Temperature dependence of the samarium oxidation state in SmB6 and Sm1-xLaxB6

Abstract

2014 The cubic lattice parameter temperature dependence of SmB6 between 300 K and 4.2 K as well as the X-ray absorption at the LIII edge measured in the same temperature range, give direct evidence of the average samarium valence change which goes from 2.60 at 300 K to 2.53 at 4.2 K. Previous work claimed that the Sm2+ : Sm3+ ratio was temperature independent in SmB6. As for SmB6 a samarium valence change has been observed with decreasing temperature below 300 K in the Sm0.75La0.25B6 solid solution. J. Physique 41 (1980) 1141-1145 OCTOBRE 1980, Classification Physics Abstracts 78 . 70D Samarium hexaboride 5mB6 has attracted much experimental and theoretical attention in the last few years. SmB6 is a homogeneous mixed valent compound in which the Sm2+ : Sm3+ ratio at room temperature has been estimated to be about 4 : 6 from magnetic susceptibility, Mossbauer resonance measurements, LIII X-ray absorption, and X-ray photoelectron spectroscopy (XPS) [1-10]. Furthermore previous Mossbauer resonance and X-ray absorption experiments performed respectively down to l.l K and 150 K had not indicated significant variations of the SM2, : Sm 3 + ratio [3, 4, 5]. On the other hand, the lattice-parameter measurements carried out previously by us between 300 K and 4.2 K suggested a samarium valence change in SmB6 [5, 11]. In the dynamic (homogeneous) mixed valence state, each rare earth ion can be viewed as fluctuating between two configurations with the 4f shell occupation number differing by one. The charge fluctuation time ip is such that fast measurements like X-ray absorption or XPS detect both configurations separately, while slow measurements such as Mossbauer isomer shift detect only an average time, in the homogeneous mixed valent compounds. In order to determine quantitatively the samarium valence change in SmB6 as a function of temperature, X-ray absorption measurements have been carried out. The results obtained by direct observation will be compared with those deduced from lattice parameter temperature dependence. 1. Experimental. SmB6 has been prepared according to the reaction Density measurements, X-ray and chemical analysis indicate an atomic ratio B/Sm = 6. The cubic lattice parameter temperature dependence of SmB6 and SMO.7.5LaO.2.5B6 have been studied between 300 K and 4.2 K with a X-ray powder diffractometer using a monochromatic CoKa X-ray radiation [ 13]. The X-ray absorption experiments have been performed at the French synchrotron in Orsay (LURE). The X-ray beam is emitted by 1.72 GeV electrons in the ring D.C.I. The radiation was monochromatized with the help of a (220) silicon crystal. The (*) Department of Physics, Tohoku University, Sendai, Japan. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198000410100114100