dependence of the thermophysical properties of the ZrTe3O8 single crystal has been systematically analyzed. The ZrTe3O8 single crystal exhibits a wide transparency window, as the UV and IR absorption cutoff edges are respectively 278 and 7788 nm. The refractive indices over the region from the visible to the near-IR have been determined and manifested relatively large values of 2.0889−2.0370 over a wavelength range of 632.8−1553 nm. Furthermore, the fundamental physical characteristics of the ZrTe3O8 single crystal associated with its distinctive 3D framework structure have been evaluated with density functional theory (DFT) calculations.

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INTRODUCTION The tellurite compounds have drawn vast and increasing attention for a large variety of applications, such as nonlinear optics,1 modulators and Q switches,2 and mobile telecommunications,3 due to their abundant configurations, outstanding visible and IR transmittance, large linear and nonlinear refractive indices, and excellent chemical stability and durability. The tellurites can form various unusual structural topologies attributed to the presence of the variable coordination modes of Te atoms such as tricoordinate TeO3 trigonal pyramids, tetracoordinate TeO4 trigonal bipyramids, and pentacoordinate TeO5 square pyramids.4 These polyhedra adopt asymmetric coordination environments with stereochemically active lone-pair electrons occupying opposite nonbonding space (Figure 1).5 In addition to lone-pair Te4+ cations, most transition-metal cations (Cr3+, Zr4+, V5+, W6+, etc.) in the d block of the

periodic table can be bonded to various types of ligands, allowing for construction of a wide variety of transition-metal compounds.6 Accordingly, materials combining lone-pair Te4+ cations with transition-metal cations can present a rich structural chemistry with flexible frameworks. In particular, zirconium tellurite, ZrTe3O8, is regarded as the only thermodynamically stable form identified in the phase diagram of the ZrO2−TeO2 system in air.7 It was first reported to have a cubic structure with Ia3̅ space group in 1971.8 Since then, gradually increasing research passions have been devoted to this ternary tellurite: for example, as a radioactive inorganic ion exchanger in nuclear technology,9 as a useful catalyst in the conversion of propan-2-ol into acetone,10 and as a promising dielectric ceramic used in modern microelectronic packaging.11 However, research to analyze the physical properties of the zirconium tellurite thus far has been restricted to the polycrystalline forms.11,12 Consequently, the quest for intrinsic characteristics without grain boundary and surface effects inspired our exploration of experimental strategies for the growth of sizable ZrTe3O8 single crystals with high crystalline quality. The ZrTe3O8 single crystal could offer a fascinating platform for revealing and elucidating fundamental physical characteristics associated with its distinctive 3D framework structure.

Figure 1. Diverse coordination modes of Te atoms. The lone-pair regions on Te atoms are drawn schematically and marked in purple. © XXXX American Chemical Society

