Flux growth and liquid phase epitaxy of undoped and Mn-doped sulfates, tungstates, and molybdates

The Mn ion is a promising activator ion for tunable and short-pulse laser materials because of its broadband luminescence in the spectral region 850-1600 nm and its simple 3d electronic configuration, which excludes an occurrence of undesirable exited-state absorption into higher 3d levels. However, hexavalent manganese can be stabilized only in the tetrahedral oxo-coordination and easily reduces to Mn or Mn at temperatures above 600°C. Recently, flux [1] and liquid-phase epitaxy (LPE) [2] growth of Mn-doped sulfates has been reported, while except for BaMoO4:Mn [3] investigations on the mechanically more stable alkaline-earthmetal molybdates and tungstates as possible host materials for efficient Mn incorporation have as yet not been reported. We investigated the growth conditions of undoped and Mn-doped MAO4, with M = Ca, Sr, Ba and A = S, Mo, W, from the ternary NaCl-KCl-CsCl solvent at temperatures 480-600°C. The growth rates increase in the series tungstates < molybdates < sulfates and depending on the cation, in the series Ca < Sr < Ba. The dopant ion Mn can be easily incorporated into BaSO4, less well into BaMoO4 and BaWO4, whereas for Caand Sr-containing tungstates and molybdates no significant doping was found, independent on the concentration of Mn in the liquid solution. Moreover, reduction of the Mn ion cannot be avoided, even at the presence of oxidizing additives such as K2CO3 or NaOH. LPE was employed for growing Mn-doped layers of BaAO4 compounds. Growth velocities of 3-5 μm/h in the temperature interval from 490-540°C from chloridic solution, containing 0.3-1mol% of K2MnO4 with respect to the solute, delivered dark-pink BaSO4 and slightly green BaMoO4 and BaWO4 layers up to 200 μm in thickness. With respect to high Mn doping levels, BaSO4 is the most suitable host material and its further investigation under different initial concentrations of manganese is currently underway.