Thermal Expansion and Phase Stability Investigations on Cs-Substituted Nanocrystalline Calcium Hydroxyapatites

Abstract

The high-temperature phase stability of Ca 102x Cs x (PO 4 ) 6 (OH) 2 , (x = 0-3) compositions synthesized by various wet chemical methods was investigated. The thermal expansion property of Ca 10 (PO 4 ) 6 (OH) 2 (abbreviated as CaHAp) and Cs-substituted CaHAp was measured by high-temperature XRD and dilatometry. The average crystallite size of the powders synthesized by wet chemical methods was found to be 10-50 nm range as shown by XRD and TEM. Up to 30 mol% Cs loading was observed to show only the apatite phase by XRD when the apatite powder was nanocrystalline in nature. However, high-temperature stability of the Cs-substituted system is limited to £5 mol%. Cs 3 (PO 4 ) is observed to be separated out on heating the material above 773 K for compositions substituted with more than 5 mol% of Cs in the Ca-sublattice. The coefficient of thermal expansion measured by HTXRD is a a = 12.42 3 10 26 K 21 , a c = 14.98 3 10 26 K 21 ; and a a = 12.62 3 10 26 K 21 , a c = 12.57 3 10 26 K 21 for CaHAp and Ca 9.78 Cs 0.2 (PO 4 ) 6 (OH) 1.96 , respectively, in the temperature range of 298-1083 K. Bulk thermal expansion measurements are seen to be in agreement with the lattice expansion results.