Spectroscopic study of Cr³⁺ in new elpasolites

Abstract

New elpasolites like A₂KMF₆ doped with Cr³⁺ have been investigated by means of optical and EPR techniques. It is shown that though the structural phase transition undergone by Rb₂KGaF₆ at T = 129 K is much stronger than that corresponding to Rb₂KInF₆, the emission band at 10 K of the latter is much broader than that for the former. This inhomogeneous broadening is correlated with the increase experienced by the EPR bandwidth below 150 K reflecting only local changes around Cr³⁺. An analysis of zero phonon lines through the fluoroelpasolite series reveals that they are very sensitive to changes of the Cr³⁺-F⁻ distance, R. It is shown that changes δR_o in the perfect lattice induce changes on R given by δR = fR_o where f is only 0.15. At variance with the findings for MnF₆ ^4-, the present results indicate that an increase of R tends to decrease the Stokes shift. The origin of this fact is discussed. Finally, the present results indicate that R increases (about 0.2 pm) in the phase transition of Rb₂KGaF₆ although the lattice volume decreases.

Key Words:

• fluoroelpasolites doped with C^r3+
• luminescence of C^r3+
• EPR of Cr³⁺
• Phase transitions
• Influence of the Cr-F distance.