EPR and dielectric spectroscopy of reorienting Cr³⁺-Li⁺ pair centres in Li₂Ge₇O₁₅ crystal
Earlier it has been shown that in the lithium heptagermanate Li₂Ge₇O₁₅ structure the chromium doping ions substitute the germanium hosts and create Cr³⁺-Li⁺ pair centres. In the paper EPR and dielectric spectra of Li₂Ge₇O₁₅:Cr³⁺ crystal are studied in the temperature range 300 ÷ 500 K. The therma...
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| Zitieren: | EPR and dielectric spectroscopy of reorienting Cr³⁺-Li⁺ pair centres in Li₂Ge₇O₁₅ crystal / M.D. Volnianskii, M.P. Trubitsyn, Yahia A.H. Obaidat // Condensed Matter Physics. — 2007. — Т. 10, № 1(49). — С. 75-78. — Бібліогр.: 9 назв. — англ. |
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irk-123456789-1180702017-05-29T03:05:27Z EPR and dielectric spectroscopy of reorienting Cr³⁺-Li⁺ pair centres in Li₂Ge₇O₁₅ crystal Volnianskii, M.D. Trubitsyn, M.P. Obaidat, Yahia A.H. Earlier it has been shown that in the lithium heptagermanate Li₂Ge₇O₁₅ structure the chromium doping ions substitute the germanium hosts and create Cr³⁺-Li⁺ pair centres. In the paper EPR and dielectric spectra of Li₂Ge₇O₁₅:Cr³⁺ crystal are studied in the temperature range 300 ÷ 500 K. The thermally activated reorientation of Cr³⁺-Li⁺ pairs has been detected through EPR line width broadening and dielectric relaxation. Ранiше було показано, що у структурi кристалiв гептагерманату лiтiю Li₂Ge₇O₁₅ домiшковi iони хрому замiщують iони германiю i створюють Cr³⁺-Li⁺ парнi центри. У роботi ЕПР i дiелектричнi спектри кристалiв Li₂Ge₇O₁₅: Cr³⁺ дослiдженi у температурному дiапазонi 300 ÷ 500 K. Вперше термiчно активована реорiєнтацiя дипольних моментiв Cr³⁺-Li⁺ пар зареєстрована шляхом вимiрювання поширення ЕПР лiнiй i дiелектричної релаксацiї. 2007 Article EPR and dielectric spectroscopy of reorienting Cr³⁺-Li⁺ pair centres in Li₂Ge₇O₁₅ crystal / M.D. Volnianskii, M.P. Trubitsyn, Yahia A.H. Obaidat // Condensed Matter Physics. — 2007. — Т. 10, № 1(49). — С. 75-78. — Бібліогр.: 9 назв. — англ. 1607-324X PACS: 76.30.Da, 76.30.-v, 77.22.Gm DOI:10.5488/CMP.10.1.75 http://dspace.nbuv.gov.ua/handle/123456789/118070 en Condensed Matter Physics Інститут фізики конденсованих систем НАН України |
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Earlier it has been shown that in the lithium heptagermanate Li₂Ge₇O₁₅ structure the chromium doping ions
substitute the germanium hosts and create Cr³⁺-Li⁺ pair centres. In the paper EPR and dielectric spectra
of Li₂Ge₇O₁₅:Cr³⁺ crystal are studied in the temperature range 300 ÷ 500 K. The thermally activated reorientation
of Cr³⁺-Li⁺ pairs has been detected through EPR line width broadening and dielectric relaxation. |
| format |
Article |
| author |
Volnianskii, M.D. Trubitsyn, M.P. Obaidat, Yahia A.H. |
| spellingShingle |
Volnianskii, M.D. Trubitsyn, M.P. Obaidat, Yahia A.H. EPR and dielectric spectroscopy of reorienting Cr³⁺-Li⁺ pair centres in Li₂Ge₇O₁₅ crystal Condensed Matter Physics |
| author_facet |
Volnianskii, M.D. Trubitsyn, M.P. Obaidat, Yahia A.H. |
| author_sort |
Volnianskii, M.D. |
| title |
EPR and dielectric spectroscopy of reorienting Cr³⁺-Li⁺ pair centres in Li₂Ge₇O₁₅ crystal |
| title_short |
EPR and dielectric spectroscopy of reorienting Cr³⁺-Li⁺ pair centres in Li₂Ge₇O₁₅ crystal |
| title_full |
