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NEUTRON INELASTIC SCATTERING AT STRUCTURAL PHASE TRANSITIONS

R. Cowley

To cite this version:

R. Cowley. NEUTRON INELASTIC SCATTERING AT STRUCTURAL PHASE TRANSITIONS.

Journal de Physique Colloques, 1972, 33 (C2), pp.C2-7-C2-10. �10.1051/jphyscol:1972202�. �jpa- 00214938�

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JOURNAL DE PHYSIQUE Colloque C2, supplkment au no 4, Tome 33, Avril 1972, page C2-7

NEUTRON INELASTIC SCATTERING AT STRUCTURAL PHASE TRANSITIONS

R. A. COWLEY

Department of Physics, University of Edinburgh, Edinburgh, Scotland

RCsumC. - Nous passons en revue les mesures par diffusion inklastique de neutrons dans les materiaux qui ont des transitions de phase structurales. Dans certains materiaux comme SrTi03 et KTaO, on observe un mode normal bien defini, dont le carr6 de la frCquence dCcroit presque lineairement avec la temperature. Dans d'autres materiaux on observe un amortissement apprkiable et dans BaTi03 et KTa03 cet amortissement est suffisamment large pour que la diffusion de ces modes soit entikrement quasi elastique. La diffusion quasi Clastique est aussi observee dans les ferro- electriques du type ordre dksordre comme NaNOz mais dans ce cas un mode de phonon et prati- quement independant de la temperature est ainsi observe. Le ferroelectrique a liaison hydro- gkne DKDP donne aussi lieu a une diffusion quasi Clastique mais dans ce cas on comprendmoins bien son origine. Nous discutons brievement des recents developpements qui contiennent l'aniso- tropie des courbes de dispersion, les effets critiques et la determination des vecteurs propres.

Abstract. - Neutron inelastic scattering measurements in materials making structural phase transitions are reviewed. In some materials SrTiO3 and KTa03 a well defined normal mode is observed whose frequency squared decreases almost linearly with temperature. In other materials appreciable damping is observed and in BaTiO3 and KNbO3 this damping is sufficiently large that the scattering from these modes is entirely quasi-elastic. Quasi-elastic scattering is also observed in order-disorder ferroelectrics such as NaNOz but in this case a well-defined almost temperature independent phonon mode is also observed. The hydrogen bonded ferroelectric DKDP gives rise to quasi-elastic scattering also but in this case its origin is less clearly understood. Recent developments including the anisotropy of the dispersion curves, critical effects and eigenvector determination are briefly discussed.

Neutron inelastic scattering measurements provide one of the most detailed ways of studying the micro- scopic properties which give rise t o structural phase transitions. The scattering is governed by the laws of conservation of energy and wavevector and since thermal neutrons have energies and wavevectors which are compatible with those of the normal modes of vibration of the atoms in crystals, the frequencies and wavevectors of these modes may be readily measured.

These techniques have now been applied t o the study of several phase transitions and in this review we shall not attempt a comprehensive cover of the whole field but merely t o highlight the most important features.

The intensity of the one-phonon part of the scatter- ing with a frequency transfer, CO, and wavevector transfer, Q, is proportional to

where n(o) is the Bose occupation number, A(Q - q) is a delta function expressing that Q - q = z, a reciprocal lattice vector, while F(Q j) is the structure factor of the mode with wavevector q and branch j.

where b, is the coherent neutron scattering length,

U(K ( qj) the displacement in the normal mode, W(f0 the Debye-Waller factor and R(K) the position in the unit cell of the atom of type K. The one-phonon response function is given for a harmonic crystal by

where o ( q j ) is the harmonic frequency of the normal mode. By using neutron scattering i t is possible t o measure S(Q, o ) and hence to deduce the form of both the norms1 mode eigenvectors U(K I q j ) and the res- ponse function o(qj).

I n a crystal which undergoes a structural phase transition Cochran [l] suggested that the frequency of a normal mode decreased t o zero as the phase transition was approached :

~ ( q j ) ~ = K(T - Tc) . (3) For a ferroelectric phase transition q = 0 but for a structural phase transition in which the unit cell size increases q corresponds to a zone boundary or possibly some other wavevector normal mode. Eq. (3) was shown t o be applicable for the near ferroelectric phase transition in SrTiO, [2] and KTaO, [3]. More recently the frequency of the zone-boundary normal mode responsible for the phase transition in SrTiO, a t 105 OKhas been studied over a very wide tempera- ture range as shown in figure 1 [4]. It may be seen from this that eq. (3) is a very reasonable first approximation.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972202

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C2-8 R. A. COWLEY

0 Shlraw on3 Yonudo Cowley et al 'This wcrk

0 1 A a, L a . L 7 b 1 1

IT-T; 1 K

-

FIG. 1. -The square of the frequency of the r25soft mode frequency as a function of temperature in SrTiO3. The dotted line is a straight line a(T- Tc) and other lines more complex theories

as described in [4].

