EPR in Mn2+ doped betaine calcium chloride dihydrate
single crystals
J.L. Ribeiro, J.C. Fayet, J. Emery, M. Pdzeril, J. Albers, A. Klöpperpieper, A.
Almeida, M.R. Chaves
To cite this version:
J.L. Ribeiro, J.C. Fayet, J. Emery, M. Pdzeril, J. Albers, et al.. EPR in Mn2+ doped betaine
calcium chloride dihydrate single crystals. Journal de Physique, 1988, 49 (5), pp.813-817.
<10.1051/jphys:01988004905081300>. <jpa-00210758>
HAL Id: jpa-00210758
https://hal.archives-ouvertes.fr/jpa-00210758
Submitted on 1 Jan 1988
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J.
Phys.
(1988)
France 49
813-817
MAI
1988,
813
Classification
Physics
64.70K
Abstracts
76.30
-
EPR in
-
77.80
Mn2+ doped betaine calcium chloride dihydrate single crystals
J. L. Ribeiro (1), J. C. Fayet (2), J.
A. Almeida (1) and M. R. Chaves
(1)
(2)
(3)
Emery (2), M. Pdzeril (2), J. Albers (3), A. Klöpperpieper (3),
(1)
C.F.U.P. (INIC), Universidade do Porto, 4000 Porto, Portugal
Université du Maine, route de Laval, 72017 Le Mans, France
Universität des Saarlandes, 6600 Saarbrücken, F.R.G.
(Requ
le 7 décembre 1987,
accepté le
2
f6vrier 1988)
Dans ce travail nous présentons une étude de RPE dans le BCCD dopé au Mn2+ . Les mesures ont
Résumé.
été effectuées entre 10 K-300 K dans la bande de fréquences 9,45 GHz avec un champ magnétique qui varie de
0 à 104 G. À la température ambiante les résultats sont décrits par un hamiltonien qui explique l’anisotropie
observée. Les axes magnétiques principaux des défauts sont identifiés par rapport aux axes critallographiques
du système. Les spectres aux basses températures permettent l’identification des différentes phases
commensurables et incommensurables du BCCD.
2014
In this paper some results concerning an EPR study of Mn2+ doped BCCD crystals are reported.
Abstract.
The measurements were done in the temperature range of 10 K-300 K using an X-band frequency of 9.45 GHz
and a magnetic field in the range 0-104 G. The high temperature data can be described by a simple Hamiltonian
which allows the understanding of the anisotropy of the spectra. The principal magnetic axes of the defects are
identified in the crystallographic coordinate system. At low temperatures the analysis of the structure of the
hyperfine lines for a particular favourable direction of the applied magnetic field allows the visualisation of
several phase transitions to different commensurate and incommensurate phases.
2014
Introduction.
Betaine
changing continuously between 0.320 and
0.285. Below this temperature down to 125 K the
modulation remains commensurate (q
2/7). For
125 &#x3E; T &#x3E; 116 K a second incommensurate phase
occurs in which the wave vector changes continuously between 0.285 and 0.25. At lower temperatures
the commensurate phases q = 1/4, q = 1/5 and
q = 1/6 are observed in the temperatures ranges 116
to 73 K, 73 to 47 K and T
47 K, respectively. This
behaviour can be described as an incomplete devil’s
staircase.
In this paper we report a study of Electronic
Paramagnetic Resonance of Mn2 + doped BCCD
crystals. Mn2 + ions replace Ca2 + in the crystalline
structure. The doped crystals were grown from a
solution with a molar ratio of Mn2 + /Ca2 + of the
order of 10- 3.
In the first part the EPR spectrum of the high
temperature reference phase is shortly described.
The principal magnetic axes of the defects are
identified and a simplified spin Hamiltonian allows
vector
calcium
chloride
dihydrate-
(CH3)3NCH2COO-CaCI2-2 H20- crystals, grown
by isothermal solvent evaporation [1, 2], exhibit at
temperature an orthorhombic structure described by the space group Pnma [3]. The unit cell,
with the dimensions a = 10.97 A, b
10.15 A,
c = 10.82 A, has four molecules. The structure of
this phase is shown in figure 1.