EPR and dielectric spectroscopy of reorienting Cr³⁺-Li⁺ pair centres in Li₂Ge₇O₁₅ crystal |
| title_fullStr |
EPR and dielectric spectroscopy of reorienting Cr³⁺-Li⁺ pair centres in Li₂Ge₇O₁₅ crystal |
| title_full_unstemmed |
EPR and dielectric spectroscopy of reorienting Cr³⁺-Li⁺ pair centres in Li₂Ge₇O₁₅ crystal |
| title_sort |
epr and dielectric spectroscopy of reorienting cr³⁺-li⁺ pair centres in li₂ge₇o₁₅ crystal |
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Інститут фізики конденсованих систем НАН України |
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2007 |
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http://dspace.nbuv.gov.ua/handle/123456789/118070 |
| citation_txt |
EPR and dielectric spectroscopy of reorienting Cr³⁺-Li⁺ pair centres in Li₂Ge₇O₁₅ crystal / M.D. Volnianskii, M.P. Trubitsyn, Yahia A.H. Obaidat // Condensed Matter Physics. — 2007. — Т. 10, № 1(49). — С. 75-78. — Бібліогр.: 9 назв. — англ. |
| series |
Condensed Matter Physics |
| work_keys_str_mv |
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2025-07-08T13:18:53Z |
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1837084949801336832 |
| fulltext |
Condensed Matter Physics 2007, Vol. 10, No 1(49), pp. 75–78
EPR and dielectric spectroscopy of reorienting
Cr3+– Li+ pair centres in Li 2Ge7O15 crystal
M.D.Volnianskii, M.P.Trubitsyn, Yahia A.H.Obaidat
Department of the solid state physics, Dnipropetrovsk National University,
13 Naukova Str., Dnipropetrovsk 49050, Ukraine
Received September 4, 2006
Earlier it has been shown that in the lithium heptagermanate Li2Ge7O15 structure the chromium doping ions
substitute the germanium hosts and create Cr3+– Li+ pair centres. In the paper EPR and dielectric spectra
of Li2Ge7O15:Cr3+ crystal are studied in the temperature range 300 ÷ 500 K. The thermally activated reori-
entation of Cr3+– Li+ pairs has been detected through EPR line width broadening and dielectric relaxation.
Key words: impurity ions, electron paramagnetic resonance, dielectric loss and relaxation
PACS: 76.30.Da, 76.30.-v, 77.22.Gm
1. Introduction
It is known that Cr3+ doping ions in the structure of Li2Ge7O15 (LGO) crystal substitute
the Ge4+ host ions within oxygen octahedral complexes [1]. Incorporation of chromium ions into
oxygen octahedra changes the local symmetry of the lattice site from monoclinic C2 group to
triclinic C1 group. The local symmetry lowering is attributed to the effect of the additional defect
compensating the charge misfit of Cr3+ ion at the Ge4+ site. Taking into account a weak coupling
of lithium ions with the germanium – oxygen lattice framework [2], the interstitial Li+ is considered
to be the most probable charge compensating defect, located within the structural cavity near the
octahedral CrO6 complex. Subsequent measurements of optical spectra have confirmed the model
of Cr3+– Li+ pair centres in the LGO crystal structure [3,4].
The available data make it possible to assume that electric dipole moments of Cr3+– Li+ pairs
are directed along the crystal axis a. Interstitial Li+ ions locally break the symmetry axis C2‖b of
the sites within the oxygen octahedral complexes [2]. As a result, there are two equivalent configu-
rations of the pair centres which are conjugated by broken C2 axis and have dipole moments with
opposite orientations. It may be assumed that pair centres can reorient due to thermal activation.
