Electron paramagnetic resonance and relaxation study of copper (II) and
silver (II) in CsCdF 3 single crystals
E. Minner, D. Lovy, and H. Bill
Uniuersite de Geneue, Departement de Chimie Physique, Sciences IL 30, quai E. Ansermet,
1211 Geneue 4, Switzerland
(Received 13 May 1993; accepted 22 July 1993)
Copper and silver, respectively, were introduced into single crystals of CsCdF3. Our detailed
electron paramagnetic resonance (EPR) study showed that both elements enter the Cd lattice
site-copper as Cu2+, silver as Ag+, which then was converted into AgH by x raying the
corresponding samples. Cu 2+ and Ai+ were shown to present in their ground state a pseudostatic J ahn-Teller effect. Motional effects were observed in the respective EPR spectra and
studied in some detail for Cu2+ as they are seen over a wide temperature range. Predictions of
a stochastic Kubo model [J. Phys. Soc. Jpn. 9, 935 (1954)] were compared with the temperature
dependent linewidths of the motionally averaged EPR spectrum. A power law (Tn with n"", 1.9)
was determined for the temperature dependence of the reorientation frequency between 30 and
90 K.
INTRODUCTION
The AMF3 family is an almost ideally suited set of
ternary fluorides for the study of the electronic structure of
transition metal ions introduced at low concentrations, and
certain members are promising solid state laser hosts.
Some of the compounds have cubic structure at all temperatures, whereas other ones present a rich variety of
phase transitions (e.g., Ref. 1). We introduced copper and
silver at an impurity level into single crystals of several
members of this family. The present paper reports results
on CuH and Ag2+ in CsCdF3 , a cubic perovskite. Both
AgH and Cu2 + show a pseudostatic E®e Jahn-Teller
(JT) effect and for both systems, the interpretation of the
EPR spectra necessitates the application of the three-state
model. 2,3 These systems were investigated with the aim to
have a reference frame for our study (in progress) of the
joint action of phase transitions and the JT effect onto
some ncf ions. In a pioneering paper, Muller et al. 4 studied
such effects on the NiH ion (3d7 ) in SrTi03.
The electron paramagnetic resonance (EPR) spectra
of both ions show pronounced motional effects due to dominant spinless reorientation transitions, and particularly
CuH EPR presents this in a wide temperature range. This
is a nearly model-like situation for the application of the
Kubo relaxation equation. The temperature dependence of
the jump frequency may be obtained by correlation of the
predictions of this model with the experimental spectra.
Additional motivation came from the interesting
cristallo-chemical behavior of the ACdF 3 host-silver impurity system (A=Cs, Rb).
While there are few studies about silver (II) impurities, this is not so for copper (II). In particular, several
authors published spectroscopic results about this ion incorporated into K2ZnF4 and BaZnF4 (for instance, Refs. 5,
6, and others). These systems have the interesting property
that their tetragonal crystal structure involves compressed
fluorine octahedra, thereby forcing the copper ion (on the
Zn site) into a d3z2-il ground state.
EXPERIMENT
Good quality transparent single crystals of 0.5-2 cm3
were grown in our Bridgman furnace. The powders which
were contained in ultrapure graphite crucibles were maintained under a fluorine-argon atmosphere. The crucibles
were heated by a rf coil coupled to a 18 k W high-frequency
generator. Though all of the host lattice chemicals were of
Optran quality, CdF2 was found to contain approximately
30 ppm of iron and sometimes manganese. CuF2 and AgF2
had 99.9% purity. Typical nominal mole fractions between
0.005 and 0.009 were added to the starting powder mixture. Part of the dopant decomposed and evaporated. The
as-grown crystals were rather inhomogeneously doped and
samples had to be selected from the lower part of the cylindrical crystal.
EPR experiments (normal, under uniaxial stress applied to the sample, and pulsed) were performed on a modified Varian E-line X-band spectrometer. Its magnetic field
calibration and scan facilities and the data acquisition are
computer controlled. The possibility to do signal averaging
is fully implemented. Our pulsed time-domain EPR spectrometer is described in Ref. 7. The K-band experiments
were performed on a home-built homodyne spectrometer
with Gunn-diode source. Optical spectra were collected on
a Cary 2300 spectrometer.
EPR results
In addition to signals due to the omnipresent Fe (and
sometimes Mn) impurities (e.g., Ref. 8), rather strong
EPR spectra arising from two different copper centers were
found in the as-grown nonirradiated samples (Fig. I).
