The superionic electrical conductor Ag4SSe is reported to undergo an unusual first-order structural phase transition at ∼260K with concomitant anomalous electronic properties. Single-crystal x-ray diffraction reveals that the crystal undergoes a structural transition from monoclinic α-Ag4SSe to an orthorhombic crystal structure below 260 K. The new lowerature phase is denoted as δ-Ag4SSe, and it is found to be isostructural to α-Ag2Se. The diamagnetic magnetic susceptibility is of larger magnitude in the lowerature δ phase than in α-Ag4SSe. 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 δ-Ag4SSe to be metallic with a much lower resistivity than semiconducting α-Ag4SSe. As a consequence, the α-δ structural phase transition of Ag4SSe causes an unusual Fermi surface reconstruction with an increased mobility of charge carriers and unexpected metallic behavior in δ-Ag4SSe. 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.
Bibliographical noteFunding Information:
Single crystals of Ag 4 SSe have been synthesized by Kerstin Küspert at the Laboratory of Crystallography in Bayreuth. DTA measurements were performed by A. Awasti (UGC-DAE Centre at Indore, India). We thank Anil Kumar and D. Buddhikot (TIFR, Mumbai, India) for their help with the measurements of temperature-dependent physical properties. We thank Franz Fischer for assistance with the dc resistivity measurements in Bayreuth. Sponsorship has been provided by the Alexander-von-Humboldt foundation in the framework of the Research Group Linkage Program funded by the German Federal Ministry of Education and Research. Part of this research has been funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - 265092781.
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