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.

汉蓝（Han Blue，化学式BaCuSi₄O₁₀）是西周时期（约公元前1200年）首次制备的一种颜料，研究表明其在低温下（87 K和130 K）会发生两次结构相变，表现为轴向热膨胀系数的不连续性和热容异常。在常温下的硅钡铜石（gillespite）结构相中，汉蓝的格吕内森常数（Grueneisen constant）极低，仅为0.10。这些观测结果出乎意料，与我们此前在ISIS对硅钡铜石结构相Ba₀.₅Sr₀.₅CuSi₄O₁₀开展的研究不一致——该研究中，在室温以下或高达1073 K的高温范围内均未观测到结构相变。此外，我们发现该相的格吕内森常数虽低，但显著更大，为0.45。我们建议对端元相汉蓝中的这些结构相变开展晶体学研究，不仅要表征两个新相的晶体结构，还要测量晶胞的演化过程，以进一步揭示相变本身的本质。