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 αL = −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+.
The crystallographic parameters of fluoro-apatite A 5 (VO 4 ) 3 F (A = Pb, Sr, and Ba) were refined by the Rietveld method. The refined parameters indicate that the F ¹ anions in Pb 5 (VO 4 ) 3 F occupy sites different from those in Sr 5 (VO 4 ) 3 F and Ba 5 (VO 4 ) 3 F. The F ¹ anions in Pb 5 (VO 4 ) 3 F are located around center of the Pb 6 octahedra, while those for Sr 5 (VO 4 ) 3 F and Ba 5 (VO 4 ) 3 F are located around the center of Sr 3 or Ba 3 triangle. Our structural analysis suggested that this difference is attributed to the over-bonding state of F ¹ anion in Pb 5 (VO 4 ) 3 F. The electron density distribution for F ¹ anions suggests the potential ionic conductivity along the c axis in Pb 5 (VO 4 ) 3 F.
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