Optical Properties of the (CrF6)3- Complex in A2BMF6:Cr3+ Elpasolite Crystals: Variation with M-F Bond Distance and Hydrostatic Pressure

This work investigates the photoluminescence (PL) properties of the Cr3+-doped and Cr3+-pure fluoroelpasolites along the A(2)BMF(6) series and as a function of pressure. In particular, we focus on the variation of the crystal-field spectrum and the associated PL. The results are explained on the basis of the octahedral (CrF6)(3-) complex subjected either to external pressure or the internal pressure exerted by different crystal hosts. We have established structural correlations between the crystal-field parameter 10Dq and the Cr-F distance, RCr-F, from which we have determined the local structure around the Cr3+ impurity, allowing the host material effect on the Cr-F bonds to be studied. As salient features, we show, first, a weak dependence of the first excitation energy, El, usually identified as 10Dq, with RCr-F as E-1 = KRCr-F-3.3 and, second, an increase of the Stokes shift upon RCr-F reduction or with increasing pressure. We associate this unusual behavior with the existence of state mixing among T-4(2g)(F), E-2(g)(G), and T-2(1g)(G) states in the first excitation band of Cr3+. Finally, high-pressure experiments performed on Rb2KCrF6 indicate that the excited-state spin crossover, E-2(g)(G) <-> T-4(2g)(F), takes place around 7 GPa. The results indicate the suitability of the selected A(2)BMF(6):Cr3+ elpasolites to establish structural correlations between PL and RCr-F.