Apatite-type Pr_9K(SiO_4)_6O_2-a potential oxide ion conductor

摘要

Pr_9K(SiO_4)_6O_2, (hexagonal, P6_3/m (No. 176), a = 6 = 9.6466(8), c = 7.1136(6) A, Z=1) was synthesised in a potassium fluoride flux. The chemical composition was proven by EPMA/EDX and the structure refined from X-ray single crystal diffraction data (R1(F~2>2 sigma(F~2))=0.0204) The compound crystallises in the regular apatite structure. Potassium fills one (4f) out of two metal positions present in the structure with an occupancy factor of 25 percent. The remaining places of this site (Pr2/K2) are occupied by praseodymium Oxygens of the silicate groups form a coordination polyhedron (CN=9) in the shape of a distorted threefold capped trigonal prism These face sharing [(Pr2/K2)O_9]-polyhedra build up chains, which are inter-connected via SiO_4-groups. The resulting channel framework accommodates sevenfold oxygen-coordinated praseodymium (Pr1), attached to the inside of the tubes that are aligned parallel to the c-axis The oxide ions O4, located on the longitudinal axis of the channels, exhibit anomalously light atomic displacement parameters along the c-direction. It can be shown that this property is in agreement with literature structural data as well as oxide ion conductivity of similar rare earth apatites.