Heat capacity, thermal expansion and heat transport in the Han Blue (BaCuSi₄O₁₀): Observation of structural phase transitions

Structural phase transitions at 87 K and 103 K are reported for single-crystalline Han Blue (BaCuSi₄O₁₀) by means of high-resolution thermal-expansion, thermal conductivity and heat capacity measurements. The phase transition at 103 K results in differing lengths of the a and b lattice parameters, and thus a lowering of the crystallographic symmetry. Negative thermal-expansion coefficients are observed perpendicular to the c-axis over a wide temperature range (108K<T<350K). The thermal conductivity is small, and decreases with temperature, both of which suggest strong scattering of heat-carrying phonons. The principle Grüneisen parameter within the plane and perpendicular to it was determined to be γ₁=-1.09 and γ₃=1.06 at room temperature; the bulk Grüneisen parameter is γ=0.10. The results are consistent with the presence of low-energy vibrations associated with the collective motions of CuO₄ and Si₄O₁₀ polyhedral subunits.