Coexistence of three types of sodium motion in double molybdate Na9Sc(MoO4)6: 23Na and 45Sc NMR data and ab initio calculations.

The rechargeable Na-ion batteries attract much attention as an alternative to the widely used but expensive Li-ion batteries. The search for materials with high sodium diffusion is important for the development of solid state electrolytes. We present the results of experimental and ab initio studies of the Na-ion diffusion mechanism in Na9Sc(MoO4)6. The ion conductivity reaches the value of 3.6 × 10-2 S cm-1 at T ∼ 850 K. The 23Na and 45Sc NMR data reveal the coexistence of three different types of Na-ion motion in the temperature range from 300 to 750 K. They are activated at different temperatures and are characterized by substantially different dynamics parameters. These features are confirmed by ab initio calculations of activation barriers for sodium diffusion along various paths.

The double molybdate Na9Sc(MoO4)6 refined from powder XRD data.

Na9Sc(MoO4)6 {nonasodium scandium hexakis[tetraoxidomolybdate(II)]} was synthesised by a solid-state method. The basic structure units are polyhedral clusters composed of an ScO6 octahedron and three NaO6 octahedra sharing total edges. The clusters are connected by sharing vertices with bridging MoO4 tetrahedra, forming a three-dimensional framework where the cavities are occupied by the other two crystallographically independent Na atoms.

Novel Red Phosphor of Gd3+, Sm3+ co-Activated AgxGd((2-x)/3)-0.3-ySmyEu3+0.30☐(1-2x-2y)/3WO4 Scheelites for LED Lighting.

Gd3+ and Sm3+ co-activation, the effect of cation substitutions and the creation of cation vacancies in the scheelite-type framework are investigated as factors influencing luminescence properties. AgxGd((2-x)/3)-0.3-ySmyEu3+0.3☐(1-2x)/3WO4 (x = 0.50, 0.286, 0.20; y = 0.01, 0.02, 0.03, 0.3) scheelite-type phases (AxGSyE) have been synthesized by a solid-state method. A powder X-ray diffraction study of AxGSyE (x = 0.286, 0.2; y = 0.01, 0.02, 0.03) shows that the crystal structures have an incommensurately modulated character similar to other cation-deficient scheelite-related phases. Luminescence properties have been evaluated under near-ultraviolet (n-UV) light. The photoluminescence excitation spectra of AxGSyE demonstrate the strongest absorption at 395 nm, which matches well with commercially available UV-emitting GaN-based LED chips. Gd3+ and Sm3+ co-activation leads to a notable decreasing intensity of the charge transfer band in comparison with Gd3+ single-doped phases. The main absorption is the 7F0 → 5L6 transition of Eu3+ at 395 nm and the 6H5/2 → 4F7/2 transition of Sm3+ at 405 nm. The photoluminescence emission spectra of all the samples indicate intense red emission due to the 5D0 → 7F2 transition of Eu3+. The intensity of the 5D0 → 7F2 emission increases from ~2 times (x = 0.2, y = 0.01 and x = 0.286, y = 0.02) to ~4 times (x = 0.5, y = 0.01) in the Gd3+ and Sm3+ co-doped samples. The integral emission intensity of Ag0.20Gd0.29Sm0.01Eu0.30WO4 in the red visible spectral range (the 5D0 → 7F2 transition) is higher by ~20% than that of the commercially used red phosphor of Gd2O2S:Eu3+. A thermal quenching study of the luminescence of the Eu3+ emission reveals the influence of the structure of compounds and the Sm3+ concentration on the temperature dependence and behavior of the synthesized crystals. Ag0.286Gd0.252Sm0.02Eu0.30WO4 and Ag0.20Gd0.29Sm0.01Eu0.30WO4, with the incommensurately modulated (3 + 1)D monoclinic structure, are very attractive as near-UV converting phosphors applied as red-emitting phosphors for LEDs.

Resolving old problems with layered polytungstates related to hexagonal tungsten bronze: phase formation, structures, crystal chemistry and some properties.

