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INVESTIGATION OF THE CRITICAL SCATTERING AT THE STRUCTURAL PHASE TRANSITION IN
RbCaF3 USING MÖSSBAUER DIFFRACTION
J. Maetz, N. Butt, H. Jex, M. Müllner
To cite this version:
J. Maetz, N. Butt, H. Jex, M. Müllner. INVESTIGATION OF THE CRITICAL SCATTERING AT THE STRUCTURAL PHASE TRANSITION IN RbCaF3 USING MÖSSBAUER DIFFRACTION.
Journal de Physique Colloques, 1979, 40 (C2), pp.C2-337-C2-338. �10.1051/jphyscol:19792119�. �jpa-
00218489�
JOURNAL DE PHYSIQUE Colloque C2, suppldment au n O 3, Tome 40, mars 1979, page C2-337
I N V E S T 1 G A T 1 ON O F T H E C R I T I C A L S C A T T E R I N G A T T H E STRUCTURAL PHASE T R A N S I T I O N I N RbCaF3 U S I N G MOSSBAUER
D1
F F R A C T I O NJ. Maetz, N.M. ~utt*, H. Jex and M. Miillner
I n s t i t u t fiir Kemphysik der J.W. Goethe Uniuersitat, Frmzkfurt am Main, Federal Republic of Germany
"A. v. Hwnboldt research f e l l m from PINSTECH, R-Zpindi, Pakistan
Rdsum6.- Les spectres de diffusion au voisinage de la transition de la phase structurale cubique O$(~m3m) tdtragonale ~'~(14/mcm) Z 196 K sont observds pour RbCaFs
.
En utilisant les rayons gamma d une source ~iissbauert~nous sdparons les diffusions dlastique et indlastique avec une rdsolution d'dnergie de 60 neV. Les contributions des domaines sont ddtermindes par diffusion des rayons X.Abstract.- The critical scattering near the phase transition of RbCaFs from its cubic ok(~m3m) to the tetragonal ~:a(l4/mcm) structure at Tc = 196 K is investigated. Mijssbauer diffraction is ap- plied to separate elastic and inelastic scattering intensities with energy resolution of 60 neV.
The influence of domains is shown from X-ray diffraction.
1. Introduction.- The high energy resolution of g-a-rays from a 57~o-source has been utilized for the separation of elastic and inelastic components /l/ from the scattering near structural phase tran- sitions. The tetragonally distorded perovskites exhibit intensities arising at the 1/2(311) reflec- tion in cubic notation. We have investigated RbCaF3.
A large twisting angle (7.1 degree at 77 K) was de- duced from V -center EPR spectra 1 2 1 . Inelastic neu-
K
tron scattering 13.41 has been applied in the criti- cal region to analyse the central peak with an energy resolution of the spectrometer around 0.165 meV /3/.
Tc was determined as 193 K 131 and 196 K /4/.
2. Experiments.- In our experiment using a M;jssbauer
l
gamma-ray diffractometer the recoilless radiation from a 200 mCi 57~o-source in a Pd-matrix was scat- tered from a RbCaF3 single crystal and then analysed by a black absorber of ammonium lithium fluoferrate with an energy solution of 60 neV. The radiation was detected by a 80 m2 Si(Li) detector (resolution 300 eV at 14 keV). In our set-up the Si(Li) is fixed in its position, while the source is moving around the 28-axis of the diffractometer. This arrangement avoids microphonic disturbances of the Si(Li) during motion.
The crystal of RbCaF3 was cut with a (31 1) surface plane, it was polished and kept heated at 450 C for 24 hours to reduce the concentration of defects at the surface. The active area was 12x8 mm2, the thickness 2 mm. The mosaic spread was less than 0.2 degree FWtlM. The crystal was cooled using a
3. Results and Discussion.- The measured elastic intensities from the 1/2(311) superlattice reflection are presented in figure 1. Below Tc the data fit sufficiently well to a temperature behaviour which was assumed I ~ ~ ~ ( T ~ - T ) ~ B , k0.25, To=199 K. This agrees with the early birefringence results /5/
e0.25 in a range T -T<lO K, while the Landau value 0~0.5 was approached at lower temperatures.
closed cycle He-refrigerator and the temperature was controlled to 5 0.5 K.