Received: February 13, 2019

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DOI: 10.1021/acs.inorgchem.9b00424 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 2. (a) Schematic illustration of the synthesis procedure of polycrystalline ZrTe3O8 through standard solid-state reaction techniques. (b) Experimental (blue) and calculated (red) PXRD patterns confirming the phase purity of ZrTe3O8. Inset: image of a polycrystalline ZrTe3O8 pellet. (c) TGA and DSC curves for ZrTe3O8. The two colors represent the stable and unstable temperature regions for ZrTe3O8, respectively. above the saturation temperature, which was maintained for 20 min to remove the outer surface defects. After that, the temperature was rapidly reduced to the saturation temperature, and then a fairly slow cooling program of approximately 0.02 °C h−1 was adopted to carry out the growth procedure. When the sizable ZrTe3O8 crystal was harvested, the furnace temperature was slowly cooled at a rate ranging from 10 to 20 °C h−1. Single-Crystal X-ray Diffraction. A colorless and transparent ZrTe3O8 single crystal (0.02 × 0.03 × 0.05 mm3) was selected and subjected to single-crystal X-ray diffraction analysis. The diffraction intensity data were recorded at 298 K using a Bruker AXS SMART diffractometer with monochromatic Mo Kα radiation.13 The data reduction and integration, as well as unit cell refinement, were carried out using the INTEGRATE program incorporated in the APEX II software.13 The semiempirical absorption corrections were performed with the SCALE program for area detector.13 The initial structure of ZrTe3O8 was determined by direct methods and then refined by fullmatrix least-squares methods.14 HRXRD. The crystallinity of the as-grown ZrTe3O8 crystal was determined by HRXRD on a Bruker D8 Discover diffractometer operated at 40 mA and 40 kV. A (100) oriented ZrTe3O8 wafer (4 × 4 × 1 mm3) was mechanically polished as the sample. Density Measurement. The experimental density of the ZrTe3O8 single crystal was estimated by the Archimedes method with deionized water as the immersion liquid at room temperature. The experimental density is defined by the relationship ρexp = ρwaterm0/m1, in which ρwater is the density of deionized water at 25 °C, m0 is the crystal sample weight in air, and m1 is the crystal sample weight in deionized water. The density value was accurately obtained through averaging the results of three crystal samples. Thermophysical Property Measurements. The thermal expansion of the ZrTe3O8 single crystal along the a axis was derived at 25−500 °C using a thermomechanical analyzer (TMA/SDTA 840, Mettler-Toledo Inc.). The specific heat of the ZrTe3O8 single crystal was measured through the differential scanning calorimetry method on a TGA/DSC1/1600HT simultaneous thermal apparatus, over the temperature region 25−250 °C, with a sapphire as a standard sample. The ZrTe3O8 single crystal was cut and polished into a plate perpendicular to the a axis with dimensions of 4 × 4 × 1 mm3 for the measurements of thermal diffusivity on a NETZSCH LFA 457 instrument in the range 25−250 °C.

In this work, we present a colorless and transparent ZrTe3O8 single crystal with a size of 35 × 32 × 21 mm3 employing the top-seeded solution growth (TSSG) method. Additionally, a structural analysis, high-resolution X-ray diffraction (HRXRD), thermophysical and optical properties, and overall structure− property relationship of ZrTe3O8 are investigated and discussed in detail, to bring out its uniquely intrinsic characteristics.

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EXPERIMENTAL SECTION

Synthesis. ZrO2 (3.5 × 10−3 mol, 99.99% purity, Alfa Aesar) and TeO2 (1.05 × 10−2 mol, 99.99% purity, Alfa Aesar) were utilized as starting materials. Polycrystalline ZrTe3O8 was prepared through a one-step solid-state reaction in air. The stoichiometric mixture was thoroughly mixed in a smooth agate mortar and further cold-pressed together to obtain a cylindrical pellet. Then the pellet was sintered at 700 °C for 2 days in a small corundum crucible with three intermediate regrindings. The purity of the resultant ZrTe3O8 sample was verified by powder X-ray diffraction (PXRD). PXRD. The PXRD data were collected using a Bruker D8 Discover diffractometer (monochromatic Cu Kα radiation, λ = 1.54178 Å). The 2θ range was from 10° to 90°. Thermal Stability. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were implemented on a Mettler-Toledo TGA/DSC1/1600HT apparatus between room temperature and 1050 °C at a temperature program of 10 °C min−1 using flowing nitrogen gas. Single-Crystal Growth. Single crystals of ZrTe3O8 were grown by the TSSG method, which was performed in a platinum crucible placed in the central region of an ohmic heating furnace. With excess Li2CO3 and TeO2 serving as fluxes, the molar ratio of Li2CO3, ZrO2, and TeO2 is 2:1:4. The reaction mixture was subsequently heated to 800 °C and maintained at that temperature for 40 h to guarantee an entirely uniform solution. Initially, a platinum wire with a diameter of 0.5 mm was used as a nucleation center to obtain small crystals of ZrTe3O8. A regular-shaped ZrTe3O8 seed crystal was carefully selected and further utilized for the large-sized and high-quality single crystal growth. In addition, the saturation temperature of the solution was obtained by the testing seed crystal method. A regularshaped crystal seed was gradually introduced into the solution at 2 °C B

