New set of crystals with off-center impurity ions:
cubic oxides SrO and BaO doped by paramagnetic 3d-ions.
L. S. Sochava
Motivation
By the early 1970s crystals with off-center impurity ions (OCII) possessing
a number of peculiar physical properties have attracted continuous attention.
First of all, OCII have been used as model objects for the investigation
of tunnel movement of atomic particles and quantum diffusion mechanisms
in solid state which were pressing problems in solid state physics and
chemistry. Besides, a crystal with OCII was a good model for the
investigation of cooperative properties of the system of local centers
possessing internal degrees of freedom, especially in connection with the
probability of inducing some new phases in the matrix by means of OCII.
To the moment of our investigation beginning (1970), only a limited
number of crystals with OCII were known, almost all of them had been produced
on the basis of alkali halide crystals. It was appropriate to try to expand
the range of crystals with OCII essentially. Moreover, we intended to use
EPR as a main experimental method. On this basis, we believed that alkaline
earth oxide crystals BaO and SrO doped by paramagnetic 3d-ions could
be promising candidates for the planned investigation. Indeed,
-
little size of 3d-ions as compared with one of Ba2+ and
Sr2+ ions made off-center position of the impurities rather
probable;
-
cubic symmetry of the host crystals had to facilitate an interpretation
of EPR spectra.
New set of crystals with off-center impurity ions
We succeeded in growing in our lab SrO and BaO crystals doped by 3d-ions.
The crystals were grown by slow cooling of a melt produced with an electric
arc. High-quality SrO crystals were obtained, with maximal size up to 1
cm.
In this investigation we applied EPR as a main method, including EPR
in the external electric field. As a second method we used an investigation
of Stark effect on zero-phonon lines of impurity ion optical absorption.
Both methods allow to clarify the impurity center symmetry and, besides,
to ascertain the presence of reorienting electrical dipole moment p associated
with the center and to find its value, to elucidate the structure of multiwell
potential and the nature of ion hopping motion under the influence of such
a potential.
Structure of the multiwell potential
The off-center positions, i.e., positions displaced from cation site, were
ascertained for following ions: for Mn2+ in BaO and for 5 ions
in SrO – Co2+, Ni2+, Ni+, Cu2+,
Fe3+ (see the Table). In this investigation we applied mainly
EPR, together with Stark effect on zero-phonon lines of impurity ion optical
absorption.
| Off-center ion |
Number of potential wells |
Site symmetry |
Effective dipole moment |
References |
| Mn2+ in BaO |
8 |
C3v |
|
|
| Ni2+ in SrO |
8 |
C3v |
13 D |
|
| Cu2+ in SrO |
24 |
Cs |
6.5 D |
|
| Ni+ in SrO |
48 |
C1 |
8.3 D |
|
| Co2+ in SrO |
8 |
C3v |
7.5 D |
|
| Fe3+ in SrO |
12 |
C2v |
10.5 D |
|
The impurity displacement from the lattice site leads to the arising of
multiwell potential, which in a cubic crystal will have equivalent energy
minima in either the six <100>, the eight <111> or the twelve equivalent
<110> directions. Among six detected off-center ions five are shifted
along <111> directions and only one ion – Fe3+ – along <110>
directions. The symmetry of a separate energy minimum is C2v
for Fe3+ and C3v for Mn2+, Co2+
and Ni2+ (see the Table). For the two rest of cases – Cu2+,
Ni+ - the symmetry of separate minimum is lower than C3v
(Cs and C1, respectively) due to the additional deformation
of the off-center ion closest environment caused by static Jahn-Teller
effect. So, we observed for the first time the Jahn-Teller off-center ions,
i.e., the ions, environments of which undergo at the same time the polar
deformation associated with the off-center position and the tetragonal
Jahn-Teller deformation.
Hopping motion of the off-center ion in the multi-well potential
We studied off-center impurities in the states localized in separate wells
of the multiwell potential. This situation is fundamentally different from
the tunneling states of the most investigated off-center impurity Li+
in KCl. The dipole moment of the Li+ ion is equal to zero for
the stationary tunneling states, the impurity being equally distributed
between the potential wells. However the off-center ion interaction with
phonons and intrinsic imperfection fields in the crystal may result in
the localization of the ion in separate wells.
All investigated off-center ions are not in the tunneling, but in the
localized states. This is evident from following experimental result: EPR
spectra of the ions can be described in detail by common spin-Hamiltonian
(tunnel splitting does not reveal itself).
Hopping motion of OCII in multiwell potential reveals itself in two
kinds of experiments:
-
arising of preferential orientation of off-center displacements under the
action of external electric field;
-
motional narrowing of the EPR spectrum, i.e., transformation of initial
spectrum into a new one, exhibiting higher symmetry.
In particular, observing of Cu2+ spectrum motional narrowing
allowed us to determine the temperature dependence of the rate of Cu2+
ion hopping motion in the range 1.6 – 25 K. As a result we have come to
the conclusion that the reorientation relaxation of Cu2+ in
SrO at low temperatures is caused by two tunneling processes: phonon-induced
tunneling and tunnel-controlled process.
Last edited in Apr, 2001
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