At lower temperatures the system undergoes a
sequence of structural phase transitions to different
structures modulated along c [4]. The temperature
dependence of the modulation wave vector is described in reference [4]. Between 164 K and 127 K
the modulation is incommensurate with the wave
room
=
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01988004905081300
=
814
b)
Experimental spectra for Hllb (a) and for
In the first case all the centers in the unit cell
(b).
HII Zmag
are equivalent and the spectrum is rather simple showing a
typical S 5/2, I = 5/2 structure. In the second case the
four molecules form two sets of nonequivalent centers
which produce a superposition of resonances.
Fig.
2.
-
=
Molecular structure of BCCD (top) and the unit
Fig. 1.
cell of the high temperature reference phase (bottom)
showing the positions of the four molecules. Mn2 + replaces
Ca2 + at the center of the distorted octahedron generated
by the Cl-03’, Cl’-03 and 01-02 chemical axis (from
Ref. [3]).
-
centers. In figure 2a the experimental
fo
H//b is shown. For this particular
spectrum
direction of the applied magnetic field the spectrum
is simple and only five fine structure lines (S
5/2)
splitted into six hyperfine lines (I = 5/2) can be
clearly observed. The crystallographic b axis is a
principal magnetic axis common to the two sets of
resonant centers. The five spin resonances are
located at 1 717 G, 2 283 G, 3 033 G, 3 850 G and
5 548 G. The average hyperfine splitting is of the
order of 90 G. A super hyperfine structure due to
the two equivalent protons of the two water
molecules can also be resolved.
With the magnetic field applied in the ac plane the
most enlarged spectrum is observed for the directions
H
HO * ( ± cos (39), 0, cos (51)) (Fig. 2b). If we
consider only a reduced spin Hamiltonian
bo 2 0° + b2 02 this means that these directions correspond to the principal magnetic axes of two nonequivalent centers [5]. The third principal directions are
therefore defined on the ac plane by the vectors
(± sin 39, 0, sin 51).
Figure 3 shows a projection on the ac plane of two
BCCD molecules. It is clear that the principal
magnetic axes of the defect are approximately defined by the projections of Mn- Cl and Mn- 0
chemical axes on the ac plane and by the orthogonal
direction b.
Without considering second order effects due to
the hyperfine coupling As.I it is possible to
describe reasonably well the observed behaviour by
the Hamiltonian (5) :
equivalent
=
the description of the observed anisotropic behaviour
and the calculation of the spin energy levels as a
function of the applied magnetic field. In the second
part, the results concerning the lower temperature
region will be reported. The detailed structure which
appears on the initial hyperfine lines is particularly
sensitive to the several phase transitions, revealing
in addition the existence of a new phase, non
modulated (q
0), at low temperatures.
=
=
Experimental.
The measurements were done using a Bruker spectrometer with a double axes goniometer allowing a
fine orientation of the samples in the applied magnetic field. A X-band frequency of 9.5 GHz was
used with a dc magnetic field in the range of
0-104 G. For the low temperature measurements a
standard Oxford-Instruments quartz cryostat adaptable to the resonance cavity was used. The temperature was measured with a thermocouple in the coolgas flow, 1.5 cm away from the sample. Due to the
hygroscopy of the material the samples were protected with a thin varnish film.
Experimental
results.
As can be seen in figure 1 the unit cell of the
reference phase has four molecules forming two non
815
Projection of the
Fig. 3.
allowing the identification of
in this plane.
-
on the ac plane
principal magnetic axes
structure
the
with
The curves of anisotropy generated by this Hamiltonian agree quite well with the experimental ones and
the energy levels can be calculated as functions of
the applied magnetic field allowing the identification
of the several resonance lines.
For the study of some essential features of the
sequence of phase transitions it was choosen the
simpler orientation H//b. The choice of any other
direction for the applied magnetic field would require much more than a nearly visual examination of
the lines. The analysis was focused at the temperature dependence of the structure of the hyperfine
lines, which is rather sensitive to local changes of
symmetry. For simplicity the description of the
results will be made by considering simple and
representative resonances centered at different
values of the magnetic field.
