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Phase transition and conduction mechanism in the ionic conductor Cu2HgI4

The ionic conductor copper(I)-tetraiodomercurate(II) has been known for several decades due to its near-room temperature insulator-conductor transition and striking colour change at the transition temperature [1]. However, the transition details and conduction mechanism have been debated [2]. We present the first combined NMR and conductivity study of this material, revealing extensive pretransition fluctuations and suggesting a new conduction mechanism in the conductive phase. 63Cu spin-lattice relaxation times show an anomalous decrease starting  50K below the transition temperature and a sharp drop of two orders of magnitude at the transition, accompanied by substantial line broadening ; careful relaxation measurements indicate that a fast process is already present several K below the transition. Pretransitional disordering also influences the 63Cu chemical shift, which shows power-law behaviour close to the transition. The 199Hg line is, however, significantly narrowed in the conducting phase, unexpectedly implying that the large and heavy Hg2+ diffuses through the material contributing to the ionic conduction process. This was confirmed by spin-spin relaxation measurements on 199Hg in a constant field gradient, which produced a faster-than-exponential decay function characteristic for spin diffusion. This work has been done using the equipment purchased within FP7 project no. 229390 SOLeNeMaR. [1] J. A. A. Ketelaar, Z. Kristallogr. 87, 436 (1934) [2] S. Hull, D. A. Keen, J. Phys.: Cond. Matt. 12, 3751-3765 (2000)

ionic conductors; nuclear magnetic resonance; nonlinear conductivity

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