Structural characterization of quaternary selenites of tungsten(VI), A 2W3SeO12 (A = NH4, Cs, Rb, K or Tl)

The quaternary A 2W3SeO12 (A = NH4, Cs, Rb, K or Tl) selenites have been prepared in the form of single crystals by hydrothermal and novel solid-state reactions. They were characterized by X-ray diffraction, thermal and spectroscopic studies. All of them have a hexagonal tungsten oxide (HTO) related [W3SeO12]2− anionic framework with pyramidally coordinated Se4+ ions.


Chemical context
Non-centrosymmetric (NCS) compounds are widely studied as they have potentially useful symmetry-dependent properties such as piezoelectricity, ferroelectricity and second-order non-linear optical (NLO) behaviour (Halasyamani & Poeppelmeier 1998). Many crystalline selenites and tellurites containing d 0 transition-metal ions such as V 5+ , Mo 6+ , W 6+ are non-centrosymmetric compounds. The solid-state chemistry of these oxides is interesting from the point of view of both structural diversity and second harmonic generation (SHG) activity. They have two types of second-order Jahn-Teller (SOJT) distortion. One is the distorted octahedral coordination of the d 0 transition-metal ion and the other is pyramidal, disphenoidal and square-pyramidal coordinations of Se 4+ and Te 4+ , which have stereoactive lone pairs. Both SOJT distortions lead to acentric coordination environments that are conducive for NCS structures (Halasyamani 2004). For example, Cs 2 Mo 3 TeO 12 (Vidyavathy Balraj & Vidyasagar, 1998) and YVSe 2 O 8 (Kim et al., 2014) have non-centrosymmetric layered structures with these SOJT distortions and exhibit SHG activity. It needs to be mentioned that quaternary selenites and tellurites containing d 0 transition-metal ions, such as YVTe 2 O 8 (Kim et al., 2014), are also known to have centrosymmetric structures and exhibit no SHG activity.
In this context, the structural characterization of new and known quaternary A 2 W 3 SeO 12 (A = NH 4 , Cs, Rb, K, Tl) selenites of tungsten(VI) by single-crystal X-ray diffraction was considered necessary for their complete structural study and, therefore, was undertaken. This report is concerned with crystal growth by solid-state reactions and structural characterization of the known compounds A 2 W 3 SeO 12 [A = NH 4 (1), Cs (2) and Rb (3)] and new compounds K 2 W 3 SeO 12 (4a) and Tl 2 W 3 SeO 12 (5).
As an illustrative example, the structure of Rb 2 W 3 SeO 12 (3) is discussed. Its asymmetric unit content of Rb 2/3 WTe 1/3 O 4 has two, one, one and four crystallographically distinct rubidium, tungsten, selenium and oxygen atoms, respectively. The tungsten atom is octahedrally coordinated to the apical O1 and O2 atoms and two each of equatorial O3 and O4 atoms (Fig. 1). The WO 6 octahedron resides near the threefold rotation axis located at the Wyckoff site 2a and shares its two cis O3 equatorial oxygen atoms with two such octahedra to form a W 3 O 15 moiety. Such trinuclear moieties are connected to one another through sharing of equatorial O4 atoms, forming a hexagonal-tungsten-oxide (HTO) layer of composition WO 4 or W 3 O 12 . In other words, the HTO layer of WO 4 is formed from the sharing of four equatorial O3 and O4 atoms of every WO 6 octahedron with four such octahedra. The HTO layer of WO 4 has three-ring holes made of either O3 or O4 atoms and six-ring holes made of alternating O3 and O4 atoms. The selenium atom resides on a threefold rotation axis located at the 2a site and has a pyramidal coordination of C 3V symmetry, with three equivalent Se-O1 bonds. Thus, only three-ring holes of O3 are capped on one side of the layer, by Polyhedral representation of (left) the unit-cell structure viewed along the a axis and (right) a (W 3 SeO 12 ) 2À layer along with the Rb + counter-cations, viewed along the c axis, of Rb 2 W 3 SeO 12 (3). A W 3 O 15 moiety with a pyramidal selenium atom is indicated by a dashed red line and the net dipole directions of the WO 6 octahedra and pyramidal SeO 3 are shown.