Both centers have similar g values. Detailed results of only
the IT center are presented in this paper.
J. Chern.
Phys. 99in(9),
November
1993
© 1993 American Institute of Physics
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Arbilrarv Units
CsCdF3:Cu
l:!lOK
38K
33K
1
-'>~\ I/IA,-----V\!\MI
.... 21<
~~~~1~________~~~________~~~
250
300
350
Magnetic Field [rnT]
FIG. J. EPR spectra ofCu 2 + in CsCdF3 as a function of T, recorded with
BII C4 • 'V,.;::::9.25 GHz (1) due to the IT center; (2) due to the static
tetragonal Cu 2 +; (3) due to the cubic Fe3+.
At 4.2 K, its angular dependence shows tetragonal
symmetry of the g tensor-two g values for BII C4 and two
for BII C2 • The number and relative intensities of the wellresolved hyperfine (hf) and super hyperfine (shf) structure
components agree with a planar CuF~- complex comprising four mutually equivalent fluorine ions. In particular,
maximum shf splitting was observed when the magnetic
field was aligned along a F--Cu2+ bond oriented parallel
to the C.(x) [or the C4 (Y)] axis defined in Fig. 2. A detailed study of the angular dependence of the EPR spectra
was realized which complemented, and fully confirmed,
these facts. These results prove that the copper ion sits on
a cadmium lattice site and they justify the model of an
elongated octahedron shown in Fig. 2. No clearly resolved
shfinteraction due to the two axial fluorine ions was found,
y
~
4
o
F
FIG. 2. The figure presents the model of the (Imp F 6 )4- cluster and
defines the labels.
6379
however, but the results of the uniaxial stress experiments
(see below) and the temperature dependence of the spectra
indirectly prove their presence.
The system (in fact, both the Cu and the Ag JT centers) still undergoes reorientation in spite of the apparently
static tetragonal EPR spectrum. Spectra were recorded
(between 2 and 5 K) with uniaxial stress being applied
along a C4 axis of the sample. The static magnetic field was
always perpendicular to this direction and along a C2 crystal axis. The elongations perpendicular to the stress field
are favored. The proportionality constant (below 5.107 P)
was determined through In (population ratios 1 III )
= (aa)/(kT), where a is the applied stress. The constant
is a= 1.636 X 10- 7 (cm- 1 p-l) at 4.2 K. A small systematic variation of the principal g values was observed as a
function of applied stress, but as the numbers lie within the
experimental errors, no detailed investigation was undertaken.
At the X band, the axial EPR spectrum disappeared
reversibly above - 28 K; at the K band, this happened
- 65 K. A motionally averaged one was observed up to
approximately 210 K with our spectrometers.
The static tetragonal Cu2+ center is observed convenientlyat T>38 K (Fig. 1). Below 25 K, it is discriminated through saturation. Its spin Hamiltonian parameters
are given in Table I.
CsCdFa:A!I+
As-grown nonirradiated samples only presented the
EPR spectra due to Fe3+ and sometimes Mn2+, but EPR
spectra of typically SIN = 50--90 (at 4.2 K) due to one
Ai+ species were obtained from crystals which had been
x rayed (typically 30--50 min at room temperature). These
spectra reflect tetragonal symmetry of the g tensor. Their
angular variation is presented in Fig. 3. This plot further
shows that the angular dependence of the well-resolved
hyperfine (hf) and shf structure splitting is similar to the
one observed for Cu2+ when the fact is taken into account
that the silver nucleus with spin 1= 1/2 (two isotopes, but
effect not resolved) is present, instead of 1=312 of the
copper nucleus (two isotopes). Indeed, one observes for
BII C4 resolved interaction with four equivalent F- neighbors in the part of the spectrum labeled gil of Fig. 3,
whereas that part labeled gl shows dominant shf interaction with two mutually equivalent F- neighbors and a
weaker one with a second pair. This is a clear signature of
the fact that the four nearest F- neighbors "sit" on C4 axes
perpendicular to the one along the g tensor axis, fully similar to copper. If Ag2+ were located on a Cs lattice site, this
orientation of the magnetic field would produce splitting
due to four equivalent F- ions as is observed for
RbCdF3 :Ag2+. Therefore, silver substitutes in this compound for a Cd ion and Fig. 2 likewise represent this center.
The temperature dependence of the EPR spectrum is
similar to the one of copper (II) ion with a transition from
the static to the dynamic spectrum between 25 and 30 K
(at the X band), but the averaged spectrum broadens and
disappears -100 K.