A 60-year-old problem with the atomic arrangements and exact compositions of alkali polytungstates related to hexagonal tungsten bronze (HTB) was solved. The systems A2WO4-WO3 (A = K, Rb) were restudied and the average monoclinic layered structures of stoichiometric polytungstates A4W11O35 (A = K, Rb, Cs, Tl) and A2W7O22 (A = K, Rb, Cs) were first successfully determined. The structures resemble those of "MoW11O36" and "MoW14O45" (J. Graham and A. D. Wadsley, Acta Crystallogr., 1961, 14, 379-383) and are derived from HTB by breaking into slabs parallel to (100) due to the ordered omission of some [WO]∞ chains along the hexagonal tunnels. The slabs in A4W11O35 (A = Cs, Tl) and A2W7O22 (A = Rb, Cs) are mutually shifted by the a/2 HTB unit cell axis. These data mainly confirmed our preliminary structural models of HTB-like alkali polytungstates (S. F. Solodovnikov, N. V. Ivannikova, Z. A. Solodovnikova and E. S. Zolotova, Inorg. Mater., 1998, 34, 845-853) and revealed a new similar thallium polytungstate. The structures of the HTB-like polytungstates and related compounds form a homologous series of layered complex oxides or fluorides An+2-xM3n+2X9n+8 where n = 2, 3 and 4 are equal to the numbers of HTB hexagonal tunnels across the polytungstate slab width for Tl2W4O13, A4W11O35 and A2W7O22 (A = K, Rb, Cs or Tl), respectively. The structures of the HTB-like polytungstates seem to intergrow with HTB-type AxWO3 to form, in particular, higher homologues of the series. Our group-supergroup analysis, measurements of nonlinear optical activity and electrical conductivity, and calculations of the bond-valence site energy barriers indicate possible ferroelectric/ferroelastic properties and moderate 2D oxide-ion mobility within the HTB-type slabs of the studied polytungstates.

Rb9-xAg3+xSc2(WO4)9: a new glaserite-related structure type, rubidium disorder, ionic conductivity.

A new triple tungstate Rb9-xAg3+xSc2(WO4)9 (0 ≤ x ≤ 0.15) synthesized by solid state reactions and spontaneous crystallization from melts presents a new structure type related to those of Cs7Na5Yb2(MoO4)9 and Na13Sr2Ta2(PO4)9. The title compound in centrosymmetric space group Cmcm contains dimers of two ScO6 octahedra sharing corners with three bridging WO4 tetrahedra. Three pairs of opposite terminal WO4 tetrahedra are additionally linked by AgO2 dumbbells to form {Ag3[Sc2(WO4)9]}9- groups, which together with some rubidium ions are packed in pseudohexagonal glaserite-like layers parallel to (001), but stacking of the layers is different in these three structures. In the title structure, the layers stack with a shift along the b axis and their interlayer space contains disordered Rb+ cations partially substituted by Ag+ ions. Almost linear chains of incompletely filled close Rb3a-Rb3d positions (the shortest distances Rb-Rb are 0.46 to 0.64 Å) are found to locate approximately along the b axis. This positional disorder and the presence of wide common quadrangular faces of Rb2 and Rb3a-Rb3d coordination polyhedra favor two-dimensional ionic conductivity in the (001) plane with Rb+ and Ag+ carriers, which was confirmed with bond valence sum (BVS) maps. Electrical conductivity measurements on Rb9Ag3Sc2(WO4)9 ceramics revealed a first-order superionic phase transition at 570 K with a sharp increase in the electrical conductivity. The conductivity σi = 1.8 × 10-3 S cm-1 at 690 K is comparable with the value of 1.0 × 10-3 S cm-1 (500 K) observed earlier for rubidium-ion transport in pyrochlore-like ferroelectric RbNbWO6.

New triple molybdate Rb2AgIn(MoO4)3: synthesis, framework crystal structure and ion-transport behaviour.

A new triple molybdate, Rb2Ag1+3xIn1-x(MoO4)3 (0 ≤ x ≤ 0.02), was found in the course of a study of the system Rb2MoO4-Ag2MoO4-In2(MoO4)3 and was synthesized as both powders and single crystals by solid-state reactions and spontaneous crystallization from melts. The structure of Rb2Ag1+3xIn1-x(MoO4)3 (x ≉ 0.004) is of a new type crystallizing in the centrosymmetric space group R-3c [a = 10.3982 (9), c = 38.858 (4) Å, Z = 12 and R = 0.0225] and contains (In,Ag)O6 octahedra and distorted Ag1O6 trigonal prisms linked by common faces to form [Ag(In,Ag)O9] dimers connected to each other via MoO4 tetrahedra into an open three-dimensional (3D) framework. Between two adjacent [Ag(In,Ag)O9] dimers along the c axis, an extra Ag2O6 trigonal prism with about 1% occupancy was found. The Ag1O6 and Ag2O6 prisms are located at levels of z ≉ 1/12, 1/4, 5/12, 7/12, 3/4 and 11/12, and can facilitate two-dimensional ionic conductivity. The 12-coordinate Rb atoms are in the framework cavities. The structure of Rb2AgIn(MoO4)3 is a member of the series of rhombohedral 3D framework molybdate structure types with a ≉ 9-10 Šand long c axes, which contain rods of face-shared filled and empty coordination polyhedra around threefold axes. Electrical conductivity of ceramics is measured by impedance spectroscopy. Rb2AgIn(MoO4)3 undergoes a `blurred' first-order phase transition at 535 K with increasing electrical conductivity up to 1.1 × 10-2 S cm-1 at 720 K. Thus, the compound may be of interest for developing new materials with high ionic conductivity at elevated temperatures.

