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

Abstract

The growth of undoped and Mn⁶⁺-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 Ca²⁺≈Sr²⁺<Ba²⁺. The Mn⁶⁺ ion tends to be reduced to Mn⁵⁺/Mn⁴⁺ with time in the chloride solution, but can be partly stabilized by the addition of alkali-metal carbonates or hydroxides. The incorporation of Mn⁶⁺ is governed by the coordination of the MnO₄ ^2- tetrahedron in the crystal. No significant doping was found for Ca and Sr compounds and only small amounts of Mn⁶⁺ were incorporated into BaMoO₄ and BaWO₄. Crystals with orthorhombic space group Pnma such as BaSO₄ exhibit significantly higher doping levels. The Mn⁶⁺ distribution in each crystal varies due to manganese reduction with growth time. Temperature-, time-, and concentration-dependent spectroscopy of BaSO₄:Mn⁶⁺ was performed.