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 ( x Bi 2 O 3 -(100 - x )TeO 2 ) and bismuth niobium tellurite ( x Bi 2 O 3 - x Nb 2 O 5 -(100 - 2 x )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.