MECHANISMS OF FERROELECTRIC PHASE TRANSITION IN CRYSTALS WITH REORIENTATING STRUCTURAL GROUPS

HAL Id: jpa-00214953

https://hal.archives-ouvertes.fr/jpa-00214953

Submitted on 1 Jan 1972

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

MECHANISMS OF FERROELECTRIC PHASE TRANSITION IN CRYSTALS WITH

REORIENTATING STRUCTURAL GROUPS

I. Aleksandrova, V. Yuzvak, V. Shabanov

To cite this version:

I. Aleksandrova, V. Yuzvak, V. Shabanov. MECHANISMS OF FERROELECTRIC PHASE TRAN-

SITION IN CRYSTALS WITH REORIENTATING STRUCTURAL GROUPS. Journal de Physique

Colloques, 1972, 33 (C2), pp.C2-63-C2-65. �10.1051/jphyscol:1972217�. �jpa-00214953�

JOURNAL

Colloque C2, supplt!ment au no 4, Tome 33, Avril 1972, page C2-63

MECHANISMS OF FERROELECTRIC PHASE TRANSITION IN CRYSTALS WITH REORIENTATING STRUCTURAL GROUPS

I. P. ALEKSANDROVA, V. I. YUZVAK and V. F. SHABANOV L. V. Kirensky Institute of Physics, Siberian Department of USSR Academy of Sciences,

Krasnoyarsk, 36, USSR

Rbumb. -

La transition ferroklectrique de NaNH4Se04.2 Hz0 est etudiee par plusieurs mkthodes spectroscopiques.

Nous discutons les faits caractkristiques du mkcanisme de polarisation par rkorientation des groupes polarises.

Abstract. -

Ferroelectric transition in NaNH4Se04.2 Hz0 crystals was investigated

various spectroscopical methods.

Characteristic features of polarization mechanism in crystals with reorientating polar groups are discussed.

Among ferroelectrics of order-disorder type there exists the group of crystals with very specific peculia- rities of the spontaneous polarization (P,) mechanism.

To this group belong (NH,),SO,, (NH,)BeF, and to some extent NaNH,SeO, . 2 H,O crystals. Ferroelec- tric properties of the last crystal have been recently found at this laboratory [I], [2].

Earlier by NMR method it has been established that ammonia groups in ammonium sulphate and ammonium fluoroberillate crystals have non-zero electrical dipole moments (p). These groups are in the state of cr hindered

rotation with high correlation frequencies both above and below Tc

141. The dipole moments ordering connected with the loss of symmetry plane is taking place below Tc.

The mechanism of these transitions will discussed below. Now our attention will be directed to some new data for pure and partly deuterated

crystals. Pure crystal becomes a ferroelectric below Tc

92.3 + 0.3 0C. The value of Curie-Weiss constant says for polarization mechanism of order- disorder type. Marked isotopic shift of Tc is absent here. Then it is very natural to suppose that there are structural elements having constant dipole moments in both phases.

It was established that proton-proton vectors orien- tation of the water molecules did not change below the phase transition [6]. It is therefore probable that dipole moments of the molecules are mutually compen- sated below T, too. But for the reliable conclusion it is necessary to determine the positions of H20-molecules in the structure.

In study of the second order quadrupolar effects in NMR Na23 spectra [7] and first order effects in

~ ' - s ~ e c t r u m it was found that the complicated phase transition occurs in this crystal.

In the interval of 3-40 below Tc the distribution of electric field gradient (EFG) on Na23 and ND, lattice sites is kept very close to those existing in para- electric phase. In this region the shift of central compo- nent of ~a~~ spectrum is slightly changed. The charac- ter of D2 spectrum is kept unchanged here. Then as temperature decreases sharp changes of spectra take place in the narrow (1-20) temperature range. These changes are explained by the appearance of non-equi- valent Na (I, 11) and ND, (I, 11) lattice positions (Fig. 1). Because this phenomenon occurs somewhat below

there is possibility to suppose that the second transition takes place in this temperature region.

However a different picture of transition is observed in 50 % deuterated crystals. The changes of NMR- spectra corresponding to Na (I, 11) and ND, (I, 11) appearance occur here as sharp jumps at the tempera- ture coinciding with

(-93.6 k0.6). In pure and partly deuterated crystals the continuous and large EFG changes are taking place below the transition region.

The investigations of NMR D2 spectra gave the most important information about polarization mechanism.

In the whole temperature range from - 1400

20 OC the observed quadrupolar effects correspond to dis- torted ND,-groups reorientating about several axes.

Dipole moment of a ND,-group calculated from principal values of EFG-tensor gave the value of p,

0.1 D for paraelectric phase. The p-value of ND, (I)-groups is not changed far below T,. Strongly distorted ND, (11) groups have the value p,

0.23 D at - 120 OC, where EFG-changes are nearly finished.

Maximal Ps-value calculated from these p, and p, does not exceed 0.3 pC/cm2. This value is much lower than experimental P, in saturation state

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972217

C2-64 I. P. ALEKSANDROVA, V. I. YUZVAK AND V. F. SHABANOV

FIG. 1. -Temperature dependences of satellite line splittings 2AH for NMR D2 spectra of ND4-groups in NaND4Se04.2 D20 crys- tal. (a), (b), (c) - rotation axes of crystals. Orientations in magne-

A A

ticfield : ~ - C I I H ~ ; ~ - C H O = ~ ~ ~ ; ~ - - ~ I I H ~ ; ~ - ~ H O = ~ ~ ~ ; 5 - CHO A = 45'.

