Low temperature phase separation in CeSi 1.86
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J. Pierre Laboratoire Louis Neel, CNRS 166X, 38402 Grenoble, France
O. Laborde CRTBT, CNRS 166X, 38402 Grenoble, France
J. L. Soubeyroux Institut Laue Langevin, 38402 Grenoble, France
J. Pelissier CENG, BP 85X, 38041 Grenoble, France (Received 23 March 1990; accepted 8 June 1990)
Samples of CeSii.86 which exhibit Kondo behavior are shown by neutron powder diffraction and transmission electron microscopy to consist of two closely related tetragonal phases. The primary phase is of the ThSi2 structure type with some vacancies in the silicon sublattice. The second phase presents an ordering of these vacancies. These two phases coexist at low temperature, but the abundance of the second phase increases with decreasing temperature. Neutron diffraction measurements and TEM experiments show that the phase separation occurs reversibly around 260 K, in close relation with an anomaly in the transport properties. The presence of a hysteresis indicates that we are dealing with a first order transition.
I. INTRODUCTION
Metal silicides have been extensively studied because of their applications in silicon based semiconductor technology.12 Among these compounds, some rare earth silicides are of special interest since they are characterized by the lowest Schottky barrier heights (: 0.3-0.4 eV) on n-type silicon, with potential applications as infrared detectors.3'4 These silicon-rich rare earth compounds with the general formula RSi* (x «S 2) crystallize in three different types of structure, ThSi2, GdSi2, and A1B2, which are closely related.5"7 The type of structure depends on the nature of the rare earth, the value of x, and the temperature. We have recently started a research program on this subject to get a better understanding of the relationship among these different parameters and their influence on the physical properties of these compounds as determined on bulk single crystals.7 As part of this work, we report in this paper the results obtained for CeSi* in the low temperature domain (4.2 K to room temperature). These compounds have been extensively studied, because of the occurrence of various anomalies associated with the intermediate valence or Kondo 2126
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 10, Oct 1990
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effect of Ce ions.8 16 At room temperature CeSi2 crystallizes in the tetragonal ThSi2 type of structure (S.G.: IAx/amd). The decrease of x from the value 2 leads first to the creation of silicon vacancies without modification of the structural symmetry. For x smaller than about 1.75, at room temperature, the compounds crystallize in the orthorhombic GdSi2 type of structure (S.G.: Imma). The limit of the homogeneity range is estimated to be about x = 1.6. For smaller x, samples are no longer single phase. The present study follows a previous work on the anisotropy of the magnetic and electrical properties of CeSi*,17 which gave us some indication that a "transition" occurs at low temperature in these compounds. Looking, for example, to the resistivity curve as functi
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