Syntheses and Characterization of AM2V2O11 (A = Ba, Sr, Pb; M = Nb

Oct 2, 2017 - Syntheses and Characterization of AM2V2O11 (A = Ba, Sr, Pb; M = Nb, Ta) Vanadates with Centrosymmetric and Noncentrosymmetric Structures...
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Syntheses and Characterization of AM2V2O11 (A = Ba, Sr, Pb; M = Nb, Ta) Vanadates with Centrosymmetric and Noncentrosymmetric Structures Anil Kumar Paidi,† P. W. Jaschin,‡ K. B. R. Varma,‡ and Kanamaluru Vidyasagar*,† †

Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India Materials Research Centre, Indian Institute of Science, Bangalore 560012, India

‡

S Supporting Information *

ABSTRACT: Five isomorphous AM2V2O11 vanadates of niobium and tantalum, namely, BaNb2V2O11, BaTa2V2O11, SrNb2V2O11, SrTa2V2O11, and PbTa2V2O11, were prepared by solid-state reactions and structurally characterized by singlecrystal and powder X-ray diffraction techniques. Barium and strontium compounds, respectively, have centrosymmetric and noncentrosymmetric types of layered structure, wherein [M2V2O11]2− anionic layers are interleaved with A2+ cations. Both types of layered structure are found for lead compound. The strontium and lead compounds are type I phase-matching materials with second-harmonic-generating efficiencies of 33−50% of LiNbO3, and their dielectric properties were evaluated. A three-dimensional structural variant was also identified for strontium compounds, which crystallize in noncentrosymmetric orthorhombic space group C2221.

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Pb2+ ions, respectively, have similar ionic radii of Ca2+ and Sr2+, and, therefore, they are normally considered along with alkaline earth metal ions for a series of isomorphous compounds for any systematic study; moreover, Pb2+ ion with stereoactive lone pair of electrons could have irregular and asymmetric coordination and probably leads to structures with lower symmetry. Thus, ten quaternary AM2V2O11 (A = Ba, Sr, Ca, Pb, Cd; M = Nb, Ta) vanadates of niobium and tantalum could be formulated, and only BaNb2V2O11, BaTa2V2O11, SrNb2V2O11, SrTa2V2O11, and PbNb 2 V2 O 11 were reported. 1,2 The powder XRD patterns1,2 of SrNb2V2O11, SrTa2V2O11, and PbNb2V2O11 compounds do not provide any structural information. The reported 1 single-crystal X-ray structure of BaNb 2 V 2 O 11 compound warrants redetermination to improve the structure agreement factor value of 8.2%. A synthetic and structural study of known and new quaternary AM2V2O11 (A = Ba, Sr, Ca, Pb, Cd; M = Nb, Ta) vanadates is considered worthy and, therefore, undertaken from the point of view of (i) isolation of new isomorphous compounds, (ii) their structure determination for identification of probable new structure types, and (iii) evaluation of the SHG responses of those compounds, if any, with noncentrosymmetric structures. Moreover, such a study is in line with our previous synthetic and structural investigation8 on AMV2O8 (A = K, Rb, Cs, Tl; M = Nb, Ta) vanadates of niobium and tantalum. The present study pertains to four

INTRODUCTION BaNb2V2O11, BaTa2V2O11, SrNb2V2O11, SrTa2V2O11, and PbNb2V2O11 are five isomorphous AM2V2O11 (A = Ba, Sr, Pb; M = Nb, Ta) vanadates of niobium and tantalum, reported1,2 first by Trunov et al. and Murashova et al. The similarity of powder X-ray diffraction (XRD) patterns of BaNb2V2O11 and BaTa2V2O11 and the single-crystal X-ray structure of BaNb2V2O11 have established3 that both are isostructural layered compounds, which crystallize in centrosymmetric R3m ̅ space group with ah ≈ 5.53 Å and ch ≈ 28.1 Å. Their layered structure4 and energy band gap value of ∼2.22 eV are considered to be the desired features for visible-light-driven photocatalysts for “splitting of water”, and a study of evaluation of photocatalytic performance of BaNb2V2O11 was indeed reported5 recently. The powder XRD patterns of the other three vanadates, SrNb2V2O11, SrTa2V2O11, and PbNb2V2O11, were indexed1,2 on orthorhombic super unit cells with a0 = 9.506−9.636, b0 = 5.527−5.453, and c0 = 26.82−27.25 Å. Their photocatalytic activity has not been investigated, probably due to lack of their structural information. A subsequent report6 of noncentrosymmetric C2221 space group with a0 = 10.95, b0 = 15.54, and c0 = 5.52 Å for SrTa2V2O11, on the basis of powder XRD pattern, indicates probable second-harmonic-generating (SHG) property of this compound. It is surprising that calcium and cadmium analogues have not been reported so far. In this context, it needs to be mentioned that three phosphate analogues, CaNb2P2O11 with a unique three-dimensional structure7 and BaNb2P2O11 and BaTa2P2O11 with a similar layered structure2 of BaNb2V2O11, are known. The Cd2+ and © XXXX American Chemical Society

