Unusual electronic properties of a low-temperature phase of Ag4SSe

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The superionic electrical conductor ${\mathrm{Ag}}_{4}\mathrm{SSe}$ is reported to undergo an unusual first-order structural phase transition at $\ensuremath{\sim}260\phantom{\rule{4pt}{0ex}}\mathrm{K}$ with concomitant anomalous electronic properties. Single-crystal x-ray diffraction reveals that the crystal undergoes a structural transition from monoclinic $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ag}}_{4}\mathrm{SSe}$ to an orthorhombic crystal structure below 260 K. The new low-temperature phase is denoted as $\ensuremath{\delta}\text{\ensuremath{-}}{\mathrm{Ag}}_{4}\mathrm{SSe}$, and it is found to be isostructural to $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ag}}_{2}\mathrm{Se}$. The diamagnetic magnetic susceptibility is of larger magnitude in the low-temperature $\ensuremath{\delta}$ phase than in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ag}}_{4}\mathrm{SSe}$. A diamagnetic susceptibility of larger magnitude is usually related to a lower density of states at the Fermi level which leads to a decrease in the Pauli paramagnetic susceptibility. However, we find $\ensuremath{\delta}\text{\ensuremath{-}}{\mathrm{Ag}}_{4}\mathrm{SSe}$ to be metallic with a much lower resistivity than semiconducting $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ag}}_{4}\mathrm{SSe}$. As a consequence, the $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\delta}$ structural phase transition of ${\mathrm{Ag}}_{4}\mathrm{SSe}$ causes an unusual Fermi surface reconstruction with an increased mobility of charge carriers and unexpected metallic behavior in $\ensuremath{\delta}\text{\ensuremath{-}}{\mathrm{Ag}}_{4}\mathrm{SSe}$. Band structure calculations substantiate such a claim. In addition, we also observe spontaneous voltage generation (SVG) at the phase transition, which has previously been observed only in a few magnetic materials.