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 Cr (3+)-doped and Cr (3+)-pure fluoroelpasolites along the A 2BMF 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 (CrF 6) (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, R Cr-F, from which we have determined the local structure around the Cr (3+) 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, E 1, usually identified as 10Dq, with R Cr-F as E 1 = KR Cr-F (-3.3), and, second, an increase of the Stokes shift upon R Cr-F reduction or with increasing pressure. We associate this unusual behavior with the existence of state mixing among (4)T 2g(F), (2)E g(G), and (2)T 1g(G) states in the first excitation band of Cr (3+). Finally, high-pressure experiments performed on Rb 2KCrF 6 indicate that the excited-state spin crossover, (2)E g(G) <--> (4)T 2g(F), takes place around 7 GPa. The results indicate the suitability of the selected A 2BMF 6:Cr (3+) elpasolites to establish structural correlations between PL and R Cr-F.