Received: August 23, 2017

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DOI: 10.1021/acs.inorgchem.7b02170 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

noncentrosymmetric space groups, Cm and C2, led to structure agreement factor values of ∼2% and ∼6%, respectively, and, therefore, the space group Cm was chosen. The thermal parameters of oxygen atoms of SrNb2V2O11 (3) and SrTa2V2O11 (4) compounds were refined isotropically, whereas those of all other atoms of compounds 1−5 were refined anisotropically. The TOPAS-V4.2 (AXS) (Bruker AXS GmbH, Karlsruhe, Germany) program14 was used for the Le Bail fit of powder XRD data of PbTa2V2O11 (5). The refined parameters were scale factor, zero error, and sample displacement (mm), and background as Chebyshev polynomial of fifth grade and 1/x function, crystallite size, microstrain, and unit-cell parameters. The slow-scan powder XRD data for four compounds 1−4 were gathered with a step size of 0.02° and a step time 3 s, in the 2θ range of 5−75°. The GSAS-EXPGUI program15 was used for the Rietveld structure refinement from the powder XRD data. The refined parameters were scale factor, background as Chebyshev polynomial, unit-cell parameters, profile function (Gaussian and Lorentzian parameters, sample displacement), and atomic positions. For the four compounds 1−4, the initial structural models based on their single-crystal X-ray structures turned out to be correct ones. For all atoms, the isotropic thermal parameters from single-crystal X-ray structure were used and not refined. The positional parameters and profile functions were refined in alternate cycles, until no substantial changes were observed in the positional parameters. In only the first refinement cycle of positional parameters, the values of Nb/Ta−O and V−O bond lengths were constrained to be in the respective ranges of 1.849−2.088 Å and 1.619−1.755 Å, with tolerance of 0.05 Å. The structure refinements proceeded smoothly to yield acceptable agreement factors. Diffuse Reflectance Spectroscopy. The diffuse reflectance spectra were recorded on powder samples of compounds 2−5, at room temperature, on a JASCO UV−visible spectrophotometer with a diffuse reflectance accessory, over the range from 200 to 900 nm. BaSO4 was used as a 100% reflectance standard. The absorption was calculated data using the Kubelka−Munk equation,16 α/S = (1 − R)2/ 2R, where R, α, and S are the reflectance, absorption coefficient, and scattering coefficient, respectively. Spectroscopic, SEM, and EDAX Study. The polycrystalline samples of AM2V2O11 vanadates 2−5 were ground with dry KBr and pressed into transparent disks. The infrared spectra were measured on these transparent disks from 400 to 4000 cm−1 on JASCO FT/IR4100 spectrometer. The Raman spectra of powder samples of compounds 2−5 were measured from 50 to 4000 cm−1 with a Bruker RFS27 Fourier transform Raman spectrometer (wavelength: 1064 nm). Scanning electron microscope (SEM) images and energydispersive X-ray spectroscopy (EDAX) data of the vanadates 1−5 were recorded on FEI Quanta-200 and Quanta-450 scanning electron microscopes. Thermal Analysis. Thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) studies of polycrystalline samples of four compounds, 2−4 and 5pow, were performed, in the 35−1000 °C range under dynamic nitrogen flow, on SDT Q600 V20.9 Build-20 TA Instruments. The value of heating and cooling rates employed was 10 °C min−1. SHG Measurement. The polycrystalline samples of SrNb2V2O11 (3), SrTa2V2O11 (4), PbTa2V2O11 (5pow), and LiNbO3 compounds were ground and sieved into less than 25, 25−53, 53−105, 105−150, 150−203, 203−250, and 250−300 μm ranges of particle size. The sieved LiNbO3 powder samples were used as a reference. The powder SHG measurements were performed by modified Kurtz-Perry method.17 A fundamental wave of pulsed Nd:YAG laser operating at 1064 nm (Q-switched mode), producing 10 ns pulses at 10 Hz, was used for the optical second harmonic studies. Second harmonic signal at 532 nm, emanating from the sample, was recorded by using a photomultiplier tube, in conjunction with a monochromator. Ferro- and Dielectric Studies. The samples of SrNb2V2O11 (3), SrTa2V2O11 (4), and PbTa2V2O11 (5pow) compounds were pressed into pellets of ∼1 cm diameter and ∼3 mm thickness and sintered at 750 °C for 3 d. The pellets were cured at 150 °C for 12 h, after applying silver paste to both sides. Xylene was used as the liquid

known compounds, BaNb2V2O11 (1), BaTa2V2O11 (2), SrNb 2 V 2 O 11 (3), and SrTa 2 V 2 O 11 (4), and one new PbTa2V2O11 (5) compound. This paper reports (i) syntheses and three structural variants of compounds 1−5 and (ii) SHG, dielectric, and ferroelectric properties of strontium and lead compounds, 3−5, with noncentrosymmetric structure.

