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; TK⪢300 K and TK≅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) Å and c=4.258(5) Å. 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