The lattice parameters a and c of CeM2 Sn2 show an anomalous temperature
dependence compared to the analogous variations for LaM 2 Sn2 . The
anomaly Δa of thermal expansion for CeCu2 Sn2 has a maximum around
90 K, whereas Δc has a maximum near 140 K, near to the maximum of the
magnetic resistivity Δp

Kaczmarska, Krystyna

Pierre, J.

Ślebarski, Andrzej

Acta Physica Polonica A

English

Repozytorium Uniwersytetu Śląskiego RE-BUŚ

             Title: Structural properties of REM2Sn2 (RE=La, Ce and M=Ni, Cu)   Author: Andrzej Ślebarski, Krystyna Kaczmarska, J. Pierre   Citation style: Ślebarski Andrzej, Kaczmarska Krystyna, Pierre J. (1994). Structural properties of REM2Sn2 (RE=La, Ce and M=Ni, Cu). "Acta Physica Polonica. A" (Vol. 85, nr 2 (1994), s. 267-270).   Vol. 85 (1994)	 ACTA PHYSICA POLONICA A 	 No. 2Proceedings of the European Conference "Physics of Magnetism 93", Poznań 1993STRUCTURAL PROPERTIES OF REM 2 Sn2(RE = La, Ce and M = Ni, Cu)A. ŚLEBARSKI, K. KACZMARSKAInstitute of Physics, Silesian University, Uniwersytecka 4, 40-007 Katowice, PolandAND J. PIERRELaboratoire Louis Neel, C.N.R.S. 166X, 38042 Grenoble, FranceThe lattice parameters a and c of CeM2 Sn2 show an anomalous temper-ature dependence compared to the analogous variations for LaM 2 Sn2 . Theanomaly Δa of thermal expansion for CeCu2 Sn2 has a maximum around90 K, whereas Δc has a maximum near 140 K, near to the maximum of themagnetic resistivity Δp.PACS numbers: 65.70.+y, 75.30.Mb1. IntroductionNumerous rare earth (RE)—transition metal (M) intermetallics with the com-position REM2X2 crystallize with the CaBe2 Ge2 tetragonal structure (space groupP4/nmm). A large number of compounds with Si and Ge has been studied. Morerecently, stannides with the same structure have been prepared. One main goalof investigations in these series is to study the influence of the transition metalon the magnetic interactions between rare earth or on the hybridization state ofCe compounds. In the following, we present structural properties of La and Cestannides with Cu and Ni.Two crystallographic phases have been identified for CeNi 2 Sn2 , the parenttetragonal structure and a monoclinic variant [1]. CeNi2 Sn2 is found to be a heavyfermion system with a γ value of 0.6 J/mole K 2 . It orders antiferromagneticallyat 2.2 K, exhibits a maximum of resistivity at 9 K and a minimum at 30 K.CeCu2 Sn2 belongs to the tetragonal structure; Ce has a smaller hybridizationthan in the former compound. It orders at 2.1 K and does not exhibit any resistivityminimum; however, its magnetic resistivity passes through a maximum at about100 K.(267)268	A. Ślebarski, K. Kaczmarska, J. Pierre2. Crystallographic propertiesAll measurements at different T were performed with the X-ray powderdiffractometer using Fe Ka radiation. To obtain the profile of a line at roomtemperature and below, every reflection was measured by a scan method in whichthe counter moved in steps of 0.01° and the counting time for every point was 10 s.Next, the measured intensity lines were approximated by the Pearson function ofthe type j21:where I0k is the maximum intensity at 290k for each of the k lines in the regionof overlap, 200k is the calculated position of the Bragg peak corrected for thezero-point shift of the counter, .6k is the half width at half height, mk is the shapefactor.The best results were obtained within the interval 1 < m < 2. For theKai profile the intensity was a half of the value for the K a i, whereas all otherparameters were the same as used in Kai profile.Pearson-type distributions were fitted to the experimental profiles by a leastsquares method. The best fit to the shape of every (hkl) line with deviation lessthan 10% was obtained. The lattice parameters a and c of CeCu2Sn2 show ananomalous temperature dependence comparing with the analogous temperatureFig. 