Figure 4 displays the temperature dependence of
the hyperfine structure centered at 3 850 G between
170 K and 100 K. In figure 4a a typical I
5/2
hyperfine sextuplet is observable. At 158 K the
structure becomes incommensurate and each line
gives rise to two edge singularities as expected
(Fig. 4b), [6]. For T 140 K this structure cannot be
described assuming a pure sinusoidal modulation,
and a multisoliton regime is observed (Fig. 4c). The
P-INC phase transition (at T = 158 K) is marked by
the rise of nuclear transitions (Ami
± 1 ) which are
particularly evident within the sextuplet (Ami = 0 )
centered at 3 033 G. This means that the BCCD
molecules have lost the (010) mirror symmetry and
that the displacement mode is antisymmetric with
respect to this mirror plane. Therefore the local
lineshifts are an even function of the amplitude and a
microscopic dipolar moment along b is allowed
without prejudice to the macroscopic polarization.
T 121 K the distortion wave locks into
For 119
2/7. This corresponds
the commensurate value q
to figure 4d.
=
=
=
4.
Sequence of hyperfine structures of the resocentered at 3 850 G for Hllb, between 170 K and
100 K. Spectra a) to h) correspond to T
170 K (Pphase), T = 152 K, T = 135 K (INC-phase 1), T 120 K
(COM-phase 2/7), T = 118 K, 116 K, 113 K (INCphase 2) and T = 100 K (COM-phase 1/4), respectively.
Fig.
-
nance
=
=
The second incommensurate phase appears in the
T 119 K. The temperature dependence of a hyperfine sextuplet in this
region is described in figure 4e, f, g. A typical
lineshape corresponding to a pure sinusoidal modulation is never observed clearly and some additional
singularities in the spectral density are observed.
This can be due to either an additional symmetry
breaking or to metastable q 2/7 regions. As the
temperature decreases the spectrum changes indicating a multisoliton regime percursor of the commensurate q = 1/4 phase (Fig. 4g). Figure 4h shows a
typical spectrum observed for the q = 1/4 commen-
temperature range 112
=
phase.
phase and at lower temperatures, this
particular resonance is obscured by the overlap of
adjacent hyperfine lines. The low field resonance at
surate
In this
1 717 G, which is less sensitive becomes suitable for
the continuation of the analysis at lower temperatures. Figure 5a and 5b show the equivalent spectra
for the q
2/7 and q = 1/4 commensurate phases
which are identical to those already described.
Figure 5c and 5d display the hyperfine lines at
=
816
5.
Structure of the initial hyperfine lines for the low temperature commensurate phases : a) corresponds to
120 K, b) to 100 K, c) to T 65 K and d) to 40 K for the resonance centered at 1 717 G. The experimental (full
lines) and computer constructed curves using equal components with the same intensity, shape and width (dotted lines)
are shown ; in c), the last two lines belong to the adjacent hyperfine structure ; in d) the last four lines are not significant
due to the confuse overlap with the adjacent structure.
Fig.
T
-
=
=
to reference [4], two
different commensurate phases should be expected.
As can be seen in figure 5a, 5b and 5c the change
of the modulation wave vector in the different low
temperature commensurate phases is reflected on
the structure of the hyperfine lines, with the exception of the q = 1/6 phase which cannot be clearly
resolved (Fig. 5d). The structure in figure 5a can be
fitted by the superposition of seven single lines, i.e.
2/7. A sharp change leads to the
7/2 * 2 from q
structure in figure 5b, which at first sight represents
a quadruplet (q = 1/4). The computer reconstruction clears out extra unresolved doublets. A second
sharp modification leads to the structure in figure 5c
(q =1/5 ) which is obscured by the partial overlap
between adjacent hyperfine lines. The computer
reconstruction clears out a set of five doublets.
Figure 5d shows a typical hyperfine line observed in
the temperature range for which the q = 1/6 commensurate phase is reported to exist [4]. As can be
seen no sharp change in the structure is observed.
The computer reconstruction (dotted line) shows
only five doublets, being possible that the extra one
is merged on the adjacent lines.
At about T = 15 K the spectrum changes drastically as can be seen in figures 6a (full structure) and
6b (detail of the sharpest line). From figure 6a and
by comparison with figure 4a, we may infer that the
size of the initial cell is restored (Z 4, q
0), but
with a low symmetry. Indeed satellite lines associated
to nuclear transitions (Amit
± 1 ) indicate that b is
temperatures where, according
=
=
=
=
Hyperfine structure just below T = 15 K :
the full hyperfine structure of the resonance
centered at 1717 G ; in b) the reconstruction of an
intense line (Ami = 0 ) reveals an underlying quadruplet.
Fig. 6.
a) shows
-
no
longer a magnetic axis.