DOI: 10.1021/acs.inorgchem.9b00424 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. (a) Small crystals of ZrTe3O8 obtained by spontaneous crystallization-driven growth. (b) As-grown ZrTe3O8 single crystal (35 mm × 32 mm × 21 mm) by the TSSG technique. (c) Theoretical morphology of the ZrTe3O8 single crystal (Figure 3c),22 on the basis of Bravais−Friedel−Donnay−Harker (BFDH) theory.23 The theory suggests that a more prominent crystal facet expressed in the external crystal morphology reflects a relatively slower growth rate. Therefore, it is obvious that the {002} facets have the slowest growth rate. UV−Vis Diffuse Reflectance Spectroscopy. The UV−vis diffuse reflectance spectra in the 200−800 nm wavelength region were collected for ground samples of ZrTe3O8 crystals by a Shimadzu UV-2550 spectrophotometer. BaSO4 was utilized as a reflectance standard. The reflectance data were converted to absorbance using the Kubelka−Munk equation.15 Transmission Spectroscopy. The optical transmission spectra in the wavelength region of 200−3000 nm were collected on a Hitachi U-3500 UV−vis−IR spectrometer, and the spectra in the 3000−8000 nm region were obtained on a Nicolet NEXUS 670 FTIR spectrometer. A thin (100) crystal plate (4 × 4 × 1 mm3) cut from the as-grown ZrTe3O8 crystal was used to carry out the measurements. Refractive Index Measurements. The refractive index is regarded as a basic physical quantity for optical crystals. A (100) wafer of ZrTe3O8 was polished to conduct the refractive index measurements. The refractive index values of ZrTe3O8 at different wavelengths were measured by an automized prism coupler (Metricon 2010/M) at room temperature. Computational Descriptions. The first-principles calculations were performed by density functional theory (DFT) in the Vienna ab initio simulation package (VASP).16 Single-crystal structural parameters of ZrTe3O8 were employed for the calculations. The generalized gradient approximation (GGA) with the Perdew−Burke−Eruzerhof modification for solids (PBEsol) was adopted to evaluate the exchange-correlation potential of electron−electron interactions.17 The following valence electron configurations were involved: 4d2 5s2 for Zr, 5s2 5p4 for Te, and 2s2 2p4 for O within the projectoraugmented wave (PAW) code.18 The Brillouin-zone integrations19 were carried out by sampling on a 7 × 7 × 7 Monkhorst−Pack k-point mesh with a 450 eV plane-wave cutoff energy.

