Optical studies ofCr3+inKMgF3:

Abstract

Three different sites of the Cr 3ϩ ions in the fluoride perovskite KMgF 3 have been identified by absorption, selective optical excitation, and time-resolved emission spectroscopy of the 4 T 2 ↔ 4 A 2 transition. Highpressure measurements showing the crossover from low-crystal field to high-crystal field, allows us to situate the Dq/B values of the different chromium sites clearly below 2.3. The different spin-orbit components associated with the zero-phonon lines of the 4 T 2 ↔ 4 A 2 transition of each type of site are clearly shown on the optical spectra and identified by group-theory analysis. The decay profiles of the 4 T 2 level are exponential and the lifetimes at 15 K are 903, 919, and 473 s for the cubic, quadratic, and trigonal sites, respectively. The trigonal center which presents the lowest energy levels was peculiarly studied. Emission and excitation spectra are compared; their evolution versus temperature is followed and explained by the thermal population of the different spin-orbit sublevels. The phonon sideband of the trigonal site is compared with our previous lattice dynamic studies of the pure compound. The different peaks of the emission broadband are described in terms of phonons of the matrix and normal modes of the ͓CrF 6 ͔ 3Ϫ complex. ͓S0163-1829͑97͒03430-9͔

Structure and spectroscopy of Cr3+ defects in KMgF3, KZnF3, and CsCaF3 crystals. An ab initio model potential embedded cluster study

“…We have calculated the potential energy surfaces of the ground and excited states of the CrF 3− 6 cubic point defects, which result in the local structural parameters, the absorption, emission, and excited state absorption spectra. The results, which are presented in the third section, reproduce the experimental structural and spectroscopic data when available and definitely support the assignment of the emission spectrum of the KMgF 3 :Cr 3+ crystal by Mortier et al [21]. The calculated excited state absorption spectra, which have not been measured, show that the laser activity of all three materials should be free from ESA loss if the pumping process is done through selective excitation below the 2 E g excited state.…”

“…These calculations, labeled AIMP/SM( 4 A 2g ), include lattice relaxation and polarization as described in the Method section. The results of the analysis of vibrational progressions in the emission spectra measured at low temperatures have also been included in the table for comparison [15,20,21]. The frequency of the a 1g phonon appears to be common to the vibrational progressions built on zero-phonon lines that have been assigned to cubic, trigonal, and tetragonal sites [15,20,21] [20].…”

“…In parallel and probably related to the poor performance of V 2+ , the study of the spectroscopic properties of Cr 3+ -doped KMgF 3 , KZnF 3 , and CsCaF 3 gained increasing interest and led to continued experimental structural and spectroscopic studies [1, 2, 11 -21], which have proven that the Cr 3+ defects (and, consequently, the interpretation of their spectroscopy) are far more complicated due to the necessary charge compensation when they substitute for Mg 2+ , Zn 2+ , and Ca 2+ ions, respectively. In particular, two significantly different assignments of the emission spectra of KMgF 3 :Cr 3+ have been proposed [18,21]. They correspond to different assumptions about the occurrence of point defects of different site symmetry, the relative position of the lowest excited states, and the spin-orbit interactions between them.…”

Pressure-induced changes inCr3+-doped elpasolites andLiCaAlF6: Interpretation of macroscopic data

“…This spin crossover transition produces a strong variation in the emission pattern going from a broad band to a sharp one. [7][8][9][10] Among the host lattices used for incorporating Cr 3+ impurities, a great deal of work has been carried out on those involving a trivalent cation [10][11][12][13][14][15][16][17][18][19] such as Al 2 O 3 , K 2 NaScF 6 , K 2 NaYCl 6 , or LiCaAlF 6 . Indeed in these cases, the Cr 3+ impurity enters the trivalent site thus giving rise to a center without any close charge compensation, a situation which is thus markedly different from that appearing in systems such as KMF 3 :Cr 3+ or K 2 MF 4 :Cr 3+ ͑M = Mg and Zn͒ where different centers are simultaneously observed.…”

“…Indeed in these cases, the Cr 3+ impurity enters the trivalent site thus giving rise to a center without any close charge compensation, a situation which is thus markedly different from that appearing in systems such as KMF 3 :Cr 3+ or K 2 MF 4 :Cr 3+ ͑M = Mg and Zn͒ where different centers are simultaneously observed. 8,9,20 When a hydrostatic pressure, P, is applied on a halide lattice doped with Cr 3+ it is crucial to look into the variations induced on the cubic-field splitting parameter, 10Dq. Indeed this parameter alone governs the position of the first spin allowed transition 4 A 2 → 4 T 2 in octahedral symmetry, and, in particular, its separation with respect to the sharp 4 A 2 → 2 E transition as the later is much less sensitive to an applied pressure.…”