Triple Molybdate Research Articles

Stoichiometry and topological features of triple molybdates AxByCz(MoO4)n with the heteropolyhedral open MT-frameworks: Synthesis, crystal structure of Rb5{Hf1.5Co0.5(MoO4)6}, and comparative crystal chemistry

In the study of the Rb2MoO4–CoMoO4–Hf(MoO4)2 system a new compound, Rb5Hf1.5Сo0.5(MoO4)6, was obtained, which was synthesized by ceramic technology at 530°С for 80 h in air. Rb5Hf1.5Сo0.5(MoO4)6 single crystals were grown by spontaneous crystallization from a melt solution. Synthesized molybdate melt incongruently at 670 °C. The crystal structure has been refined using single-crystal X-ray diffraction data. Triple molybdate Rb5Hf1.5Сo0.5(MoO4)6 (a = 10.6169(4) Å, c = 38.1282(5) Å. V = 3722.02(2) Å3; space group R3‾c, Z = 6) is characterized by the heteropolyhedral open MT-framework formed by two types of MO6-octahedra and one type of MoO4-tetrahedron. Both MO6-octahedra are predominantly occupied by hafnium with the admixture of cobalt, but characterized by the different ratio between them. Large Rb + cations fill the large cavities and occupy A(1)O9- and A(2)O12-polyhedra. The topological features of the heteropolyhedral open MT-framework in related molybdates with the general formula AxByCz(MoO4)n are discussed.

Synthesis, thermal and dielectric characteristics of Rb<sub>5</sub>Li<sub>1/3</sub>Zr<sub>5/3</sub>(MoO<sub>4</sub>)<sub>6</sub>

This work addressed the directed synthesis of a new phase Rb5Li1/3Zr5/3(MoO4)6, along with the determination of its crystallographic, thermal and electrophysical properties. The directed synthesis of the Rb5Li1/3Zr5/3(MoO4)6 phase was carried out using the solid-state reaction in the temperature range of 350–470 °C. According to differential scanning calorimetry, the synthesised compound Rb5Li1/3Zr5/3(MoO4)6, crystallised in trigonal form (space group R3c, Z = 6), undergoes a diffused first-order phase transition. The structure of triple molybdate Rb5Li1/3Zr5/3(MoO4)6 comprises MoO4 tetrahedra and octahedrally coordinated MO6-polyhedra. This structure is characterised by a statistical distribution of lithium and zirconium atoms in the M position (M1 = 0.790 Zr + 0.210 Li, M2 = 0.877 Zr + 0.123 Li). Rb atoms are located in the large voids of the tetrahedronoctahedral framework. The electrophysical properties of triple molybdate Rb5Li1/3Zr5/3(MoO4)6 having a scaffold structure favourable for ion transport, were studied. The correlation between dielectric and thermal characteristics in the high-temperature region near the phase transition was revealed. The temperature and frequency dependences of electrical conductivity were measured at 473–873 K in heating and cooling modes in the frequency range of 1–10 kHz. The compound exhibited a high thermally activated conductivity, reaching 1.48·10-2 Cm K/cm with activation energy in the range of 0.6–0.8 eV at a temperature of 480 °C. Well-shaped semicircles in the low-frequency region and unresolved arcs in the high-frequency region changing with increasing temperature were observed in the impedance spectra of ceramic Rb5Li1/3Zr5/3(MoO4)6 sample at various temperatures. The evolution of the imaginary part (Z'') as a function of the real part (Z') of the complex impedance resembled that of the complex impedance for compounds having ionic conductivity.

Crystallographic Insights into the Crystal Structure and Intrinsic Properties of Ca12Al14O33-Type Rb3LiZn2(MoO4)4 Single Crystals.

