Study of the crystalline electric field in PrxLa1-xAl3

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Study of the crystalline electric field in PrxLa1-xAl3

P. Alekseev, I. Sadikov, A. Filipov, E. Savitskii, V. Terekhova, I. Markova, O.

Chistyakov

To cite this version:

P. Alekseev, I. Sadikov, A. Filipov, E. Savitskii, V. Terekhova, et al.. Study of the crystalline electric field in PrxLa1-xAl3. Journal de Physique Colloques, 1979, 40 (C5), pp.C5-147-C5-148.

�10.1051/jphyscol:1979555�. �jpa-00218972�

JOURNAL DE PHYSIQUE

Colloque C5, supplément au n° 5, Tome 40, Mai 1979, page C5-147

Study of the crystalline electric field in PrXa^Al,

P. A. Alekseev (*), I. P. Sadikov (*), A. A. Filipov (*),

E. M. Savitskii (**), V. F. Terekhova (**), I. A. Markova (**) and O. D. Chistyakov (**)

I. V. Kurchatov Institute of Atomic Energy (*) and A. A. Baykov Institute of Metallurgy (**), Moscow, U.S.S.R.

Résumé. — Nous avons réalisé des expériences de diffraction inélastiques de neutrons sur des échantillons poly- cristallins de PrJLa

^t

.^A^ (x = 1,0 ; 0,75 ; 0,50 ; 0,25) dans le but d'étudier la corrélation entre le champ cristal- lin (CEF) et les caractéristiques du comportement des électrons de conduction dans les intermétalliques de terres rares. Nous faisons une analyse qualitative des relations observées en tenant compte de l'effet de la variation du paramètre de réseau sur la base du modèle de charges ponctuelles, de la présence possible de distorsions locales de symétrie et d'un certain nombre de mécanismes de relaxation.

Abstract. — Inelastic neutron scattering experiments were performed on Pr^La^^A^ polycrystalline specimens (x = 1.0 ; 0.75 ; 0.50 ; 0.25). Relationships of energetic positions and widths of a number of peaks in neutron scattering spectra with Pr concentration were obtained. A qualitative analysis of observed relationships is made by taking into account the effect of the lattice parameter variation on the basis of the effective point charges (EPC) model, the possible presence of local distortions of symmetry and a number of relaxation mechanisms.

At the basis of a variety of physical properties of rare-earth compounds are a number of phenomena typical for a majority of these compounds. Splitting of the ground state into a number of sublevels under the action of crystalline electric field (CEF effects) [1]

should also be classed with these phenomena. The CEF levels have a width which in metals is basically related to some types of relaxation processes [2].

The object of this study was to investigate the effects of a CEF and to obtain experimental information on relaxation processes in PrJLa^^A^ compounds with the variation of magnetic ion concentration.

Experiments to study the magnetic scattering of neutrons were carried on PrAl

3

, Pr

075

La

0

.

25

A1

3

, Pr

0 5

La

0 5

A1

3

, and Pr

0

.2sLa

0 75

A1

3

specimens. Poly- crystalline specimens of size 120 x 60 x 6 mm were obtained by arc melting in purified helium medium [3].

Specimens were annealed at 800 °C for 25 hours to remove internal stresses and for homogenization. A subsequent control of phase composition (micro- structural, X-ray and neutron) revealed the presence of no more than 1-2 wt % of the second phase in PrAl

3

and 6-8 wt % in Pr^La^^A^. Measurements were taken by a time-of-flight spectrometer with a source of cold neutrons [4]. The resolution of the spectrometer was 0.6-1.0 meV [5], the specimens were kept at 90 K. The experimental data of about 30 spectra for P^La^^Ala specimens at all scattering angles were processed as per the programme taking account into the resolution function (see Fig. 1), and were found the energetic positions of peaks corres- ponding to transitions between CEF levels and their widths (Figs. 2, 3) as a function of x. Effective

i e-!S-tV)-B* na «to 3 & i s s J, i ? <

fax. ' M r«90/c (aj

550 1 °^1— ^r « . .1 K

/rt*K,r'90K (b)

...~^_- r.~~*Cm»r^z 3 t •Vi-.-1..

g l . . ' ' . ' »

200 300 4tai TOr(cti> '

Fig. 1. — Spectra of inelastic scattered neutrons on PrAl

3

(a) and Pr

0 25

La

0 75

A1

3

(b) specimens at T = 90 K for 6 = 45°.