incongruently, which decomposes before reaching its melting point. Single-Crystal Growth and Morphology. With respect to crystal growth, the flux method must be adopted because of the incongruent melting nature of ZrTe3O8. The selection of the flux system is extremely important to the growth of highquality crystals. As attributed to low melting point, excellent reactivity, and superior solubility, TeO2 has been used widely in the growth of new oxide crystals.20 Moreover, Li+ cations can enlarge the size of grown crystals substantially.21 Hence, we have sought to harvest the ZrTe3O8 crystal with Li2CO3 and TeO2 serving as fluxes. Using small crystals obtained by spontaneous crystallization-driven growth (see Figure 3a), a colorless and transparent ZrTe3O8 single crystal with a size of 35 × 32 × 21 mm3 (with smooth surfaces and well-shaped borders) was successfully grown by the TSSG method (see Figure 3b). More importantly, this method is quite conducive to the practical applications of the ZrTe3O8 single crystal due to its bulk growth habit and environmental friendliness (neither a rigorous atmosphere nor toxic ingredients are involved). The as-grown crystal exhibits quite prominent facets, that is, {002} and {112}, which match well with the theoretical morphology predicted by the Mercury program. Crystal Structural Analysis. The structure of ZrTe3O8 was solved by single-crystal X-ray diffraction. At 298 K, it belongs to a cubic structure with Ia3̅ space group, consistent with the structure reported in 1971.8 The crystallographic parameters and details of the structure refinement for ZrTe3O8 are clearly recorded in Table 1. Cubic ZrTe3O8 (Figure 4d) can be considered as a distorted fluorite-type structure with Zr4+ cations occupying F− anion sites in CaF2 (Figure 4e).8 There is only one independent Te4+ cation, one independent Zr4+ cation, and two independent O2− anions in the asymmetric unit of ZrTe3O8. The Te4+ and Zr4+ cations occupy the 24d and 8b sites of the ZrTe3O8 crystal lattice, respectively, surrounded by O2− anions lying in the 16c and 48e positions. (In contrast, the Ca2+ cations and F− anions occupy the 4a and 8c sites of the CaF2 crystal lattice, respectively.) As typically observed in paratellurite TeO2,24 each Te atom in ZrTe3O8 is coordinated by four O atoms (d(Te−O1) = 2.1709(13) Å (×2) and d(Te−O2) = 1.861(5) Å (×2)) to form a distorted disphenoid (Figure 4a) as a result of its second-order Jahn−Teller activity. 1b The TeO 4 disphenoid units further construct Te3O8 groups by a cornersharing scheme. Each TeO4 disphenoid is linked to two ZrO6 octahedra and four other TeO4 disphenoids through four

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RESULTS AND DISCUSSION Synthesis and Thermal Stability. Polycrystalline ZrTe3O8 was prepared through standard solid-state reaction techniques (Figure 2a). In order to identify and confirm the phase purity of the polycrystalline sample, the PXRD analysis was performed at room temperature (see Figure 2b). The analysis reveals that the obtained polycrystalline ZrTe3O8 is phase pure, whose PXRD pattern matches well with the calculated pattern. As presented in Figure 2c, the TGA and DSC curves indicate that ZrTe3O8 can remain stable up to ∼950 °C (stable and unstable temperature regions highlighted by two different colors). During the heating process, ZrTe3O8 shows obvious weight loss at 948.7 °C in the TGA curve; simultaneously, it displays a small endothermic signal in the DSC curve. The residues after the measurements are confirmed as ZrO2, as indicated by the PXRD analysis (see Figure S1). These results suggest that ZrTe3O8 melts C

DOI: 10.1021/acs.inorgchem.9b00424 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Parameters and Details of the Structure Refinement for ZrTe3O8 empirical formula formula wt temp (K) wavelength (Å) cryst syst space group a (Å) V (Å3) Z ρcalcd (g cm−3) μ (mm−1) F(000) crystal size (mm3) no. of data/restraints/params R(int) GOF (F2) final R indices (I > 2σ(I)) R indices (all data) CCDC

ZrTe3O8 602.01 298(2) 0.71073 cubic Ia3̅ (No. 206) 11.340(4) 1458.2(16) 8 5.484 13.282 2080.0 0.02 × 0.03 × 0.05 215/0/21 0.043 1.400 R1 = 0.0284, wR2 = 0.0616 R1 = 0.0285, wR2 = 0.0619 1886093

Figure 5. X-ray diffraction rocking curve of the ZrTe3O8 single crystal.

