On the structure of the disordered Bi2Te4O11 phase

Abstract The structure of the disordered metastable Bi 2 Te 4 O 11 phase has been investigated using both neutron powder diffraction and reverse Monte Carlo (RMC) modelling. The average structure, of fluorite-type (space group Fm 3 m ), is characterized by very high Debye-Waller parameters, especially for oxygen. Whereas the cations form a fairly well-defined FCC lattice, the oxygen sublattice is very disordered. It is shown that the local order is similar to that present in the stable monoclinic Bi 2 Te 4 O 11 phase. Clear differences are observed for the intermediate range order. The present phase is analogous to the “anti-glass” phases reported by Tromel in other tellurium-based mixed oxides. However, whereas Tromel defines anti-glass as having long range order but no short range order, it is shown here that this phase is best described as an intermediate state between the amorphous and crystalline states, i.e. having short and medium range order similar to that of tellurite glasses and a premise of long range order with the cations only.