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Crystal-field effects and phase transitions in Rocksalt-type cerium compounds
F. Hulliger, H. Ott
To cite this version:
F. Hulliger, H. Ott. Crystal-field effects and phase transitions in Rocksalt-type cerium compounds.
Journal de Physique Colloques, 1979, 40 (C5), pp.C5-128-C5-129. �10.1051/jphyscol:1979546�. �jpa- 00218963�
JOURNAL DE PHYSIQUE Colloque C5, supplément au n° 5, Tome 40, Mai 1979, page C5-128
Crystal-field effects and phase transitions in Rocksalt-type cerium compounds
F. Hulliger and H. R. Ott
Lahoratorium fur Festkorperphysik, ETH-H6nggerberg, 8093 Zurich, Switzerland
Résumé. — Nous présentons une étude systématique du champ cristallin et de l'interaction d'échange dans les monopnictides et les monochalcogénures de Ce.
Abstract. — We discuss the general behaviour of crystal-field and exchange interactions in Ce monopnictides and Ce monochalcogenides.
Studies on cubic Ce compounds are especially attractive because the single 4f electron of the Ce3 +
ions is in a J = 5/2 ground state and possible crystal- electric-field effects lead to a simple energy level scheme, namely a T7 doublet and a T8 quartet state, separated by an energy splitting A. Assuming non- interacting ions the thermal and magnetic properties can be expressed in simple equations and an analysis of experimental data should be straightforward.
Already some early experiments on Ce monopnic- tides [1-4], however, revealed several complications such as anomalously small crystal-field interactions, and unexpected and complicated magnetic structures below the magnetic ordering temperature, both most pronounced in the heavy Ce monopnictides CeSb and CeBi.
When investigating the two series of Ce monopnic- tides and monochalcogenides we searched for cha- racteristic trends and changes of the crystal-field and exchange interactions when varying the anions. CeP, CeAs, CeSb, CeBi; CeS, CeSe and CeTe order anti- ferromagnetically at low temperatures [5, 6]. In the pnictide series the Neel temperature increases with increasing principal quantum number of the anion from ~ 7 K (CeP) to ~ 25 K (CeBi), just the contrary to what is observed for the chalcogenides, where TN decreases from ~ 8 K for CeS to ~ 2 K for CeTe.
In both series the crystal-field energy splitting A decreases with increasing size of the anion and we note that the splittings are almost equal in both series, a rather surprising result in view of the additional free electron per cation in the chalcogenides. In all compounds the T7 doublet is the ground state, with some reservations to be made in the cases of CeSb and CeBi. The pnictides show type I antiferromagnetic order, again with the exception of CeSb [7] (compli- cated behaviour) and partly CeBi, where the type I ordering changes to type IA at ~ TN/2. In CeS neutron diffraction experiments revealed an antiferromagnetic
order of type II [8]. We note that only in CeSb and CeBi the magnetic ordering is accompanied by a large
distortion from cubic to tetragonal symmetry [9], most probably due to a quadrupole driven structural transition involving the low-lying T8 quartet state.
First we consider the magnetic phases of Ce mono- pnictides and we discuss the variation of the nearest- neighbour (Jj) and the next-nearest-neighbour (J2) exchange parameters as they appear in the molecular- field calculation of TN and 0p, using the experimental values of A, TN and 0p [5, 6, 10], assuming a type I ordering in all cases, including several data from ter- nary compounds like CeAs^Sb^^ (figure la).
Although admitting some scattering in the data due to deviations from stoichiometry we recognize the clear trend of J1 and J2 to remain constant over the series. The assumption of type I ordering gives reaso- nable values for Jt and J2 also in the case of CeSb where no such ordering has been observed and the complicated behaviour of CeSb is therefore even more puzzling. At this point we may also mention that a
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5.8 60 62 64 a[A] 58 60 62 a [A]
a) b) Fig. la. — Jt and J2 for Ce monopnictides calculated from rN,
09 and A, assuming type I ordering. The broken frames denote the values for CeSb.
Fig. lb. — Calculated values for Jy and J2 of Ce monochalcoge- nides. 1 : Jl (type I), 2 : /t (type II), 3 : J2 (type I), 4 : J2 (type II).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979546
CRYSTAL-FIELD EFFECTS AND PHASE TRANSITIONS IN ROCKSALT-TYPE CERIUM COMPOUNDS C5-129
small amount of As in CeSb (CeAs,.,Sb,.,) already destroys the tetragonal distortion at T,. The analogous plot is quite different for the chalcogenides (figure Ib), where we assume either type I or type I1 ordering.
For CeS the type I1 ordering is well established [7] and it may be seen that this type of ordering is also plau- sible for all other compounds in the series, with some uncertainty towards CeTe, where a possible type I ordering would require a sign change for both J, and J, with respect to the rest of the series. If J , remains more or less constant in the chalcogenides as well, then a type I1 ordering is predicted for CeTe.
We note a strong variation of J,, quite in contrast to the pnictides, as well as the possibility of a non- ordering compound in the vicinity of a = 6.25
A.
A very interesting behaviour is also expected for ternary compounds between the two binaries CeP and CeS, where a change of the type of ordering is anticipated. The relevant features are shown in figure 2.
It is again the strong variation of J , which dominates the behaviour and again we recognize the possibility of a non-ordering compound (- CeP,.,,S,.,,). The gradual decrease of charge carriers is evident from the change of colour of the compounds. Starting from the yellow CeS we obtain wine-red CePo,25So.,5 (similar
CeP CeS
Fig. 2. - Exchange parameters for Ce(P, S).
to CeSe) and blue CePo.,So,5 (similar to CeTe). It may therefore be stated that the type I1 ordering is most stable for a comparatively large number of free carriers and a tendency towards type I ordering may be noted for a decreasing free-electron concentration.
References
[I] FURRER, A., BUEHRER, W., HEER, H., HAELG, W., BENES, J., VOGT, O., Neutron Inelastic Scattering (Vienna) 1972, p. 563.
[2] BIRGENEAU, R. J., BUCHER, E., MAITA, J. P., PASSELL, L., TURBERFIELD, K. C., Pkys. Rev. 8 8 (1973) 5345.
[3] TSUCHIDA, T., NAKAMURA, Y., J. Phys. Soc. Japan 22 (1967) 942.
[4] LEBECH, B., FISCHER, P., RAINFORD, B. D., Proc. Conf. Rare Earths and Actinides, Durham 1971.
[5] HULLIGER, F., OTT, H. R., Z. Phys. B 29 (1978) 47.
[6] HULLIGER, F., NATTERER, B., OTT, H. R., J. Magn. Mag.
Mat. 8 (1978) 87.
[7] FISCHER, P., LEBECH, B., MEIER, G., RAINFORD, B. D., VOGT, O., J. Phys. C 11 (1978) 345.
[8] SCHOBINGER-PAPAMANTELLOS, P., FISCHER, P., NIGGLI, A., KALDIS, E., HILDEBRANDT, V., J. Phys. C 7 (1974) 2023.
[9] HULLIGER, F., LANDOLT, M., OTT, H. R., SCHMELCZER, R., J. Low Temp. Phys. 20 (1974) 269.
[lo] TSUCHIDA, T., WALLACE, W. E., J. Chem. Phys. 43 (1965) 2087.
