a b s t r a c t Structural phase transitions at 87 K and 103 K are reported for single-crystalline Han Blue (BaCuSi 4 O 10 ) 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 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 γ 1 ¼ À1.09 and γ 3 ¼1.06 at room temperature; the bulk Grüneisen parameter is γ¼0.10. The results are consistent with the presence of low-energy vibrations associated with the collective motions of CuO 4 and Si 4 O 10 polyhedral subunits
