Thermal stability of osmium mixed oxides I. CaOsO 3 decomposition products: A new orthorhombic phase Ca 2Os 2O 7

Abstract

The thermal decomposition of CaOsO 3 by differential thermal analyses, thermogravimetry and X-ray powder diffraction has been studied. In nitrogen CaOsO 3 decomposes at 880 ± 10°C into CaO, osmium metal and oxygen due to the reaction CaOsO 3 → CaO + Os + O 2. In static air the decomposition occurs in three stages: 2CaOsO 3 + 1 2 O 2 → Ca 2Os 2O 7 (in region 775–808°C), Ca 2Os 2O 7 → Ca 2Os 2O 6,5 + 1 4 O 2 (at a temperature interval of 850–1000°C) and in the third stage Ca 2Os 2O 6,5 → 2CaO + OsO 4 ÷ 1 4 O 2 (at 1005 ± 5°C). The first intermediate Ca 2Os 2O 7 is isostructural with orthorhombic Ca 2Nb 2O 7 and its cell parameters are: a 0 = 3.745 Å, b 0 = 25.1 Å, c 0 = 5.492 Å, Z = 4, space group Cmcm or Cmc2. Ca 2Os 2O 7 exhibits metallic conductivity and its electrical resistivity is 4.6 × 10 −2 ohm-cm at 296K.