From intermediate valence to magnetic behavior without long-range order by hydrogenation of the ternary gallide CeNiGa

We have studied both the crystal chemistry and magnetic, transport, and thermal properties of the hydride CeNiGaH1.1(1). This compound crystallizes in the hexagonal AlB2-type structure with a random distribution of nickel and gallium atoms on the B site, which has an important influence upon the macroscopic properties. Its thermoelectric power versus temperature indicates that cerium is in a trivalent state. The electrical resistivity displays two minima, which could be expected for the Kondo-type interactions in the presence of crystal field effects. Specific heat measurements up to 300K allow us to determine the splitting energies Δ1=100K and Δ2=159K. A broad maximum is observed around 4K in the specific heat in the low temperature region. This maximum and its evolution with the applied magnetic field, are discussed in the framework of the existing theories, which point toward the existence of short-range magnetic correlations and spin glasslike freezing below 1.8K. This study reveals: (i) that the hydrogenation of the intermediate valence gallide CeNiGa induces a valence transition for cerium which is purely trivalent in the hydride and (ii) the absence above 1.8K of long-range magnetic ordering resulting from structural disorder around Ce atoms. © 2005 The American Physical Society.