Reorientation of the pair centres should be accompanied by: i) shortening of the configuration life
time and ii) switching of defect dipole moments.
The paper is devoted to the detection of thermally activated Cr3+– Li+ reorientation in
LGO:Cr3+ crystal through measurements of EPR spectra and dielectric permittivity.
2. EPR lines width broadening
Angular variations of LGO:Cr3+ EPR spectra [1,5] have shown that for arbitrary orientations of
static magnetic field B with respect to the crystal axes there are four (km = 4) conjugated spectra
of triclinic symmetry C1. For magnetic field B rotating in the main crystal planes two structurally
equivalent Cr3+ spectra are registered. If Cr3+ centres kept the monoclinic local symmetry C2 of
the perfect lattice sites, then the magnetic multiplicity of the spectrum would be km = 2 with
unique direction along axis b [6]. In this case, for a magnetic field rotating in (ab) and (bc) planes
one spectrum would be observed, whereas in (ac) plane two conjugated spectra would be detected.
Therefore, doubling of the spectra in (ab) and (bc) planes results from lowering of the positional
c© M.D.Volnianskii, M.P.Trubitsyn, Yahia A.H.Obaidat 75
M.D.Volnianskii, M.P.Trubitsyn, Yahia A.H.Obaidat
symmetry C2 −→ C1 due to the formation of Cr3+– Li+ pair centres. Assuming that at high
temperatures the probability of reorientation between equivalent states should increase, it may
be supposed that in (ab) and (bc) planes EPR spectra should broaden on heating [7]. It may be
expected that a fast enough reorientation between equivalent configurations should average the
local distortions produced by pair centres, and effectively restore point symmetry C2 of the perfect
lattice site.
Figure 1. Fragment of LGO:Cr3+ EPR spec-
trum: four conjugated lines corresponding to
electronic transition |MS | = 1/2←→ 3/2.
At low enough temperatures the so-called
slow reorientation limit is valid [7]. The spec-
tra from equivalent configurations are observed
separately. EPR lines broaden on heating and
the line width is inversely proportional to the
life time τ ′ of the equivalent configurations
δB = δB0 +
h
gβ
· 1
2
√
3π
(τ ′)−1, (1)
where δB0 is the background line width contri-
buted by all other mechanisms except for the
pair centres reorientation, h is Planck constant,
g is the spectroscopic factor, β is Bohr magne-
ton, the life time τ ′ and relaxation rate (τ)−1
exponentially depend on the inverse tempera-
ture (τ ′/2) = τ = τ0 exp(U/kT ), (τ0)
−1 is fre-
quency of oscillations near potential minima, U
is activation energy, k is Boltzmann constant.
Figure 2. Temperature dependencies of the line
width of the spectral components depicted in the
figure 1: a) the total peak-to-peak line width; b)
the line width, contributed by Cr3+– Li+ centres
reorientation.
EPR spectra of LGO:Cr3+ were measured
using a conventional X-band spectrometer. The
temperature of the samples was regulated by
a standard gas flow cryostat. To detect the
assumed local kinetics, the magnetic field B
was rotated from c axis to b axis by 260 and
then tilted from (bc) plane by ≈ 0.80. For
such an orientation, all four conjugated spec-
tra are distinctly resolved. The group of con-
jugated EPR lines corresponding to the elec-
tronic transition |MS | = 1/2 ←→ 3/2 (fig-
ure 1) was measured in the temperature inter-
val 290÷ 500 K. The line shape was described
by a Lorentzian, the temperature dependencies
of the line width are depicted in figure 2a. For
all components the studied linewidths demon-
strate non-monotonous temperature dependen-
cies δB(T ). The decrease of δB on heating in
the interval 290÷ 380 K is obviously connected with ferroelectric phase transition (TC = 283.4 K)
which is accompanied by an EPR line width anomaly [5]. The anomalous line width broadening
has been described by the Curie-Weiss law, and for the background line width in (1) the following
expression was used
δB0 = δB0
0 +
CCW
T − TC
. (2)
The temperature dependence δB0 in (2) being taken into account enables us to derive the line
width contributed by the pair centres reorientation. The temperature dependencies of the line
width originated from the pair centres dynamics are presented in figure 2b.