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Minner, Lovy, and Bill: Copper (II) and silver (II) in CsCdF3
TABLE I. Spin Hamiltonian parameters of copper and silver in severailluoride host crystals.'
Ion
Type
T (K)
gil
Cu(II)
63CU' (II)
65Cu'(II)
Ag(II)
Cu(II)
Cu(II)
Ag(II)
Cu(II)
Ag(II)
Jahn-Teller
Static
Static
Jahn-Teller
Jahn-Tellerb
4.2
77
77
4.2
4.2
77
77
77
4.2
2.6453( 10)
2.5543( 10)
2.5543(10)
2.5650( 10)
2.614O( 10)
2.5150(10)
2.5190( 10)
2.5665(8)
2.5185(5)
Lattice
CsCdF 3
CsCdF3
CsCdF3
CsCdF3
RbCdF 3
RbCdF3
RbCdF3
NaF
NaF
Jahn-Telle~
Jahn-Teller",d
Staticd
Jahn-Teller
gi
IAIII
IAI I
IQI
IA1x(F)
2.1205(10)
83(5)
102(5) 10(2)
2.0923( 10) 235(5)
52(8)
56(8)
2.0923(10) 252(5)
67(3)
54(3)
2.1058(10)
2.1180(5)
129(5) 106 ( 10)
11.6(9)
80(5) 75(10)
5(2)
2.1705(5)
2.0765(10) 98(10) 93( 10)
226( 11) 240(12)
12.6(3)
2.093(8)
62(1)
2,0965(15)
85(1)
I
312(5)
360(8)
360(8)
578(3)
310(10)
290( 10)
619( 10)
137(2)
581
IA1yCF)
84(5)
105(8)
105(8)
109(3)
95( 10)
80( 10)
73(10)
43(3)
63
I
IA1ztF)
83(5)
100(8)
100(8)
79(3)
90(5)
70(5)
81(10)
39(5)
83
I
Footnote
e
e
e
e
f
f
f
g
h
"All hyperfine and quadrupole structure constants in megahertz.
"With tetragonal field due to phase transition.
cxy axes are rotated by 45°.
dA1x(F) and A1z(F) are rotated by an angle of 27" in the (xz) plane.
<nus work.
fReference 18.
l!R.eference 19.
hReference 20.
Parametrization of the EPR spectra
Our experimental results prove that when external
stress is absent, the adiabatic warped Mexican hat potential
has three equivalent angular minima with a shallow angular barrier separating them. Both impurities indeed present
a moderately strong E ® e JT effect. Thus, the ground state
properties of both of them may be analyzed with the aid of
the three-state mode1. 2,3,9 Several places present this model;
we use Ref. 10. We observe elongated octahedra. Thus,
angular minima of the adiabatic potential are at q;=O,
2'IT/3, and 4'IT/3 and maxima which are shifted by 'IT/3 from
these positions. The electronic wave function associated
with this lowest sheet of the potential is Iv+) = sin (q;/
2) 13~-,:z) +cos(q;/2) Ix2 -r). The tetragonal symmetry of the observed spectra and the fact that the g values
are practically independent of external stress justify a base
change from the vibronic states {I A 2 ), IEo), IE,)} of the
three state model to localized vibronic states {,I'x' '11Y' '11 z}. 2
CsCdF3:Ag
The diagonal elements of the transformed matrix (still operators in the electronic and nuclear spin spaces, respectively) are
(1)
with
TJ=q-v1r,
where
q={Olcosq;IO)
and
r= (A 2 1cos q; 10) are the Ham reduction factors 2 and with
Go = gt{3oB . S + U;S . Ii,
i
4.2K
+ V..eE'
320
300
280
260
o
10
20
30
40
Angle
FIG. 3. Angular dependence of the EPR spectra of AgH (T=4.2 K),
v"z9.25 GHz. (Crosses) experimental points. Equation (2) was adjusted
to the points, the lines represent the result of the best fit.
The symbols related to the spin have their usual
meaning-3r is the A 2-E level splitting, 0 0 and OE are
nuclear quadrupole operators, and the terms V..eo and V..eE
are the 0 and € strain coupling terms. The angular dependence of the experimental EPR spectra was parametrized
by fitting it to these diagonal Hamiltonians with the aid of
a least-squares computer algorithm. We initially included,
by perturbation, contributions from the nondiagonal part,
but within the precision of our experiments, these effects
were negligible, in agreement with the finding that the g
values and line shapes are independent of applied stress.
