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