Abstract: A compound of the general formula (I): A3BF2M1?xTxO2?2xF4+2x doped with Mn(IV), in which A is selected from the group consisting of Li, Na, K, Rb, Cs, Cu, Ag, Tl, NH4, NR4 and mixtures of two or more thereof, where R is an alkyl or aryl group, B is selected from the group consisting of H and D and mixtures thereof, where D is Deuterium, M is selected from the group consisting of Cr, Mo, W, Te, Re and mixtures of two or more thereof, T is selected from the group consisting of Si, Ge, Sn, Ti, Pb, Ce, Zr, Hf and mixtures of two or more thereof, and 0?x?1.