Synopsis of SYNTHESES, STRUCTURAL VARIANTS ...

Synopsis of SYNTHESES, STRUCTURAL VARIANTS AND PROPERTIES OF QUATERNARY VANADATES OF NIOBIUM AND TANTALUM, AMV2O8 and AM2V2O11 (A = K, Rb, Cs, Tl, Ba, Sr, Pb; M = Nb, Ta)

A THESIS

to be submitted by

ANIL KUMAR PAIDI

for the award of the degree of DOCTOR OF PHILOSOPHY

DEPARTMENT OF CHEMISTRY INDIAN INSTITUTE OF TECHNOLOGY MADRAS CHENNAI – 600 036 APRIL 2017

SYNTHESES, STRUCTURAL VARIANTS AND PROPERTIES OF QUATERNARY VANADATES OF NIOBIUM AND TANTALUM, AMV2O8 AND AM2V2O11 (A = K, Rb, Cs, Tl, Ba, Sr, Pb; M = Nb, Ta) 1.

INTRODUCTION The solid state and material chemistry of crystalline oxides of group V metals,

namely, vanadium, niobium and tantalum, has been important from the point of view of applications based on their redox, electrochemical, photocatalytic, magnetic and nonlinear properties (Whittingham et al., 1996, Ishizawa et al., 1975, Korili et al., 1996, Kudo et al., 2000, Halasyamani and Poeppelmeier, 1998). In oxides, vanadium is found to be stable in +3, +4 and +5 oxidation states, whereas niobium and tantalum are mostly found in +4 and +5 oxidation state. The three metal ions are found to have octahedral coordination. However, vanadium in +5 oxidation state is found to be also five coordinated with mostly squarepyramidal geometry and four coordinated with tetrahedral geometry. In fact, tetrahedral geometry is more prevalent than the other two geometries for pentavalent vanadium in oxides. In view of three possible coordination geometries of V5+ in oxides, the vanadates have structural diversity and a wide range of compositions. The coordination geometries of V5+ ions found in vanadium phosphates (Boudin et al., 2000) are shown in Figure 1.

Figure 1. The different environments of V5+ ion encountered in the vanadium phosphates. The V–O distances are given in Å. (Reproduced from Boudin et al., 2000). V5+, Nb5+ and Ta5+ ions are d0 transition metals ions, which exhibit second order JahnTeller distortions of their coordination polyhedra in oxides. These distortions could lead to

non-centrosymmetric structures and possibly second-harmonic generating (SHG) property (Halasyamani and Poeppelmeier, 1998). Moreover, semi-conducting layered oxides of Nb5+ and Ta5+ metals ions are known to function as photocatalytic materials. The well-known examples are K4Nb6O17 and KTaO3 (Kudo et al., 2000). Therefore, quaternary vanadates of niobium and tantalum would be interesting from the point of view of wide ranging compositions, structural diversity and possible properties due to their structural features. Only eight such quaternary vanadates are known. On the other hand, thirty five quaternary phosphates of niobium and tantalum are reported. Four quaternary ASbV2O8 (A = K, Rb, Tl, Cs) vanadates were isolated in previous exploratory synthetic and structural investigations for quaternary vanadates (Devi and Vidyasagar, 1998). The cesium compound has layered yavapaiite type structure, whereas the potassium, rubidium and thallium compounds have a related three-dimensional structure. Therefore, there is a definite scope for an exploratory synthesis of similar A+M5+V2O8 (A = K, Rb, Tl, Cs; M = Nb, Ta) quaternary vanadates. Their structural elucidation would help to understand the common structural relation, if any. There are only few reports of quaternary vanadates containing alkaline earth metals. BaNb2V2O11, BaTa2V2O11, SrNb2V2O11, SrTa2V2O11 and PbNb2V2O11 are five isomorphous AM2V2O11 (A = Ba, Sr, Pb; M = Nb, Ta) vanadates of niobium and tantalum, reported first by Trunov et al., (1985) and Murashova et al., (1986). The similarity of powder X-ray diffraction (XRD) patterns of BaNb2V2O11 and BaTa2V2O11 (Shpanchenko and Antipov, 2000) and the single crystal X-ray structure of BaNb2V2O11 have established (Trunov et al., 1985) that both are isostructural layered compounds. Their layered structure (Villars et al., 2007) and energy band gap value of ~2.22 eV are considered to be the desired features for visible-light-driven photo catalysts for “splitting of water” and a study of photocatalytic degradation of methylene blue by BaNb2V2O11 was indeed reported (Qin et al., 2016) recently. A subsequent report (Shpanchenko and Antipov, 2000) of a non-centrosymmetric C2221 space group for SrTa2V2O11(4), on the basis of powder XRD pattern, indicates probable second-harmonic-generating (SHG) property of this compound. Syntheses and structural elucidation of the remaining AM2V2O11 compounds and a study of their SHG and photocatalytic properties is desirable for complete characterization of the series of quaternary AM2V2O11 vanadates.

