Journal of Magnetism and Magnetic Materials 86 (1990) 293-295 293 North-Holland RESISTIVrI~ ANOMALIES DUE TO CHARGE FLUCTUATIONS OF Ce IN LaCu 6 A. SLEBARSKI Instytut Fizyki, UniwersytetSlaski, 40-007 Katowice, Poland Received 4 October 1989 Measurements of the electricalresistivity of polycrystalline samples of Lal_xRExCu 6 (RE= Ce, Pr, Gd) are presented. This paper presents a qualitative analysis of the resistivity anomalies correlate with the temperature-dependent fluctuation temperature Tf. 1. Introduction The main goal of this work was to enlarge the empirical basis of the resistivity anomaly A0(T) -- Palloy -- Pmatrix of dilute rare earth (RE) impurities with unstable 4f shells in metals and in alloys. This anomaly is usually interpreted for Ce impuri- ties in terms of the Kondo effect, namely the effective impurity cross section increases over the normal cross section due to potential scattering by an instability of the 4f shell (positive resistivity anomaly). However, it was recently found [1,2] that the resistivity anomalies of dilute Ce in ZrIr 2 are negative, i.e. that the Ap resistivity increment becomes much smaller than that of the stable impurities. It was suggested in refs. [1,2] that, at T = 0, the resistivity increment A0 comes from the potential scattering due to the charge density contrast of the impurity against the matrix, which follows from the fractional valence of both impurity and ma- trix. In this paper the resistivity anomalies of La l_x- CexCu 6 are analysed by using the dynamic alloy model [3]. A qualitative description of such resistivity anomalies is proposed here by calculating the scattering of conduction electrons (ce) on valence fluctuations (charge fluctuations) of Ce impurities in the LaCu 6 lattice. The LaCu 6 matrix was cho- sen for the following reasons. Ce in CeCu 6 is in a mixed valence state according to the resistivity (v ffi 3.05 [4]) and L m X-ray absorption data (v ffi 3.11 [5]). On the other hand the susceptibility and resistivity analogy of LaCu 6 to LaAl 2 suggests that the possibility of a 4f instability of La in LaCu 6 should be considered. 2. Experiment and discussion Polycrystalline samples of 2% and 3% of Ce, Pr and Gd in LaCu 6 were prepared by melting the RECu 6 compounds in a levitation furnace under an argon atmosphere. For LaCu 6 and for the Ce, Pr and Gd alloys the monoclinic structure were found [6]. The X-ray examination performed by the powder method, using FeK a radiation, has shown that the lattice parameter b of the Ce alloy is much smaller than that of the remaining alloys, while the a and c parameters of these alloys are comparable. The resistivity of the Lal_xRExCu 6 alloys was measured by the Van der Panw (dc) method. Fig. 1 shows the resistivity and the susceptibility of LaCu 6 matrix. Even though the residual resistivity of LaCu 6 is small (p(4 K)= 2 t~fl cm), its sus- ceptibility strongly depends on the temperature, which seems to be characteristic for the known La alloys. At room temperature the susceptibility equals 1.3 × 10 -4 emu/mol and is comparable with the literature data of single crystals [7]. Fig. 2 shows the resistivity increment Ap/x of 2% and 3% of Gd, Pr and Ce alloys. The Ap/x vs. T depends on the kind~ of impurity and is nearly temperature independent for Gd because of the 0304-8853/90/$03.50 © 1990 - ElsevierSciencePublishers B.V. (North-Holland)