Fig. 1 : Elastic intensity from the
4
(311) reflec- tion of RbCaFa. The data (two runs) belowlabove To are specified by full/open symbols. Theoretical cur- ves are scaled at T/To=0.9 and 1.05 to fit the e r periments.However, later analysis 161 yielded P0.29 and T0=196.9 K. In X-ray single crystal experiments we have studied the influence of domains in the distor- ded low temperature phase of RbCaF3. Figure 2 shows the squared intensities which should be proportional
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792119
C2-338 JOURNAL DE PHYSIQUE
to the rotation angle 4' = const.(To-T) of the CaF6- sity (Fig. 3) we recall that below Tc a new super
octahedra. lattice reflection is generated under which thermal
diffuse scattering must occur.
Fig. 2 : Squared X-ray intensity of the synrmetry equivalent 1/2(311) reflections according to their domain formation (o,~, A) and 119 of the total SUM
squared (+). Full lines indicate a unique power law while broken ones connect deviating points.
The data have been obtained from the full set of cubic equivalent 1/2(311) reflections splitting in- to 3 domains according to a tetragonal distortion in one of the cubic axis x,y,z. It is evident that a unique power law holds in the range To-T<30 K. At lower temperatures, however, the three branches switch off to follow some other law; only the total intensity 119
itot
remains on the same line over the full range. The deviation of our Gssbauer data fromtheory at low temperatures is obviously caused by domains and not by changes of
B.
Above Tc an elastic contribution grows rapidly at about 200 K. This tem-Fig. 3 : Inelastic intensity at the 1/2(311) reflec- tion of RbCaF3. The data (two runs) belowlabove To arespecified by full/open symbols. Theoretical cur- ves are scaled at T/To=0.9 and 1.05 to fit the expe- riments.
Its temperature dependence should be Iin%T(To-T) B
,
where the first factor arises from the averaged pho- non energy, while the second one comes from the ine- lastic structure factor. We assume the same tempera- ture dependence as for the elastic structure factor.
Figure 3 shows reasonable agreement. Above Tc, howe- ver, the inelastic scattering arises from the over- damped soft mode at the R-point of the cubic Bril- louin zone. The temperature law /7/ is I ~ ~ % T w ~ ~ which fits reasonably with the experiments.
Ref erences
/I/ O'Connor, D.A., Butt, N.M., Phys. Lett.
2
(1963) 233./2/ Halliburton, L.E., Sonder, E., Solid State C o r mun.
2
(1977) 445./3/ Almairac, R., Rousseau, M., Gesland, J.Y.. Nouet, J., Hennion, B., J. Physique
2
(1977) 1429.141 Kamitakahara, W.A., Rotter, C.A., Solid State Commun.
17
(1975) 1350./S/ Modine, F.A., Sonder, E., Unruh, W.P., Finch, C.B., Westbrook, R.D., Phys Rev. (1975) 1623.
perature range has been investigated/3,4/ by inelas- /6/ Bates, J.B., Major, R.W., Modine, F.A., Solid State Commun.
11
(1975) 1347.tic neutron scattering. A central peak and an over-
/7/ Darlington, C.N.W., O'Connor, D.A., J. Phys.
2
damped phonon were found, whose characteristic pa-
(1976) 3561.
rameters W-, uo, 6,
r
were given 131. Taking these values one expects an elastic intensity 171 I e l %~ 6 ~ u ~ - ~ u ~ - ~ , (Fig. 1). This law does not fit to our data. Especially it is not clear, whether the rest intensity is due to doping or unresolved (dEs60 neV) inelastic scattering.
For an interpretation of the inelastic inten-