the basis for scientific investigations of its intrinsically physical properties. Thermal Properties. Of particular importance are the thermal properties of a crystal not only in crystal growth and processing but also in practical applications. Consequently, the temperature dependence of the thermophysical properties (thermal expansion, specific heat, thermal diffusivity, and thermal conductivity) of the ZrTe3O8 single crystal has been systematically analyzed. The thermal expansion coefficient can be represented by a second-rank symmetric tensor [αij]. For the ZrTe3O8 crystal with a cubic crystal system, the tensor [αij] in the principal coordinate system can be expressed as

bridging O atoms (see Figure 4c). Meanwhile, it is particularly mentioned here that asymmetric oxide coordination conditions always exist in octahedrally coordinated d0 transition metals (for example, Zr4+, Ta5+, W6+).25 Abnormally, the ZrO6 units in ZrTe3O8 containing the d0 transition metal, Zr4+, are completely regular octahedra (d(Zr−O) = 2.082(5) Å (×6)). The reason is that the “pre-distorted” nature of the TeO4 disphenoids serving as blocking groups restricts ZrO6 octahedra to be in peculiarly undistorted coordination environments.26 As shown in Figure 4b, six oxide ligands of ZrO6 octahedra are all bonded to a Te4+ cation. The additional crystallographic data of ZrTe3O8 are clearly given in Tables S1−S3 in the Supporting Information. HRXRD. The full-width at half-maximum (fwhm) of the Xray diffraction rocking curve is an important factor to demonstrate the crystalline perfection of single crystals.27 The relatively low value of fwhm (measured as ∼39 arcsec) on the (004) reflection, as shown in Figure 5, reflects that the crystallinity of the ZrTe3O8 single crystal is reasonably excellent. The high-quality ZrTe3O8 single crystal furnishes

ij α 0 0 yz jj z jj 0 α 0 zzz jj zz jj zz k0 0 α { There is only one independent thermal expansion coefficient α, which can be determined by measuring the crystal along the physical X axis (i.e., crystallographic a axis), in accordance with the IEEE standard. The thermal expansion of the ZrTe3O8 crystal (25−500 °C) is presented in Figure 6a. It is shown that the thermal expansion length (ΔL/L) increases linearly with

Figure 4. (a) TeO4 disphenoid and ZrO6 octahedron. Coordination environments of ZrO6 octahedron (b) and TeO4 disphenoid (c) in the a−c plane. Overview of the crystal structures of ZrTe3O8 (d) and CaF2 (e) in the a−c plane. D

DOI: 10.1021/acs.inorgchem.9b00424 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. Thermophysical properties of the ZrTe3O8 crystal. (a) Temperature dependence of the thermal expansion for the ZrTe3O8 crystal. (b) Temperature dependence of the unit cell parameters for the ZrTe3O8 crystal. Inset: variations of the densities for the ZrTe3O8 crystal with an increase in temperature. (c) Thermal expansion ellipsoid for the ZrTe3O8 crystal. (d) Temperature dependence of the specific heat for the ZrTe3O8 crystal. (e) Temperature dependence of the thermal diffusivity for the ZrTe3O8 crystal. (f) Temperature dependence of the thermal conductivity for the ZrTe3O8 crystal.

to tolerate more thermal energy with increasing temperature. The thermal diffusivity λ of the ZrTe3O8 crystal in the a axis direction is given in Figure 6e from 25 to 250 °C. The thermal diffusivity λ tends to decrease with increasing temperature, being on the order of 0.456 and 0.496 mm2 s−1 at 25 and 250 °C, respectively. Similar to the thermal expansion coefficient, the thermal conductivity of a crystal can be also represented by a second-rank symmetric tensor, [κij]. There is only one independent thermal conductivity for the ZrTe3O8 crystal with a cubic crystal system. The thermal conductivity coefficient κ of the ZrTe3O8 crystal can be described by κ = ρλCp, in which ρ represents the density, λ represents the thermal diffusivity, and Cp represents the specific heat. From Figure 6f, we can clearly see that the thermal conductivity of the ZrTe3O8 crystal increases as the temperature increases. The thermal conductivity coefficients are evaluated to be 1.38 and 1.67 W m−1 K−1 at 25 and 250 °C, respectively. More importantly, the ZrTe3O8 crystal with high symmetry can enjoy the merits of resistance to thermal anisotropies, making it easier for crystal processing and device design. Density. The density is an important quantity that describes the compactness degree of a crystal. The experimental density ρexp of the ZrTe3O8 crystal estimated by the Archimedes method is 5.432 g cm−3, which is very close to the theoretical value (5.484 g cm−3) derived from the structural parameters by the expression ρcal = MZ/NAV, in which M denotes the molar weight, Z denotes the molecular number in each unit cell, NA denotes Avogadro’s constant, and V denotes the unit cell volume. The size of the crystal sample also varies from 25 to 500 °C as a result of the thermal expansion effect. Accordingly, the temperature dependence of the density can be expressed as