(-

0.7 p ~ / c m 2 ) [S]. Thus, 1800-ordering of ammonia group is not a single mechanism of P,-appearance.

Information about Se04-group distortions was obtained from Raman- and IR-spectra. In accordance with selection rules for regular tetrahedral groups 4 normal vibration must be manifested in Raman- spectrum

v,(A,), v2(E), v,(I;,), v4(F2). The last two are active in IR-spectrum as well. In IR-absorption the forbidden frequencies were found too

degenerated lines were splitted in IR- and Raman-spectra. This corresponds to the vibrations of distorted Se04- groups. Additional distortions of these groups below

T, are very small.

A new line of 840 cm-' appears in Raman-spectrum near T,. Its intensity in maximum sharply increases with temperature decrease. This line appearance is probably associated with decreasing of water molecules mobility in Tc-region. Correlation with NMR-data allows to suppose that this type of mobility is a 1800- flipping of water molecules around bisector of angle formed by 0-H bonds.

Thus, in the whole group of crystals [3], [4] behind outer simplicity of polarization mechanism (the-order- ing of permanent dipoles) a complicated picture of transition connected with dynamics of polar groups and their interaction with structural distortions is hidden.

The most difficult for experimental investigations are the questions connected with the ammonia groups dynamics. The characteristic property is nonequal to zero the mean over time p-value of the distorted NH4-groups that are in the state of reorientation.

The distorted configurations of the ammonia groups were fixed (e. g. in ammonium sulphate) by neutron diffraction method that

integrate

the information at a long period. The p-value of the group being calculated from the coordinates of protons is nearly equal to the same value deduced from the NMR-data [6]. These results can be correlated with the hindered rotation of the groups if the significant differences of the life times corresponding to the diffe- rent tetrahedral orientations in the lattice would be assumed.

If the phase transition is governed by the collective interaction of these rotators, the correlation frequency or the type of motion must be abruptly changed at T,.

According to NMR-data these phenomena are absent in ammonium sulphate and ammonium fluoroberillate crystals. On the other hand the appreciable changes of potential function and of hindrance of ammonia groups in (NH4),S04 below Tc were detected by the neutron scattering method that probably is more sensi- tive in this case [lo].

In 20 %-deuterated crystals of NaNH,Se04.2 H 2 0 near

100

an anomaly behaviour of ND4-groups EFG-tensor was observed 181. These data allow to suppose that there are deep changes of deuteron dynamics in the transition region. Intergroup deuteron exchange could not be excluded here. Thus, the ammonia dynamics itself requires more detailed inves- tigation and more parameters are need for its descrip- tion. The notions

reorientation

or

hindered rotation

are too general.

Now it is clear, that ammonia group dynamics is

one of the active mechanisms of transition. But the

change in group dynamics and its interactions with

other transition mechanisms require further investiga-

tion.

MECHANISMS OF FERROELECTRIC PHASE TRANSITION IN CRYSTALS

References

[I] ALEKSANDROV (K. S.), ALEKSANDROVA (I. P.), ZHE-

REBTSOVA

(L. I.), ROSTUNTSOVA (A. I.), LEIBOVICH (T. A.), ZAITSEVA (M. P.), Fiz. Tverd. tela,

[2]

ALEKSANDROV (K. S.), ALEKSANDROVA (I. P.), ZHE-

REBTSOVA

(L. I.), ZAITSEVA (M. P.), ANISTRATOV (A.

[3]

O'REILLY (D. E.), TSANG (T.),

[4] ALEKSANDROVA (I. P.), ALEKSANDROV (K. S.), ZHE-

REBTSOVA

(L. I.), IZV.

[5]

ZAITSEVA (M. P.), ZHEREBTSOVA (L. I.), SHABANOVA (L. A.), Zzv. Acad. Nauk, SSSR, Ser. fiz.,

[6]

ALEKSANDROVA (I. P.), ALEKSANDROV (K. C.), KRUPNAYA (V. P.), Fiz. Tvevd. tela,

[7]

YUZVAK (V. I.), ALEKSANDROVA (I. P.), TCHERBAKOV (V. N.), Fiz. Tverd. tela,

[8]

ALEKSANDROVA (I. P.), YUZVAK (V. I.), TCHERBAKOV

N.), Izv. Acad. Nauk SSSR, Ser., fiz.,

[9]

SCHLEMPER (E.

HAMILTON

C.),

[lo]

DAHLBORG (U.), LARSSON (R.), PIRKMAJER