Open circles — experiment, points — after corrections for the spectrometer resolution function. The arrows indicate the peaks corresponding to transitions between CEF levels. In the margin are shown a scheme of CEF levels for PrAl

3

as per the data of [6], and the transitions observed in the spectrum.

charges Zj? for 12 Al ions surrounding Pr

3+

ions that correspond to 2nd, 4th and 6th order CEF para- meters [1, 6] have been determined by using the EPC model : Zjg> = 0.85; Z # = 3.37 and ZJg> = 15.

Assuming that on replacing Pr by La in PrAl

3

the effective charges of Al

3 +

ions do not change and by using the X-ray data about a and c lattice parameters dependence on Pr concentration for Pr^Laj _.,A1

³

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979555

C5- 148 P. A. ALEKSEEV et al.

were found the positions of peaks corresponding to transitions shown by arrows on figure 1 on the basis of EPC model expressions for CEF Hamiltonian.

The resulting concentration dependence of calculated positions of peaks is shown in figure 2, by solid lines.

The least square fitting of calculated EPC model spectra to the experimental ones for Pr,La,-41, enabled to find CEF parameters and corresponding splitting schemes for each of the studied compounds (see Fig. 2). They were found to differ from the calculated ones on the assumption that Z g ) remains constant.

dependence of peak positions upon La concentration.

But, along with this are found differences in the definite type of dependence. The attempts to achieve better agreement between experiment and theory lead to a concentration dependence of effective charges of aluminium.

From figure 3 it may be seen that the change in level widths is not monotonic as the specimen is freed from the nonmagnetic La. The following factors can contribute to the widths of transitions between the CEF levels

:

impurities, deviation from the given stoichiometry, static shift of ions and spin-phonon and exchange interactions. The content of impurities and the second phase were checked in the specimens during their preparation, and it was quite low. The spin-phonon interaction also varies too little as Pr is substituted by La. The question on static shifts that would lead to additional degeneracy and shift of levels was studied by X-ray diffraction analysis method. Within the limits of experimental errors (about 10 %) was not noticed additional contribution to the thermal shift of rare-earth ions.

Fig. 3. - Dependence of average widths of peaks y, over the scat- tering angle on La concentration.

Fie. 2.

.,

- Relationshiv between the avera~ed ^- over scattering angles ^{- -}

positions of peaks ci corresponding to transitions between CEF

~ h it is most likely that basic contribution to ~ ~ ,

levels and the composition of specimen. Points show experimental

data; solid lines

-

the

EPC

model calculations with values

of width variation of transitions with concentration is

effective charges independent of x ; dotted lines

-

the least square

made by exchange interaction possibly through con-

fitting procedure. -

current mechanisms.

The authors wish to express their gratitude to Thus, despite the imperfection of the EPC model it Yu. L. Shitikov for making available the programme covers basic contribution in CEF and enables in the to take into account the resolution function and for first approximation to forecast qualitatively a reliable useful discussions.

References

[I] WALLACE, W.

E.,

Rare-Earth Zntermetaiiics (Acad. Press, SYRYKH, G. F., CHERNOPLEKOV, N. A., SHITIKOV,

Yu. L.,

New York) 1972. Prib. i Tekh. Eksper. 5 (1973) 34.

[2] RAINFORD,

B.

D.,

D.

Phil. Thesis, Oxford (1969). [S] SHITIKOV,

Yu. L.,

ZEMLYANOV, M. G., MIRONOV, S. P., Prib. i [3] SAVITSKII,

E.

M., TEREKHOVA,

V.

F., BUROV, I. V., MAR- Tekh. Eksper. 5 (1977) 35.

KOVA,

I.

A., NAUMKIN, 0. P., Compounds of Rare-Earth [6] ALEKSEEV, P. A., SADIKOV,

I.

P., MARKOVA, I. A., SAVIT- Metals (Nauka Publishers) 1962. SKII, E. M., TEREKHOVA, V. F., CHISTYAKOV, 0. D., [4] ZEMLYANOV, M. G., GOLOVIN, A. E., MIRONOV,

S.

P., Fiz. tverd. Tela

18

(1976) 676.