Composition and structure of phases formed in the process of hydrated antimony pentoxide thermolysis

Abstract

The composition and structure of phases formed in the thermolysis of hydrated antimony pentoxide (HAP) have been studied by means of X-ray diffraction, isothermal thermogravimetric, volumetric, and NMR methods. It has been shown that (a) the HAP thermolysis at 400–1110 K proceeds in four steps: at each step a phase of specific composition is formed (P i -phases: i = 2–5). The P 5-phase, composition Sb 6O 13, is known. Phases P 2, P 3, and P 4 have been identified in the present study. (b) The HAP → P 2 and P 2 → P 3 transformations are accompanied only by dehydration; the P 5 → Sb 2O 4 transformation is accompanied only by reduction processes; the P 3 → P 4 and P 4 → P 5 transformations are accompanied by both dehydration and reduction. (c) The HAP of formula Sb 2O 5 · nH 2O, with 2 ≤ n ≤ 4, is a phase of variable composition, ranging from Sb 2O 5 · 2H 2O to Sb 2O 5 · 4H 2O. (d) The protons in HAP are present in the form of hydrogen and/or hydronium ions, as well as water molecules of crystallization; in P i -phases ( i = 2–4) they exist only in the form of hydrogen ions. The H + and (H 2O)H + ions are statistically distributed over positions 16 d, while antimony and oxygen atoms and water molecules of crystallization occupy positions 16 c, 48 f, and 8 b, respectively.