EPR in Mn2+ doped betaine calcium chloride dihydrate single crystals

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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 be- taine calcium chloride dihydrate single crystals. Journal de Physique, 1988, 49 (5), pp.813-817.

�10.1051/jphys:01988004905081300�. �jpa-00210758�

(3) Saarlandes, Saarbrücken,

(Requ le 7 décembre 1987, accepté ^{le 2} f6vrier 1988)

Résumé.

Dans

travail

présentons

étude de RPE dans le BCCD dopé

Mn2+ . Les _mesures ont été effectuées entre 10 K-300 K dans la bande de fréquences ^{9,45 GHz}

champ magnétique qui ^{varie de}

0 à 104 G. À la température ambiante les résultats sont décrits par

hamiltonien qui explique l’anisotropie

observée. Les

magnétiques principaux des défauts sont identifiés par rapport

critallographiques

du système. ^Les spectres

^basses températures permettent l’identification des différentes phases

commensurables et incommensurables du BCCD.

Abstract.

In this paper

results concerning

^EPR study ^of Mn2+ doped ^BCCD crystals

reported.

The measurements

done in the temperature range of 10 K-300 K using

^X-band frequency of 9.45 GHz and

magnetic field in the range 0-104 G. The high temperature ^data

^{be described} by

simple Hamiltonian which allows the understanding ^{of the} anisotropy ^{of the} spectra. ^The principal magnetic

of the defects

identified in the crystallographic coordinate system. ^{At low} temperatures ^the analysis ^{of the structure of the} hyperfine lines for

particular favourable direction of the applied magnetic field allows the visualisation of several phase transitions to different commensurate and incommensurate phases.

Introduction.

Betaine calcium chloride dihydrate- (CH3)3NCH2COO-CaCI2-2 H20- crystals, grown

by isothermal solvent evaporation [1, 2], ^{exhibit at}

room temperature ^an orthorhombic structure de- scribed 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 de- scribed in reference [4]. Between 164 K and 127 K the modulation is incommensurate with the wave

vector changing continuously ^{between 0.320 and}

0.285. Below this temperature ^{down to 125 K the} modulation remains commensurate (q

2/7). ^For

125 > T > 116 K a second incommensurate phase

occurs in which the wave vector changes ^continu- ously ^{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

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01988004905081300

814

Fig. ^1.

^{Molecular structure of BCCD} (top) ^{and the unit}

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]).

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 spec- trometer with a double axes goniometer allowing ^a

fine orientation of the samples ^{in the} applied mag- netic 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 adapt-

able to the resonance cavity ^{was used. The} tempera-

ture was measured with a thermocouple in the cool- gas flow, ^{1.5 cm} away from the sample. ^{Due to the} hygroscopy of the material the samples ^were pro- tected 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}

b)

Fig. ^2.

Experimental spectra ^for Hllb (a) ^{and for}

HII Zmag ^{(b). In the first}

^{all the centers} in the unit cell

are

equivalent ^{and the} spectrum ^{is rather} simple showing

typical S

^{5/2, I = 5/2} structure. In the second

the four molecules form two sets of nonequivalent ^centers

which produce

superposition ^of

equivalent ^{centers. In} figure ^{2a the} experimental spectrum ^fo H//b is shown. For this particular

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 corre-

spond ^{to the} principal magnetic ^{axes of two} nonequi-

valent 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 ^de-

fined 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) :

with

The curves of anisotropy generated by this Hamilto- nian 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 re-

quire ^{much more than a} nearly visual examination of the lines. The analysis ^{was focused at the} tempera-

ture 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.

For 119 T 121 K the distortion wave locks into the commensurate value q

2/7. This corresponds

to figure ^4d.

Fig. ^4.

Sequence ^of hyperfine structures of the

centered at 3 850 G for Hllb, between 170 K and 100 K. Spectra a) ^to h) correspond ^{to T}

^{170 K} (P- phase), ^{T = 152 K, T = 135 K} (INC-phase 1), ^T

^{120 K} (COM-phase 2/7), ^{T = 118} K, 116 K, 113 K (INC- phase 2) and T = 100 K (COM-phase 1/4), respectively.

The second incommensurate phase appears in the temperature range 112 T 119 K. The tempera-

ture dependence ^{of a} hyperfine sextuplet ^{in this} region is described in figure 4e, f, g. A typical lineshape corresponding ^{to a} pure sinusoidal modu- lation 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 ^indicat- ing ^a multisoliton regime percursor of the ^commen- surate q ^{= 1/4} phase (Fig. 4g). Figure ^{4h shows a} typical spectrum ^{observed for} the q ^{= 1/4 commen-}

surate phase.

In this phase ^{and at lower} temperatures, ^this particular ^resonance is obscured by ^the overlap ^of adjacent hyperfine lines. The low field resonance at 1 717 G, which is less sensitive becomes suitable for the continuation of the analysis ^{at lower} tempera-

tures. 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

Fig. ^5.

Structure of the initial hyperfine lines for the low temperature commensurate phases : a) corresponds ^to

T

120 K, b) ^{to 100 K,} c) ^{to T}

^{65 K and} d) ^{to 40 K for the}

^{centered at} 1 717 G. The experimental (full lines) ^and computer constructed

using equal components ^{with the}

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

not significant

due to the confuse overlap ^{with the} adjacent ^structure.

temperatures where, according ^{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} excep- tion 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.

7/2 * 2 from q

^{2/7. A} sharp change ^{leads to the}

structure in figure 5b, ^{which at first} sight represents

a quadruplet (q = 1/4). ^{The computer} reconstruc- tion 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 com-}

mensurate 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 ^drasti- cally ^{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

Fig. ^6.

Hyperfine ^structure just below T = 15 K :

a) shows the full hyperfine ^{structure of the resonance}

centered at 1717 G ; ⁱⁿ b) the reconstruction of

intense line (Ami = 0 ) ^reveals

underlying quadruplet.

no longer ^a magnetic axis. As shown in figure ^{6b, the}

intense lines (demi = 0 ) ^{may represent an unresolved} quadruplet ^which would indicate that all the four sites in the cell are unequivalent. On the other hand the superhyperfine ^{structure of the} protons ^is again

resolved on other lines, ^{which is} indicative of a well

ordered lattice.

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 tran-} sitions, 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}

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 progress- ive 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 non-} equivalence ^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 stimulat-

ing discussions.

References

[1] ROTHER, H.J., ALBERS, J., KLÖPPERPIEPER, A., Ferroelectrics 54 (1984) ^107.

[2] KLÖPPERPIEPER, A., ROTHER, ^H. J., ALBERS, J., MÜSER, ^H. E., Jpn. ^J. Appl. Phys. ²⁴ Suppl. ^24-

2 (1985) ^829.

[3] BRILL, W., SCHILDKAMP, W., SPILKER, J., ^{Z. Kris-} tallgr. 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 Paramagne-

tic Resonance of Transition Ions (Clarendon Press, Oxford) ^1970.

[6] BLINC, R., Phys. Rep. ⁷⁹ (1981) ^331.