Twinned tetragonal structure and equation of state of NaTh 2F 9

Abstract

The crystal structure and stability of NaTh 2F 9 have been studied using thermal analysis, powder X-ray diffraction at atmospheric conditions, and single-crystal X-ray diffraction at high pressure. Sodium dithorium fluoride is stable at least up to 5.0 GPa at room temperature and to 954 K at ambient pressure. In contrast to earlier investigations, which have reported the structure to be cubic ( I 4 ¯ 3 m , Z=4), we observe a tetragonal distortion of the lattice. The actual crystal structure ( I 4 ¯ 2 m , Z=4) is twinned and composed of corner-sharing distorted ThF 9 tricapped trigonal prisms and distorted NaF 6 octahedra. The twinning element is a three-fold axis from cubic symmetry. The ThF 9 polyhedra are rigid and it is the volume changes of the octahedra around the Na atoms that have the major contribution to the bulk compressibility. The zero-pressure bulk modulus B 0 and the unit-cell volume at ambient pressure V 0 are equal to 99(6) GPa and 663.1(1.0) Å 3, respectively, with the fixed first pressure derivative of the bulk modulus B′=4.00. An inspection of the known crystalline phases in the system NaF-ThF 4 reveals that their bulk moduli increase with the increasing ThF 4 content.