Molybdate oxide materials have attracted considerable academic interest owing to their multifunctional optoelectronic properties and applications. However, to date, studies on the intrinsic properties of multiple molybdates have rarely been implemented. Herein, a prospective triple molybdate crystal, Rb3LiZn2(MoO4)4, with high crystalline quality was successfully grown using top-seeded solution growth (TSSG) approaches. Intriguingly, it affords a cage-like structure with the I4̅3d space group, analogous to that of Ca12Al14O33 (C12A7). The Rb3LiZn2(MoO4)4 crystal exhibits excellent thermal stability up to 603 °C, accompanied by a congruent melting nature. Simultaneously, it preserves the optical merits of a large band gap of 4.10 eV and a wide transmission window of 0.29-5.4 μm, which are superior to those of most molybdate crystals. More importantly, Raman spectroscopic measurements demonstrated that the title compound possesses an intense Raman shift located at 925 cm-1 and narrow line width, facilitating a stimulated Raman laser. In addition, first-principles calculations were also implemented to elucidate the structure-property relationships of Rb3LiZn2(MoO4)4. These observations provide an empirical platform for intuitively comprehending the underlying properties of multiple molybdates and pave the way for exploiting Raman crystals.

Synthesis, crystal structure and photocatalytic activity of a new NaLi1.07Co2.94(MoO4)5 nanoparticles for real tannery wastewater treatment

The present study aimed to contribute the photocatalytic degradation of Acid Brown dye (AB) found in leather industry wastewater in presence of new triple molybdate NaLi1.07Co2.94(MoO4)5 under sunlight. The new photocatalyst has been synthesized by a solid state reaction method and characterized using X-ray Powder Diffraction (XRPD), Raman, FTIR, Scanning Electron Microscopy (SEM), UV–vis–NIR spectrophotometry. It crystallizes in the triclinic system, space group P-1with the unit cell parameters are: a ​= ​6.881(2) Å, b ​= ​6.968(2) Å, c ​= ​17.696(6) Å, α ​= ​87.540(5)°, β ​= ​87.785(5)° and γ ​= ​78.403(7)°. The structure can be described by the succession of different types of layers connected by sharing vertices and edges to lead a three-dimensional framework with tunnels in which the Na ​+ ​cations reside. Raman and IR studies confirm the existence of MoO42− functional groups. The title compound is a wide-band-gap semiconductor with a value around 3.3 ​eV. The photocatalytic decomposition of Acid Brown organic dye load in tannery wastewater was studied. Significant results for chemical oxygen demand (COD) and color removal were achieved. A maximum of 56% removal dyes was attained after 5 ​h. With the addition of hydrogen peroxide as an oxidant, the degradation efficiency increased to 90% and an important reduction in the COD were observed. The photodegradation kinetics favorably followed the pseudo-first-order with a constant k of 2.9 10−1 h−1 in presence of H2O2 and 1.5 10−1 h−1 without adding any oxidant. The proposed leather wastewater treatment process is an attractive alternative highly recommended for wastewater treatment.

Emerging near-unity internal quantum efficiency and color purity from red-emitting phosphors for warm white LED with enhanced color rendition

A robust and bright red emitter is recognized as a key enabler for high-quality warm white light-emitting diode (w-LED). Herein, triple molybdate scheelite-type NaCaLa1-x(MoO4)3:xEu3+ (NCLM:xEu3+) phosphors derived from NaCaLa(MoO4)3 host by the equivalent-valent substitution strategy are successfully synthesized and systematically studied. Impressively, the NCLM:Eu3+ phosphors (λem = 615 nm) can be effectively stimulated by 395 and 466 nm light, exhibiting an ultra-high color purity and quantum efficiency close to 100%. The structure-related emission characteristics were analyzed based on the Judd-Oflet theory. The packaged white LED device used tri-color phosphors displays a superior color rendering index of 86.0 and low correlated color temperature of 2797 K. Moreover, the NCLM-based polydimethylsiloxane film maintains an impressive luminescent performance when it is folded or bent, demonstrating the potential applications in the field of flexible display and other light-emitting devices.

