ON THE PHASE-TRANSITIONS OF BI2TE4O11

The polymorphic phase transitions of Bi2Te4O11 have been investigated using X-ray powder diffraction (XPD), selected area electron diffracti on (SAED), and differential scanning calorimetry (DSC) in the 25-730 d egrees C range. The metastable cubic modification, which forms under f ast crystalization of the Bi2Te4O11 melt, 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 pha se is cation ordering along a cubic [111] direction. The ordering proc ess has a small activation energy, but the structure reordering itself is exotherm. The final stage of this ordering is the separation of th e cations into the triplets of planes forming two types of structural slabs with composition Bi2Te2O7 and TeO2. Every third plane contains o nly 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 s tructure, 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-containi ng layers in accordance with the fluorite-based structural model of th e Bi2Te2O7 layers. The result is monoclinic Bi2Te4O11 with P2(1)/n sym metry. 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 alternat ive stage of the phase transition, in which the Bi2Te2O7 and TeO2 slab s are already formed, but the oxygen coordination in the TeO2 layer is still fluorite-type hexahedral. The formation of the rutile-type TeO2 slabs can be a next step of the transition. The boundary between the two observed phases is irregular. The solid state first order phase tr ansformation can be assumed at the grain boundaries. (C) 1998 Academic Press.