Twinned tetragonal structure and equation of state of NaTh2F9

The crystal structure and stability of NaTh2F9 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 (image, Z=4), we observe a tetragonal distortion of the lattice. The actual crystal structure (image, Z=4) is twinned and composed of corner-sharing distorted ThF9 tricapped trigonal prisms and distorted NaF6 octahedra. The twinning element is a three-fold axis from cubic symmetry. The ThF9 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 B0 and the unit-cell volume at ambient pressure V0 are equal to 99(6) GPa and 663.1(1.0) Å3, 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-ThF4 reveals that their bulk moduli increase with the increasing ThF4 content.