Charge compensation and the luminescence of Cr3+ in KMgF3

Abstract

Studies of the photoluminescence spectra of Cr3+ ions in KMgF3 crystals co-doped with Cr3+ and Ni2+ ions are reported. Several crystal field sites are identified by the different R-line spectra due to the 2E→4A2 transition and broadband luminescences associated with the 4T2→4A2 transitions. Cr3+ ions substituting without local charge compensation on the octahedral Mg2+ site give rise to a low temperature R line in photoluminescence at λ=702.3 nm with a radiative decaytime of 3 ms at T=14 K. At T=300 K this isotropic centre gives rise to an unpolarized broadband 4T2→4A2 emission, which results from the thermal occupancy of an excited 4T2 state just above the 2E level which, at lower temperature, gives rise to emission in the R-line. Other crystal field sites are due to some Cr3+ ions having Mg2+ or K+ vacancies in nearest-neighbour positions, these vacancies being required to maintain charge neutrality in doped fluoride perovskites. The Cr3+−K+ vacancy complex results in the centre having trigonal symmetry, and low temperature, photoluminescence via R1 and R2 lines at 716.8 nm and 716.0 nm, respectively. Finally, Cr3+ ions having a nearest neighbour Mg2+ vacancy have tetragonal symmetry, experiencing weak crystal fields. In consequence, the 4T2 level lies below 2E and the photoluminescence spectrum at low temperature takes the form of a polarized broad 4T2→4A2 band with peak at 760 nm and radiative decaytime of 54 μs.