increasing temperature, simultaneously giving rise to continuous a axis and volume expansion (see Figure 6b). The linear thermal expansion coefficient α is obtained according to the relation α = ΔL/LΔT, in which ΔL denotes the length change with increasing temperature, L denotes the initial length at room temperature, and ΔT denotes the change in temperature. The calculated linear thermal expansion coefficient of the ZrTe3O8 crystal in the a axis direction is α = 10.15 × 10−6 K−1 from 25 to 500 °C, slightly larger than that of TiTe3O8 (8.56 × 10−6 K−1).2 The thermal expansion of a crystal manifests the anharmonic nature of the interatomic interactions directly. The reason the ZrTe3O8 crystal has a positive thermal expansion coefficient is that the interatomic bond lengths increase with increasing temperature. The stronger interatomic bonds which have narrower and steeper potential wells result in a slower rate of increase in the interatomic distances (Ti−O bands in TiTe3O8 stronger than Zr−O bands in ZrTe3O8), and hence the ZrTe3O8 crystal has a larger thermal expansion coefficient in comparison to that of TiTe3O8. The orientation relationship between the thermal expansion ellipsoid and the ZrTe3O8 crystal is demonstrated in Figure 6c. The thermal expansion ellipsoid is isotropic and can be conducive to reduce cracking phenomena that arise in the practical and complicated scenarios. According to the thermal expansion coefficient of the ZrTe3O8 crystal, the temperature dependence of its unit cell parameters is obtained. The length of the a axis increases from 11.3400 Å at 25 °C to 11.3947 Å at 500 °C, and the volume of the unit cell increases from 1458.35 to 1479.55 Å3. The variations are found to be 0.48 and 1.45%, respectively. The specific heat Cp of the ZrTe3O8 crystal is plotted against temperature shown in Figure 6d. The curve shows the increasing trend from 0.548 J g−1 K−1 at 25 °C to 0.611 J g−1 K−1 at 250 °C, which means that the ZrTe3O8 crystal tends E

DOI: 10.1021/acs.inorgchem.9b00424 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ρT = =

