Phase transitions in jalpaite, Ag3CuS2

Ag 3 CuS 2 is comprehensively studied by applying synchrotron and neutron powder diffraction as well as thermal analysis in the range from 2 K up to the melting point around 960 K.The unique sequence of the reversible phase transitions PbasedFm 3 m was detected prior to the sample's melting.The transitions at 110, 387 and 483-549 K are found to be of 1 st order, whereas the transition at 250 K is a 2 nd order one.The major change in the structure of jalpaite resulting from theI4 1 /a phase transition is a modification in the coordination geometry of the silver atoms.The large degree of structural disorder is stated for Fm 3 m and Im 3 m structured Ag 3 CuS 2 which correlates with the high ionic conductivity within both cubic polymorphs.The thermal expansion of jalpaite shows some unusual features, i.e., a negative expansion along the c-direction in the I4 1 /amd phase, a non-linear expansion within the Im 3 m polymorph and an expansion increase upon entering the mixed Im 3 m+Fm 3 m region.Low temperature specific heat data confirm 1 st and 2 nd order anomalies at 110 K and 250 K, respectively, and illustrate a pronounced non-Debye-like behaviour of jalpaite.