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EXPERIMENTAL SECTION

Synthesis and Crystal Growth. Nb2O5, Ta2O5, V2O5, Nb, and Ta metal powders, ACO3 (A = Sr, Ba), Pb(NO3)2, Pb3(CO3)2(OH)2, and ABr2·xH2O (A = Ba, Sr, and Pb) and known ternary SrNb2O6, SrTa2O6, and TaVO5 oxides were employed for the syntheses and crystal growth of the title compounds 1−5. The polycrystalline samples of compounds 1−5 were prepared, on a scale of 1 g, by heating stoichiometric mixtures of appropriate reactants in open air at 750 °C for 48 h, with two intermittent grindings. For barium and strontium compounds 1−4, ACO3 (A = Sr, Ba), M2O5 (M = Nb, Ta), and V2O5 were employed. The polycrystalline lead analogue, represented as PbTa2V2O11(5pow), was obtained by heating TaVO5 with either Pb(NO3)2 or PbBr2. It could also be obtained from a reactant mixture of Pb3(CO3)2(OH)2, Ta2O5, and V2O5. Similarly the two tantalum compounds, 2 and 4, were obtained from ACO3 (A = Sr, Ba) and TaVO5 as well. Single-crystal growth of BaNb2V2O11 (1) and BaTa2V2O11 (2) compounds was achieved in two steps. 1:2:2 molar reactant mixtures of BaCO3, M2O5 (M = Nb, Ta), and V2O5 were first heated in an alumina boat in open air at 600 °C for 10 h. The resulting solid products were mixed with 1 mol of BaBr2·2H2O, heated further at 900 °C for 1 d, and then cooled to 300 °C at 2 °C/h, and the furnace was finally turned off. Single crystals of SrNb2V2O11 (3) and SrTa2V2O11 (4) compounds were obtained, when 1:1:2:2 molar reactant mixtures of SrM2O6 (M = Nb, Ta), SrBr2·6H2O, Nb/Ta metal powders, and V2O5 were heated, in quartz tubes, in open air at 900 °C for 30 h and then cooled to 300 °C at 2 °C/h, and the furnace was finally turned off. Similarly single crystals of lead analogue, represented as PbTa2V2O11 (5xal), were obtained by heating stoichiometric reactant mixture of PbBr2 and TaVO5 in quartz tubes, in open air at 1000 °C for 1 d and then turning off the furnace. All of these crystal growth attempts led to formation of plate-shaped crystals of the desired vanadates 1−5, along with polycrystalline mixture of AM2O6, M9VO25 (A = Ba, Sr, Pb; M = Nb, Ta), and some unidentified phases. In the case of the strontium compounds, block-shaped single crystals of an orthorhombic structural modification, represented as SrNb2V2O11 (3ortho) and SrTa2V2O11 (4ortho), were additionally obtained. X-ray Diffraction and Crystal Structure. The powder XRD patterns of compounds 1−5 were recorded on a Bruker D8 Advanced powder X-ray diffractometer using Cu Kα (λ = 1.5406 Å) radiation. The powder XRD patterns were simulated, using the MERCURY program,9 on the basis of their single-crystal X-ray structures. Suitable single crystals of compounds 1−5 were selected and mounted on thin glass fiber with epoxy glue and optically aligned on a Bruker APEX II charge coupled device X-ray diffractometer using a digital camera. Intensity data were measured at 25 °C using Mo Kα (λ = 0.7103 Å) radiation. APEX II software (Bruker AXS) was used for preliminary determination of the cell constants and data collection control.10 The determination of integral intensities and global refinement were performed using SAINT-plus (Bruker AXS). A semiempirical absorption correction was subsequently applied using SADABS. Space group determination, structure solution, and leastsquares refinement were performed using SHELXTL program.11 DIAMOND and ORTEP were the graphic programs12,13 employed to draw the structures. The crystal structures of compounds 1−5 were solved by direct methods and refined by full matrix least-squares on F2. The final Fourier difference maps did not show any chemically significant feature, and the Fourier difference peaks with an electron density of greater than 1 e/Å3 were found to be ghosts. For both strontium compounds 3 and 4, the structure refinements in the two possible B

DOI: 10.1021/acs.inorgchem.7b02170 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. SEM images of (a) BaNb2V2O11(1), (b) BaTa2V2O11(2), (c) PbTa2V2O11(5xal), (d) SrNb2V2O11(3), (e) SrTa2V2O11(4), (f) SrNb2V2O11(3ortho), and (g) SrTa2V2O11(4ortho) compounds.

Table 1. Pertinent Crystallographic Data for AM2V2O11 (A = Ba, Sr, Pb; M = Nb, Ta) Compounds compound formula weight crystal system space group (No.) a (Å) b (Å) c (Å) β (deg) V (Å3) Z ρcalcd (g/cm3) μ(Mo Ka) (mm−1) crystal size (mm3) λ (Mo Kα) (Å) temperature (K) θ range (deg) total/independent reflections reflections with I > 2σ (I) Flack factor parameters refined R1/wR2 goodness of fit

BaNb2V2O11(1) 601.01 hexagonal R3̅m (166) 5.5301(4) 5.5301(4) 28.1637(14)

BaTa2V2O11(2) 777.10 hexagonal R3̅m (166) 5.5380(3) 5.5380(3) 28.1600(14)

PbTa2V2O11(5xal) 873.22 hexagonal R3̅m (166) 5.5297(3) 5.5297(3) 27.933(3)

745.91(8) 3 4.311 10.774 0.10 × 0.10 × 0.10 0.710 73 293(2) 2.17−27.89 1981/258 220

747.94(7) 3 5.176 27.599 0.08 × 0.10 × 0.12 0.710 73 293(2) 2.17−27.07 1209/248 237

739.70(10) 3 5.881 46.948 0.08 × 0.12 × 0.15 0.710 73 293(2) 4.32−26.39 2074/224 206

