Negative Thermal Expansion in Fluoroapatite Pb 5 (VO 4 ) 3 F Enhanced by the Steric Effect of Pb 2 .

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 Pb 5 (VO 4 ) 3 F 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 ( P 6 3 / m ) phase to the monoclinic ( P 2 1 / b ) phase. Such a structural transition has been found in other apatite-type compounds, but the magnitude of the volume change in Pb 5 (VO 4 ) 3 F 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 6s 2 lone-pair electrons of Pb 2+ .