Flux Growth of [NaK6F][(UO2)3(Si2O7)2] and [KK6Cl][(UO2)3(Si2O7)2]: The Effect of Surface Area to Volume Ratios on Reaction Products

Abstract

Single crystals of two new salt-inclusion uranium(VI) silicates, [NaK6F][(UO2)3(Si2O7)2] and [KK6Cl][(UO2)3(Si2O7)2], were grown from mixed alkali halide salt fluxes. Both compounds crystallize in the [NaRb6F][(UO2)3(Si2O7)2] structure type in the orthorhombic space group Pnnm with lattice parameters a = 11.0819(5) A, b = 13.1149(6) A, and c = 7.8418(4) A ([NaK6F]) and a = 11.0830(7) A, b = 13.5850(4) A, and c = 7.8693(5) A ([KK6Cl]). The surface area to volume (sa/vol) ratio of the reaction, controlled by the reaction vessel dimensions, has a strong influence on the reaction products with small sa/vol ratios favoring the growth of the salt-inclusion phases. Both compounds exhibit the typical luminescence of the uranyl group with the luminescence of [KK6Cl][(UO2)3(Si2O7)2] being less intense than [NaK6F][(UO2)3(Si2O7)2], likely due to quenching by the Cl– ions.