V. I. Voronkova

Oxygen-conducting crystals of La2Mo2O9: Growth and main properties

Single crystals of the anionic conductor La2Mo2O9 are grown by crystallization from a nonstoichiometric melt. Their polymorphism and domain structure, as well as the temperature dependences of conductivity and dielectric permittivity, are studied. In the temperature range 750–600°C, the conductivity of these crystals is as high as 10−1–10−2 Ω−1 cm−1.

Synthesis and electrical properties of Bi2V1 − x Ge x O5 + y solid solutions

A continuous series of Bi2V1 − xGexO5 + y solid solutions has been prepared by solid-state reactions, and their polymorphism and electrical properties have been studied. The solid solutions with 0 < x ≤ 0.2 are isostructural with the monoclinic phase α-Bi2VO5.5, and those with 0.2 < x ≤ 0.3 are isostructural with the orthorhombic phase. In the range 0.6 ≤ x < 1, the solid solutions have the orthorhombic Bi2GeO5 structure. The solid solutions with 0.4 ≤ x ≤ 0.5 have a tetragonal structure. Increasing the germanium content suppresses the ferroelectric phase transition in the Bi2VO5.5-based solid solutions, without changing the transition temperature. With increasing vanadium content, the conductivity of the solid solutions gradually increases, from 3 × 10−5 (x = 1) to 3 × 10−1 S/cm (x = 0) at 550°C.

Crystal structure of the metastable cubic βms phase of La2Mo2O9 single crystal at T = 33 K

The structure of the cubic metastable βms phase of La2Mo2O9 single crystal has been precisely investigated by X-ray diffraction at 33 K for the first time. The measurement of the unit-cell parameter of this crystal in the range from room temperature to 33 K showed that the unit-cell parameter and volume change continuously in this range. The crystal has a similar structure at T = 33 K and at room temperature. A local lowering of the symmetry for La and Mo atoms, caused by their displacement, is confirmed, and a similar displacement (which was not observed at room temperature) is revealed for O(1) atoms. The thermal parameters for O(2) and O(3) atoms do not change with a decrease in temperature, in contrast to the thermal parameters of Mo, La, and O(1) atoms. This fact indicates that the O(2) and O3 atoms in this crystal are statically disordered.

Structure of the metastable cubic B1 phase of the La2Mo2O9 single crystal

The structure of the metastable B1 phase of the La2Mo2O9 single crystal is investigated using X-ray diffraction. It is established that the crystal structure of the compound under investigation is described by the cubic unit cell with the parameter a = 7.158(5) Å, which makes it possible to index approximately 84% of the reflections measured for this single crystal. The structure of the metastable cubic B1 phase is characterized by a local lowering of the symmetry for the La and Mo atoms, which are displaced from their positions on the threefold axis, thus forming three sites around it with an occupancy of 0.333(2). The O(1) atom in the structure of the metastable cubic B1 phase remains in the 4a position on the threefold axis but occupies it by only 86%. The O(2) and O(3) atoms located in a general position occupy their own sites with occupancies of 0.77(2) and 0.35(2), respectively. The final R factor of the refinement of this structural model is 2.52%.

Structure of compound Pr5Mo3O16+δ exhibiting mixed electronic—ionic conductivity

The structure of Pr5Mo3O16 + δ single crystals is studied by X-ray diffraction, EDXS microanalysis, transmission microscopy, and XANES spectroscopy. It is found that in the structure Pr and Mo cations mutually replace each other, atomic positions of oxygen are split into several additional positions, and structural voids accommodate interstitial oxygen atoms (which make the main contribution to the conductivity). The disorder of the oxygen sublattice is responsible for the continuity of the framework of the ways of migration of oxygen ions.

