We report magnetization, specific heat, and resistivity measurements of the metal-rich hydride halides $\mathrm{Y}\mathrm{I}{\mathrm{H}}_x$ and $\mathrm{Gd}\mathrm{I}{\mathrm{H}}_x(0.6<x<1.0)$ as a function of temperature and applied magnetic field. A strong dependence of the electrical and magnetic properties on the hydrogen content $x$ is observed. Isostructural nonmagnetic samples $\mathrm{Y}\mathrm{I}{\mathrm{H}}_x$ show metallic behavior at room temperature with increased resistivity values as $x$ approaches its lower limit 0.61(3). Upon cooling, the resistivity passes through a smooth minimum, suggesting a transition from an itinerant to a localized electronic state at low temperatures. The presence of magnetic Gd ions leads to significant changes of the electrical transport properties and anomalous magnetic behavior. By reducing the hydrogen content in $\mathrm{Gd}\mathrm{I}{\mathrm{H}}_x$ a metal-insulator transition occurs at a critical concentration $x=0.78\left(2\right)$. Magnetization and specific heat measurements indicate competing ferromagnetic and antiferromagnetic interactions which result in a partial antiferromagnetic ordering below $T_{\mathrm{N}}$, varying from $50\mathrm{K}$ for $x=0.86$ to $25\mathrm{K}$ for $x=0.69$, respectively. At lower temperatures, the system $\mathrm{Gd}\mathrm{I}{\mathrm{H}}_x(x⩽0.78)$ exhibits characteristics of a spin glass. For the semiconducting samples $(x⩽0.78)$, a colossal negative magnetoresistance as large as 3 orders of magnitude for $x\approx 0.7$ is observed at $2\mathrm{K}$. The metallic $\mathrm{Gd}\mathrm{I}{\mathrm{H}}_{0.86}$ phase exhibits a complex magnetoresistance which is positive around the Néel temperature and becomes negative at temperatures well below $T_{\mathrm{N}}$. The observed correlations can be described in terms of a mobility edge scenario and formation of bound magnetic polarons. To gain a better insight into the electronic structure of $\mathrm{Ln}\mathrm{I}\mathrm{H}$ first principles tight-binding linear muffin-tin orbital atomic-sphere-approximation band structure calculations have been performed.

• Received 15 August 2005

DOI:https://doi.org/10.1103/PhysRevB.73.035114