Synthesis, crystal structures and properties of the new compounds K7-xAg1+x(XO4)4 (X = Mo, W).

Two new isostructural compounds, namely heptapotassium silver tetrakis(tetraoxomolybdate), K7-xAg1+x(MoO4)4 (0 ≤ x ≤ 0.4), and heptapotassium silver tetrakis(tetraoxotungstate), K7-xAg1+x(WO4)4 (0 ≤ x ≤ 0.4), have been synthesized and found to crystallize in the polar space group P63mc (Z = 2) with the unit-cell dimensions a = 12.4188 (2) and c = 7.4338 (2) Šfor K6.68Ag1.32(MoO4)4 (single-crystal data), and a = 12.4912 (5) and c = 7.4526 (3) Šfor K7Ag(WO4)4 (Rietveld analysis data). Both structures represent a new structure type, with characteristic [K1(XO4)6] `pinwheels' of K1O6 octahedra and six XO4 tetrahedra (X = Mo, W) connected by common opposite faces into columns along the c axes. The octahedral columns are linked to each other through Ag1O4 tetrahedra along with the K2 and K3/Ag2 polyhedra, forming the polar rods (...Ag1O4-X1O4-empty octahedron-Ag1O4...). Ag1 is located almost at the centre of the largest face of its coordination tetrahedron and seems to have some mobility. The new structure type is related to the Ba6Nd2Al4O15 and CaBaSiO4 types, and to other structures of the α-K2SO4-glaserite family. The differential scanning calorimetry (DSC) and second harmonic generation (SHG) results show that both compounds undergo first-order phase transformations to high-temperature centrosymmetric phases.

Cs3LiZn2(WO4)4 and Rb3Li2Ga(MoO4)4: different filled derivatives of the cation-deficient Cs6Zn5(MoO4)8 structure.

Two new compounds, namely cubic tricaesium lithium dizinc tetrakis(tetraoxotungstate), Cs3LiZn2(WO4)4, and tetragonal trirubidium dilithium gallium tetrakis(tetraoxomolybdate), Rb3Li2Ga(MoO4)4, belong to the structural family of Cs6Zn5(MoO4)8 (space group I-43d, Z = 4), with a partially incomplete (Zn5/6□1/6) position. In Cs3LiZn2(WO4)4, this position is fully statistically occupied by (Zn2/3Li1/3), and in Rb3Li2Ga(MoO4)4, the 2Li + Ga atoms are completely ordered in two distinct sites of the space group I-42d (Z = 4). In the same way, the crystallographically equivalent A+ cations (A = Cs, Rb) in Cs6Zn5(MoO4)8, Cs3LiZn2(WO4)4 and isostructural A3LiZn2(MoO4)4 and Cs3LiCo2(MoO4)4 are divided into two sites in Rb3Li2Ga(MoO4)4, as in other isostructural A3Li2R(MoO4)4 compounds (AR = TlAl, RbAl, CsAl, CsGa, CsFe). In the title structures, the WO4 and (Zn,Li)O4 or LiO4, GaO4 and MoO4 tetrahedra share corners to form open three-dimensional frameworks with the caesium or rubidium ions occupying cuboctahedral cavities. The tetrahedral frameworks are related to that of mayenite 12CaO·7Al2O3 and isotypic compounds. Comparison of isostructural Cs3MZn2(MoO4)4 (M = Li, Na, Ag) and Cs6Zn5(MoO4)8 shows a decrease of the cubic lattice parameter and an increase in thermal stability with the filling of the vacancies by Li+ in the Zn position of the Cs6Zn5(MoO4)8 structure, while filling of the cation vacancies by larger Na+ or Ag+ ions plays a destabilizing role. The series A3Li2R(MoO4)4 shows second harmonic generation effects compatible with that of ß'-Gd2(MoO4)3 and may be considered as nonlinear optical materials with a modest nonlinearity.