Structural basis for the anomalous low-temperature thermal expansion behaviour of the gillespite-structured phase Ba_0.5Sr_0.5CuSi₄O₁₀

Authors: Knight, Kevin S.; Henderson, C. Michael B.

Source: European Journal of Mineralogy, Volume 19, Number 2, March-April 2007 , pp. 189-200(12)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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Abstract:

The crystal structure of the gillespite-structured phase Ba_0.5Sr_0.5CuSi₄O₁₀ has been refined from powder neutron time-offlight data at eight temperatures between 5K and 300K. No structural phase transitions were observed on cooling to the lowest temperature. The temperature dependence of the crystal structure in the a - b plane has been found to be dominated by low energy lattice modes involving cooperative anti-phase rotations of the square-planar CuO₄ group with the Si₄O₁₀ polyhedral unit which results in non-Grüneisen behaviour for the a lattice parameter and negative volume thermal expansion to 100K. Concomitant with these rotations is an associated tilting of SiO₄ tetrahedra about the bridging anions resulting in an increasing aplanarity of the Cu site with temperature. The behaviour of the c-axis with temperature is conventional and appears to be related to the tilting of SiO₄ tetrahedra and an increase in the Ba/Sr bond distances resulting in a decrease of the overbonded bond valence sum at the Ba/Sr site.

Keywords: GILLESPITE; WESSELSITE; EFFENBERGERITE; POWDER NEUTRON DIFFRACTION; CRYSTAL STRUCTURE; THERMAL EXPANSION

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2007/0019-1711

Publication date: 2007-03-01

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