Dielectric anomalies in the incommensurate phase of urea-doped thiourea

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Dielectric anomalies in the incommensurate phase of urea-doped thiourea

A. Onodera, F. Denoyer, J. Godard, M. Lambert

To cite this version:

A. Onodera, F. Denoyer, J. Godard, M. Lambert. Dielectric anomalies in the incommen- surate phase of urea-doped thiourea. Journal de Physique, 1988, 49 (12), pp.2065-2069.

�10.1051/jphys:0198800490120206500�. �jpa-00210887�

Dielectric anomalies in the incommensurate phase ^of urea-doped ^thiourea

A. Onodera (1,*), ^F. Denoyer (1,**), ^{J. Godard} (1) and M. Lambert (2)

(1) Laboratoire de Physique des Solides (***), ^Bâtiment 510, Université Paris-Sud, ⁹¹⁴⁰⁵ Orsay ^Cedex,

France

(2) Laboratoire Leon-Brillouin (+), CEN-Saclay, 91191 Gif-sur-Yvette Cedex, ^France

(Requ ^{le 18} juillet 1988, accepté le 23 aogt 1988)

Résumé.

La constante diélectrique d’un monocristal de thiourée dopée ^urée

été mesurée. On peut s’attendre à

que l’introduction de molécules dopantes perturbe fortement les chaines de liaisons hydrogènes

de telle sorte que l’onde de modulation

^trouvera affectée. Dans la phase incommensurable une nouvelle sorte d’effet mémoire est observée et quand ^la température varie très doucement

série d’anomalies quasi- périodiques ^{est mise en évidence.}

Abstract.

Complex dielectric constant has been measured in the modulated phase ^of

OC(ND2)2-doped SC(ND2)2 single crystal. ^One

expect that the introduction of doping ^molecules strongly disturb the chains of hydrogen bonds in such

way that the modulated

should be affected. In the incommensurate phase

new

kind of memory effect is observed and when the variation of temperature is very slow

series of

quasiperiodic anomalies is detected.

Classification

Physics ^Abstracts

61.70T

64.70R

77.20

Many ^modulated crystals have been known to exhibit interesting ^{kinetic behaviour such as thermal} global hysteresis and memory effects in the incom- mensurate phase [1, 2]. The thermal global hysteresis phenomena in modulation wave vector [3, 4], ^dielec-

tric constant [5-7] ^and birefringence [7-10] ^{on several} polar ^or non-polar incommensurate systems ^have been reported : ^the heating ^and cooling ^{curves do}

not coincide. Furthermore, ^{when a} crystal ^is kept during ^a few hours at an arbitrary temperature ( Tm ) ^{within the} incommensurate phase, ^{the sub-} sequent measurement shows an unexpected ^small anomaly ^around Tm. ^{When the} crystal is annealed in the paraelectric phase ^{for a} sufficient time, ^no

memory effects can be detected anymore. These effects are believed to be closely ^{related to defects,}

such as impurities, vacancies, ^or dislocations, ^in- teracting ^{with the} incommensurate modulation

wave.

dressed.

(***) Laboratoire associ6

CNRS.

(+) Laboratoire commun CEA-CNRS.

Thiourea SC(NH2)2 and its deuterated homolog-

ous SC(ND2)2 (abbreviated ^as d-thiourea) ^{are well-}

known ferroelectrics [11] which show an inter- mediate-modulated phase [12-14] ^{over a wide tem-} perature ^interval [T;, Tc], between the high-tem-

perature paraelectric ^{and the} low-temperature ^fer-

roelectric phases. ^The paraelectric phase, ^or-

thorhombic with space group Pnma (at ^{room tem-} perature : a

7.655, ^b

8.537, ^c

^5.520 A) pos-

sesses four molecules per unit cell, arranged ^{in an} antiparallel ^manner [15]. The intermediate phase ^is

characterized by ^{a modulated} polarization ^wave,

with a wavevector q directed along ^{the b} direction : q

6b* (8

^wave number). ^{In first} approximation,

in the intermediate and ferroelectric phases, ^each

molecule rotates around a molecular axis parallel ^to

the b direction. The modulated _wave, incommensur- ate between Ti( cS 17 ) ^and T1( 8

0.115) ^locks-in discontinuously ^onto the rational values 5=1/9, giving ^{rise to a} high-order commensurate phase

stable on a narrow temperature interval, ^between Tl ^and Tc [16].