Growth, crystal structure, and properties of the Li3Ba2Ln3(MoO4)8 (Ln = Er, Tm) molybdates

The compounds Li3Ba2Ln3(MoO4)8 (Ln = Er, Tm) have been obtained by the high-temperature flux technique using the slow cooling method. Crystals grown spontaneously were used to characterize the materials and investigate their properties using X-ray diffraction (XRD), Raman spectroscopy, and magnetic measurements. Single crystal structures were solved and refined in the monoclinic space group C2/c. Lattice dimensions are a = 5.1935, b = 12.6364, c = 19.1022 Å, β = 91.399° for Er-based triple molybdate (pink crystals) and a = 5.1846, b = 12.5766, c = 19.1557 Å, β = 91.525° for Tm-molybdate (green crystals). The Ln cations, mainly at general position 8f, are coordinated by eight oxygen atoms and adopt a distorted antiprismatic geometry. The distribution of atoms over the three cationic sites is discussed comparatively to literature reports. Unpolarized Raman characterization supports both the disordered cation distribution, and the distortion of the MoO4 units. From the magnetic susceptibility measurements, it is found that the two triple molybdates follow a Curie-Weiss law in the 10–300 K domain, with a negative paramagnetic temperature θCW. The antiferromagnetic character is highlighted by the field-dependent magnetization curves.

Structure, characterization, and properties of BaMoO4 and BaKYb(MoO4)3 flux-grown single-crystals

Crystals of the barium molybdates BaKYb(MoO4)3 and BaMoO4 were obtained by spontaneous nucleation from high-temperature flux experiments. The compounds were characterized by energy-dispersive spectrometry (EDS) and Raman spectroscopy, and their structure was solved from single-crystal X-ray diffraction data at room temperature. BaMoO4 crystallizes in the tetragonal I41/a symmetry with a ​= ​b ​= ​5.5723(4) and c ​= ​12.7954(9) Å. With the scheelite-type structure, BaMoO4 displays sharp and intense lines in its FT-Raman spectrum registered at room temperature. The new triple molybdate BaKYb(MoO4)3 is described in a monoclinic lattice C2/c of parameters a ​= ​17.3517(5), b ​= ​12.1477(4), c ​= ​5.2481(2) Å, β = 105.344(2)°. The Yb3+ ions are surrounded by oxygen leading to YbO8 coordination polyhedra while atomic disorder is observed for the cations Ba2+ and K+, statistically sharing their positions inside 8-vertex coordination polyhedra. The Raman spectrum confirms the presence of distorted MoO4 tetrahedra for the BaKYb(MoO4)3 molybdate. Investigation of its magnetic properties was undertaken in order to check whether a magnetic ordering occurs at low temperature.

Potassium and thallium conductors with a trigonal structure in the M2MoO4–Cr2(MoO4)3–Hf(MoO4)2 (M = K, Tl) systems: Synthesis, structure, and ionic conductivity

The triple molybdates M5CrHf(MoO4)6 (M = K, Tl) and TlCrHf0.5(MoO4)3 were found upon studying the corresponding ternary molybdate systems M2MoO4–Cr2(MoO4)3–Hf(MoO4)2 (M = K, Tl) in the subsolidus region using X-ray powder diffraction. The crystal structures of M5CrHf(MoO4)6 (M = K, Tl) and TlCrHf0.5(MoO4)3 are refined by Rietveld method. M5CrHf(MoO4)6 (M = K, Tl) crystallizes in space group R3¯c with unit cell parameters: a = b = 10.45548 (5), c = 37.24614 (3) Å, V = 3526.14 (4) Å3, Z = 6 for K5CrHf(MoO4)6and a = b = 10.53406 (12), c = 37.6837 (5) Å, V = 3621.39 (9) Å3, Z = 6 for Tl5CrHf(MoO4)6. TlCrHf0.5(MoO4)3 crystallizes in space group R3¯ with unit cell parameters: a = b = 12.9710 (2), c = 11.7825 (2) Å, V = 1716.78 (6) Å3, Z = 6. The thermal stability and electrical conductivity of the new compounds were investigated. Electrical conductivity measurements gave high values for the triple molybdates M5CrHf(MoO4)6 (M = K, Tl) (σ = 5.22 × 10−4 S / cm for K5CrHf(MoO4)6, σ = 1.1 × 10−2 S / cm for Tl5CrHf(MoO4)6 at 773 K) and relatively low values for the triple molybdate TlCrHf0.5(MoO4)3 (σ = 4.42 × 10−6 S / cm at 773 K).

