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PHONON DISPERSION AND TRANSVERSE MODE SOFTENING IN RbFeCl3
D. Petitgrand, B. Hennion, C. Escribe-Filippini, S. Legrand
To cite this version:
D. Petitgrand, B. Hennion, C. Escribe-Filippini, S. Legrand. PHONON DISPERSION AND TRANS- VERSE MODE SOFTENING IN RbFeCl3. Journal de Physique Colloques, 1981, 42 (C6), pp.C6- 782-C6-784. �10.1051/jphyscol:19816231�. �jpa-00221316�
JOURNAL DE PHYSIQUE
CoZZoque C6, supple'ment au n o 12, Tome 42, de'cembre 1981 page C6-782
PHONON D I S P E R S I O N AND TRANSVERSE MODE SOFTENING I N RbFeC13
* **
D. Petitgrand, B. Hennion, C. Escribe-Filippini and S. Legrand Laboratoire Le'on BrilZouin, CEN-Saclay, 91191 Gif-sur-Yvette, France
*~nstitut Laue-Langevin, 156X, Centre de Tri, 38042 GrenobZe Cedex, fiance
** DPhG/PSEZM, CEN-Sac lay, 91 191 Gif-sur-Yve tte, Zrance
Abstract.- Phonon dispersion curves of RbFeC13 have been measured by neutron scattering. For wave vectors in the hexagonal plane the transverse phonons polarized along Z are found to have very low energies. Furthermore the shape of this branch changes drastically as the temperature is lowered from room temperature to 20K, the zone boundary energy at the K point being decreased by a factor of two. However no Bragg reflection indicative of a phase transi- tion has been detected at this point. These features are interpreted as sho- wing evidence that RbFeClj is at the border of a phase transition for which a microscopic mechanism will be discussed.
I. Introduction.- Phonon softening has been extensively studied in AMX3 compaunds of cubic perovskite structure in connect ion with structural phase transitions.
More recently it has been reported that some AMX3 crystals with hexagonal structures also undergo structural phase transition^'^). In this paper we pive the first expe- rimental report of the observation of a phonon softening in the hexagonal RbFeC13 compound.
2. Experiments and Results.- The RbFeC13 single crystals used in these experiments are the same as those grown for the spin wave in~esti~ation'~). The inelastic neutron scattering experiments have been carried out on the triple-axis spectrometer IN 3 at the Institut Laue-Langevin. The experiments were performed with incident neutron
O- 1
wave-vectors of 2.3 or 2.66 A and collimations of 30'/20'/20'/20'. The sample was mounted in a pumped 4 ~ e cryostat with the c-axis horizontal so that the [hot] plane was in the scattering plane.
The dispersion curves of acoustic phonon measured at room temperature are shown in Fig. 1.
Since our measurements in the c direction do not extend to very small q, the sound velocity of the TA (001) branch (broken line in Fig.]) was set equal to 'that of the TA (110) branch as required by elastic theory. Obviously this sound velocity is smaller than the phase velocity derived from our lowest experimental ooint, which means that this transverse mode must have an upward curvature. This peculiarity, unexpected from classical elastic theory, is similar to that first observed in
~raphite'~) and more recently in CSN~F~('). This is a consequence of the inhomope- neous structure which can be regarded as made up of strongly coupled FeC16 octahedra making infinite fibres along the c-axis. These fibres are much more loosely coupled to the neighbouring ones via Rb ions. The bending of a fibre requires an elastic
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19816231
energy EI 3 a4 (E being the Young modul s and I the momentum of inertia of the fibre) in additioiz to the shear energy C - a Y of the FeC16-Rb-FeC16 bonds. Thus the trans- verse phonons dispersion takes the 44 2 4
form u2 = p-I (C k + El kz) , which explains t 44 z
the positive curvature at low q .
For phonons propagating in the hexagonal plane, the most striking feature is the very low value of the transverse mode (e//Z) energies compared with those of the longitudinal mode. From the slope at small q we deduce C11/C44= 17, a value much larger than typical values ( 4 < C 1 1 / C 4 4 < 6) for ionic crystals. We have carried out a systematic investigation of this transverse phonon between 1.5 and 300 K the results of which are illustrated in Fig.2. The whole branch gradually flattens as the tempera- ture is lowered from 300 K to 20 K and then becomes almost temperature independent at lower temperatures. Furthermore, the decrease in energy appears to be stronger at the K point with the consequence that this point becomes a minimum for T < 1 5 O K, ins- tead of a maximum at room temperature. Down to 50 K, the lowering of the phonon ener- gies is strongly reminiscent of the behaviours observed in cubic crystals SrTiO (1)
3 '
KM~F~'~). ~bcaF~(~)... for which the lowering is achieved by a complete softening of the mode giving rise to a structural phase transition. Thus we have carefully checked the elastic scattering, but we found no evidence of Bragg scattering at the K point.
3.- Discussion.- In the above mentioned cases of cubic AMX3 conpounds, the soft mode has been identified as a coherent rotation of the ?fX octahedral.But in the hexagonal
6
A ? ? structure two neighbouring octahedra share a triangular face and a coherent ro- 3
tation of the octahedra thus involves a strong coupling with internal modes of MX6 giving rise to a high frequency mode. In fact the only possibility which let the in- ternal modes frozen and is compatible with our observations is a mode involving trans-
1 1
lations of the whole fibre along c. For the special case of q = (730),which is the sensitive point, the motions of the 3 fibres situated on the vertex of a triangle are out of phase by 120°. The fact that this mode is of low energy can be explained by repulsive interactions between chains in their vertical motion. Indeed we anticipate that such a repulsion arises from the mismatch of the ionic radii of ~ b + and cl-.~his interpretation is strongly supported by the fact that below 120K RbFeBr3 undergoes a structural phase transition which just has the point K as a superlattice reflection.
Thus one must conclude that RbFeC13 would be slightly below and RbFeBr3 beyond the stability threshold.
One of the remaining problems is the description of the anharmonic behaviour of such a system. We plan to do accurate measurements of the temperature dependenceof the energies and widths of these unusual phonons in order to investigate this point.
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C6- 784 JOURNAL DE PHYSIQUE
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Fig.l- Dispersion of acoustic phonons polarized in the [hot] plane at 300 K.
I I I I I
0.1 0.2 0.3 0.4 0.5
c
Temperature dependence of the q = (550). e = (001) transverse phonon.
