Structure and thermal behavior of metastable sillenites prepared by mechanochemical synthesis

The structure and thermal behavior of metastable solid solutions with nominal compositions Bi₂PbO₄, Bi₄HgO₇, Bi₁₂TlO_19.5, and Bi₁₂Cd_0.7O_18.7 with the sillenite structure obtained by mechanochemical treatment in a planetary mill (thermal decomposition of the metastable Bi₄CdO₇ phase with a bcc structure above 700 K) have been studied. The large Pb²⁺ atoms occupy up to ~30% positions in the sillenite structure (Hg²⁺ and Hg(1) occupy up to ~15% of (24f) positions). The (2a) sites are partially occupied by bismuth with a minor shift along the 111] axis and are transformed into the (8c) site, forming an umbrella structure with c.n. 3. The relatively small Cd²⁺and Tl³⁺ cations partially occupy the (2a) sites. During annealing, the metastable Pb-sillenites are partially stabilized with a loss of oxygen and increasing content of bismuth. Sillenite with a high content of lead retains its structure until it melts at 923 K. Cadmium sillenite reacts with CdO, passing into a rhombohedral structure at 900 K (30 K below the melting point). Mercury sillenite Bi₄HgO₇ decomposes with isolation of metallic mercury at ~600 K.Original Russian Text Copyright © 2004 by V. V. ZyryanovTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 3, pp. 480–490, May–June 2004.