Phase formation in the Ag2MoO4–Rb2MoO4–Hf(MoO4)2 system

Systematic studies of the subsolidus structure of ternary molybdate systems allow expanding the representation of ternary molybdates. In this paper we studied the solid phase interaction in the Ag2MoO4–Rb2MoO4–Hf(MoO4)2 system for the first time using X-ray phase analysis.To determine the quasi-binary sections, we use the method of “intersecting cuts”. It helped to reveal the formation of new Rb5Ag1/3Hf5/3(MoO4)6 and Rb3AgHf2(MoO4)6 phases. We also determined their thermal characteristics using differential scanning calorimetry. The ternary molybdate Rb5Ag1/3Hf5/3(MoO4)6 crystallised in the trigonal syngony with the followingunit cell parameters: a = 10.7117(1), c = 38.5464(5) Å (space group R3с, Z = 6). The Ag2MoO4–Rb2MoO4–Hf(MoO4)2 system is characterised by the existence of ten quasi-binary cross sections.The experimental data obtained in this work complement the information on phase equilibria in condensed ternary systems containing molybdates of tetravalent elements and two different monovalent elements. This provides opportunities for the combination of the compositions of ternary molybdates due to cationic substitutions, which will allow controlling their properties.

Triple molybdates K3–x Na1+x M 4(MoO4)6 (M = Ni, Mg, Co) and K3+x Li1–x Mg4(MoO4)6 isotypic with II-Na3Fe2(AsO4)3 and yurmarinite: synthesis, potassium disorder, crystal chemistry and ionic conductivity

The triple molybdates K3–x Na1+x M 4(MoO4)6 (M = Ni, Mg, Co) and K3+x Li1–x Mg4(MoO4)6 were found upon studying the corresponding ternary molybdate systems, and their structures, thermal stability and electrical conductiviplusmnty were investigated. The compounds crystallize in the space group R 3 c and are isostructural with the sodium-ion conductor II-Na3Fe2(AsO4)3 and yurmarinite, Na7(Fe3+, Mg, Cu)4(AsO4)6; their basic structural units are flat polyhedral clusters of the central M1O6 octahedron sharing edges with three surrounding M2O6 octahedra, which combine with single NaO6 octahedra and bridging MoO4 tetrahedra to form open three-dimensional (3D) frameworks where the cavities are partially occupied by disordered potassium (sodium) ions. The split alkali-ion positions in K3–x Na1+x M 4(MoO4)6 (M = Ni, Mg, Co) give their structural formulae as [(K,Na)0.5□0.5)]6(Na)[M1][M2]3(MoO4)6, whereas the lithium-containing compound (K0.5□0.5)6(Mg0.89K0.11)(Li0.89Mg0.11)Mg3(MoO4)6 shows an unexpected (Mg, K) isomorphism, which is similar to (Mn, K) and (Co, K) substitutions in isostructural K3+x Li1–x M 4(MoO4)6 (M = Mn, Co). The crystal chemistry of the title compounds and related arsenates, phosphates and molybdates was considered, and the connections of the cationic distributions with potential 3D ionic conductivity were shown by means of calculating the bond valence sum (BVS) maps for the Na+, Li+ and K+ ions. Electrical conductivity measurements gave relatively low values for the triple molybdates [σ = 4.8 × 10−6 S cm−1 at 390°C for K3NaCo4(MoO4)6 and 5 × 10−7 S cm−1 at 400°C for K3LiMg4(MoO4)6] compared with II-Na3Fe2(AsO4)3 (σ = 8.3 × 10−4 S cm−1 at 300°C). This may be explained by a low concentration of sodium or lithium ions and the blocking of their transport by large potassium ions.

A new Tl4.86Fe0.82Hf1.18(MoO4)6 ternary molybdate: crystal structure and properties

Single crystals of Tl4.86Fe0.82Hf1.18(MoO4)6 [a = b = 10.5550 (3), c = 37.7824 (9) Å, γ = 120°] are obtained by the self-flux method in the Tl2MoO4–Fe2(MoO4)3–Hf(MoO4)2 system. On the differential scanning calorimetry curve in the temperature range 320–350 K and at T ∼ 690 K, endothermic peaks are observed. The second harmonic generation test shows an excess of the signal of the quartz standard by almost three times at room temperature. In the range 320–340 K its intensity decreases by almost three times and at T ∼ 700 K it drops to zero. In the same interval, the temperature dependences of the unit-cell parameters and volume show stepwise changes. According to the X-ray diffraction data, the crystal structure consists of nonpolar and polar domains with different local symmetries. The structure is a three-dimensional framework consisting of alternating (Hf,Fe)O6 octahedra connected by MoO4 tetrahedra. Hf and Fe atoms occupy mixed Hf/Fe positions with different probabilities: 0.77:0.23, 0.50:0.50 and 0.32:0.68. Tl cations are located inside the framework in zigzag channels extended along the a and b axes. The thallium arrangement is disordered, i.e. it involves additional positions and vacancies. The complex crystal structure has been solved using the nonstandard space group R1, taking into account the local symmetry R 3 c for the Mo atoms and mixed Hf/Fe positions mainly occupied by Hf atoms. The possible paths of ion transport are analyzed. The energy required to overcome the potential barrier between sites of Tl cations to migrate, which corresponds to the activation energy of conductivity, is estimated. The ion current is shown to be most probable in the ab plane.