22 0.0253/0.0602 1.073

22 0.0211/0.0496 1.170

22 0.0540/0.1516 1.221

medium to measure the densities of the sintered pellets by the Archimedes’ method. The capacitance (Cp) and dielectric loss (D) were recorded simultaneously as a function of frequency (40−110 MHz) at room temperature, at a signal strength of 0.5 V (Vrms) using an Agilent

SrNb2V2O11(3) 551.30 monoclinic Cm (8) 9.5148(6) 5.5175(3) 9.5049(5) 109.700(3) 469.78(5) 2 3.897 9.970 0.10 × 0.12 × 0.15 0.710 73 293(2) 2.28−26.75 2102/990 858 0.34(3) 59 0.0521/0.1084 1.085

SrTa2V2O11(4) 727.40 monoclinic Cm (8) 9.5242(3) 5.5275(2) 9.5116(4) 109.715(2) 471.39(3) 2 5.125 30.700 0.08 × 0.10 × 0.15 0.710 73 293(2) 2.27−28.28 2032/892 844 0.33(4) 59 0.0307/0.0610 1.070

4294A Impedance Analyzer. From the capacitance data, the dielectric constant (ε′r ) of the samples was evaluated using the generic equation ε′r = Cpt/ε0A, where A and t are, respectively, the surface area and thickness of the ceramic pellet, and ε0 is the permittivity of free space (8.854 × 10−12 F m−1). The P−E hysteresis loops were recorded using C

DOI: 10.1021/acs.inorgchem.7b02170 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) Simulated powder XRD pattern of PbTa2V2O11 (5xal), (b) simulated and (c) observed powder XRD patterns of PbTa2V2O11 (5pow).

The single-crystal X-ray structures of all five compounds 1−5 were determined, and their pertinent crystallographic data are presented in Table 1. The positional and thermal displacement parameters and bond lengths are provided in the Supporting Information (Tables S1−S7). The structures of these five compounds are of three types, namely, centrosymmetric layered structure of BaTa2V2O11 (2), noncentrosymmetric layered structure of SrTa2V2O11 (4), and three-dimensional structure of SrTa2V2O11 (4ortho). The polycrystalline samples of barium and strontium compounds 1−4 were ascertained to be homogeneous by comparing their powder XRD patterns with the simulated ones and also from successful structure refinement of slow-scan powder XRD data (Figure S1) by Rietveld method.15 The values of refined fractional coordinates, bond lengths, and bond valence sums obtained from the powder X-ray diffraction data reasonably agree with those obtained from the single-crystal X-ray data (Tables S1−S7). In the case of PbTa2V2O11 compound, the polycrystalline PbTa2V2O11 (5pow) and single-crystal PbTa2V2O11 (5xal) samples have the structures of SrTa2V2O11 (4) and BaTa2V2O11 (2), respectively. Thus, the observed powder XRD pattern of PbTa2V2O11 (5pow) is distinctly different from the pattern simulated from its crystal structure PbTa2V2O11 (5xal) (Figure 2). PbTa2V2O11 (5xal) is a metastable phase formed in minute quantities at high temperature of 1000 °C, and PbTa2V2O11 (5pow) is a stable phase, for which crystal growth at high temperatures of greater than 750 °C is not successful. The single-crystal X-ray structure unit-cell parameters of SrTa2V2O11 (4) were used for Le Bail fit of powder XRD data of PbTa2V2O11 (5pow) vanadate by employing TOPASV4.2 (AXS) program (TOPAS-V4.2, 2008). The Le Bail fit led to the values of 9.4804(2) Å, 5.5767(5) Å, 9.6142(1) Å, 109.240(5)°, and 479.9140(5) Å3 for its monoclinic unit-cell parameters a, b, c, β, and V, respectively. These unit-cell parameters and the positional parameters of strontium compound were used for simulation of PbTa2V2O11 (5pow) powder XRD pattern, which is reasonably similar to its observed one (Figure 2). The powder XRD patterns of barium

a modified Sawyer−Tower circuit (Automated PE loop tracer; Radiant Technologies Precision Premier II Tester attached with Precision 10 kV−HVI-II amplifier) and analyzed for ferroelectric characteristics such as remnant polarization (Pr), maximum polarization (Pmax), and coercive field (Ec).

