Unusual increase of the Kondo effect by hydrogenation: case of the ternary silicide CeNiSi

The ternary silicide CeNiSi, which crystallises in the tetragonal LaPtSi-type structure (a = 4.070(2) Angstrom, c = 14.030(5) Angstrom), absorbs hydrogen at P(H-2) = 2 MPa and T = 393 K to form the stable hydride CeNiSiH0.8(1). The hydrogenation induces (i) an anisotropic variation of the unit cell parameters (a = 4.040(1) Angstrom and c = 14.253(4) Angstrom) without significant increase of the unit cell volume (+0.1%) and (ii) a strong increase of the Kondo interaction in comparison with the initial ternary silicide. This last evolution is observed for the first time during insertion of H-atoms into CeNiX (X = np(1) or np(2) element) compounds. X-ray diffraction analysis, magnetic susceptibility and Ce L-III-edge X-ray absorption spectroscopy measurements performed on CeNiSi and CeNiSiH0.8(1) are reported and compared with those obtained on the isostructural RENiSiHi(1.0(1)) hydrides with RE = La and Nd. (C) 2004 Elsevier B.V. All rights reserved.