The application of pressure and electric-field has revealed the existence of supplementary high-order

commensurate phases ^inside the incommensurate

region (i.e. ⁸ = 1/7 and 6 = 1/8) ^{in the} (pressure,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198800490120206500

2066

temperature) [17] ^and (electric-field, temperature) [18] phase diagrams.

This study ^comes from after an intensive study ^of

the irradiation effects on incommensurate d- thiourea [19]. ^The main motivation for studying

radiation damage ^{was to} understand the discrepan-

cies on the modulation wavevector behaviour as

resulting ^{from the} comparison ^{between a} high ^inten- sity X-ray diffraction study [20] ^{and a thermal}

neutron investigation [21]. ^To illustrate this different behaviour it has been show, ^for example, ^{that under}

an applied ^{electric field of} 1 200 V/mm the modu- lation wave number as a function of decreasing

temperature goes by steps ^{in the} X-ray experiment (pinning ^on commensurate values 5

4/29, 2/15, 1/8) [20] whereas in the neutron study ^{the wave}

vector smoothly decreases down to lock-in at the 1/8 commensurate value [21]. ^{Even if EPR} experiments [22] performed ^on X-ray irradiated single crystals

have given ^some informations on the radiation

damage, the defects have never been identified. An alternative _{way of} creating specific ^{defects in}

thiourea was to introduce impurities ^{in a well-con-}

trolled _{manner ;} urea molecules appeared ^well-

suited. The molecular configurations ^{of urea} (OC(NH2)z) ^{and thiourea} (SC(NH2)2) ^are very similar except for C = 0 (1.260 A) ^{and S = C}

lengths (1.746 ^{A for thiourea} and 1.738 A for d-

thiourea). This difference plays ^a fundamental role in the building ^{of the} crystalline phases : pure

crystals of thiourea and urea have very different structures. Introduction of urea molecules inside thiourea crystals (solid solutions) ^{is a} way of creating

defects affecting mostly ^the hydrogen bonds inside the crystal. ^{Therefore such defects can be a} pertinent parameter ^to study the effect of impurities ^{on the}

modulation wave. This paper reports ^new interesting

features obtained by dielectric measurements on

Sl-xOxC(NDz)2.

Single crystals ^were grown from a mixed alcoholic solution of d-thiourea and d-urea (3 %) by ^slow evaporation. ^{The content of d-urea was estimated to}

be approximately ^{1 %} [23]. ^{The electrodes} (Cr ⁺ Au) ^were evaporated ^on the cleaved faces

((b, c) plane) ^and gold ^{lead wires were fixed with} platinum-based lacquer. ^The sample ^was placed ^{in a} large cryostat ^and temperatures ^{were controlled} within ± 0.005 K by ^a computer-controlled ^{PID unit.}

The dielectric constant was measured by ^{an HP-}

4275 A LCR meter at a frequency ^{of 10 kHz. All}

measurements were performed ^{on the same} sample.

The temperature dependence ^of dielectric constant

s§ ^is given ⁱⁿ figure ^{1 both on} cooling ^and heating

runs at the constant temperature ^rate 0.4 K/minute.

The dielectric anomalies of this sample, ^{on the} cooling ^run (the heating run) ^{are found at} Ti ^{= 217.45 K} (217.48 K), T3 ^{= 198.59 K} (199.5 K), T2 ^{= 194.5 K} (195.1 K), Tl ^{= 193.3 K} (194.05 K)

Fig. ^1.

Temperature dependence of dielectric constant in d-urea-doped d-thiourea on the cooling ^{and the} heating

runs at the temperature scanning ^rate of 0.4 K/minute.