The proportionality constant a obtained from the stress
experiments determines Vs and also V e , This latter quantity is easily obtained when the relation given by Ham1 is
used
J. of
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Phys., Vol.
99, No.9,
November
1993
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6381
CsCdF3:Cu
and
V~= (v'3/d) Vs
(d=4.465
A
at RT).
Within the probably not very good assumptions that
'TJ = 3/2 and that the bulk elastic compliance constants can
be used, we found Vs =5971 (cm-1/strain) and Ve =34.7
(cm-I/pm).
Relaxation and statistical dynamics of the Impurity
The EPR spectra of both impurities present strong motional effects. Especially the copper (II) ion represents a
model situation where spinless transitions dominate over a
sizeable temperature interval the dynamics of the EPR
spectrum (Fig. 1). The linewidths determined from the
motionally averaged experimental spectra are useful parameters to follow this evolution. The peak-to-peak widths
of the first derivative EPR spectra were determined as a
function of temperature. At every temperature selected, the
experimental spectrum was accumulated until a good SIN
ratio was reached. This is important near the transition
region (25-38 K), where the intensity of the signals is low.
Decomposition of the spectrum with the aid of Voigt type
profiles, by using a least-squares minimization procedure,
yielded the homogeneous contribution to the experimental
linewidths. This one is presented in Fig. 4(a). The Gaussian part reflects static inhomogeneous contributions due to
isotropic shf and residual random static strain effects. Between 40 and 70 K, the points follow the empirical law
aBpp=bra with a= -0.914 and b=232.1. The really dramatic narrowing is well exemplified in the 73 K spectrum
of Fig. 1 and by Fig. 4(a). They illustrate two features.
First, the narrowing is not symmetrical. This fact is fully
reproduced by the Kubo model used below. For this reason, linewidths of the central part at g=2.3 were determined and compared with corresponding linewidths of the
model. Then, the spectra between 70 and 90 K present
partly resolved shf structure. We obtained the points of
Fig. 4(a) by neglecting this secondary splitting. These values thus represent an upper limit of the linewidths. When
the supplementary splitting was included, an experimental
linewidth a~~p = 3.0 mT was for instance obtained from
the 73 K spectrum. A log-log plot of these "corrected"
linewidths against T was used to determine appropriate
new parameters of the power law. We found b=2044 and
a = - 1.5. These figures represent a lower, but probably
more realistic limit of the linewidths. Correspondingly, the
temperature dependence of the jump rate determined below was evaluated for both cases.
The nonstatic JT effect is at the origin of the motional
effects observed. They are specific for many E ® e JT systems, as reported for instance by Williams et al. II for
Cu(II) in lanthanum double nitrates. The strain coupling
operator ensures strong interaction between the orbital
part of the degenerate electronic state and the time dependent host strains of eg symmetry, and it has nonzero matrix
elements between the slightly nonorthogonal localized vi-
FIG. 4. The linewidth of the motionaly averaged spectra. (a) Experimental results as a function of T. Best fit to the central part of the spectra.
(b) Results of Eq. (2) as a function of the transition frequency (see the
text). The points at 1800 and 2000 MHz are ±2 mT as the spectra show
sizeable overlap between the lines.
bronic states [Eq. (1)], thereby enabling reorientation
transitions between the minima of the adiabatic potential.
Depending on the orientation of the B part of the reorientation, transitions are accompanied by a change in the
splitting of the spin levels due to the anisotropic g, A cu ,
A Fi. Within this mechanism, no sudden phase modulation
of the precessing spin accompanies reorientation, and spinflip transitions have a much lower transition probability
than the spinless reorientations. Formally, to each rotational transition may be associated a rotation R (C3 )
= ±21T/3. An equivalent rotation operator may be defined
in the associated spin space (for our DI12 ® E ® e). There
is, however, no 1: I coupling between them. The main formal reason is that no first order matrix elements of the
spin-orbit coupling exist within the electronic E doublet.
This is an essential difference to cubic DII2 ® T ® e JT systems, where the nonzero spin-orbit matrix elements within
the T triplet state establish an immediate association with
such an operator in spin space. There, each reorientation
transition within the ground vibronic triplet causes a sudden jump of the spin phase by an angle ±21T/3 fad. An
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Minner, Lovy, and Bill: Copper (II) and silver (II) in CsCdF3
important consequence of this fact is that no motionally
averaged EPR spectrum is observed for T ® e, under usual
experimental conditions, whereas no such immediate coupling besets the E ® e case.
