Structure of bismuth tellurite and bismuth niobium tellurite glasses and Bi2Te4O11 anti-glass by high energy X-ray diffraction.

ABSTRACT

Glass and anti-glass samples of bismuth tellurite (xBi2O3-(100 - x)TeO2) and bismuth niobium tellurite (xBi2O3-xNb2O5-(100 - 2x)TeO2) systems were prepared by melt-quenching. The bismuth tellurite system forms glasses at low Bi2O3 concentration of 3 to 7 mol%. At 20 mol% Bi2O3, the glass forming ability of the Bi2O3-TeO2 system decreases drastically and the anti-glass phase of monoclinic Bi2Te4O11 is produced. Structures of glass and the anti-glass Bi2Te4O11 samples were studied by high-energy X-ray diffraction, reverse Monte Carlo simulations and Rietveld Fullprof refinement. All glasses have short short-range disorder due to the existence of at least three types of Te-O bonds of lengths 1.90, 2.25 and 2.59 Å, besides a variety of Bi-O and Nb-O bond-lengths. The medium-range order in glasses is also disturbed due to the distribution of Te-Te pair distances. The average Te-O co-ordination (N Te-O) in the glass network decreases with an increase in Bi2O3 and Nb2O5 mol% and is in the range 4.17 to 3.56. The anti-glass Bi2Te4O11 has a long-range order of cations but it has vibrational disorder and it exhibits sharp X-ray reflections but broad vibrational bands similar to that in glasses. Anti-glass Bi2Te4O11 has an N Te-O of 2.96 and is significantly lower than in glass samples.