Low temperature structural variations and molar heat capacity of stolzite, PbWO4

The crystal structure, thermal expansion and heat capacity of PbWO₄ (mineral name stolzite) scintillator material were comprehensively studied over a wide temperature range. No phase transitions were found down to 2 K (I4₁/a, scheelite structure type). A distinct feature of the temperature induced structural variations in PbWO₄ are the different thermal elongations of shorter and longer Pb-O distances. The low-temperature thermal expansion of PbWO₄ was parameterized on the basis of the 1st order Grüneisen approximation using a Debye function for the internal energy with a Debye temperature of 237 K, a bulk modulus of 67 GPa and a Grüneisen parameter of 1.08. The expansion along the c-axis is about 2.5-3 times higher in the range 23-290 K than along the a-direction. This pronounced anisotropy of the thermal expansion arises from the arrangements of rigid tetrahedral WO₄²⁻ units along 〈100〉-directions while Pb²⁺ cations occupy the sites between WO₄²⁻ in 〈001〉-directions.