2.

OBJECTIVE AND SCOPE

a) In continuation of the previous study on ASbV2O8 (A = K, Rb, Tl, Cs) compounds reported from this laboratory (Devi and Vidyasagar, 1998), a similar synthetic and structural study of their niobium and tantalum analogues, AMV2O8 (A = K, Rb, Tl, Cs; M = Nb, Ta) is undertaken to understand their structural variants and, if any, common structural motif amongst them. b) In the context of a report of a non-centrosymmetric C2221 space group for SrTa2V2O11 (Shpanchenko and Antipov, 2000), a synthetic and structural study of known and possible new quaternary AM2V2O11 (A = Ba, Sr, Ca, Pb, Cd; M = Nb, Ta) vanadates is undertaken from the point of view of (i) new isomorphous compounds, (ii) probable new structure types and (iii) SHG responses of those compounds, if any, with non-centrosymmetric structures. c) As BaNb2V2O11 was reported (Qin et al., 2016) to exhibit photocatalytic activity in terms of degradation of methylene blue (MB) solution, AM2V2O11 (A = Ba, Sr, Ca, Pb, Cd; M = Nb, Ta) vanadates are considered to be worthy of investigation for their photocatalytic activity for hydrogen generation. 3

DESCRIPTION OF THE RESEARCH WORK

3.1

A+M5+V2O8 (A = K, Rb, Cs, Tl; M = Nb, Ta) vanadates: Syntheses and crystal growth of ANbV2O8 (A = K(1), Rb(2), Tl(3), Cs(4)) and ATaV2O8

(A = K(5), Rb(6), Tl(7), Cs(8)) compounds have been carried out by conventional solid state method of heating stoichiometric mixtures of appropriate reactants in open air. Single crystals of seven vanadates, ANbV2O8 (A = Rb(2), Tl(3), Cs(4)) and ATaV2O8 (A = K(5), Rb(6), Tl(7), Cs(8)), were obtained for X-ray diffraction study. The structure of KNbV2O8(1) compound was established from Rietveld structural refinement of their slow-scan powder XRD data (Figure. 2) only. Two isostructural cesium compounds, 4 and 8, possess a new structure type with a monoclinic unit cell, whereas the remaining six compounds have the structure of known (Devi and Vidyasagar, 1998) orthorhombic KSbV2O8. The three-dimensional [(MV2O8)−]∞ (M = Nb, Ta) anionic frameworks of both structure types of compounds 1–8 could be conceived to be built from one-dimensional M2V4O18 ribbons as follows. Two MV2O10 kröhnkite chains are fused, whereby the yellow tetrahedral row of one chain is corner connected to the pink octahedral row of the other chain, to give rise to one-dimensional

M2V4O18 ribbon (Figure 3). Two- or three-dimensional [(MV2O8)−]∞ structural frameworks of vanadates are obtained by linking M2V4O18 ribbons to one another, through sharing rows of unshared oxygen atoms (Figure. 3) of two types of polyhedral rows, i.e., blue tetrahedral and pink octahedral rows.

Figure. 2. Final Rietveld XRD data plots of KNbV2O8(1) compound with the values of agreement factors and χ2 (red, observed; green, calculated; black, vertical bars – positions of the Bragg reflections; pink, difference between observed and calculated intensities).

Figure. 3. End-on (left) and side (right) views of the segment the M2V4O18 ribbon. The arrows point to the rows of oxygen atoms involved in the interconnectivity of M2V4O18 ribbons. In the structure type of six compounds, 1-3 and 5-7, blue tetrahedral and pink octahedral rows of one M2V4O18 ribbon are respectively corner-connected to pink octahedral and blue tetrahedral rows of other (Figure. 4) ribbons. In the new type of three-dimensional

[(MV2O8)−]∞ framework of cesium compounds, 4 and 8, blue tetrahedral and pink octahedral rows of one M2V4O18 ribbon are respectively corner-connected to blue tetrahedral and pink octahedral rows of other (Figure. 4) ribbons. This mode of connectivity of each M2V4O18 ribbon to four such ribbons accounts for the existence of pink bioctahedral M2O11, blue pyrovanadate V2O7 and yellow tetrahedral VO4 moieties in the [(MV2O8)−]∞ structural framework.