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RESULTS AND DISCUSSION BaNb 2 V 2 O 11 (1), BaTa 2 V 2 O 11 (2), SrNb 2 V 2 O 11 (3), SrTa2V2O11 (4), and new PbTa2V2O11 (5) compounds were successfully prepared in polycrystalline and single-crystal forms, by conventional solid-state reactions. Barium and strontium compounds 1−4 could be prepared in polycrystalline form from reactant mixtures of ACO3 (A = Sr, Ba), M2O5 (M = Nb, Ta), and V2O5. Similar reactant mixture of Pb(NO3)2/PbO, Ta2O5, and V2O5 for PbTa2V2O11 (5) compound always led to formation of known competitive phases such as PbTa2O6 and Ta9VO25. Polycrystalline PbTa2V2O11 (5pow) could be conveniently prepared by heating TaVO5 with Pb(NO3)2 or PbBr2 at 750 °C, above which the compound is found to undergo thermal decomposition. A reactant mixture of Pb3(CO3)2(OH)2, Ta2O5, and V2O5 also gives rise to polycrystalline PbTa2V2O11 (5pow). CaNb2V2O11, CaTa2V2O11, CdNb2V2O11, and reported PbNb2V2O11 vanadates could not be prepared under the various experimental conditions employed in this study. The powder XRD pattern of CdTa2V2O11 shows some resemblance to that of CaNb2P2O11 compound,7 but it could not be satisfactorily indexed. Single-crystal growth of compounds 1−5 was achieved by employing barium, strontium, and lead bromides at high temperatures of 900−1000 °C. The SEM images (Figure 1) show the plate morphology of their crystallites, and the results of EDAX analyses correspond to ∼1:2:2 ratio of A/M/V elements in these five compounds. Another structural modification of strontium compounds, represented as SrNb2V2O11 (3ortho) and SrTa2V2O11 (4ortho), was observed to have formed additionally as block-shaped single crystals (Figure 1). D

DOI: 10.1021/acs.inorgchem.7b02170 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry compounds 1 and 2 differ from those of strontium and lead compounds 3, 4, and 5pow, in that they contain a high intense (108̅) reflection. The powder XRD pattern of PbTa2V2O11 (5pow), when compared to those of strontium compounds 3 and 4, has more reflections (Figure. 2). These five AM2V2O11 compounds, 1−5, have centrosymmetric and noncentrosymmetric types of layered structure, wherein [M2V2O11]2− anionic layers are interleaved with A2+ cations. The [M2V2O11]2− anionic layer could be conceived to be built from MO6 octahedra and VO4 tetrahedra as follows. A ⟨111⟩-oriented ReO3 type of M2O9 layer of two-octahedra-thick is connected to VO4 tetrahedra on both sides (Figure. 3). In

Figure 4. Polyhedral representation of (top) the unit cell and (bottom) a segment of (Ta2V2O11)2− layer of (left) BaTa2V2O11 (2) and (right) SrTa2V2O11 (4) compounds.

Ba1/12Ta1/6V1/6O11/12, and all of these atoms reside on special Wyckoff sites. Ta is octahedrally coordinated to three each of O(1) and O(2) oxygen atoms, and each TaO6 octahedron is corner-connected to three such TaO6 octahedra and three VO4 tetrahedra through interior O(1) and exterior O(2) atoms, respectively (Figure 5). The vanadium of VO4 tetrahedron is bonded to three O(2) atoms and one O(3) atom. The Ta, O(1), and O(2) atoms constitute the Ta2O9 slab, in which the plane of interior O(1) atoms and the two adjacent planes of exterior O(2) atoms are parallel to the (001) plane of hexagonal unit cell. Thus, the [M2V2O11]2− anionic layers are parallel to the (001) plane of hexagonal unit cell. The respective Ta−O(1) and Ta−O(2) bond length values of 1.887(2) and 2.045(5) Å clearly indicate C3 octahedral distortion, wherein Ta is displaced from the center of octahedron toward the triangular face composed of three interior O(1) atoms. The electric dipoles due to these displacements in TaO6 octahedra point away from the plane of O(1) atoms and toward the plane of O(2) atoms. Therefore, they are aligned in the opposite [001] and [001̅] directions within the Ta2O9 layer (Figure 5), and thus their overall effect is null. The vanadium atom resides on the 6c Wyckoff site with threefold axis symmetry and forms three long V−O(2) and one short V−O(3) bonds of 1.728(6) and 1.625(9) Å lengths, respectively. As the short V−O(3) bond is parallel to the c-axis, the dipoles of VO4 tetrahedra with C3v symmetry are aligned in the opposite [001] and [001̅] directions toward the plane of O(2) atoms, and, therefore, they have no net effect. The barium ion resides on 3a Wyckoff site and bonds to six each of O(2) and O(3) oxygen atoms, and the values of Ba−O(2) and Ba− O(3) bond lengths are 2.8048(1) and 3.2055(2) Å, respectively. Its 12-coordination geometry is icosahedral (Figure. 6).

Figure 3. Polyhedral representation of (top) three-dimensional ReO3 framework, (middle) a ⟨111⟩-oriented ReO3 type of Ta2O9 layer of two-octahedra-thick, and (bottom) Ta2O9 layer connected to VO4 tetrahedra in Ta2V2O11 layer of BaTa2V2O11(2).

other words, each MO6 octahedron is corner-connected to three MO6 octahdera and three VO4 tetrahedra in the interior and exterior regions, respectively, of the [M2V2O11]2− anionic layer. Each VO4 tetrahedron is corner-connected to three MO6 octahedra, and its unshared fourth oxygen atom points toward the interlayer space, wherein the A2+ cations reside and bond to 11 or 12 oxygen atoms of VO4 tetrahedra of adjacent [M2V2O11]2− anionic layers. Crystal Structure of Centrosymmetric AM 2V 2O 11 Vanadates. The isostructural BaNb2V2O11 (1), BaTa2V2O11 (2), and PbTa2V2O11 (5xal) compounds crystallize in R3̅m space group, and their structure is described here by taking BaTa2V2O11 (2) as an example. It contains centrosymmetric [Ta2V2O11]2− anionic layers stacked along the crystallographic c-axis (Figure 4), and there are three such layers per unit cell. The three crystallographically independent oxygen atoms O(1)−O(3) and one each of barium, tantalum, and vanadium atoms constitute the asymmetric unit content of E

DOI: 10.1021/acs.inorgchem.7b02170 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. ORTEP diagrams of a segment of layered (Ta2V2O11)2− structural framework in (top) BaTa2V2O11 (2) and (bottom) SrTa2V2O11 (4) compounds. The arrows point toward the interior and adjacent exterior planes of oxygen atoms in the Ta2O9 slab. All atoms are represented with 50% probability.