As shown in
figure 6b, the
unresolved
intense lines (demi = 0 ) may represent
that
all
the four
quadruplet which would indicate
hand
On
the
other
sites in the cell are unequivalent.
the superhyperfine structure of the protons is again
resolved on other lines, which is indicative of a well
ordered lattice.
an
817
Discussion.
Our results show that Mn2 + replacing Ca2 + in the
crystal structure is a good EPR probe for the study
of the structural phase transitions in BCCD. The
room temperature spectra can be understood by
considering the lower order quadrupolar terms for
the crystal field and the principal magnetic axes of
the Mn2 + defects can be reasonably well related
with chemical axes in the molecular structure.
At low temperatures the spectra show important
changes in correlation with phase transition sequence. The first worthnote point in the analysis of
the results concerns the values of the different
critical temperatures, which are found to be slightly
shifted from those reported in [1, 3] for the pure
system. In order to check a possible effect of the
Mn 2, impurities on the values of these temperatures
we try to confirm these shifts by other studies.
Preliminar measurements of pyroelectric coefficients
and dielectric constants on samples with the same
molar rate of Mn 2+ /Ca2 + did not reveal any
appreciable changes in the critical temperatures for
the
higher temperature phase transitions
P -+ INCl -+ q
2/7 -+ INC2. For the other transitions, the values found are not quite reproducible
[2] and therefore no firm conclusion can be drawn. It
is possible that the observed differences may be
partially due to experimental limitations on the
measurement of the absolute values of the tempera=
ture.
In the first incommensurate phase the analysis of
the hyperfine lines shows that a pure sinusoidal
regime is observed over a large temperature range
indicating that the pinning of the distortion wave by
the Mn impurities is not relevant.
The analysis of the hyperfine structure allows also
the identification of several commensurate phases.
When the modulation wave vector takes a rational
value q m/n each non-equivalent center splits in,
at most, n different centers. If the amplitude of the
=
distortion is small compared with the reference unit
cell dimensions and if the magnetic field is applied in
a suitable symmetry direction of the reference structure (as it is the case here with H//b) this splitting
can be equivalently described as a modulated line
shift given by :
wherei refers to the number of the unit cell.
Applying this simple expression one can account
for the position of the lines satisfactorily by assuming
four magnetically equivalent sets of seven centers for
H//b and q 2/7. For q = 1/4 this simple model
does not hold neither for the line positions nor for
2/7 and q = 1/4 exhibit
their number. The phases q
therefore essential differences in agreement with the
existence of small spontaneous polarizations along b
for q 2/7 and along a for q = 1/4 [1, 2].
The transition from q = 1/4 to q = 1/5 can be
observed by the rise of an extra doublet in the
hyperfine superstructure, which reflects the increase
of the unit cell. At lower temperatures, the progressive overlap between the different adjacent hyperfine
lines, already observed within the phase q 1/5,
prevents a simple evidence of the transition to the
phase q 1/6.
An additional transition to a nonmodulated phase
at low temperatures, not reported in [4], was clearly
observed. This new phase produces a sharp change
of the spectrum and each intense hyperfine line
reveals an underlying quadruplet showing the nonequivalence of the four sites in the restored unit cell.
=
=
=
=
=
Acknowledgments.
The authors are greatly indebted to Dr. A. Leble
(Lab. de Spect. du Solide, Univ. du Maine) for the
program used to simulate the spectra and to Prof.
Dr. H. E. Mfser (Univ. of Saarlandes) for stimulating discussions.
References
[1] ROTHER, H.J., ALBERS, J., KLÖPPERPIEPER, A.,
Ferroelectrics 54 (1984) 107.
A., ROTHER, H. J., ALBERS, J.,
KLÖPPERPIEPER,
[2]
MÜSER, H. E., Jpn. J. Appl. Phys. 24 Suppl. 242 (1985) 829.
[3] BRILL, W., SCHILDKAMP, W., SPILKER, J., Z. Kristallgr. 172 (1985) 281.
[4] BRILL, W., EHSES, K. H., Jpn. J. Appl. Phys. 24
Suppl. 24-2 (1985) 826.
[5] ABRAGHAM, A., BLEANEY, B., Electron Paramagnetic Resonance of Transition Ions (Clarendon
Press, Oxford) 1970.
BLINC,
R., Phys. Rep. 79 (1981) 331.
[6]