Phase transition in Pb5Al3F19

A study of the PbF₂-AlF₃ system has resulted in a new material Pb₅Al₃F₁₉ with a tetragonal structure at room temperature closely related to that of Pb₅Cr₃F₁₉. A phase transition in Pb₅Al₃F₁₉ has been detected by means of X-ray diffraction, calorimetric and dielectric measurements. The coexistence of the high and low-temperature Pb₅Al₃F₁₉ phases over the range 270 to 300 K and the abrupt 0.7% increase in the a-axis and 1.5% decrease in the c-axis, on heating through the phase transition, are characteristic of a first-order phase change. The increase in symmetry from point group 4 below to point group 4/m above the phase transition is consistent with a ferroelectric to paraelectric transition, as found previously in Pb₅Cr₃F₁₉.