Opto-structural study of A2BMF6: Cr3+ elpasolite crystals. variation of the (CrF6)3--complex optical properties with Cr-F bond distance, hydrostatic pressure and excited-state crossover

Abstract

This work investigates the optical properties of the Cr3+-doped and Cr3+-pure elpasolites A2BMF6 along compound series and as a function of pressure. We are interested in establishing structural correlations between the electronic structure and the actual Cr-F distance, RCr-F, from which we have been able to reveal the local structure around the Cr3+impurity. As salient features, we show, firstly, a weak dependence of the first excitation band energy, E1, usually identified as 10Dq, with RCr-F as E1 = K RCr-F−3.3, and, secondly, an increase of the Stokes shift upon RCr-F reduction or with increasing pressure. We associate this unusual behavior with the existence of Fano resonance in the first excitation band of Cr3+. High-pressure experiments performed on Rb2KCrF6 indicate that the excited-state spin crossover, 2Eg(G) ⌆ 4T2g(F), takes place around 7 GPa. Short lived broad-band luminescence changes into long-lived ruby-like narrow lines The reported results point out the suitability of the selected A2BMF6: Cr3+ elpasolites to establish structural correlations between PL and RCr-F.