New triple molybdate K5ScHf(MoO4)6: Synthesis, properties, structure and phase equilibria in the M2MoO4–Sc2(MoO4)3–Hf(MoO4)2 (M = Li, K) systems

Subsolidus phase relations in the M2MoO4–Sc2(MoO4)3–Hf(MoO4)2 (M ​= ​Li, K) systems have been studied by the method of “intersecting cuts”. No new triple molybdates have been identified in the Li2MoO4–Sc2(MoO4)3–Hf(MoO4)2 system and a new triple molybdate K5ScHf(MoO4)6 is formed in the K2MoO4–Sc2(MoO4)3–Hf(MoO4)2 system. The structure of K5ScHf(MoO4)6, have been determined in space group R3‾c through Rietveld analysis of X-ray powder diffraction data. The melting point of molybdate is 999 ​K. The compound has high ion conductivity (about 10−3 ​S ​cm−1).

Synthesis, Structure, and Conductivity of Alluaudite‐Related Phases in the Na2MoO4–Cs2MoO4–CoMoO4 System

Phase formation study of the Na2MoO4–Cs2MoO4–CoMoO4 system resulted in new cesium‐containing alluaudite‐related phases. The solid solution Na4–2x‐yCsyCo1+x(MoO4)3 (0 ≤ x, y ≤ 0.30), based on the alluaudite‐type Na4–2xCo1+x(MoO4)3, and triple molybdate Na10(Cs4‐xNax)Co5(MoO4)12 (0 ≤ x ≤ 0.30) were found, and their structures were solved. In the structure of Na3.21Cs0.37Co1.21(MoO4)3 (a = 13.0917(8) Å, b = 13.5443(8) Å, c = 7.1217(4) Å, space group C2/c, β = 112.331(2), Z = 4), the cesium ions partially substitute the Na+ in the channels running along the c‐axis. The structure of Na10(Cs3.77Na0.23)Co5(MoO4)12 (a = 13.6572(3) Å, b = 11.5063(3) Å, c = 27.9898(5) Å, space group Pbca, Z = 4) was proved to be the aristotype for the pseudo orthorhombic Na25Cs8R5(MoO4)24 (R = Fe, Sc, In). The compounds contain alluaudite‐like layers of MoO4 tetrahedra and pairs of edge‐shared (Co, Na)O6 or (R, Na)O6 and NaO6 octahedra, which are connected by bridging MoO4 tetrahedra to form 3D frameworks differing from the alluaudite type. The frameworks contain channels along the c‐axis filled by Cs+ and Na+ ions. Bond valence sum (BVS) maps show that the alluaudite‐related molybdates can have a 2D sodium‐ion conductivity at elevated temperatures in contrast to the alluaudite‐type cathode material Na2+2xFe2‐x(SO4)3 with a 1D conductivity. The measured ionic conductivity of Na4–2xCo1+x(MoO4)3, Na4–2x‐yCsyCo1+x(MoO4)3, and Na10Cs4Co5(MoO4)12 reaches 10–3–10–2 S cm–1 at 500 °C.

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 Å and 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.

Crystal Structure and Electrical Properties of New Triple Molybdate Na0.45K1.55Cu3(MoO4)4

Single crystals of new triple molybdate - sodium potassium copper(II) tetra-[molybdate(VI)] Na0.45K1.55Cu3(MoO4)4 - are synthesized by a solid-state reaction and its crystal structure is determined. The compound crystallizes in the monoclinic space group P21/c with a = 5.097(2) A, b = 14.594(3) A, c = 19.905(3) A, β = 94.00(2)°, V = 1476.9(7) A3, Z = 4. K2 and Na1 cations are located at the same general site with occupancies of 0.55 and 0.45 respectively. The structure is formed by infinite Cu3Mo3O23-type ribbons. These ribbons are connected by a tridentate tetrahedron sharing vertices to build layers parallel to the (001) plane. All these layers are connected by vertices to form a three-dimensional framework. The structural model is supported by the bond valence sum. The electrical properties of the title compound are investigated using complex impedance spectroscopy.