bonding of the selenium atom to apical O1 oxygen atoms, to give rise to an asymmetric (W 3 SeO 12 ) 2À layer. These layers are stacked, as shown in Fig. 1, along the crystallographic c-axis direction in the ABAB . . . fashion because adjacent layers are rotated with respect to each other such that the six-ring hole of one layer is above the uncapped three-ring hole of the next layer. As the other apical oxygen O2 atoms are not bonded to selenium, the Se-O bonding is described as intra-layer bonding and, therefore, the structure is two-dimensional. The pyramidal SeO 3 moieties and the lone-pair of electrons of Se 4+ are respectively parallel and perpendicular to the HTO layers of WO 4 . The selenites 1-5 of the present study are found to contain the same staggered stacking of the HTO-related WO 4 layers. K 2 W 3 SeO 12 (b) was reported (Huang et al., 2014a,b) to be obtained under hydrothermal conditions and found to contain similar non-centrosymmetric HTO-related [W 3 SeO 12 ] 2À layers with intra-layer Se-O bonds. On the other hand, K 2 W 3 SeO 12 (4a) of the present study was prepared by solidstate reaction and is isostructural with the reported K 2 W 3 TeO 12 (Goodey et al., 2003). Its centrosymmetric, threedimensional HTO-related [W 3 SeO 12 ] 2À framework contains inter-layer Se-O bonds (Fig. 2) and its asymmetric unit has one formula unit. The three W1-W3 atoms are octahedrally coordinated to six apical O1-O6 and six equatorial O7-O12 oxygen atoms. The three WO 6 octahedra in the trinuclear W1W2W3O 15 moieties share equatorial O7-O9 oxygen atoms and these moieties are connected to one another through the other equatorial O10-O12 oxygen atoms to form the WO 4 layer. The Se atom forms interlayer Se-O bonds, by bonding to the apical O7, O10 and O12 oxygen atoms of W1W2W3O 15 moieties of adjacent HTO layers (Fig. 2) and thus the [W 3 SeO 12 ] 2À framework is three-dimensional in nature.
Tl 2 W 3 SeO 12 (5) has an orthorhombic unit cell with a o = 11.5962 (10) Å , b o = 12.7206 (5) Å and c o = 7.2362 (9) Å . The structure refinements in the non-centrosymmetric Pna2 1 and centrosymmetric Pnam space groups led to the respective structure agreement factor values of 6.37% and 15.98%; the structure refinements were unsatisfactory, mostly due to X-ray absorption. Its single crystal X-ray structure solution model is found to be same as the three-dimensional structure of K 2 W 3 SeO 12 (4a) and its observed powder XRD pattern ( Figure S1b in the supporting information) agrees reasonably with the one simulated on the basis of this model structure. Moreover, the powder XRD patterns and unit-cell parameters of these two compounds are similar. The orthorhombic unitcell parameters of the thallium (5) compound are related to the monoclinic unit-cell parameters of the potassium (4a) compound as follows: The single-crystal X-ray data for the thallium compound (5) in the centrosymmetric P2 1 /n space group, corresponding to the potassium compound (4a), led to the same structure model and a high value of 19.18% for the structure-agreement factor. It is inferred from these observations that the Tl 2 W 3 SeO 12 compound (5) has the same three-dimensional structure as K 2 W 3 SeO 12 (4a).
