A detailed study of the variation in lattice parameter and structure with temperature and dilution in yttrium-substituted holmium hexachloro-elpasolite CS2NaYxHo1-xCl6

The lattice parameters of Cs2NaHoCl6 at 25 K, 120 K, and 294 K determined in this work are 10.6389(3) angstrom, 10.6666(2) angstrom, and 10.7220(2) angstrom, respectively, and those of Cs2NaYCl6 at 120 K and 294 K are 10.6722(2) angstrom and 10.7275(2) angstrom, respectively, by Rietveld refinement using space group Fm3m. The variation in structure of Cs2NaYxHo1-xCl6 with yttrium substitution for x = 0 to 1 at 120 K and 294 K has been determined. Given the highly-absorbing samples examined by PXRD, great care is needed with the alignment of the diffractometer and the inclusion of suitable corrections in the Rietveld refinement model. The variation of the lattice parameter of Cs2NaHoCl6 with temperature between 100 K and 475 K has been determined. The mean value for the effective, coordination-number-6, ionic radius of trivalent yttrium/holmium in this elpasolite structure at 120 K determined in this work is 0.764(2) angstrom and at 294 K is 0.766(3) angstrom, i.e. no change within experimental limits was observed in this value, assuming cubic close-packing of Cl- and Cs+ ions. No long-range ordering was found in the 50: 50 mixture consistent with random substitution of holmium by yttrium for all compositions. The process is charge-driven as trivalent yttrium replaces the corresponding holmium without displacing monovalent sodium. Thermoluminescence is observed from samples irradiated at low temperature during their return to room temperature.