Synthesis, revised crystal structures, and refractive indices of ABW-type CsMTiO4 (M = Al, Fe, Ga) and ANA-type CsTi1.10Si1.90O6.50, and the determination of the electronic polarizability of 4-coordinated Ti4+

Abstract Single crystals of ABW-type CsAlTiO 4 ( CAT ), CsFeTiO 4 ( CFT ), CsGaTiO 4 ( CGT ), and ANA-type CsTi 1.1 Si 1.9 O 6.5 ( CST ) were grown and characterized by electron microprobe analyses, single-crystal X-ray diffraction, thermal analyses, and spindle-stage optical investigations to determine the electronic polarizability of 4-coordinated Ti 4+ , α ( [4] Ti 4+ ). The crystal structure of CAT was confirmed to crystallize in the highest possible topological symmetry Imma ( a = 8.9677(2) Å, b = 5.7322(1) Å, c = 9.9612(3) Å) with tetrahedrally coordinated Al and Ti equally distributed on Wyckoff position 8i. Twinning by reticular merohedry with a twin index of 2 was observed for most of the crystals resulting in a hexagonal twin lattice ( a = 11.487(3) Å, c = 8.968(2) Å) with Laue symmetry 6 / mmm . Refractive indices measured by immersion methods on an untwinned specimen are n x = 1.716(5), n y = 1.725(2), and n z = 1.727(1) with 2 V z = 127.1(6)°. The diffraction patterns of CFT and CGT clearly showed superstructure reflections causing a symmetry lowering of index 4 with a transformation according to 2 a , b , c from Imma to Pmab w ith a = 18.3054(7) Å, b = 5.8083(2) Å, c = 9.9938(4) Å for CFT , and a = 18.2921(6) Å, b = 5.7636(2) Å, c = 9.9210(3) Å for CGT . Refractive indices for CGT are n x = 1.750(3), n y = 1.772(3), and n z = 1.776(2) with 2 V z = 132(1)°. The crystal structure of the ANA-type CsTi 1.1 Si 1.9 O 6.5 was confirmed to crystallize in space group Ia 3 ¯ $\overline{3}$ d ( a = 13.8333(4) Å). The extra 0.5 O atoms are needed for charge compensation and to allow the sum of electronic polarizabilities to give a total electronic polarizability calculated from the refractive index n = 1.718(4). The electronic polarizability of [4] Ti 4+ was calculated from the difference between the observed total polarizabilities (derived from the mean refractive indices of CAT and CGT ) and the sum of electronic polarizabilities of cations and anions omitting the polarizability of Ti resulting in α ( [4] Ti 4+ ) = 5.15(5) Å 3 .