Crystal structure of new triple molybdate AgMg3Ga(MoO4)5 from Rietveld refinement

A polycrystalline sample of a new triple molybdate AgMg3Ga(MoO4)5 was obtained by solid-state reaction techniques. Structural refinement based on X-ray powder diffraction data showed that the crystal structure is isotypic with NaMg3In(MoO4)5 (sp. gr. P$\bar 1$). In the structure pairs of edge-shared (Mg, Ga)O6, octahedra are connected by common vertices to form a three-dimensional framework. Large framework cavities involve Ag+ cations. The title compound was found to melt at 1079 K.

Triple molybdate scheelite-type upconversion phosphor NaCaLa(MoO4)3:Er3+/Yb3+: structural and spectroscopic properties.

Triple molybdate NaCaLa(1-x-y)(MoO4)3:xEr3+,yYb3+ (x = y = 0, x = 0.05 and y = 0.45, x = 0.1 and y = 0.2, x = 0.2 and y = 0) phosphors were successfully synthesized for the first time by the microwave sol-gel method. Well-crystallized particles formed after heat treatment at 900 °C for 16 h showed a fine and homogeneous morphology with particle sizes of 2-3 μm. The structures were refined by the Rietveld method in the space group I41/a. The optical properties were examined comparatively using photoluminescence emission and Raman spectroscopy. Under excitation at 980 nm, the NaCaLa0.7(MoO4)3:0.1Er3+,0.2Yb3+ and NaCaLa0.5(MoO4)3:0.05Er3+,0.45Yb3+ particles exhibited a strong 525 nm emission band, a weaker 550 nm emission band in the green region, and three weak 655 nm, 490 nm and 410 nm emission bands in the red, blue and violet regions. The pump power dependence and Commission Internationale de L'Eclairage chromaticity of the upconversion emission intensity were evaluated in detail.

Phase equilibria in the Cs2MoO4–ZnMoO4–Zr(MoO4)2 system, Crystal structures and properties of new triple molybdates Cs2ZnZr(MoO4)4 and Cs2ZnZr2(MoO4)6

Subsolidus phase relations in the Cs2MoO4–ZnMoO4–Zr(MoO4)2 system were determined and two new compounds, Cs2ZnZr(MoO4)4 and Cs2ZnZr2(MoO4)6, were obtained. The structure of Cs2ZnZr(MoO4)4 (a=5.7919(1) Å, c=8.0490(3) Å; space group P3¯m1; Z=0.5; R=0.0149) belongs to the layered glaserite-like KAl(MoO4)2 structure type where the octahedral Al3+ positions are statistically occupied by 0.5 Zn2++0.5 Zr4+. The second triple molybdate, Cs2ZnZr2(MoO4)6 (a=13.366(1) Å, c=12.253(3) Å, space group R3¯, Z=3, R=0.0324), is isostructural to Cs2MnZr2(MoO4)6 and Cs2M2Zr(MoO4)6 (M=Al, Fe) and contains a mixed 3D framework built of МоO4 tetrahedra and (Zn, Zr)O6 octahedra sharing common vertices. Cesium cations are located in large channels of the framework. The latter compound undergoes a first-order phase transition at 723 K with considerable increasing its ionic conductivity.

Structure cristalline de type alluaudite K0.4Na3.6Co(MoO4)3.

