Two new members of the A2B′Cu3F12 family of kagome-related materials have been prepared, in order to further understand the crystal-chemical relationships, phase transitions and magnetic behaviour within this family of potentially frustrated S = ½ two-dimensional quantum magnets. Cs2TiCu3F12 adopts a crystal structure with the ideal kagome lattice topology (space group R m) at ambient temperature. Diffraction studies reveal different symmetry-lowering structural phase transitions in single crystal and polycrystalline forms at sub-ambient temperatures, with the single crystal form retaining rhombohedral symmetry and the powder form being monoclinic. In both cases, long-range antiferromagnetic order occurs in the region 16–20 K. Rb2TiCu3F12 adopts a distorted triclinic structure even at ambient temperatures.

The system Cs2−xRbxSnCu3F12 exhibits several different distorted variants of the kagome lattice, which are probed in detail by powder diffraction methods.

A new fluorite-like solid solution, II-Bi1 − x Te x (O,F)2 + δ, was produced by solid-phase synthesis at 873 K with subsequent annealing, its concentration boundaries were determined, and a scheme of an isothermal (873 K) section of the BiF3-BiOF-TeO2 system was proposed. The new phase was characterized by X-ray powder diffraction, electron microscopy, and impedance spectroscopy. Making heterovalent substitutions simultaneously in the cation and anion sublattices, Te4+ ⇒ Bi3+ and O2− ⇒ F− allowed one to vary the tellurium cation content x (at constant anion nonstoichiometry δ) or the anion nonstoichiometry δ (at constant tellurium cation content x or constant fluoride ion content), which enabled one to describe the effect of these parameters on the properties of the solid solution. The anion excess δ was found to dominate the unit cell parameter of the solid solution and its ionic conductivity. The conduction within the studied temperature range was proven to be mainly by fluoride ions. It was assumed that the ordering of superstoichiometric anions, or clustering, can manifest itself as the structural modulations of the phase II-Bi1 − x Te x (O,F)2 + δ that were detected in this work.

Tysonite solid solutions Bi1−x Ba x O y F3−x−2y in the BiF3-BiOF-BaF2 system were obtained by solid-phase synthesis in sealed copper tubes in an argon atmosphere at 873 K with subsequent quenching. The solid solutions were studied by X-ray diffraction, electron diffraction, and impedance spectroscopy. On the basis of X-ray powder diffraction data, the homogeneity ranges of the tysonite solid solutions were determined and the scheme of their location in the BiF3-BiOF-BaF2 system at 873 K was suggested. Aliovalent substitutions in both the cation and anion sublattices Ba2+ → Bi3+ and O2− → F− made it possible to vary the concentration of anion vacancies. It was found that, at a high concentration of anion defects at 873 K, the hexagonal tysonite modification with space group P63/mmc is stable. With a decrease in the defect concentration, the trigonal tysonite modification with space group \(P\bar 3c1\) becomes stable. An ordered monoclinic tysonite-type modification BiO y F3 − 2y (0.13 < y < 0.23) was revealed. For the homogeneity ranges of all tysonite phases, dependences of the unit cell parameters and conductivity on the composition along the sections with a constant barium or oxygen content were reported. The most probable location of oxygen anions and anion vacancies in the tysonite structure is discussed.

Isothermal anneals (at 873 K) and powder X-ray diffraction were used to study isothermal sections of phase diagrams of the NdF3-Nd2O3-MF2 (M = Ba, Sr) systems. In studying the NdF3-Nd2O3-BaF2 system, classical solid-phase synthesis was supplemented with mechanochemical activation of feedstock mixtures or BaF2 activated with gaseous hydrogen fluoride was used. In both systems, a solid solution with the fluorite structure based on MF2 and NdOF phases, a solid solution with the tysonite structure based on NdF3, and an ordered fluorite-related phase Ba4Nd3F17 were found. The NdOF-based solid solutions were shown to have polymorphism: βtrig ai αcub at ≈800 K; a new trigonal phase of these solid solutions has been discovered. The effect of a dimensional factor \(\left( {R_{Ba^{2 + } } > R_{Sr^{2 + } } } \right)\) on phase formation and unit cell parameters of the solid solutions was traced.