1. Volume anomaly of CeCu2Sn2 as a function of temperature.Structural Properties of REM2Sn2 ...	 269dependence of LaCu 2 Sn 2 . These anomalies are clearly seen in the plots of |Δa| =la(CeCu2 Sn2 )—a(LaCu 2 Sn 2 )I and |Δc| = Ic(CeCu2Sn2) — c(LaCu2Sn2)| vs. T.We observe an anisotropy of lattice thermal expansion of CeCu2 Sn 2 . |Δa|shows a maximum around T = 90 K but |Δc| shows a maximum atT =140 K.We note also that volume increment |ΔV| = |V(CeCu 2Sn2) — V(LaCu2Sn2)| plot-ted vs. T (Fig. 1) is proportional to resistive increment Δp = p(CeCu2 Sn2 ) —p(LaCu2 Sn2 ) [1].Fig. 2. Intensity of the Bragg line (1,1,2) for Ce and La compounds as function oftemperature.In Fig. 2 we show the temperature dependence of the Bragg (1,1,2) lineintensities I of CeCu2 Sn2 and LaCu2Sn2 . Here a weak temperature change ofthe intensity of X-ray diffraction line for a small diffraction angle is expected.The intensity of (1,1,2) Bragg's line of LaCu 2 Sn2 (Fig. 2) nearly does not dependon temperature, however, comparing this I—T plot with the 1(T) dependence ofCeCu2 Sn2 of the same (h, k,1) Bragg reflection one clearly sees an anomalousdecrease in the intensity of the line for Ce-compound below 130 K. This resultcorrelates with the abnormal lattice parameters temperature dependencies andwith the abnormal Δp(T) of CeCu 2 Sn 2 .3. DiscussionAnomalous behaviour of thermal expansion has been observed in severalheavy fermion systems. For instance, a negative thermal expansion may occurat very low temperatures (i.e. in CeCu6 [3]), which is explained within the Fermiliquid model. The present anomalies occur at higher temperatures, but are believedto be related to cerium hybridization.270 	 A. Ślebarski, K. Kaczmarska, J. PierreFirst, the Kondo behaviour or mixed-valent character of Ce may result ina difference in cohesion energy and Debye temperature between La and Ce com-pounds. We also remark that the maxima of Δa and Δc occur in a temperaturerange close to the maximum of magnetic resistivity, which is a temperature repre-sentative of crystal field splitting and where also lattice correlations between Cesites begin to set up.The temperature dependence of some Bragg intensities was found anomalousfor CeCu2 Sn2 , which implies a strong change in the structure factor. Such changemay arise from variations in the Ce scattering factor, in the Debye—Waller factors,or may be due to the shifts of atomic positions in the cell as well as to an incipientdistortion of the cell, as observed in CeNi 2 Sn2 and CePt 2 Sn2 [4]. These phenomenaare linked to Ce atomic volume which is a function of valency, and possibly tosome redistribution of the electronic density due to the anisotropy of the groundstate wave function. All these modifications in the 4f configuration are stronglycorrelated to the lattice cohesion, and may explain the different behaviours ofLaCu2 Sn2 and CeCu2 Sn2 .References[1] K. Kaczmarska, J. Pierre, A. Ślebarski, R.V. Skolozdra, J. Alloys Comp., in press.[2] A. Ratuszna, K. Majewska, Powder DUP.. 5, 41 (1990).[3] A. de Wisser, A. Lacerda, P. Haen, J. Flouquet, F.E. Kayzel, J.J.M. Franse, Phys.Rev. B 39, 11301 (1989).[4] M. Latroche, M. Selsane, C. Godart, G. Schiffmacher, J.D. Thompson, W.P. Bey-ermann, J. Alloys Comp. 178, 223 (1992).

Structural properties of REM2Sn2 (RE=La, Ce and M=Ni, Cu)

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Structural properties of REM2Sn2 (RE=La, Ce and M=Ni, Cu)

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