Abstract
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.
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