Figure 6. ORTEP diagram of icosahedral geometry of 12 oxide ions around (top) Ba2+ in BaTa2V2O11 (2) and (bottom) Sr2+ in SrTa2V2O11 (4) compounds, viewed along (left) [001] and (right) [100] directions. All atoms are represented with 50% probability.

The values of individual dipole moments of Ta−O1, Ta−O2, V−O2, and V−O3 bonds were calculated18 to be 18.22, 14.72, 18.87, and 20.03 D, respectively (Table S.8). The net dipole moment values of 0.0028 D for TaO6 octahedron and 3.96 D for VO4 tetrahedron were obtained from vector summation of the dipole moments of six Ta−O bonds and four V−O bonds, respectively. The TaO6 octahedral dipoles, like the VO4 tetrahedral dipoles, are antiparallel to one another in the [Ta2V2O11]2− anionic layer, and, therefore, the net dipole moments have no net effect, and the structure is macroscopically not polar (Figure 7). This is true for the other two

isostructural compounds BaNb2V2O11 (1) and PbTa2V2O11 (5xal) (Table S10). Crystal Structure of Noncentrosymmetric AM2V2O11 Vanadates. SrNb2V2O11 (3), SrTa2V2O11 (4), and polycrystalline PbTa2V2O11 (5pow) are the other type of isostructural layered compounds, which crystallize in polar Cm space group. SrTa2V2O11 (4) is chosen as an example to describe their structure, which consists of noncentrosymmetric [Ta2V2O11]2− anionic layers stacked along the monoclinic c-axis (Figure 4). There is only one such layer per unit cell. The asymmetric unit content of Sr0.5TaVO5.5 contains 1, 2, 2, and 8 crystallographically independent atoms of strontium, tantalum, F

DOI: 10.1021/acs.inorgchem.7b02170 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. Electric dipole moment vectors (blue arrows) in (top) the TaO6 octahedron and (bottom) the VO4 tetrahedron in (left) BaTa2V2O11 (2) and (middle, right) SrTa2V2O11 (4) compounds.

D along [001] direction is obtained (Table S10) for noncentrosymmetric (Nb2V2O11)2− layer of SrNb2V2O11 (3). Within a sphere of 3.7 Å radius around strontium ion in compounds 3 and 4, there are 12 oxygen atoms, which form an icosahedron. However, this icosahedron is not as symmetric (Figure 6) as the one found for barium and lead ions in compounds 1, 2, and 5xal. Strontium ion is coordinated to only 11 of those icosahedral oxygen atoms; two Sr−O(8) bonds are ∼3.47 Å long, and the other nine Sr−O bonds are less than 3.00 Å long (Table S4). This difference in the coordinations of divalent lead, strontium, and barium ions in compounds 1−4 and 5xal seems to determine whether or not the [M2V2O11]2− layer is puckered and the structure is noncentrosymmetric. For these five layered compounds 1−5, the bond valence sums for niobium, tantalum, and vanadium are calculated19 to be in the range of 5.17−5.69. The second-order Jahn−Teller distortions of octahedrally coordinated d0 metal ions, Nb5+, and Ta5+ ions are moderate, because the values of octahedral distortion,20 Δd, are calculated to be in the range of 0.3804−0.5838. Noncentrosymmetric C222 1 Modification of SrM2V2O11 (M = Nb, Ta) Vanadates. The third structural variant of this study refers to three-dimensional structural modification of strontium compounds SrNb2V2O11 (3ortho) and SrTa2V2O11 (4ortho), which crystallize in noncentrosymmetric orthorhombic space group C2221. They are obtained, in the crystal growth attempts, as block-shaped crystals (Figure 1) along with platelike crystals of SrNb2V2O11 (3) and SrTa2V2O11 (4), and the results of their EDAX analyses correspond to the expected 1:2:2 ratio of Sr/(Nb or Ta)/V atomic content. The values of orthorhombic unit-cell parameters a, b, and c, respectively, are 10.949(4), 15.488(5), and 5.4822(15) Å for SrNb 2 V 2 O 11 (3 ortho ) and 10.9950(3), 15.5617(2), and 5.5095(7) Å for SrTa2V2O11 (4ortho) (Table S5). The space group and the values of unit-cell parameters of SrTa2V2O11 (4ortho) are found to be same as those reported.6 The quality of single-crystal X-ray data of these two compounds was good enough to obtain a structure model, which could only be refined to yield unsatisfactory values of ∼15% for structure agreement factors. The three-dimensional structure of SrTa2V2O11 (4ortho) is imagined to consist of Ta2V2O13 columns that are aligned parallel to the c-axis and located at the origin and C-center (Figure S.3). They are linked to one another by connecting VO4 tetrahedra and TaO6 octahedra of one column with TaO6 octahedra and VO4 tetrahedra of four