In the structurally characterized compounds 1-4a of the present study, the WO 6 octahedra have C 3 distortion as three W-O bonds are <1.9 Å long and their three trans W-O bonds are >1.9 Å long; the values of WO 6 intraoctahedral distortions (Halasyamani 2004), Á d , are calculated to be in the 0.73-0.86 range (Table S1). The Se 4+ ions have pyramidal coordination. The W-O and Se-O bond-length values are in the 1.703 (17)-2.184 (9) Å and 1.695 (10)-1.739 (10) Å ranges, respectively. The ammonium and alkali metal ions are found to be six-to nine-coordinated ( Figure S2), when the cut-off value of 3.6 Å is considered for NÁ Á ÁO non-bonding distances and A-O bond lengths. The calculated values (Brese & O'Keeffe 1991) of bond-valence sums for W 6+ , Se 4+ and monovalent alkali metal ions are in the 6.079-6.283, 3.807-3.975 and 0.060-1.275 ranges, respectively. The respective values of 3.210, 3.322 and 3.207 Å for the shortest interlayer OÁ Á ÁO non-bonding distances of compounds 1-3 with intralayer Se-O bonds are significantly higher than the corresponding value of 2.563 Å for compound 4a with interlayer Se-O bonds.
The net dipole moment values for the WO 6 and SeO 3 polyhedra were calculated by vector summation of the dipole moments (Maggard et al., 2003;Ok & Halasyamani 2005;Galy et al., 1975) of six W-O bonds and three Se-O bonds and found to be in the 0.79-1.85 D and 5.73-9.13 D ranges, respectively (Tables S1-S3). The net dipole for the WO 6 octahedron points towards the triangular face of three oxygen atoms with W-O bonds >1.9 Å long, whereas the net dipoles for the SeO 3 polyhedra point opposite to the lone pair of electrons of selenium. In compounds 1-3, as shown for Rb 2 W 3 SeO 12 (3) in Fig. 1, the intra-layer SeO 3 dipole is oriented along the c-axis direction and perpendicular to the HTO layer. For the WO 6 octahedra, the net dipole moment components along the a and b axes cancel one another, whereas the c-axis component is antiparallel and additive to the net dipole moment of pyramidal SeO 3 . In the case of centrosymmetric three-dimensional K 2 W 3 SeO 12 (4a), as shown in Fig. 2, the net dipole moments of the WO 6 and SeO 3 polyhedra macroscopically cancel one another and result in a zero net dipole moment.
The solid state UV-Visible absorption spectra (Fig. 3) of compounds 1-5 reveal that their band gap values are in the range 2.7-3.5 eV (Kubelka & Munk, 1931). The additional absorption edge observed for the Tl 2 W 3 SeO 12 compound (5) corresponds to band gap value of 2.0 eV. When compared to Cs 2 W 3 SeO 12 (2), the corresponding Cs 2 W 3 TeO 12 tellurite (Zhao et al., 2015) has a lower band gap of 2.89 eV.
Rb 2 W 3 SeO 12 (3), K 2 W 3 SeO 12 (4a) and Tl 2 W 3 SeO 12 (5) undergo thermal decompositions and give rise to endothermic peaks at $600, $575 and $575 C and their respective observed weight losses of 10.0%, 12.3% and 9.0% compare well with those calculated for the loss of SeO 2 ( Figure S3). The other endothermic peaks at $850 and 750 C could not be assigned. It was reported (Harrison et al., 1995) that a similar thermal loss of SeO 2 occurs in a single step between 500 and 600 C for Cs 2 W 3 SeO 12 (2) and in two steps at 350 and 450 C for (NH 4 ) 2 W 3 SeO 12 (1). When compared to the tungsten selenites 1-5, analogous A 2 W 3 TeO 12 (A = K, Rb, Cs) tellurites of tungsten (Goodey et al., 2003;Zhao et al., 2015) and A 2 Mo 3 SeO 12 (A = Rb, Tl) selenites of molybdenum (Chang et al., 2010) undergo single-step thermal decomposition at higher and lower temperatures of >700 and 300 C, respectively.