Figure. 4. Polyhedral representation of the unit cells of (left) KTaV2O8(5) and (right) CsTaV2O8(8), compounds. Blue ellipse refers to one representative Ta2V4O18 ribbon in the (TaV2O8)− framework containing corner-connected pink TaO6 octahedra and blue and yellow VO4 tetrahedra. Thus the present and previous (Devi and Vidyasagar, 1998) research findings together represent the syntheses and structural elucidation of twelve AMV2O8 (A = K, Rb, Cs, Tl; M = Sb, Nb, Ta) vanadates, which have three structure types. Nine AMV2O8 (A = K, Rb, Tl; M = Sb, Nb, Ta) and two CsMV2O8 (M = Nb, Ta) vanadates respectively have the first and second types of three-dimensional structures described above. CsSbV2O8 has the third structure type (Devi and Vidyasagar, 1998), which is a highly puckered variant of the layered yavapaiite structure (Fleck and Kolitsch, 2003). 3.2

AM2V2O11 (A = Ba, Sr, Pb; M = Nb, Ta) vanadates: BaNb2V2O11(1),

BaTa2V2O11(2),

SrNb2V2O11(3),

SrTa2V2O11(4)

and

new

PbTa2V2O11(5xal) compounds have been successfully prepared in polycrystalline and single crystal forms, by conventional solid state reactions. The single crystal X-ray structures of all five compounds 1-5 have been determined. 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 (Figure 5) of PbTa2V2O11(5pow) is distinctly different from the pattern simulated (Figure 5a) from its crystal structure PbTa2V2O11(5xal).

Figure 5. a) Simulated PbTa2V2O11(5xal), b) Simulated PbTa2V2O11(5pow) and c) observed PbTa2V2O11(5pow) powder XRD patterns. These

five

AM2V2O11

compounds,

1-5,

have

centrosymmetric

noncentrosymmetric types of layered structure, wherein [M2V2O11]

2-

and

anionic layers are

interleaved with A2+ cations. The [M2V2O11]2- anionic layer could be conceived to be built from MO6 octahedra and VO4 tetrahedra. Isostructural BaNb2V2O11(1), BaTa2V2O11(2) and PbTa2V2O11(5xal) compounds crystallize in R3̅m space group. It contains centrosymmetric [Ta2V2O11]2- anionic layers stacked along the crystallographic c-axis (Figure 6) and there are three such layers per unit cell. SrNb2V2O11(3), SrTa2V2O11(4) and polycrystalline PbTa2V2O11(5pow) are the other type of isostructural layered compounds, which crystallize in polar Cm space group. Their structure consists of noncentrosymmetric [Ta2V2O11]2- anionic layers, which are undulated and stacked along the monoclinic c-axis (Figure 6) and there is only one such layer per unit cell. Ba2+, Sr2+ and Pb2+ ions are twelve coordinated with icosahedral geometry. The Nb-O, Ta-O and V-O bond length values indicate the ~C3 distortion of NbO6 and TaO6 octahedra and ~C3v symmetry of VO4 tetrahedra. The electric

dipoles associated with these distortions cancel one another completely in centrosymmetric structures of the first type and partially in non-centrosymmetric structures of the second type.

Figure 6. Polyhedral representation of the unit cell of (left) BaTa2V2O11(2) and (right) SrTa2V2O11(4) compounds, viewed along b-axis. The third structural variant of this study refers to three-dimensional structural modification of strontium compounds, SrNb2V2O11(3ortho) and SrTa2V2O11(4ortho), which crystallize in non-centrosymmetric orthorhombic space group, C2221.

Figure 7. Polyhedral representation of the unit cell of SrTa2V2O11(4ortho) compound viewed along ~c-axis. Blue ellipse refers to one representative Ta2V4O13 column in the (Ta2V2O11)2− framework.

The three-dimensional structure of SrTa2V2O11(4ortho) is imagined to be obtained by connecting Ta2V2O13 columns. These Ta2V2O13 columns are aligned parallel to the c-axis and (Figure 7) and linked to one another by connecting tetrahedra and octahedra of one column with octahedra and tetrahedra of another column respectively. The eleven coordinated strontium ions are located in the void channels. 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 8) indicate that these three compounds exhibit type 1 phase matching behaviour (Kurtz and perry, 1968). The estimated average values of NLO susceptibility ⟨deff⟩exp for the 3, 4 and 5pow vanadates are 19.40, 20.85 and 17.61 pm/V respectively (Ok et.al., 2006).

