On the Phase Transitions of Bi2Te4O11

Abstract

The polymorphic phase transitions of Bi2Te4O11have been investigated using X-ray powder diffraction (XPD), selected area electron diffraction (SAED), and differential scanning calorimetry (DSC) in the 25–730°C range. The metastable cubic modification, which forms under fast crystalization of the Bi2Te4O11melt, has fluorite-type structure. Each cation position is filled with bismuth and tellurium in 1/3–2/3 ratio, while the anion positions are occupied by oxygen in 11/12 site occupancy (evenly distributed vacancy), representing a structure with no chemical ordering. The first process in the transition of cubic phase is cation ordering along a cubic [111] direction. The ordering process has a small activation energy, but the structure reordering itself is exotherm. The final stage of this ordering is the separation of the cations into the triplets of planes forming two types of structural slabs with composition Bi2Te2O7and TeO2. Every third plane contains only Te, and the first two are occupied by equal amounts, of Bi and Te with random distribution. The oxygen content is lower than what would be expected based on the available anion sites in the ideal fluorite structure, and these positions are populated by oxygen in a statistical (random) distribution. The next step of transition is the ordering of oxygen vacancy. The oxygen vacancy is concentrated at the Bi-containing layers in accordance with the fluorite-based structural model of the Bi2Te2O7layers. The result is monoclinic Bi2Te4O11withP21/nsymmetry. There are, however, several grains in the sample that show the coexistence of an exclusively cation-ordered, fluorite-type structure and that of Rossel’s model. This indicates an intermediate or alternative stage of the phase transition, in which the Bi2Te2O7and TeO2slabs are already formed, but the oxygen coordination in the TeO2layer is still fluorite-type hexahedral. The formation of the rutile-type TeO2slabs can be a next step of the transition. The boundary between the two observed phases is irregular. The solid state first order phase transformation can be assumed at the grain boundaries.