In other materials it has been found that the normal mode is not well defined but that it is necessary to take account of the damping of the normal mode to describe the response of the crystal. Under these conditions a classically damped simple harmonic oscillator model is frequently a reasonable first approximation when eq. (2) is replaced by

This expression has been used with at least qualitative success, as shown, for example, in figure 2, to describe the scattering from the T,, mode responsible for the phase transition in KMnF, [5].

a t . .

ZOO

FREQUENCY (IO'HZ)

FIG. 2. - Neutron scattering from the T 2 s phonon in KMno.8C00.2F3 as a function of temperature as compared with a

best fit s i m ~ l e harmonic oscitlator model.

The results of figure 2 show that the normal mode may be fairly well defined well above the transition temperature but as the temperature is reduced the scattering becomes entirely quasi-elastic and the mode is overdamped. In some materials the damping constant y,

is sufficiently large that only quasi-elastic scattering has been observed. The width of this scattering then gives a measure of the lifetime of the fluctuations

while the total scattered intensity for a given q is proportional to l/w(qj)'. Quasi-elastic scattering of this type has been observed in BaTiO, by Yamada et al. [6] and in KNbO, by Nunes et al. [7].

One of the surprising results which has been found in these perovskite materials is the behaviour of the normal modes with wavevector q , in the neighbourhood of a mode q,, which is responsible for a phase transi- tion. In figure 3 we show w ( q j ) for several different

FIG. 3. - The dispersion curves ~ ( q j ) ~ for various branches of the dispersion relations around the soft modes of SrTiO3 at 90 OK and of BaTiOs [9] at 423 OK and for the R point soft mode

in SrTi03 [8] and KMnF3 [lo], [5] at 295 OK.

directions away from q , for the ferroelectric modes in SrTiO, [8] and BaTiO, [lo], [5]. In the case of BaTiO, the dispersion curve is very anisotropic with very low frequency and overdamped modes when the polarisa- tion vector is along the [loo] direction. Little anisotropy is observed in SrTiO,. Around the T , , modes both crystals show anisotropy when q - q , is along [loo]

but it is more marked for KMnF,. These anisotropies are related to the streaks observed in the X-ray scatter- ing of these materials [I 11, but at present their origin is not understood microscopically. Since both KMnF, and BaTiO, show larger anisotropies than SrTiO, and the modes are also more heavily damped in these materials, the anisotropy may arise in part from anharmonic interactions.

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NEUTRON INELASTIC SCATTERING AT STRUCTURAL PHASE TRANSITIONS C2-9

A rather different picture has been obtained from studies of order-disorder ferroelectric materials such as NaNO, [12]. Above the ferroelectric phase transition in this material the NO, groups may align with the N ion along either the f b or - b direction, but below the transition temperature they align along one or other of these directions. The normal modes of vibra- tion are then found to have frequencies which are relatively independent of temperature but in addition there is quasi-elastic scattering associated with the fluctuations in the positions of the NO, groups. The former oscillations have frequencies of 5 x 1012 Hz while the hopping from one equilibrium position to the other occurs on a time scale of 109Hz. These two types of motions are quite uncoupled from one another and give rise to different features in the scattering. In practice the transition in NaNO, is complicated because there is a sinusoidal ferroelectric phase which exists for a few degrees above T,. In figure 4 we show the intensity of the quasi-elastic scattering in the para- electric phase, sinusoidal phase, and ferroelectric phase showing that it increases in magnitude as the tempera- ture is decreased towards T, particularly in the region of wavevectors near the periodicity of the sinusoidal phase.

120- 1) SPIRAL

100- 160°C

-

80- Z

60- EXPTL.

>- RESOLUTION

a FWHM

a 40- H

n e m

20-b

a a

-

>- FERROELECTRIC

W Z

O o.'~ 012 d:3 d.4 '

WAVEVECTOR =aq/2,

FIG. 4. - The intensity of quasi-elastic scattering in NaNOz as a function of temperature. The wavevector transfer is along the line (1,2.0) in reciprocal space and the results show the critical scattering in the paraelectric phase, the satellite reflection in the sinusoidal phase and further reduced critical scattering in the

ferroelectric phase [12].

