SOFT PHONON VALLEY NEAR THE TRANSITION TO AN INCOMMENSURATE PHASE IN BARIUM SODIUM NIOBATE

HAL Id: jpa-00221295

https://hal.archives-ouvertes.fr/jpa-00221295

Submitted on 1 Jan 1981

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.

SOFT PHONON VALLEY NEAR THE TRANSITION TO AN INCOMMENSURATE PHASE IN BARIUM

SODIUM NIOBATE

J. Schneck, J. Toledano, J. Aubree, B. Joukoff, C. Joffrin

To cite this version:

J. Schneck, J. Toledano, J. Aubree, B. Joukoff, C. Joffrin. SOFT PHONON VALLEY NEAR THE

TRANSITION TO AN INCOMMENSURATE PHASE IN BARIUM SODIUM NIOBATE. Journal de

Physique Colloques, 1981, 42 (C6), pp.C6-734-C6-736. �10.1051/jphyscol:19816215�. �jpa-00221295�

JOURNAL DE PHYSIQUE

CoZZoque C6, suppZe'ment a u n o 1 2 , Tome 42, de'cembre 1981 p a g e C6-734

SOFT PHONON VALLEY NEAR THE TRANSITION TO AN INCOMMENSURATE PHASE IN BARIUM SODIUM NIOBATE

J. Schneck, J.C. Toledano, J. Aubree, B. Joukoff and C. ~offrin*

C. N. E.T., 1 9 6 rue d e Paris, 9 2 2 2 0 Bagneux, F r a n c e

*

Abstract

The dynamic characteristics of the tetragonal to incom mensurate transition in Barium Sodium Niobate has been investiga-

ted. The precursor effects consist, in the tetragonal phase, of a soft phonon and a central peak.

Barium Sodium Niobate was known to undergo in the vicinity of 300°C

ferroelastic transition with the symmetry change 4mn+mm2

However it has been recently shown that there were actually two transitions in the 300°C range(2). The upper one nearby 300°C which is continuous corresponds to the onset of an incommensurate phase with the modulation vector 2

(aX+b*) (1+6) /4+cX/2 (referred to the tetra- gonal lattice).

The stability range of this phase is

30°C, where 6 varies on heating from 8

to 12.5

The orthorhombic symmetry previously obser- ved is only that of the "average" cristalline structure. The lower transition which is discontinuous leads to another incommensurate phase with identical point-symmetry an8 the same direction of modulation but whith a different periodicity very close to a commensurate one (6%

By means of inelastic neutron scattering the dynanics of the upper transition has been studied in the high temperature tetragonal phase up to 660°C. In the investigated sample this transition occurs at

'L %

TI% 2 8 8 " ~ , the lower one being at

I I 250°C. Above TI, energy scans has been performed near the vector (4 - (1+6) /4,4 - ^(1+S)/4 , 1/2) which corresponds to one of the strongest satellites observed, below TI, in the

scattering plane.

The critical wavevector has been found at k

(3.72, 3.72, 0.5)

C

and is temperature independent. For T<TI, corresponds to the tempe-I

C

rature dependent position of the satellite reflection.

For T>T inelastic scans reveal the presence of a damped soft

phonon mode as well as that of a central compcnent with a critical be- havior. The soft phonon becomes overdamped below 400°C. The central component exhibits a steep increase of intensity on approaching TI whe- reas its width is determined by the instrumental resolution

0.15 Thz)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19816215

The contribution of the twc former features have been separa-

Fig.1

Variations of the squared

turation of wm is probably due to freauencies of the undamved renor-

0.

O-l;

w _h

0.5

9 a4 k 0.3 0.2 0.1

malized

mode, (filled the interaction of the spft-phonon triangles), and wo2wysquares).

The inserc shows wn2 in a ranne with a relaxing degree of freedom ted through a fit of the enerqy 0

. 1 3 ,

'

scans

.The plot of the fitted squared frequency of the soft pho- non wm2 is shown on fig. 1. wm de-

- creases markedly from

to

- 350°C.

Below

wm remains near-

- T* _1.-

.:: _,

ly constant because below this tem-

of

above TI. piving rise to a central peak.

250 350 450 550 650

perature the phonon contribution

('C) seems to remain constant. This sa-

The inverse of the static sus- ceptibility x

l/wo *

^{has been}

obtained through the measurements of the total intensity I scattered

in both the central peak and the soft mode peak. We have w

T/I .

As it is shown On fig.1 the temperature variations of wo2 :re in good agreement with a linear law below

The set of fitted wm frequencies obtained for various k vectors

along the 11101 direction gives the dispersion curve represented on fig.2.

f

Fig.2

Dispersion curve of

the soft-phonon branch along

the

direction.

C6-736 JOURNAL DE PHYSIQUE

Previous X-ray experinents

have shown in the high temperature phase precursor effects of the transition at TI as strong rods of dif- fused scattering approximately along YO] . In these rods a strong temperature dependence of the soft mode is observed showing that the dispersion curve along the direction is very flat.

On the basis of the preceding results, the nature of the diffused scattering observed in the X-ray experiment is clarified. Close to the crltical wavevector and near T it is essentially due to the cen-

C

I

tral peak distinct from the overdamped phonon contribution. On going away from 3k ^{in the} [lie] direction the contribution of the soft pho-

C

non increases and makes an important fraction of the scattering. Over 100°C above TI the diffused rods essentially reflect the valley shaped dispersion surface of the soft phonon branch.

These diffused rods reveal the occurence of strong correlations in the (ac) orthorhombic plane. A possible structural basis for a correlation along the c directicn could be in the fact that at micros- copic level the soft mode has been assigned to a collective shearing of the oxygen octahedra in the structure

This feature could esta- blish strong correlations in the chains of oxygen octahedra parallel to c, through the corner sharing of the consecutive octahedra.

By contrast no obvious mechanism accounts for the correlation along the [110] direction.

REFERENCES

I. J.C. Tol6dan0, Phys. Rev.

12, 943 (1975) and references therein.

2.

Schneck and

Denoyer, Phys. Rev.

383 (1981)

A.D. Bruce and P.A. Cowley, Adv. in Physics, 29, 219 (1980) 4. B. Dorner, J.D. Axe,

Shirane, Phys. Rev. B6, 1950 (1972) 5.

Schneck, J.C. Tol6dan0, C. Joffrin,

Aubrge,

Joukoff and

A. Gabelotaud (to be published).