Apatite-type Pr9K(SiO4)6O2—a potential oxide ion conductor

Pr9K(SiO4)6O2 (hexagonal, P63/m (No. 176), a = b = 9.6466(8), c = 7.1136(6) Å, 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(F2 > 2σ(F2)) = 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%. 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)O9]-polyhedra build up chains, which are inter-connected via SiO4-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 high 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.