The Ti, T3, T2, TI, Tc temperatures ^{are shown}

^the cooling ^run.

and 7c ^{= 191.1 K} (191.9 K). Comparison ^with previ-

ous data taken with a normal d-thiourea sample ^on

the same experimental ^{device shows a} very similar dielectric behaviour as a function of temperature with anomalies at Ti

^218.1 K, T3

^198.75 K, T2

^194.40 K, T,

^{193.4 K and} Tc

^{190.9 K. The}

d-urea impurities ⁱⁿ d-thiourea seem essentially ^to

affect the Ti transition, giving ^{rise to a} slightly

reduced temperature ^interval [Ti, Tc] ^{for the modu-}

lated phase. In pure d-thiourea, the anomalies observed inside the modulated region ^at Tl, T2 ^and T3 have been assigned, ^{on the basis of neutron}

diffraction data, ^{either to a lock-in} transition at the commensurate value 6=1/9 (T

T1 ) ^{or to the}

passage of the modulation wave vector through ^the

commensurate values 5

2/17 (T

T2) ^{or 5 = 1/8} ( T = T3); ^the magnitude ^{of the} anomaly ^at T3

reflects the ferroelectric character of the incipient

8 = 1/8 phase. ^The dielectric variation being very similar for the urea doped thiourea, ^it appears reasonable to assume that under these experimental conditions, ^such impurities ^{do not} strongly ^{affect the} temperature dependence ^{of the wave vector modu-}

lation. In addition, ^{a small increase of the} global hysteresis (observed throughout ^{the modulated re-} gion) ^can be noticed.

(i) CROSSOVER-LIKE MEMORY EFFECT.

The tem-

perature dependence ^of dielectric constant, E’, ^on

both cooling ^and heating runs, is measured firstly ^at

the constant temperature ^rate of 0.3 K/minute. After

keeping ^the sample ^at Tm (213 K) ^{in the incommen-}

surate phase during ²⁴ hours, e’ a ^{does not show a} plateau-like ^broad anomaly ^at Tm ^{in contrast with}

the case of pure crystals [24]. ^{It shows} only ^a

crossover from the heating global ^{line to the} cooling

global ^{line at} Tm ^{as shown in} figure ^2.

Fig. ^2.

^{A crossover} of dielectric constants at Tm ^after writing ^a memory (Tm

^213.0 K, ²⁴ hours) in d-urea-

doped d-thiourea. The temperature scanning ^{rate is 0.3}

K/minute. The T m temperature ^{was achieved} coming ^from

the ferroelectric state. Measurements

done in the

following : starting ^from Tm ^{down to 199} K, up ^to 217 K

(just ^below T;), ^{and down to} 199 K. The temperature cycle

has been repeated ³ times and the results coincide. Global thermal hysteresis curves are shown by thin lines (results

of Fig. 1).

(ii) ^{MEMORY EFFECT AT THE SCAN RATE OF}

5 mK/minute.

The measurement has been done in the following cycles ^after writing ^a memory ^at

Tm (203.5 ^{K, 12} hours) ^{as shown in} figure ^{3. The}

usual plateau-like memory effect is observed in the

heating ^{runs. In the} subsequent cooling ^{runs, how-}

ever, ^no apparent ^{anomalies are detected. This} sample ^{seems to} remind its history only ^{on the}

Fig. ^3.

Memory effects after writing

memory

(Tm

^{203.5 K, 12} hours) ⁱⁿ d-urea-doped d-thiourea. The

T m temperature

^achieved coming from the ferroelec- tric state. Measurements

done in the following : starting ^{from the} point (*) ^at T., (a) ^{down to 200 K} (0), (b) up ^to 212 K (.), (c) ^{down to 200 K} (.), (d) up ^to 215 K (o), (e) ^{down to 200 K} (o) ^{at the constant} tempera-

ture scanning ^rate of 5 mK/minute.

heating ^run. Such effect has not been reported ⁱⁿ

pure crystals [24] and is characteristic of this doped

material. The dielectric constants of the second

heating ^and cooling cycles approach ^{to the} global

lines. The temperature region ^of plateau-like

anomalous part ^becomes narrower on the second

heating ^run.