The oscillation frequency (τ0)
−1 was estimated from the Debye temperature which has a typical
value of θD ≈ 200 K for the crystals of germanate family [8]. The activation energy is estimated
76
EPR and dielectric spectroscopy of reorienting
as U = 0.55 ± 0.05 eV from the line width of the four components registered. Evidently, in the
temperature interval studied the line width broadening is too small (∼ 7% from the background line
width) and the activation energy is determined with low accuracy. Nevertheless, one can estimate
in what temperature and frequency range the permittivity anomalies from Cr3+– Li+ reorientation
can be detected.
3. Dielectric relaxation of Cr 3+– Li+ pair centres
Figure 3. Temperature-frequency dependen-
cies of a) real ε′ and b) imaginary ε′′ parts
of LGO:Cr3+ permittivity, measured along a
axis at the following frequencies f : 0.5 kHz (1);
1 kHz (2); 5 kHz (3); 10 kHz (4).
The permittivity of pure and Cr3+ doped
(0.1 wt.) LGO crystals was measured using a
bridge method in the temperature interval 300÷
500 K and frequency range 0.5 ÷ 10 kHz. De-
pendencies of real ε′ and imaginary ε′′ parts of
LGO:Cr3+ permittivity measured along a axis
are plotted in figures 3a, b. There is a disti-
nct ε′(T ) step-like anomaly and ε′′(T ) maximum
which shift to high temperature side with the in-
creasing of measuring field frequency f . Along b
and c axes the quantities ε′ and ε′′ of LGO:Cr3+
do not demonstrate any anomalies and slightly
depend on temperature. For nominally pure LGO
crystal, the real and imaginary parts of permit-
tivity monotonously depend on temperature and
do not show any anomalous features.
The anomalies of the LGO:Cr3+ permittivity
are typical of a thermal polarization process [9]. The experimental data show that ε′, ε′′ anomalies
are contributed by chromium dipole moments directed mainly along a axis. To determine the
parameters of pairs centres reorientation, the real and imaginary parts of complex permittivity
ε∗(T ) (figure 3) are described by means of the Debye model
ε∗ = ε′ − iε′′ = ε∞ +
ε0 − ε∞
1 + iωτ
, (3)
where ε0 − ε∞ = C/T , C is Curie constant, ω is angular frequency of measuring field and τ =
τ0 exp(U/kT ) is relaxation time.
Figure 4. Reorientation rate of Cr3+– Li+ pair
centres vs inverse temperature.
Fitting of (3) to the experimental data was
carried out for the imaginary part of permittivity,
since processing of ε′′(T ) maxima enables us to
determine the parameters in (3) to a higher accu-
racy in comparison with less pronounced step-like
ε′(T ) anomalies. At high temperatures the back-
ground component of ε′′(T ) sufficiently increases
as it may be described by the expression ε′′bgr =
ε0
bgr+A exp(−W/kT ), where ε0
bgr is the tempera-
ture independent part and W = 0.85 eV is the ac-
tivation energy for the exponential growth of the
background component. The dependencies calcu-
lated by using (3) are shown in figure 3 by solid
lines, the parameters used are
ε∞ = 7.59± 0.017, C = 907± 14 K,
τ0 = (4.5± 2) · 10−13 s, U = 0.66± 0.02 eV. (4)
The comparison of the measured data and the calculated dependencies reveals that the Debye
model makes it possible to describe the experimental results. The temperature dependence of
77
M.D.Volnianskii, M.P.Trubitsyn, Yahia A.H.Obaidat
the relaxation rate τ−1, obtained from ε′′(T ), may be described by the Arrhenius law with good
accuracy (figure 4).
It may be noted that the activation energy U determined from the dielectric measurements
(4) is close enough to the value (0.55± 0.05 eV), estimated from the EPR spectra. The frequency
(τ0)
−1 (4) agrees well with independent measurements of the Debye temperature θD ≈ 200 K [8].
