A. Udovenko, E. I. Pogoreltsev, Yu.V. Marchenko, N. Laptash
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引用次数: 2
摘要
采用x射线衍射、差示扫描微量热法(DSM)和振动光谱法研究了氟化物水溶液中(NH4)3VOF5和(NH4)3VO2F4单晶的相变。这些化合物的室温(RT)相属于正交对称[Immm和I222, Z = 6,分别为(NH4)3VOF5和(NH4)3VO2F4],具有相似的单位胞参数和两个独立的钒原子。在RT以上[(NH4)3VOF5和(NH4)3VO2F4分别在350和440 K下],(NH4)3VOF5和(NH4)3VO2F4分别发生可逆相变,形成具有钒八面体6个和12个空间取向的高对称性动态无序类斜长石(Fm{\bar 3}m, Z = 4)结构。配体原子以24e + 96j的混合(分裂)位置分布,其中一个铵基在四面体32f位上无序,而另一个铵基由于其96j位氢原子的无序而形成八个空间取向。DSM和光谱数据表明,从高温到室温的相变与八面体从各向同性取向到两种不同动态状态的转变有关。
Orientational disorder and phase transitions in ammonium oxofluorovanadates, (NH4)3VOF5 and (NH4)3VO2F4
Single crystals of (NH4)3VOF5 and (NH4)3VO2F4 were obtained from aqueous fluoride solutions and phase transitions in these compounds were investigated using X-ray diffraction, differential scanning microcalorimetry (DSM) and vibrational spectroscopy. The room-temperature (RT) phases of these compounds belong to orthorhombic symmetry [Immm and I222, Z = 6, for (NH4)3VOF5 and (NH4)3VO2F4, respectively] with similar unit-cell parameters and two independent vanadium atoms. Above RT [at 350 and 440 K for (NH4)3VOF5 and (NH4)3VO2F4, respectively], the compounds undergo reversible phase transitions into high-symmetry dynamically disordered elpasolite-like (Fm{\bar 3}m, Z = 4) structures with six and 12 spatial orientations of the vanadium octahedron for (NH4)3VOF5 and (NH4)3VO2F4, respectively. The ligand atoms are distributed in a mixed (split) position of 24e + 96j, one of the ammonium groups is disordered on the tetrahedron 32f site, but another one forms eight spatial orientations due to disorder of its hydrogen atoms in the 96j position. DSM and spectroscopic data enable the phase transition from high temperature to room temperature to be connected with the transition from isotropic orientations of the octahedron to its two different dynamic states.