A new triple molybdate, potassium sodium cobalt tris-(molybdate), K0.4Na3.6Co(MoO4)3, was synthesized using solid-state reactions. The Co(2+) and one Na(+) cation are located at the same general site, each with occupancy 0.5. Another site (site symmetry 2) is occupied by Na(+) and K(+) cations, with occupancies of 0.597 (7) and 0.402 (6), respectively. The other two Na(+) cations and one of the two Mo atoms lie on special positions (site symmetries -1, 2 and 2, respectively). The structure is characterized by M 2O10 (M = Co/Na) dimers, which are linked by MoO4 tetra-hedra, forming infinite layers. The latter are connected firstly by insertion of one type of MoO4 tetra-hedra and secondly by sharing corners with the other type of MoO4 tetra-hedra. This results in an open three-dimensional framework with the cavities occupied by the Na(+) and K(+) cations. The structure is isotypic with Na3In2As3O12 and Na3In2P3O12. A comparison is made with structures such as K2Co2(MoO4)3 and β-NaFe2(MoO4)3 and their differences are discussed.

Synthesis, crystal structure, sintering and electrical properties of a new alluaudite-like triple molybdate K0.13Na3.87MgMo3O12

A new triple molybdate K0.13Na3.87MgMo3O12 was synthesized by solid state reaction.

Synthesis, crystal structure and properties of alluaudite-like triple molybdate Na25Cs8Fe5(MoO4)24

A new triple molybdate Na25Cs8Fe5(MoO4)24 was synthesized using solid state reactions and studied with X-ray powder diffraction, second harmonic generation (SHG) technique, differential scanning calorimetry, Mössbauer and dielectric impedance spectroscopy. Single crystals of Na25Cs8Fe5(MoO4)24 were obtained and its structure was solved (the space group P1¯, a=12.5814(5), b=13.8989(5), c=28.4386(9)Å, α=90.108(2), β=90.064(2), γ=90.020(2)°, V=4973.0(3)Å3, Z=2, R=0.0440). Characteristic features of the structure are polyhedral layers composed of pairs of edge-shared FeO6 and (Fe, Na)O6 octahedra, which are connected by bridging МоО4 tetrahedra. The layers share common vertices with bridging МоО4 tetrahedra to form an open 3D framework with the cavities occupied by the Cs+ and Na+ cations. The compound undergoes first-order phase transformation at 642K and above this phase transition, electrical conductivity reaches 10−3–10−2Scm−1. Thus, Na25Cs8Fe5(MoO4)24 may be considered as a promising compound for developing new materials with high ionic conductivity.

Synthesis and characterization of a NASICON phase of variable composition Na1 − x Ni1 − x Sc1 + x (MoO4)3 (0 ≤ x ≤ 0.5)

Subsolidus phase ratios in the Na2MoO4-NiMoO4-Sc2(MoO4)3 system have been studied using X-ray diffraction, differential thermal analysis, and vibrational spectroscopy. A phase of variable composition Na1 − x Ni1 − x Sc1 + x (MoO4)3 (0 ≤ x ≤ 0.5) having NASICON structure (space group \(R\bar 3c\)) and a triple molybdate crystallizing in triclinic system (space group \(P\bar 1\)) have been obtained. The high conductivity of Na1 − x Ni1 − x Sc1 + x (MoO4)3 allows the phase of variable composition to be regarded as a promising sodiumion-conductive solid electrolyte.

Vibrational spectra and electrophysical properties of a NASICON phase of variable composition Na1 − x Co1 − x Sc1 + x (MoO4)3 (0 ≤ x ≤ 0.5)

Subsolidus phase ratios of the Na2MoO4-CoMoO4-Sc2(MoO4)3 system have been studied by X-ray diffraction, differential thermal analysis, and vibrational spectroscopy. A phase of variable composition Na1 − xCo1 − xSc1 + x(MoO4)3, 0 ≤ x ≤ 0.5 having NASICON structure (space group \(R\bar 3c\)) and triple molybdate NaCo3Sc(MoO4)5 crystallizing in triclinic system (space group \(P\bar 1\)) have been obtained. The high conductivity of Na1 − xCo1 − xSc1 + x(MoO4)3 allows the phase of variable composition to be regarded as a promising sodium-ion conducting solid electrolyte.