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Selective Imaging of the Cux-sublattice Structure in the Cu2-xSe Crystal

The Cu2-xSe crystal exhibits superionic conductivity in the high temperature (HT) phase (T > 415K ; 0 <x< 0.2). The structure is cubic, but complex as being composed of two subsystems: (i) &#8220; Se-Cu immobile cage&#8221; (ZnS-type structure ; a = 0.581 nm), and (ii) &#8220; Cu-mobile subsystem&#8221; consisting of Cu cations distributed along the open &#8220; mobility paths&#8221; that connect tetragonal, via trigonal to octahedral interstitial sites in the F43m cell. In the room temperature (RT) phase the superionic conductivity disappears due to closing of these &#8220; mobility paths&#8221; and subsequent condensation of Cu cations at displaced sites. The ordering of this partial subsystem is revealed as pairing of overall layer stacking, rhombohedral distortion of the cage, superstructure formation on monoclinic lattice.

Modulated superstructure; sublattice; HREM Selective Imaging

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