(iii) ^{STEP-WISE BEHAVIOUR AT THE} SCAN RATE OF

0.5 mK/minute.

At a very slow cooling ^or heating

rate of 0.5 mK/minute, ^new interesting ^features

appear in the real part 6’ a ^{as well as in the} imaginary part Ea ^{of the complex} dielectric constant, for ^the

doped d-thiourea. The results are reported ;

-

in figure ^{4 for the} heating ^{run in between}

202 K and 212 K inside the modulated region. ^The Ea goes ^{as a} function of temperature by ^{a series of}

small anomalies. The effect is much more clearly

visible on the imaginary part ^{which varies in a} quasi- periodic way, the quasi-periodicity being ^about

0.7 K. Such effects are much less pronounced ^{on the} cooling ^{run than on the} heating ^run (approximately

four times smaller).

Fig. ^4.

Temperature dependence ^of complex ^dielectric constant, 8’ a (e) ^and E a " (0) ^{at the} temperature heating ^rate

of 0.5 mK/minute in d-urea-doped d-thiourea. The first

measuring point

^achieved coming from the ferroelec- tric state.

-

in figure 5, ^after writing ^a memory effect

during ^{24 hours at the} nearly ^same temperature previously ^mentioned Tm

203.3 K. The curve

E’a(T) ^{on the} heating ^run restricted to the small temperature ^interval [201.5 ^K-203.7 K] ^{reveals sev-}

eral anomalies which precede ^an anomaly ^at Tm ^{or in}

its vicinity. ^{This last} Tm anomaly ^{does not} appear very different from the previous ^{ones : if the} memory effect is present, ^its magnitude is very small ^com-

pared ^{to the} amplitude variations due to the quasi- periodic behaviour. It has to be quoted ^{that such a}

small rate of heating corresponds ^{to a} long ^{time of}

2068

Fig. ^5.

Temperature dependence ^of complex ^dielectric

constant, Ea (0) ^and Ea’ (e) ^{at the} heating ^{rate of 0.5}

mK/minute after writing ^a memory (Tm

^203.3 K, 24 hours).

annealing ^{for the} sample ^{which can} explain ^the

absence of any memory effect.

To conclude, the dielectric experiments ^{have re-} vealed ;

(a) ^a crossover-like memory effect ^at Tm, (b) ^{a new kind of} memory effect only observed in the course of heating ^runs,

(c) ^a quasi-periodic ^behaviour.

The quasi-periodic ^behaviour suggests ^that step-wise changes may ^occur in the modulation wave-vector

variations under the same experimental conditions.

Such steps ^could be related either to an intrinsic effect, Devil’s staircase as suggested by Aubry [26]

or to the pinning ^{of the} modulation by ^defects [27, 28] giving ^{rise to a} series of metastable states. Step-

wise changes have been also observed in normal d- thiourea under the same experimental ^conditions given ⁱⁿ figure 4, ^but they ^{are much less} pronounced.

In that view, ^{the normal} d-thiourea could also appear, in a way, like a doped crystal since defects

are also present ^{in the} deuterated system (samples

are never fully deuterated). Deuteration of samples

is nevertheless necessary ^to investigate ^{the wave-}

vector variation in future neutron diffraction _exper- iments (the expected effect will be small and the incoherent scattering by ^{H atoms has to be} suppres-

sed). ^Such experiments ^{will needs a} very good temperature definition of the sample (0.02°) : they

are essential for the explanation ^of the dielectric behaviour.

Similar effects have been recently ^{observed in the}

incommensurate phase ^of quartz [9, 29].

Acknowledgments.

The authors would like to thank J. P. Jamet, ^P.

Lederer, ^K. Parlinski for stimulating discussion.

One of the authors (A.O.) would like to express also his sincere thanks to Professor Y. Shiozaki and Dr. I. Takahashi for many kind suggestions ^and

discussions.

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