The Ham IT model implies one effective vibration of eg
symmetry to determine the quantum mechanical energy
level scheme. An immense number of crystal phonons
which transform as eg essentially contribute to the reorientation transitions, but the individual coupling constants are
small. 12 The back influence to the lattice is correspondingly
small. When the residual static strain distribution (which,
however, exists) is further neglected, then it is meaningful
to replace the detailed (JT impurity-bath) interaction by a
single Markov process and to apply the stochastic Liouville
model, first established by Kubo. See, for instance Refs.
13-16 as space does not permit a presentation of the underlying theory. This model yields as a particularly interesting function the frequency dependent susceptibility. The
equation is (with notation from Ref. 17)
bining the two fits [Figs. 4(a) and 4(b)], one obtains
p=.2.41 X 107 Tl.2 for the smoothed experimental lines,
whereas the relationp=. 1.51 X 106 Tl.9 (between 30 and 80
K) is found in the other case.
We further determined the spin-lattice relaxation
times TI and T2 of the Cu(II) center in CsCdF3 with our
time domain pulsed EPR spectrometer at 4.2 K. With
BII C4 experiments were performed on both spectral components. The 11"/2, 'T, and 11" pulse sequence gave T 211 = T 21
=0.48(2) Jl-s. The corresponding TI was determined with
the three pulse sequence 11"/2, 'T, 11"/2, 'T', and 11"/2 and
found to be Till =Tu =9.8(3) Jl-s. Unfortunately the microwave power available (10 W) did not allow us to measure the temperature dependence of the relaxation times
with this method. However, closer examination of the EPR
linewidths [Figs. 1 and 4(a)] convinced us that spin-lattice
relaxation influences them only above 90 K. Admitting a
spin-flip contribution of 1.8 mT to the experimental linewidth at 90 K, a T2 spin-lattice relaxation time of 3.2
X 10- 9 s was calculated.
(2)
DISCUSSION
This result assumes linear response theory to be valid, and
it is based on a set of levels with splitting much smaller
than kT. 13 wI-' is the (constant) microwave frequency
and LX=[H, .. ] is the Liouville operator of the Hamiltonian. According to our stress results, H =. ~Hi (i
=x, ... ,z). Pj=probability to occupy orientational state
'l'j' with P j = 1/3 (neglecting random static strain). cis
a constant
r=p
-2 1 1)
(
1
-2
1
1
1
-2
is the transition matrix between the orientational
states specified by 'l'j of Eq. (2). Finally ro is the static
linewidth. Its value is ro= 1.0 mT and it was
obtained by fitting Eq. (2) to the spectrum at 4.2 K. Equation (2) was evaluated within the operators
IMs ,Mil ,M12 ,M,J ,k) (M; ,M;I ,Mil. ,Mn ,j I which form a
complete basis for LX. We only included the electron spin
(S,Ms) and the nuclear spins of Ag (Mil), FI (Mi2 ), and
F 4 (M,J) because of time and memory limitations of the
computer available. The matrix is of dimensions (768)2.
The spin Hamiltonian parameters contained in H are
known (Table I) and the residuallinewidth was obtained
for p-+O. There is thus one parameter left, p the jump
frequency, which determines the evolution of the spectrum.
We wrote a computer program which evaluated the quantity dx,,/dB obtained from Eq. (2) (220< B<330 mT in
steps of 0.5 at the X band, 960 < B < 1260 mT in steps of 1
at 36 GHz) at T = 1 K and for an extended range of jump
frequencies. Peak-to-peak linewidths of this function were
established by computer. Figure 4(b) presents some of the
results. An empirical relation expressing the calculated linewidths by the jump frequency was determined for a range
of Iinewidths embracing those of Fig. 4(a). The relation is
aBpp=h'pr with r= -0.788 and h' = 1.5134X 108. Com-
Cristallo-chemlcal aspects of the centers
Remarkably, silver (as Ag+) is located on a Cd lattice
site in CsCdF3 , whereas this ion enters the Rb lattice site in
RbCdF3 •18 Copper enters as Cu2+ into both hosts and was
always found to enter the Cd lattice position. The Cd2+ ion
has an ionic radius of 97 pm, Rb + has an ionic radius of
147 pm, and Cs+ has an ionic radius 167 pm, whereas
Cu2+ has an ionic radius of 72 pm, Cu + has an ionic radius
of 96 pm, Ag2+ has an ionic radius of 89 pm, and Ag+ has
an ionic radius of 126 pm. These simple geometrical criteria give wrong predictions. The fact that all our CsCdF3
single crystals contained cubic Fe3+, which would favor
the presence of Ag+ on the Cd site, is not conclusive either,
because all RbCdF3 crystals contained this impurity too.