The anion-excess fluorite-like solid solutions with general composition Bi1−xTex(O,F)2+δ (x>0.5) have been synthesized by a solid state reaction of TeO2, BiF3 and Bi2O3 at 873K with following quenching. The homogeneity areas and polymorphism of the I↔IV Bi1−xTex(O,F)2+δ phases were investigated. The crystal structure of the low temperature IV-Bi1−xTex(O,F)2+δ phase has been solved using electron diffraction and X-ray powder diffraction (a=11.53051(9) Å, S.G. Ia-3, RI=0.046, RP=0.041). Glass formation area in the Bi2O3–BiF3–TeO2 (10% TiO2) system was investigated. IVBi1−xTex(O,F)2+δ phase starts to crystallize at short-time (0.5–3h) annealing of oxyfluoride glasses at temperatures above Tg (600–615K). The ionic conductivity of the crystalline Bi1−xTex(O,F)2+δ phase and corresponding glass-ceramics was investigated. Activation energy of conductivity Ea=0.41(2)eV for the IV-Bi1−xTex(O,F)2+δ crystalline samples and Ea=0.73eV for the glass-ceramic samples were obtained. Investigation of the oxyfluoride samples with a constant cation ratio demonstrates essential influence of excess fluorine anions on the ionic conductivity.

Tysonite solid solutions Bi1 − x M x (O, F)3 − d (M = Na, Sr, or Nd) based on α-BiO y F3 − 2y were prepared by solid-state synthesis at 873 K with subsequent quenching to ice-cold water. Aliovalent substitutions in both the cation and anion sublattices (M n+ → Bi3+ and O2− → F−) made it possible to vary the anion-vacancy density. The solid solutions were characterized by X-ray diffraction and impedance spectroscopy. The homogeneity regions for the tysonite solid solution were determined; triangulation schemes at 873 K were suggested for the systems BiF3-BiOF-NaBiF4 and BiF3-BiOF-SrF2, and a scheme of the subsolidus phase diagram for the system BiO0.1F2.8-NdF3 was suggested. In the system BiO0.1F2.8-NdF3, the transition temperature from the low-symmetry tysonite phase (phase II, space group P \(\overline 3 \) c1, Z = 6) to the high-symmetry one (phase I, space group P63/mmc, Z = 2) decreases with increasing anion-vacancy density. Conductivity measurements were performed in the temperature range 300–523 K and the frequency range from 5 to 1 × 106 Hz. The conductivity of samples in the system BiO0.1F2.8-NdF3 increases with increasing bismuth-ion and anion-vacancy concentrations.

The anion-excess ordered fluorite-related phase Ba2.1Bi0.9(O, F)6.8−δ has been synthesized by a solid state reaction of BaF2, BiF3 and Bi2O3 at 873K with subsequent short annealing at 573K. The crystal structure of the new phase has been solved using electron diffraction and X-ray powder diffraction (a=9.5372(1)Å, c=18.1623(3)Å, space group I4/m, RI=0.025, RP=0.029). Interstitial anions in the fluorite-based structure are considered to form isolated cuboctahedral 8:12:0 clusters. The structural relationship between the oxyfluoride phase Ba2.1Bi0.9(O, F)6.8−δ and similar rare-earth-based fluorides is discussed.

A fluorite-like solid solution Ba-1-BixOzF2 (+ - 2z) on the basis of cubic BaF2 was synthesised in the BaF2-Bi2O3-BiF3 system and the homogeneity range at 873 K was determined. The samples were studied by X-ray powder diffraction and electron diffraction, and their transport properties were measured by the complex impedance method at 300-623 K. Tendencies of variation of lattice parameters and transport properties were determined. These tendencies are discussed on the basis of a defect clustering hypothesis. Thermal treatment at 573 K of the solid solution, quenched from 873 K results in the formation of a new ordered tetragonal fluorite-like phase with lattice parameters a = 9.5355(4) angstrom, c = 18.151(1) angstrom. (c) 2005 Elsevier Ltd. All rights reserved.