Experiments have also been conducted on two materials of the KDP class. These materials are DKDP [13], [14] and DADP [15] and show a ferro- electric phase transition and an antiferroelectric phase transition respectively. In both cases only quasi- elastic scattering has been observed and its intensity

has been measured as a function of temperature and wavevector as shown in figure 5. The intensity is observed to increase as the phaselphase transition is approached and furthermore to have a shape as a function of wavevector which is characteristic of a ferroelectric phase transition. It is however still uncertain whether this quasi-elastic scattering arises from an overdamped normal mode as in BaTiO,, or from order-disorder scattering as in NaNO,, or an overdamped tunneling mode as suggested by tunneling theories of these materials.

5 0 0 0 0 -

- 10000-

I

V)

!Z 5 0 0 0 - Z 3

>-

LT (303) 1.

a 1000- BRAG6

(r REFLECTION

500-

a a

- 226 3' K

>- loo-

V)

5 0 - I- z

- -

6 2.0 2.4 2.8 3.2 3.6

WAVEVECTOR 5 =%

FIG. 5. - Intensity of the quasi-elastic scattering along the line (1, 0, 3). The left-hand side shows the ferroelectric and Bragg scattering ; the right-hand side the temperature dependence

of the ferroelectric scattering.

We have shown that the response functions a(qj) may have quite different structures dependent upon the physical system. Recently more detailed work has been started on studying the form of a(qj) in detail close to the phase transition. Experiments by Feder et al. [16] have shown that the shape in SrTiO, is in detail quite different from that discussed above and that these discrepancies are believed to arise from the failure of our simple models in the critical region.

We can expect a large amount of detailed information about the shape of the response functions to be forth- coming in the near future.

The Brookhaven group has also shown that there are strong interferences between the scattering from acoustic modes and the ferroelectric modes. The inter- ferences give rise to characteristic asymmetries in the scattered distributions. We can expect more detailed information of this type.

In addition to the form of the response function neutron scattering can also give information about the

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C2- 10 R. A. COWLEY

normal mode eigenvectors. If the scattered intensity is crystal structures [17]. Measurements of this type have measured as a function of wavevector transfer Q for been made of the modes involved in many different the same wavevectors q the eigenvectors U(K I qj) can phase transitions as described for example for DKDP be obtained by a similar process to that used in finding and DADP in this conference [IS].

References [I] COCHRAN (W.), Advan. Phys., 1960,9,387.

[2] COWLEY (R. A.), Phys. Rev. Letters, 1962, 9, 159.

[3] SHIRANE (G.), NATHANS (R.) and MINKIEVICZ (V. J.), Phys. Rev., 1967, 157, 396.

[4] OTNES (K.), RISTE (T.), SHIRANE (G.) and FEDER (J.) Solid State Commun., 1971, 9, 1103.

[5] BUYERS (W. J. L.), COWLEY (R. A.) and PAUL (G. L.), J. Phys. SOC. Japan Supp., 1970, 28, 242.

[6] YAMADA (Y.), AXE (J. D.) and SHIRANE (G.), Phys.

Rev., 1969, 177, 848.

[7] NUNES (A. C.), AXE (J. D.) and SHIRANE (G.), Ferro- electrics, 1971, 4, 261.

[8] STIRLING (W.) and COWLEY (R. A.) (this conference).

[91 HARADA (J.), AXE (J. D.) and SHIRANE (G.), Phys. Rev.

1971, B 4, 155.

[lo] GESI (K.), AXE (J. D.), SHIRANE (G.) and LINZ (A.), Phys. Rev. (to be published).

[ l l ] DENOYER (F.), COMES (R.) and LAMBERT (M.) (this conference).

[12] SAKURAI (J.), COWLEY (R. A.) and DOLLING (G.), J.

Phys. Soc. Japan, 1970,28, 1426.

[13] SKALYO (J.), FRAZER (B. C.) and SHIRANE (G.), Phys.

Rev., 1970, B 1, 278.

[14] PAUL (G. L.), COCHRAN (W.), BUYERS (W. J . L.) and COWLEY (R. A.), Phys. Rev., 1970, B 2 , 4603.

[15] MEISTER (H.), SKALYO (J.), FRAZER (B. C.) and SHIRANE (G.), PhyS. Rev., 1969, 184, 550.

[16] FEDER (J.), RISTE (T.), SAMUELSON (E. J.) and OTNES (K.) (this conference).

[17] COCHRAN (W.), Neutron Inelastic Scattering Vol. I, I. A. E. A. Vienna, 1968, p. 275.

[18] WALLACE (E. A.), COCHRAN (W.) and STRINGFEL-

LOW (M.) (this conference).

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