MZ MZ = NAVT [1 + α(T − T0)]3 NAV0 ρ0 [1 + α(T − T0)]3

in which ρ0 and ρT are the densities of the crystal at T0 and T, respectively, V0 and VT are the unit cell volumes of the crystal at T0 and T, respectively, and α is the linear thermal expansion coefficient in the a axis direction. As shown in the inset of Figure 6b, the density of the ZrTe3O8 crystal decreases linearly from 5.432 g cm−3 at 25 °C to 5.354 g cm−3 at 500 °C. Optical Properties. Probing the optical properties of a crystal (i.e., the behavior of light in optically transparent media), which refer specifically to the absorption, transmission, and refractive index, is considerably meaningful scientifically and practically. Accordingly, the UV−vis diffuse reflectance spectra on ground samples of ZrTe3O8 crystals are shown in Figure 7a. The UV absorption edge of ZrTe3O8 is extended to ∼360 nm, in comparison to the 380 nm absorption edge for TiTe3O8.2 As presented in Figure 7a (inset), the band gap Eg of ZrTe3O8 is estimated to be about 4.30 eV by fitting the corresponding Tauc plot of the Kubelka−Munk transformed UV−vis diffuse reflection data,15 while TiTe3O8 exhibits a band gap Eg of approximately 3.37 eV.2 The tendency is consistent with the calculated result by modified Becke− Johnson potential.28 The variation of the band gaps between ZrTe3O8 and TiTe3O8 stems from the difference in the transition metal d levels.29 The relatively large band gap means that the ZrTe3O8 crystal can show enhanced resistance to laser damage. Figure 7b shows the transmission spectra of a 1 mm thick ZrTe3O8 single crystal. The UV absorption cutoff of the ZrTe3O8 crystal is at 278 nm, which is much shorter than that of TiTe3O8 (440 nm).2 It can be also compared to those of tellurites, such as Bi2TeO5 (400 nm),30 α-BaMo2TeO9 (380 nm),31 Zn2MoTeO7 (300 nm),32 Li2ZrTeO6 (293 nm),20a and BaW2TeO9 (360 nm),33 and those of fluoride tellurites, such as Li7(TeO3)3F (240 nm).4d Furthermore, the ZrTe3O8 crystal is highly transparent (>70%) up to 6 μm with the IR absorption cutoff edge located at about 7788 nm. These obtained results reveal that the ZrTe3O8 crystal is suitable for large numbers of applications over wide wavelength ranges. The refractive index of a crystal is a dimensionless quantity that describes how light propagates through the transparent medium. In order to determine the dispersion of the refractive indices of ZrTe3O8 at room temperature, the prism coupling technique was used. Table S4 summarizes the refractive indices of the ZrTe3O8 crystal, where λ represents the wavelength in micrometers and n represents refractive index values. The Sellmeier equations widely adopted to describe the dependence of the refractive index on the wavelength were fitted using the least-squares-fit method:34 n2 = 4.08183 + 0.17933/(λ2 + 0.23626). The fitted values are consistent with the measured values (see Table S4 and Figure 7c), confirming that the Sellmeier equations are reliable. The refractive index of the ZrTe3O8 crystal at 1064 nm is 2.0525. The ZrTe3O8 crystal with relatively large refractive indices as well as wide transparency window could turn out to be especially applicable in the optics of modern digital cameras. Structure−Property Relationship. To better evaluate the microscopic mechanism of the optical properties associated with the structural features, first-principles calculations of

Figure 7. Optical properties of the ZrTe3O8 crystal. (a) UV−vis diffuse reflectance spectra of ground samples of ZrTe3O8 crystals. The inset gives the corresponding Tauc plot. (b) Transmission spectra of the ZrTe3O8 single crystal. The inset shows a ZrTe3O8 wafer (4 mm × 4 mm × 1 mm). (c) Refractive index dispersion curves for the ZrTe3O8 single crystal.

ZrTe3O8 were carried out in the VASP code.16 As presented in Figure S2, ZrTe3O8 has a direct band gap of 3.63 eV, which is smaller than the experimental result (4.30 eV) on account of the well-known inaccurate description by the GGA method.35 On the basis of the total and partial density of states (TDOS and PDOS), the detailed electronic structures of ZrTe3O8 are projected on the constituent elements (see Figure 8a). Owing to the optical properties of a single crystal closely relevant to the electronic transitions near the band gap,36 the bottom of the conduction band and the top of the valence band have been analyzed in detail: (1) the former (∼4 to 7 eV) mainly consists of O 2p orbitals mixed with Te 5p and Zr 4d orbitals; (2) the latter from approximately −6 to 0 eV is primarily composed of Te 5p and O 2p orbitals; (3) there exists evident hybridization between O and neighbor atoms. This demonstrates that TeO4 disphenoids and ZrO6 octahedra have F

