Low-temperature flux growth of sulfates, molybdates, and tungstates of Ca, Sr, and Ba and investigation of doping with Mn⁶⁺

The growth of undoped and $Mn^{6+}$-doped molybdates and tungstates of alkali-earth metals and BaSO4 has been investigated. Single crystals were grown by the flux method within the temperature range of 600–475 °C, using the ternary NaCl–KCl–CsCl solvent. Sizes of undoped crystals increase within the series tungstates < molybdates < sulfate and, depending on the cation, within the series $Ca^{2+}Sr^{2+} < Ba^{2+}$. The $Mn^{6+}$ ion tends to be reduced to $Mn^{5+}/Mn^{4+}$ with time in the chloride solution, but can be partly stabilized by the addition of alkali-metal carbonates or hydroxides. The incorporation of $Mn^{6+}$ is governed by the coordination of the $MnO^{2-}_{4}$ tetrahedron in the crystal. No significant doping was found for Ca and Sr compounds and only small amounts of Mn6+ were incorporated into $BaMoO{_4}$ and ${BaWO}_4$. Crystals with orthorhombic space group Pnma such as ${BaSO}_4$ exhibit significantly higher doping levels. The $Mn^{6+}$ distribution in each crystal varies due to manganese reduction with growth time. Temperature-, time-, and concentration-dependent spectroscopy of ${BaSO}_4$:$Mn^{6+}$ was performed.