Compression Studies of Single-Crystal SnO2 and PbO2 in a Diamond Cell

Abstract

Compression behaviors of two single-crystal rutile-structure dioxides, SnO2 (cassiterite) and PbO2 (plattnerite), were studied in a Merrill-Bassett type diamond cell at room temperature. The samples were compressed in a mixture of 4:1 methanolethenol solution with pressure measurements by the ruby scale. A four-circle diffractometer was used to obtain the diffraction patterns of these crystals at high pressures. Compression results on SnO2 did not show significant lattice distortion, with a slight increase in c/a up to 35 kbar. The compression data are in excellent agreement with Hazen and Finger (1981) and in reasonable agreement with Ming and Manghnani (1982). Fitting these data to the Birch-Murnaghan equation gives a bulk modulus (Ko) of 2.24 +- 0.08 Mbar with Ko'= 6.3. On the other hand, the rutile-type PbO2 was found to transform from a tetragonal to an orthorhombic phase above 5 kbar. The cell parameters a, b and c of this phase have different linear compressibility. This phase is different from the reported orthorhombic phases of lead dioxides (alpha-PbO2). It could represent an intermediate distorted phase which occurs during the transformation from the beta-PbO2 to the alpha-PbO2 phase. The bulk modulus of PbO2 was determined to be 1.34 +- 0.06 Mbar by fitting the data to the Birch-Murnaghan equation. A linear relationship was found to exist between the bulk sound velocity and mean atomic weight of the rutile-type dioxides.

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