Figure 8. SHG intensities and calculated values of AM2V2O11 vanadates and LiNbO3. 3.3

Photocatalytic activity of layered AM2V2O11 (A = Ba, Sr, Pb; M = Nb, Ta) oxides for hydrogen evolution : BaNb2V2O11(1), BaTa2V2O11(2), SrNb2V2O11(3), SrTa2V2O11(4) and PbTa2V2O11(5)

compounds were evaluated for their photocatalytic activity for hydrogen generation, under visible light irradiation. In the absence of any cocatalyst, only BaNb2V2O11(1), BaTa2V2O11(2) and SrTa2V2O11(4) compounds displayed photocatalytic activity (Figure 9), resulting in ~52, ~78 and ~158 μ mol/g H2 evolution respectively, at 4 h continuous irradiation of visible light.

Figure 9. H2 evolution from methanol−water mixture, under visible light irradiation for 4 h interval, catalyzed by AM2V2O11 compounds, 2-5 with (red) no cocatalyst, (green) platinum cocatalyst and (blue) NiO cocatalyst. BaTa2V2O11(2), SrNb2V2O11(3), SrTa2V2O11(4) and PbTa2V2O11(5) compounds showed photocatalytic activity for hydrogen generation, in presence of two cocatalysts, Pt and NiO. The platinum cocatalyst enhanced the hydrogen evolution, when compared to the situation without any cocatalyst. SrTa2V2O11(4) loaded with 0.5 wt.% of NiO shows the maximum activity of 237 μmol/g. 4. CONCLUSIONS In this doctoral work on quaternary vanadates of niobium and tantalum, thirteen AMV2O8 and AM2V2O11 (A = K, Rb, Cs, Tl, Ba, Sr, Pb; M = Nb, Ta), compounds have been synthesized by solid state reactions and structurally characterized by single crystal and powder X-ray diffraction studies. Eight of them are new compounds. Out of eight AMV2O8 (A = K, Rb, Cs, Tl; M = Ta, Nb) vanadates, the two cesium compounds possess an entirely new type of three-dimensional structure with bioctahedral M2O11, pyrovanadate V2O7 and tetrahedral VO4 moieties. The other six vanadates have the known structure of KSbV2O8. A structural correlation between the twelve A+M5+V2O8 vanadates and the nineteen A+M5+P2O8 phosphates of niobium, tantalum and antimony was made. BaNb2V2O11, BaTa2V2O11, SrNb2V2O11, SrTa2V2O11 and new PbTa2V2O11 compounds 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. All five compounds are wide band gap semiconductors. All of them, except BaNb2V2O11, function as photocatalysts for hydrogen generation, under visible light irradiation. 5. REFERENCES 1. Whittingham. M. S. (2004) Lithium Batteries and Cathode Materials, Chem. Rev., 104, 4271−4301. 2. Korili, S. A., P. Ruiz and B. Delmon (1996) Oxidative dehydrogenation of n-pentane on magnesium vanadate catalysts, Catal. Today, 32, 229–235 3. Ishizawa, N., F. Maruma, T. Kawamura and M. Kimura (1975) The crystal structure of Sr2Nb2O7, a compound with perovskite-type slabs, Acta Crystallogr, B31, 1912-1915. 4. Kudo, A., Kato, H. and S. Nakagawa (2000) Water splitting into H2 and O2 on new Sr2M2O7 (M = Nb and Ta) photocatalysts with layered perovskite structures: factors affecting the photocatalytic activity. J. Phys. Chem. B., 104(3), 571-575. 5. Boudin, S., A. Guesdon, A. Leclaire and M. M. Borel (2000) Review on vanadium phosphates with mono and divalent metallic cations: syntheses, structural relationships and classification, properties, Int. J. Inorg. Mater., 2, 561–579. 6. Halasyamani. P. S. and K. R. Poeppelmeier (1998) Noncentrosymmetric Oxides Chem. Mater., 1998, 10, 2753-2769 7. Devi. R. N. and K. Vidyasagar (1998) Synthesis and characterization of new vanadates of antimony, ASbV2O8 (A = K, Rb, Tl or Cs), J. Chem. Soc., Dalton Trans., 3013–3019. 8. Trunov, V. K., E. V. Murashova, Y. V. Oboznenko, Y. A. Velikodnyi and L. N. Kinzhibalo (1985) The BaO-Nb2O5-V2O5 system. Russ. J. Inorg. Chem., 30, 269−271. 9. Murashova, E. V., V. K. Trunov, and Y. A. Velikodnyi (1986) The Crystal Structures of the BaNb2P2O11 and NbPO5 Formed in the BaO-NbO-P2O5 System, Russ. J. Inorg. Chem., 31, 951-952. 10. Shpanchenko. R. and E. Antipov (1999) Powder Diffraction, File No. 51-0419, ICDD Grant-in-Aid. 11. Villars, P., K. Cenzual, J. Daams, R. Gladyshevskii, O. Shcherban, V. Dubenskyy, K. N. Melnichenko, O. Pavlyuk, I. Savysyuk, S. Stoyko (2007) Structure Types. Part 5: Space Groups (173) P63-(166) R-3m. Springer: Berlin and Heidelberg, Germany, 692−692.