Structure of fluorite-like compound based on Nd5Mo3O16 with lead partly substituting for neodymium

A single crystal of Nd5Mo3O16 with lead partly substituting for neodymium, which has a fluorite-like structure, was studied by precision X-ray diffraction, high-resolution transmission microscopy and EDX microanalysis. The crystal structure is determined in the space group Pn3¯n. It was found that the Pb atoms substitute in part for Nd atoms in the structure and are located in the vicinity of Nd2 positions. Partial substitutions of Mo cations for Nd positions and of Nd for Mo positions in crystals of the Ln5Mo3O16 oxide family are corroborated by X-ray diffraction for the first time. The first experimental verification of the location of an additional oxygen ion in the voids abutting MoO4 tetrahedra was obtained.

Single-crystal structure of vanadium-doped La2Mo2O9

A high-precision X-ray diffraction study of single crystals of two compositions—La2Mo1.78V0.22O8.89 and La2Mo1.64V0.36O8.82—was performed. In the vanadium-doped compounds, as in the structure of the metastable βms phase of pure La2Mo2O9, the La and Mo atoms and one of the three oxygen atoms are displaced from the threefold axis, on which they are located in the high-temperature β phase. The structure contains two partially occupied oxygen sites. It was shown that molybdenum atoms are partially replaced by vanadium atoms, which are not involved in the disordering, are located on the threefold axis, and are shifted toward one of the oxygen atoms. This is consistent with the temperature-induced changes in the structure of La2Mo2O9 and the changes in the properties of these crystals caused by the introduction of vanadium atoms into the structure.

Ferroelectric-Superionic Conductor Phase Transitions in Crystals MeNbWO6·nH2O (Me=Tl, Rb)

Crystals TlNbWO60.24H2O and RbNbWO60.13H2O with pyrochlorelike structure are obtained by flux method and their dielectric, ion-conductive and non-linear optical properties are studied. The crystals of Tl- and Rb-pyrochlore undergo phase transition from ferroelectric to superionic phase at about 50°C and 100°C respectively, the exact temperature of the phase transition depending on the water content in the crystal. The effect of water content as well as of high cationic mobility on properties of crystals and phase transitions are discussed.

X-ray diffraction study of oxygen-conducting compounds Ln₂Mo₂O₉ (Ln = La, Pr).

The La2Mo2O9 (LM) and Pr2Mo2O9 (PM) single crystals are studied using precision X-ray diffraction and high-resolution transmission microscopy at room temperature. The crystal structures are determined in the space group P2(1)3. La and Pr atoms, as well as Mo1 and O1 atoms, are located in the vicinity of the threefold axes rather than on the axes as in the high-temperature cubic phase. In both structures studied, the O2 and O3 positions are partially occupied. The coexistence of different configurations of the Mo coordination environment facilitates the oxygen-ion migration in the structure. Based on the X-ray data, the activation energies of O atoms are calculated and the migration paths of oxygen ions in the structures are analysed. The conductivity of PM crystals is close to that of LM crystals. The O2 and O3 atoms are the main contributors to the ion conductivity of LM and PM.

Synthesis and Electrical Properties of a Fluorite-Like Nd 5 Mo 3 O 16 Compound with Partial Substitution of Molybdenum by Tungsten, Niobium, or Vanadium

The phase formation of Nd5Mo3 – xWxO16.5, Nd5Mo3 – xNbxO16.5 – х/2, and Nd5Mo3 – xVxO16.5 – х/2 solid solutions based on a fluorite-like Nd5Mo3O16.5 compound (mixed conductor with interstitial oxygen conductivity) has been studied. The electrical conductivity of doped compounds obeys the Arrhenius law and, at a low impurity content, is as high as 0.03–0.08 S/cm at 800°C. Substitution of Mo6+ cations by V5+ and Nb5+ cations reduces the interstitial oxygen content, which causes a decrease in the solid-solution electrical conductivity by 1–2 orders of magnitude and a decrease in the cubic unit-cell parameter. A wide diffuse anomaly with a maximum of about 1500–4000 has been observed in the temperature dependence of the permittivity for all single-crystal and polycrystalline samples in the range of 300–900°C.