4. Conclusions
Investigations of EPR and permittivity spectra have made it possible to detect the thermally
activated reorientation of Cr3+– Li+ pair centres in LGO crystal. The reorientation of Cr3+– Li+
pairs leads to a temperature EPR line width broadening and results in relaxation anomalies of the
dielectric permittivity.
The reorientation of the pair centres is apparently realized by the hopping of interstitial Li+
ions through the channels of LGO structure. It is obvious that the spatial location of Cr3+ ions is
fixed within the oxygen octahedral complexes.
References
1. Galeev A.A., Hasanova N.M., Bykov A.B., Vinokurov B.M., Nizamutdinov N.M., Bulka G.R. EPR of
Cr3+ and Fe3+ in Li2Ge7O15 single crystal. In: Spectroscopy, a crystal chemistry and a realstructure
of minerals and their analogues. Kazan’ state university, 1990, p. 77–87 (in Russian).
2. Iwata Y., Koizumi H., Koyano N., Shibuya I., Niizeki N. Crystal structure determination of ferroelectric
phase of 5PbO.3GeO2, Journ. Phys. Soc. Jap., 1973, 35, No. 1 , 314–315.
3. Basun S.A., Kaplyanski A.A., Feofilov S.P. Dipolar centres in Li2Ge7O15 crystal activated with (3d3)
ions: a microstructure and spectroscopic effects of an internal and external electric field, Solid State
Phys., 1994, 36, No. 11, 3429–3449 (in Russian).
4. Kaplyanski A.A., Basun S.A., Feofilov S.P. Ferroelectric transition induced dipole moments in probe
ions in Li2Ge7O15 crystals doped with Mn4+ and Cr3+, Ferroelectrics, 1995, 169, 245–248.
5. Trubitsyn M.P., Volnianskii M.D., Busoul I.A. EPR study of the ferroelectric phase transition in
Li2Ge7O15:Cr3+ crystal, Solid State Phys., 1998, 40, No. 6, 1102–1105 (in Russian).
6. Meil’man M.L., Samoilovich M.I. Introduction to the EPR Spectroscopy of Activated Single Crystals.
Atomizdat, Moscow, 1977, 272 p. (in Russian).
7. Wertz J.E., Bolton J.R. Electron Spin Resonance: Elementary Theory and Practical Applications.
McGraw-Hill Book Comp., N.Y., 1972.
8. Antonenko A.M. Nonlinear elastic properties of single crystals of some composite oxides: Thesis for Ph.
D. degree. Dnipropetrovsk state university, 1980, 128 p. (in Russian).
9. Poplavko Yu.M. Physics of Dielectrics. Vyscha Shkola, Kiev, 1980, 398 p. (in Russian).
ЕПР i дiелектрична спектроскопiя Cr3+– Li+ парних центрiв,
що реорiєнтуються, у кристалах Li 2Ge7O15
М.Д. Волнянський, М.П. Трубiцин, Ях’я А.Х. Обайдат
Днiпропетровський нацiональний унiверситет, кафедра фiзики твердого тiла,
вул. Наукова 13, Днiпропетровськ 49050, Україна
Отримано 4 вересня 2006 р.
Ранiше було показано, що у структурi кристалiв гептагерманату лiтiю Li2Ge7O15 домiшковi iони хро-
му замiщують iони германiю i створюють Cr3+– Li+ парнi центри. У роботi ЕПР i дiелектричнi спе-
ктри кристалiв Li2Ge7O15:Cr3+ дослiдженi у температурному дiапазонi 300÷500 K. Вперше термiчно
активована реорiєнтацiя дипольних моментiв Cr3+– Li+ пар зареєстрована шляхом вимiрювання
поширення ЕПР лiнiй i дiелектричної релаксацiї.
Ключовi слова: домiшковi iони, електронний парамагнiтний резонанс, дiелектричнi втрати i
релаксацiя
PACS: 76.30.Da, 76.30.-v, 77.22.Gm
78
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