vanadium, and oxygen, respectively, which reside on Wyckoff sites 2a and 4b. Ta(1), Ta(2), and O(1)−O(6) atoms constitute the Ta2O9 layer, in which Ta(1) is octahedrally bonded to O(1)−O(4) atoms, and Ta(2) is similarly bonded to O(1), O(2), O(5), and O(6) atoms. Ta(1)O6 octahedron is corner-connected to three Ta(2)O6 octahedra and three V(1)O4 tetrahedra through interior O(1) and O(2) atoms and exterior O(3) and O(4) atoms, respectively. In a similar fashion, Ta(2)O6 octahedron is corner-connected to three Ta(1)O6 octahedra and three V(2)O4 tetrahedra through interior O(1) and O(2) atoms and exterior O(5) and O(6) atoms, respectively. The layer of interior O(1) and O(2) atoms and similarly the two adjacent layers of exterior O(3)−O(6) atoms are undulated, as shown in Figure 5. Therefore, the [Ta2V2O11]2− anionic layer, parallel to the ab plane of unit cell, is also an undulated one. V(1) and V(2) vanadium atoms are tetrahedrally bonded to exterior atoms, O(3)−O(6) and also, respectively, to the fourth atoms, O(7) and O(8). The values of Ta−O and V−O bond lengths have wide ranges of 1.849(2)− 2.070(17) Å and 1.62(2)−1.755(18) Å, respectively. The Ta− O and V−O bond length values (Table S4) indicate the ∼C3 distortion of TaO6 octahedra and ∼C3v symmetry of VO4 tetrahedra. The electric diploes of crystallographically distinct TaO6 and VO4 polyhedra do not cancel one another as described below. Ta(1) and Ta(2) atoms are displaced from the centers of octahedra toward the triangular faces that are composed of interior O(1)−O(2) atoms, and, therefore, the dipoles due to these displacements in TaO6 octahedra point away from this interior plane and toward the planes of exterior O(3)−O(6) atoms. Therefore, the dipoles in Ta(1)O6 octahedra are aligned in antiparallel fashion to those in Ta(2)O6 octahedra (Figure 5), and the diploe moments of 3.1458 D for Ta(1)O6 and 2.1111 D for Ta(2)O6 octahedra (Table S9) lead to the net value of 1.0347 D (Figure 7). Similarly, V(1)O4 and V(2)O4 tetrahedra are so oriented that their short V(1)−O(7) and V(2)−O(8) bonds are slightly inclined to the undulated Ta2O9 layer, and their dipoles are almost antiparallel to each other. The diploe moments of 4.3315 and 3.5217 D for V(1)O4 and V(2)O4 tetrahedra, respectively (Table S9), lead to the net value of 0.8098 D (Figure 7). Vector summation over the (Ta2V2O11)2− layer gives a net dipole moment of 0.2246 D along the [001] direction. The dipole moment value of 2.4255 G

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solidified melts, which were obtained by cooling the bulk polycrystalline samples to room temperature from 1000 °C in open air. Second-Harmonic Generation Studies. Powder SHG measurements, in the 25−53 μm particle size range, of SrNb2V2O11 (3), SrTa2V2O11 (4), and PbTa2V2O11 (5pow) compounds revealed their SHG efficiencies of ∼40%, ∼50%, and ∼33% of LiNbO3, respectively. The plots of particle size versus SHG efficiency (Figure 9) indicate that these three

adjacent columns, respectively. The positional and thermal displacement parameters, bond lengths, and the description of the unrefined structure model of SrTa2V2O11 (4ortho) are given in the Supporting Information. SrNb2V2O11 (3ortho) and SrTa2V2O11 (4ortho) could not be synthesized in polycrystalline form by the solid-state method. Their simulated powder XRD patterns, when compared to those of SrNb2V2O11 (3) and SrTa2V2O11 (4), contain more reflections (Figure S5). The centrosymmetric version of this three-dimensional structure was reported7 for CaNb2P2O11, which crystallizes in C2/c space group. Diffuse Reflectance Spectroscopic Study. The UV− visible absorption spectra (Figure. 8), derived from the diffuse

Figure 9. SHG intensities and calculated ⟨deff⟩ values of SrNb2V2O11 (3), SrTa2V2O11 (4), PbTa2V2O11 (5pow), and LiNbO3.

compounds exhibit type 1 phase-matching behavior.17 The average nonlinear optical (NLO) susceptibility ⟨deff⟩exp values of 19.40, 20.85, and 17.61 pm/V for the respective 3, 4, and 5pow vanadates are considered to be large enough for suitable NLO applications.27 They were estimated28 by using the following equation.