We observed that copper without exception entered as
Cu+ into CaF2, SrF2, and BaF2 single crystals prepared in
our laboratory. It always substituted for a (doubly
charged) host cation lattice position.
This all agrees with the conjecture that the standard
enthalpy of formation of CuF is probably weakly positive
and that CuF probably does not exist on thermodynamic
ground, but at the higher temperatures (T > 1360 ·C) necessary to grow the alkaline-earth fluorides, the entropy
term in the free Gibbs energy contributes strongly enough
(and < 0), so that CuF can exist (at any rate for the time
interval of the crystal growth). This ion is stabilized by the
Madelung potential once it has been introduced into the
solid.
Relaxation
Gilfl measured the spin-lattice relaxation of Cu2+ in
KZn tutton salt and found T 1=0.6 s at 4.2 K in samples
containing nominally 2% copper. To gain insight into the
spin-lattice relaxation mechanisms of our systems, we investigated the static Cu2+ impurity in NaF (Ref. 19) at 4.2
K with the same equipment and procedure as given above.
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Minner, Lovy, and Bill: Copper (1/) and silver (1/) in CsCdF3
The results are T 2 = 4. 5 (1) f.Ls and T 1 = 23 ( 3) f.Ls. This is
approximately 2X 104 times shorter than Gill's result and
only about ten times longer than the results on the JT
system Cu 2 + in CsCdF3• It is likely that the effective copper (II) concentration is higher in our NaF and CsCdF3
samples than the nominal one as this parameter is difficult
to control. Rapid spin-spin interaction would explain these
short spin relaxation times at 4.2 K and the globally slow
temperature dependence because, as the results of Gill
show, the classical spin-lattice coupling is weak for mutually distant copper (II) ions. One could argue that shf
interaction with the F- nuclear spins could be an essential
relaxation mechanism. The fact that Fe3+ (present in a
comparatively low concentration) have about 103 times
longer spin-lattice relaxation times, in spite of important
shf interaction, is at odds with this hypothesis. There are,
therefore, two rather well decoupled relaxation mechanisms. One is the spin-spin and "classical" spin-lattice
relaxation, with a minor contribution from those reorientation transitions which involve a spin flip. The other one,
which seems to dominate up to - 90 K essentially consists
of the reorientation relaxation due to the transitions without spin flip, with its nearly quadratic temperature dependence between 30 and 80 K (where the second relation was
used). This quite slow temperature dependence, as compared to Van Vleck processes, is due likely to low lying
vibrational energy levels, which exchange energy with the
system. As the low frequency part of the phonon spectrum
of the pure crystal has a Debye model behavior with low
phonon density, the interacting bath probably consists of
the ensemble of the lowest vibronic levels of the JT centers,
distributed by the random static strain. This bath exchanges energy during the individual reorientations. This
type of model gives a temperature dependence near to the
second power of T as has been shown, for instance, for
glasses. 22 In glasses, low lying vibrational energy levels are
present due to the structural disorder affecting the chemical bonds.
CONCLUSIONS
This paper reports EPR and relaxation measurements
on copper(II) and silver(II) incorporated into CsCdF3
single crystals. EPR under uniaxial stress onto the sample
showed that the tetragonal deformation of the coordinating
fluorine octahedra is due to the JT effect. Due to this effect,
6383
the impurities, when in sufficient concentration, produce a
bath oflow lying vibronic levels which determines the temperature dependence of the reorientation process.
Note added in proof. The phonon spectrum of the host
crystal may show enhanced low frequency density of states
if the T 19 modes involving the fluorine octahedra behave as
discussed, e.g., in Ref. 23.
ACKNOWLEDGMENTS
The authors are indebted to Dr. J. Y. Gesland and Dr.
J. J. Rousseau (University Le Mans, France) for making
available their thesis. One author (H.B.) would like to
thank Dr. M. C. M. O'Brien for helpful correspondence
about g values. D. Frauchiger contributed to crystal
growth, J.-B. Pluss made the command electronics of the
EPR spectrometers, and F. Rouge built much of the mechanical equipment. Work supported by the Swiss National Sciences Foundation.
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