12. Qin, L., P. Cai, C. Chen, H. Cheng, J. Wang, S. I. Kim and H. J. Seo (2016) Enhanced Visible Light-Driven Photocatalysis by Eu3+-Doping in BaNb2V2O11 with Layered Mixed-Anion Structure, J. Phys. Chem. C., 120, 12989−12998. 13. Shpanchenko. R. and E. Antipov (2000) Powder Diffraction, File No. 52-1582, ICDD, Grant-in-Aid. 14. Fleck. M. and U. Kolitsch (2003) Natural and synthetic compounds with kröhnkite-type chains. An update, Z. Kristallogr., 218, 553–567. 15. Kurtz S. K., and T. T. Perry (1968) A Powder Technique for the Evaluation of Nonlinear Optical Materials, J. Appl. Phys., 39, 3798-3813. 16. Ok, K. M., E. O. Chi and P. S. Halasyamani (2006) Bulk characterization methods for non-centrosymmetric materials: second harmonic generation, piezoelectricity, pyroelectricity, and ferroelectricity, Chem. Soc. Rev., 35, 710-717. 6. PROPOSED CONTENTS OF THESIS Chapter 1:

Introduction

Chapter 2:

AMV2O8 (A = K, Rb, Tl, Cs; M = Nb, Ta) vanadates

Chapter 3:

AM2V2O11 (A = Ba, Sr, Pb; M = Nb, Ta) vanadates

Chapter 4:

Photocatalytic activity of layered AM2V2O11 (A = Ba, Sr, Pb; M = Nb, Ta) oxides for hydrogen evolution

Chapter 5:

Summary and conclusions

7. LIST OF PUBLICATIONS BASED ON THIS RESEARCH WORK 7.1 Refereed journals 1. Paidi, A. K., R. N. Devi and K. Vidyasagar (2015) Synthesis and structural characterization of AMV2O8 (A = K, Rb, Tl, Cs; M = Nb, Ta) vanadates: a structural comparison of A+M5+V2O8 vanadates and A+M5+P2O8 phosphates, Dalton Trans., 44, 17399−17408. 2.

Paidi, A. K., P. W. Jaschin, K. B. R. Verma and K. Vidyasagar* (2017) Syntheses and characterization of AM2V2O11 (A = Ba, Sr, Pb; M = Nb, Ta) vanadates with centrosymmetric and non-centrosymmetric structures. (Inorg. Chem. 2017. Article ASAP)

3. Walko. P. S., A. K. Paidi, and K. Vidyasagar* (2017) Synthesis, structural characterization and ion exchange properties of A3Sb3P2O14·3H2O (A = Rb, Cs, Tl) Phosphates. (Manuscript under review).

7.2 Presentations in conferences 1. A. K. Paidi., R. N. Devi and K. Vidyasagar, “Synthesis and structural characterization of AMV2O8 (A = K, Rb, Tl, Cs; M = Nb, Ta) vanadates, Poster presented in the symposium on “ISCAS - 2015”, held at University of Delhi, Delhi, India, during May 08-10th, 2015. (ISCAS-Best poster award ) 2. A. K. Paidi., R. N. Devi and K. Vidyasagar, Synthesis and structural characterization of new quarternary vanadates of niobium and tantalum, AMV2O8 (A = K, Rb, Tl, Cs; M = Nb, Ta), Poster presented in the “15th European conference on solid state chemistry (ECSSC15)”, held at University of Vienna, Austria during 23– 26th August 2015. 3. A. K. Paidi., R. N. Devi and K. Vidyasagar, “Structural variants of quaternary vanadates of niobium and tantalum, AMV2O8 and AM2V2O11 (A = mono- and di-valent metal ions; M = Nb, Ta)”, Oral presentation in ‘Chemistry in-House Symposium (CiHS-2015)’ held at Indian Institute of Technology Madras, on Aug. 12th, 2015.