STUDY OF THE FORMATION OF BI2TE4O11

The solid state reaction in a 1:4 mole ratio mixture of Bi2O3 and TeO2 and the polymorphic phase transition of Bi2Te4O11 have been investiga ted using differential scanning calorimetry (DSC), electron microprobe , X-ray powder diffraction (XPD), and selected area electron diffracti on (SAED) analysis in the 25-730 degrees C temperature range. Upon hea ting first a 8Bi(2)Te(4)O(11) + 23TeO(2) eutectic is formed, which mel ts at 598.9 degrees C. In this melt the excess of Bi2O3 reacts further and the Bi2O3 + 4TeO(2) = Bi2Te4O11 reaction takes place. The cubic m odification is formed by fast crystallization of the Bi2Te4O11 melt. T he structure of the cubic Bi2Te4O11 can be characterized by the lattic e constant of a = 5.6397(5) Angstrom and space group Fm3m. The main pr oduct of a slow cooling is the same cubic polymorph although a subordi nate formation of the monoclinic phase is also observed. The beta-Bi2T e4O11 cubic phase undergoes a monotropic transformation into the alpha -Bi2Te4O11 monoclinic modification at temperatures higher than 400 deg rees C. The cubic --> monoclinic transition is the result of an orderi ng in one set of {111} planes and the orthogonality of the cubic phase in the [110] projection changes to monoclinic symmetry. The melting e nthalpies of the cubic beta-phase and the monoclinic alpha-phase are 3 5.9 +/- 3.3 J/g and 84.3 +/- 4.3 J/g respectively. (C) 1996 Academic P ress, Inc.