DOI: 10.1021/acs.inorgchem.9b00424 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 8. Electronic structures of ZrTe3O8. (a) TDOS and PDOS plots of ZrTe3O8. (b) Side view of the 3D EDD plot of ZrTe3O8 in the a−c plane. Red and green isosurfaces represent respectively electron accumulation and depletion regions with a value of 0.024 e Å−3. (c) Side view of the 3D ELF plot of ZrTe3O8 in the a−c plane. Yellow isosurfaces represent ELF with value of 0.8. (d) Side view of the ELF plot along [010] direction.

systematically analyzed: the thermal expansion coefficient in the a axis direction from 25 to 500 °C is α = 10.15 × 10−6 K−1, the specific heat shows an increasing trend from 0.548 J g−1 K−1 at 25 °C to 0.611 J g−1 K−1 at 250 °C, and the thermal conductivity coefficient range in the a axis direction is from 1.38 W m−1 K−1 at 25 °C to 1.67 W m−1 K−1 at 250 °C. The ZrTe3O8 crystal exhibits a wide transparency window, as the IR absorption cutoff edge can extend to about 7788 nm (highly transparent (>70%) up to 6 μm). Refractive index measurements over the wavelength range of 632.8−1553 nm reveal that the ZrTe3O8 crystal has large values of 2.0889−2.0370. The first-principles calculations reveal that the TeO 4 disphenoids and ZrO6 octahedra dominate the optical properties of the ZrTe3O8 single crystal. The results presented here shed light on inquiring into the intrinsic characteristics based upon the single crystal platforms and understanding the chemical origin of their characteristics.

important roles in the optical properties of the ZrTe3O8 single crystal. From the calculated 3D electron density difference (EDD) plot in the a−c plane (Figure 8b), we can see that there are obvious covalent characteristics between the (Zr/Te)−O bonds. The results indicate that the polyhedral configurations for the Zr and Te atoms of ZrTe3O8 depicted in the above crystal structural analysis are reasonable, which are vital to construct the 3D distorted fluorite-type structure. The electron localization function (ELF) can allow the nodal structure of the molecular orbitals (involving lone-pair electrons) to be studied by a chemically intuitive method. As anticipated, the 3D ELF plot (Figure 8c) in the a−c plane also confirms the existence of stereochemically active lone-pair electrons for the Te atoms, which appear away from their closely adjacent atoms. In addition, the lone-pair electrons seem more obvious in the red region of Figure 8d, and the Te coordination polyhedron resembles a highly distorted [TeO4E] pseudopentahedron (E designates the lone-pair electrons). Moreover, the above phenomena have also been observed and interpreted in other relevant materials with lone-pair electrons.37

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ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00424. Powder X-ray diffraction pattern of the residue, calculated band structure, selected bond lengths (Å) and angles (deg), fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2), atomic displacement parameters (Å2), and refractive indices at different wavelengths (PDF)

CONCLUSION In summary, we have successfully harvested a large-sized and high-quality ZrTe3O8 single crystal using selected Li2CO3− TeO2 fluxes by the TSSG technique. On the basis of the asgrown single crystal, the intrinsic characteristics of the 3D distorted fluorite-type zirconium tellurite have been investigated and discussed in detail. The temperature dependence of the thermophysical properties of the ZrTe3O8 crystal has been G

DOI: 10.1021/acs.inorgchem.9b00424 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Accession Codes

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CCDC 1886093 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for Z.G.: [email protected]. *E-mail for X.T.: [email protected]. ORCID

Xutang Tao: 0000-0001-5957-2271 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support provided from the Shandong Provincial Key R&D Program (No. 2018CXGC0411), the Fundamental Research Funds of Shandong University (No. 2017JC044), the Young Scholars Program of Shandong University (No. 2018WLJH67), the National Natural Science Foundation of China (NSFC) (Nos. 11504389, 51572155, and 51772170), the National Key Research and Development Program of China (No. 2016YFB1102201), and the 111 Program (No. BP2018013).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b00424 Inorg. Chem. XXXX, XXX, XXX−XXX