Phase transitions in jalpaite, Ag3CuS2

Ag3CuS2 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 P- was detected prior to the sample's melting. The transitions at 110, 387 and 483–549 K are found to be of first order, whereas the transition at 250 K is a second-order one. The major change in the structure of jalpaite resulting from the phase transition is a modification in the coordination geometry of the silver atoms. A large degree of structural disorder is stated for - and -structured Ag3CuS2, 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 I41/amd phase, a nonlinear expansion within the polymorph and an expansion increase upon entering the mixed region. Low-temperature specific heat data confirm first- and second-order anomalies at 110 and 250 K, respectively, and illustrate a pronounced non-Debye-like behaviour of jalpaite.