Optical studies of Cr
3
in KMgF
3
: Time-resolved site-selective spectroscopy
and experimental evidence of spin-orbit coupling
M. Mortier,* Q. Wang, J. Y. Buzare
´
, and M. Rousseau
Laboratoire de Physique de l’Etat Condense ´, CNRS URA 807, Universite ´ du Maine, Avenue O. Messiaen, 72017 Le Mans Cedex, France
B. Piriou
Laboratoire PCM, CNRS URA 1907, Ecole Centrale des Arts et Manufactures, Grande Voie des Vignes,
92295 Cha ˆtenay-Malabry Cedex, France
Received 27 January 1997; revised manuscript received 3 April 1997
Three different sites of the Cr
3+
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. High-
pressure 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
3-
complex. S0163-18299703430-9
I. INTRODUCTION
In the 3 d transition-metal ions, most of the visible and
near-infrared electronic transitions are strongly coupled with
phonons. Then, the emission and excitation associated spec-
tra present vibronically broadened bands. This coupling is of
a great practical interest for the manufacture of tunable solid-
state lasers operating at room temperature. However, the ex-
tent of the phonon spectrum of a material is also a strong
promoter of nonradiative transitions which are strongly nega-
tive for laser efficiency. In such a case, it is advisable to
choose compounds with a low cutoff frequency of phonons
with regard to the energy gap between the electronic levels
of the active ion. In this framework, the fluoride compounds,
which present a cutoff frequency of phonons lower than
600 cm
-1
, are good candidates.
Since the work of Brauch and Durr,
1
in 1984, which has
shown laser operation in KZnF
3
:Cr
3+
, many spectroscopic
studies have been done on the fluoride perovskites. In par-
ticular, the cubic and stable perovskites KMgF
3
and KZnF
3
have been well investigated by electron paramagnetic reso-
nance EPR for Cr
3+
.
2–7
The different studies have con-
cluded the existence of four unequivalent sites for trivalent
chromium impurities.
KMgF
3
has the standard perovskite structure belonging to
the ( O
h
1
- Pm 3
¯
m ) space group with a =3.973 Å. Its struc-
ture is scheduled stable by many criterions
8,9
and in fact no
phase transition has been observed in this compound. The
Mg
2+
ion has a ionic radius of 0.86 Å and is replaced sub-
stitutionally by the Cr
3+
ion ( r =0.76 Å) in a (1 b ) octahe-
dral site. Because of the charge difference between the
Mg
2+
ion and Cr
3+
ion, several charge compensation mecha-
nisms required for electrical neutrality lead to different
crystal-field sites for the substituted Cr
3+
ions. The first and
most probable case is a nondistorted site, the charge compen-
sation being farther in the crystal. The second most probable
case is an axially distorted site along a 111 axis due to a
K
+
vacancy in (1 a ) of the same cell, resulting in a C
3 v
symmetry site. The least probable site is distorted along the
100 axis due to a Mg
2+
vacancy in the neighboring cell
C
4 v
symmetry.
1,2
Another quadratic site is cited but not
clearly interpreted and never optically observed in KZnF
3
crystals.
3
With excitation and high-pressure emission measurements
at RT, we show that the chromium ions in KMgF
3
present a
low-crystal-field behavior, clearly below the
4
T
2
-
2
E level
crossing situated at Dq / B =2.3, even for distorted sites. Af-
ter, the usual spectroscopic parameters of the chromium ions
will be deduced.
Up to now, many optical studies have been devoted to
Cr
3+
-doped KMgF
3
Ref. 10 and KZnF
3
.
1,3,11–13
To our
knowledge, there is no unambiguous identification of the
three different Cr
3+
crystal-field sites in KMgF
3
through
their emission and excitation spectra. To clarify this situa-
tion, we present a site-selective experiment which allows us
to separate the response of the three different Cr
3+
sites.
Furthermore, the corresponding spin-orbit components of the
three sites are very well observed on emission and excitation
spectra and identified through a group-theory study. Previ-
ously, they had been wrongly attributed to R lines by Lee,
Han, and Henderson.
10
Then, we propose an opposite point of view to the inter-
pretations of Lee, Han, and Henderson about the optical
properties of Ni
2+
and Cr
3+
codoped KMgF
3
crystal. Indeed,
PHYSICAL REVIEW B 1 AUGUST 1997-II VOLUME 56, NUMBER 6
56 0163-1829/97/566/302210/$10.00 3022 © 1997 The American Physical Society