Structural Phase Transitions in Han Blue (Effenbergerite) BaCuSi4O10

Han Blue, BaCuSi4O10, a pigment first created in the Western Zhou period ~1200 BCE, has been shown to exhibit two structural phase transitions at low temperature, 87 K and 130 K, which are manifested as discontinuities in the axial thermal expansion coefficients and anomalies in the heat capacity. At ambient temperature in the gillespite-structured phase, Han Blue is found to have an extremely low Grueneisen constant of 0.10. These observations are unexpected and are in disagreement with earlier work we have carried out at ISIS on the gillespite-structured phase Ba0.5Sr0.5CuSi4O10 in which no structural phase transitions are observed below room temperature, or at high temperature up to 1073 K. Furthermore, we find a low valued, but significantly larger Grueneisen constant of 0.45. We propose to carry out a crystallographic study of these structural phase transitions in the end-member phase Han Blue characterising the crystal structures of the two new phases but also measuring the evolution of the unit cells to shed more light on the nature of the transitions themselves.