The reactants and their quantities, the temperature and duration of heating and the yields of products for the synthesis and crystal growth of compounds 1-5 are presented in Table S4. The ammonium compound (1) was synthesized by the hydrothermal method, with or without NH 4 Cl as mineralizer. The other four compounds (2-5) were obtained by solidstate reactions. The reactant mixtures were heated first in the open air and later in evacuated sealed silica ampoules. After the reaction, the solid product contents were washed with water to dissolve away the excess SeO 2 .
The hydrothermal and solid-state synthetic methods enabled the growth and isolation of single crystals of compounds 1-5. The utilization of excess SeO 2 as flux in the novel solid-state synthetic procedure facilitated the growth of single crystals of compounds 2-5. The powder XRD patterns of compounds 1-5 are presented in Figures S1a and S1b. (NH 4 ) 2 W 3 SeO 12 (1), Rb 2 W 3 SeO 12 (3) and Tl 2 W 3 SeO 12 (5) were obtained as homogeneous phases, as their observed powder XRD patterns compare reasonably well with the simulated ones. The powder XRD patterns of Cs 2 W 3 SeO 12 (2), Rb 2 W 3 SeO 12 (3) and K 2 W 3 SeO 12 (4a) contained two or three additional reflections of <10% intensity due to WO 3 or an Solid state UV-visible absorption spectra for the A 2 W 3 SeO 12 [A = NH 4 (1), Cs (2), Rb (3), K (4a) and Tl (5)] compounds. unidentified phase; however, the homogeneous polycrystalline sample of Rb 2 W 3 SeO 12 (3) could be obtained ( Figure S1b), under a different set of solid-state synthetic conditions mentioned in Table S4. Cs 2 W 3 SeO 12 (2) was prepared in polycrystalline form by the reported hydrothermal method (Harrison et al., 1995). It is evident from the scanning electron micrographs ( Figure S4) that crystallites of compounds 1 and 2 have a hexagonal prism shape and compounds 3-5 have blockshaped morphologies. The EDXA analyses confirmed the expected ratios of metal contents for all compounds 1-5.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The crystals of the ammonium (1) and rubidium (3) compounds are twinned by merohedry (Spek 2020) by the [À1 0 0 1 1 0 0 0 À 1] and [1 0 0 À 1 À1 0 0 0 À 1] twin laws and their twinned lattices are generated through twofold rotation of the primary lattices about the [120] direction and the b axis, respectively. The crystal of the potassium (4a) compound is twinned by pseudo-merohedry (Spek 2020) by the twin law [À1 0 0 0 À 1 0 0 0 1] and the twinned lattice is generated through twofold rotation of the primary lattice about the c axis, as the value of the angle of its monoclinic system is very close to 90 . The respective values of refined batch scale factor for the ammonium (1), rubidium (3) and potassium (4a) compounds are 0.029, 0.192 and 0.385. The hydrogen atoms of the NH 4 + ions in the ammonium compound (1) were not located in the difference-Fourier maps but are included in the formula. The final difference-Fourier maps did not show any chemically significant features and the Fourier difference peaks with an electron density of >1 e Å À3 were found to be ghosts. No reasonable structure solutions and refinements in the centrosymmetric P6 3 /m space group were found for compounds 1-3.

Special details
Experimental. Single crystals of compounds 1-5 were obtained along with the polycrystalline sample and the crystals were hand-picked for XRD study and mounted on thin glass fibres with epoxy glue and optically aligned on a Bruker APEXII 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. The determination of integral intensities and global refinement were performed using SAINT-plus (Bruker AXS). A semi-empirical absorption correction was subsequently applied using SADABS. Space group determination, structure solution and least-squares refinement were carried out using SHELXTL (Sheldrick 2008) program. DIAMOND 3.0 (PENNINGTON 1999) and ORTEP-3 (Farrugia 1997) for windows were the graphic programs employed to draw the structures. The structures were solved by direct methods and refined by full matrix least squares on F 2 . Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 3.94 e Å −3 Δρ min = −3.11 e Å −3 Extinction correction: SHELXL2018/3 (Sheldrick 2015b), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.00176 (12) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.  (10)