Structure of bismuth tellurite and bismuth niobium tellurite glasses and Bi(2)Te(4)O(11) anti-glass by high energy X-ray diffraction

Glass and anti-glass samples of bismuth tellurite (xBi(2)O(3)–(100 − x)TeO(2)) and bismuth niobium tellurite (xBi(2)O(3)–xNb(2)O(5)–(100 − 2x)TeO(2)) systems were prepared by melt-quenching. The bismuth tellurite system forms glasses at low Bi(2)O(3) concentration of 3 to 7 mol%. At 20 mol% Bi(2)O(3), the glass forming ability of the Bi(2)O(3)–TeO(2) system decreases drastically and the anti-glass phase of monoclinic Bi(2)Te(4)O(11) is produced. Structures of glass and the anti-glass Bi(2)Te(4)O(11) 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 Bi(2)O(3) and Nb(2)O(5) mol% and is in the range: 4.17 to 3.56. The anti-glass Bi(2)Te(4)O(11) 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 Bi(2)Te(4)O(11) has an N(Te–O) of 2.96 and is significantly lower than in glass samples.