3d impurities in normal and inverted perovskites: Differences are not explained by ligand field
theory
J. M. García-Lastra,
1
J. Y. Buzaré,
2
M. T. Barriuso,
1
J. A. Aramburu,
3
and M. Moreno
3
1
Departamento de Física Moderna, Universidad de Cantabria, 39005 Santander, Spain
2
Laboratoire de Physique de lEtat Condensé (UMR CNRS 6087), Institut de Recherche en Ingénierie Moléculaire et Matériaux
Fonctionnels (FR CNRS 2575), Université du Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France
3
Departamento de Ciencias de la Tierra y Física de la Materia Condensada, Universidad de Cantabria, 39005 Santander, Spain
Received 14 December 2006; revised manuscript received 2 February 2007; published 3 April 2007
Although the lattice constants of KMgF
3
normal cubic perovskite structure and BaLiF
3
inverted perov-
skite materials differ only by 0.3%, there is a significant difference between the measured cubic field splitting
parameters 10Dq for KMgF
3
:Ni
2+
7800 cm
-1
and BaLiF
3
:Ni
2+
8400 cm
-1
. By means of density functional
theory calculations, it is shown that such a difference similar to that observed for Co
2+
and Mn
2+
impurities
can hardly be understood within the traditional ligand field theory, which ignores the influence of the electro-
static potential V
R
r exerted by the rest of the lattice ions upon the localized electrons of the NiF
6
4-
complex.
Although V
R
r is known to be very flat for a normal perovskite structure, it is shown that this is no longer true
for an inverted perovskite. The origin of this significant difference is accounted for by simply considering, for
the two cubic lattices, the electrostatic interaction of the two first ion shells around the complex with the
electrons in the NiF
6
4-
unit. The results of this work emphasize the importance of V
R
r when comparing the
electronic properties of the same transition metal complex but embedded in two lattices that are not isomor-
phous even if both are cubic.
DOI: 10.1103/PhysRevB.75.155101 PACS numbers: 71.70.Ch, 71.55.-i, 71.15.Mb
I. INTRODUCTION
A great deal of interest is currently focused on doped
fluoride materials for their potential applications in tunable
solid state lasers,
1
storage phosphors,
2,3
scintillators,
2,4,5
lenses for optical lithography in the vacuum uv domain,
6
or
radiation dosimeters.
7
Within the realm of fluorides, particu-
lar attention has been paid to BaLiF
3
doped with transition
metal,
8–14
rare earth,
5,15
or Pb
2+
impurities.
16
BaLiF
3
crystallizes in the so-called inverse perovskite
structure
17
Fig. 1, displaying a primitive cubic lattice where
the monovalent Li
+
cations are surrounded by an octahedron
of F
-
ions. This structure is certainly unusual in trifluoride
materials where compounds like KMgF
3
or RbCdF
3
have the
normal perovskite structure
18
also depicted in Fig. 1. By con-
trast, some oxides have been reported to possess the inverse
perovskite structure.
19
While heavy cations like Ce
3+
or Pb
2+
enter BaLiF
3
at the
Ba
2+
site,
5,15,16
smaller 3d cations like Cr
3+
, Ni
2+
, or Mn
2+
go
substitutionally to the Li
+
site as unequivocally demonstrated
by the electron paramagnetic resonance EPR and electron
nuclear double resonance techniques.
8–11
This view is also
supported by extended x-ray absorption fine structure EX-
AFS results
12
on Ni
2+
-doped BaLiF
3
as well as by calcula-
tions using parameterized interatomic potentials.
20
Reported
EPR data
10
on BaLiF
3
:Cr
3+
show the existence of a close
charge compensation in terms of a Ba
2+
vacancy. By con-
trast, in the case of divalent impurities Ni
2+
and Mn
2+
, the
experimental spin Hamiltonians are fully consistent with a
local octahedral symmetry around the impurity.
8,11
The lack
of any experimental evidence showing the presence of small
tetragonal, trigonal, or orthorhombic components in the mea-
sured spin Hamiltonian thus supports that the necessary
charge compensation has a remote character indeed. It is
worth noting that cubic Fe
3+
centers have been formed in
KMgF
3
and other similar perovskites
21,22
although other cen-
ters with a close charge compensation have also been found
in these lattices.
23–25
A similar situation to this one has been
observed
26
for Rh
2+
-doped NaCl.
Within the traditional ligand field theory
27,28
the electronic
properties due to a substitutional 3d impurity M in a cubic
insulating lattice are explained only in terms of the MX
N
complex formed with the N nearest anions X. According to
this view, the changes undergone by electronic properties of
the MX
N
complex when the host lattice is varied are ascribed
to the induced modifications in the M-X distance R. This idea
has been well verified in the case of Mn
2+
, Ni
2+
, and Fe
3+
impurities in normal cubic fluoroperovskites
22,25,29,30
or Cr
3+
in cubic elpasolites.
31
For Mn
2+
and Ni
2+
impurities in nor-
F
-
K
+
Li
+
Mg
2+
Ba
2+
F
-
K
+
Li
+
Mg
2+
Ba
2+
FIG. 1. Color online 21-atom clusters used in the calculations
of the inverse BaLiF
3
left and normal KMgF
3
right perovskite
structures. Unit cells are marked with dashed lines. The lattice con-
stant a is thus equal to 2R
0
. Similar clusters were used for the
calculations of the corresponding Ni
2+
impurity centers but replac-
ing the central ion Li
+
in BaLiF
3
and Mg
2+
in KMgF
3
by Ni
2+
.
PHYSICAL REVIEW B 75, 155101 2007
1098-0121/2007/7515/1551016 ©2007 The American Physical Society 155101-1