Figure 8. Solid-state UV−visible absorption spectra of BaTa2V2O11 (2), SrNb2V2O11 (3), SrTa2V2O11 (4), and PbTa2V2O11 (5pow) compounds.

reflectance data with the use of the Kubelka−Munk relation, of 2, 3, 4, and 5pow compounds, show that they have optical bandgap values of 2.36, 2.01, 2.45, and 2.24 eV, respectively. These four compounds are wide band-gap semiconductors. Infrared and Raman Spectroscopic Study. The infrared and Raman spectral features of these vanadates 2−5 are assigned as per the literature reports on phosphates8,21−24 and vanadates.25 Raman spectra (Figure S6) contain the stretching and bending modes of vibration, both symmetric and asymmetric, of the VO4 tetrahedron in the ranges of 953− 1020 and 400−560 cm−1, respectively. The (ν1), (ν2), and (ν3) modes of vibration of the NbO6/TaO6 octahedron are also observed at 854−871, 670−694, and 492−553 cm −1 , respectively.26 Only some of these vibrational modes could be clearly identified in their infrared absorption spectra. TGA and DSC Studies. The TGA plots (Figure S7) of compounds 2, 3, 4, and 5pow show that they do not undergo any thermal weight loss or gain up to ∼1000 °C. The DSC plot (Figure S7) of compound 2 is different from those of compounds 3, 4, and 5pow. The endothermic peaks at ∼815, ∼969, and ∼815 °C observed for respective compounds 3, 4, and 5pow probably correspond to their structural phase transition from noncentrosymmetric monoclinic modification to centrosymmetric hexagonal modification. Barium compound 2 with centrosymmetric structure does not undergo phase transition. These four compounds melt in the temperature range of 900−1000 °C, and the molten samples recrystallize at ∼750 °C to their room-temperature structural modification. This was deduced from the powder XRD patterns of their

deff = [798(I2ω(AM 2V2O11)/(I2ω(LiNbO3)]1/2

Ferroelectric Studies. The plots of polarization versus applied electric field at room temperature for SrNb2V2O11 (3), SrTa2V2O11 (4), and PbTa2V2O11 (5pow) compounds are shown in Figure S8. The unsaturated polarization, even after applying high electric fields, suggests that the ferroelectric domains may not completely switch at room temperatures. Furthermore, the shapes of the loops suggest the presence of finite electrical conductivity in the systems. The values of maximum polarization Pmax and remnant polarization Pr for these three compounds are presented in Figure S8. Dielectric Studies. The variations of dielectric constant (εr) and dielectric loss (D) as a function of frequency for SrNb2V2O11 (3), SrTa2V2O11 (4), and PbTa2V2O11 (5pow) compounds are shown in Figure 10. The values of εr are higher for niobium compound 3 than the two tantalum compounds 4 and 5pow at all frequencies under study. The values of εr remain invariant over large frequency ranges from 1 × 102 to 1 × 108 Hz for tantalum compounds 4 and 5pow and from 1 × 104 to 1 × 107 Hz for niobium compound 3. The D is observed for compounds 3, 4, and 5pow in the frequency ranges from 50 to 1 × 103, from 50 to 1 × 104, and from 50 to 1 × 105 Hz, respectively. The significantly low dielectric loss in the 1 × 105 to 1 × 108 Hz frequency range is an indication of their potential for capacitor applications. H

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Figure 10. Room-temperature variation of (left) εr and (right) D of SrNb2V2O11 (3), SrTa2V2O11 (4), and PbTa2V2O11 (5pow) compounds with the frequency.

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Notes

CONCLUSION BaNb 2 V 2 O 11 (1), BaTa 2 V 2 O 11 (2), SrNb 2 V 2 O 11 (3), SrTa2V2O11 (4), and new PbTa2V2O11 (5) compounds have been prepared by solid-state reactions and structurally characterized by single-crystal and powder X-ray diffraction studies. All of them have layered structure, wherein [Ta2V2O11]2− anionic layers are interleaved with divalent barium, strontium, and lead ions. Barium compounds possess centrosymmetric structure. The strontium and lead compounds have noncentrosymmetric structure, exhibit SHG and ferroelectric properties, and undergo reversible structural phase transition at high temperature. A metastable three-dimensional structural variant has also been found for strontium compounds. A partial assignment of their infrared and Raman spectra has been performed. All five compounds are wide bandgap semiconductors.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The X-ray powder diffractometer in our department of chemistry was purchased with financial assistance, received under FIST scheme (SR/FST/CSI-158/2007), from SERC Division of Department of Science and Technology, Ministry of Science and Technology, Government of India. Indian Institute of Technology Madras provided the single-crystal X-ray diffractometer facility in the department of chemistry. We thank Mrs. S. Srividya and Mr. V. Ramkumar for the powder and single-crystal X-ray data collection, respectively. We also thank Sophisticated Analytical Instrument Facility of our institute for electron microscopic and spectroscopic facilities. Research Fellowship (Award No. F 2-12/2002(SA-I)) for A.K.P., from Univ. Grants Commission, Government of India, is acknowledged.

ASSOCIATED CONTENT

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02170. Final Rietveld XRD data plots, tables of atomic fractional coordinates, thermal parameters (Å2), and bond lengths, dipole moment calculations, structural description of SrTa2V2O11 (4ortho), P−E plots showing ferroelectricity, TGA, DSC plots, IR and Raman spectroscopic data for BaNb2V2O11 (1), BaTa2V2O11 (2), SrNb2V2O11 (3), SrTa2V2O11 (4), and PbTa2V2O11 (5) compounds (PDF)

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Accession Codes

CCDC 1565100 and 1565102−1565105 contain 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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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kanamaluru Vidyasagar: 0000-0003-2939-1255 I

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J

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