The ternary gallide CeNiGa: Polymorphism and hydrogen absorption

The ternary gallide CeNiGa presents a crystallographic transformation with temperature. The crystal structure of the high-temperature form (HTF), determined for the first time by X-ray diffraction on a single crystal, is orthorhombic TiNiSi-type, whereas the low-temperature form (LTF) adopts the hexagonal ZrNiAl-type. Electrical resistivity and magnetization measurements reveal that both (LTF) and (HTF) CeNiGa are classified as intermediate valence compounds, but their Kondo temperatures TK are strongly different; T-K much greater than300K and T-K congruent to 95(5) K for (LTF) and (HTF), respectively. Both forms react with hydrogen at room temperature and form the hydride CeNiGaH1.1(1) which crystallizes in the hexagonal AlB2-type with lattice parameters a = 4.239(4) Angstrom, and c = 4.258(5) Angstrom. Hydrogenation also induces a valence transition for cerium from the intermediate valence state (CeNiGa) to a purely trivalent state (CeNiGaH1.1(1)). This behavior is correlated to an increase of the unit cell volume after hydrogenation and is compared to that observed previously for CeNiAlH1.93. (C) 2002 Elsevier Science (USA).