Negative Thermal Expansion in Fluoroapatite Pb₅(VO₄)₃F Enhanced by the Steric Effect of Pb²

Negative thermal expansion (NTE) is an unusual thermophysical phenomenon and has gained attention as a way of controlling thermal expansion. Here, we report a substantial NTE in fluoroapatite Pb₅(VO₄)₃F in a limited temperature range. The dilatometric study revealed volume shrinkage below 150 K, giving a linear thermal expansion coefficient of α_L = -44 ppm/K in the temperature range from 140 to 120 K upon heating. The NTE behavior is associated with a structural transition from the hexagonal (P6₃/m) phase to the monoclinic (P2₁/b) phase. Such a structural transition has been found in other apatite-type compounds, but the magnitude of the volume change in Pb₅(VO₄)₃F is remarkable. Our structural analysis revealed that the structural transition is classified as an antiferroelectric-to-paraelectric transition and the volume change during the transition is enhanced by the steric effect of 6s² lone-pair electrons of Pb²⁺.