High-temperature phase transitions, dielectric relaxation, and ionic mobility of proustite, Ag₃AsS₃, and pyrargyrite, Ag₃SbS₃

Abstract

The nature of phase transitions in natural and synthetic proustite, Ag₃AsS₃, has been studied in darkness above 300 K and compared with its natural counterpart pyrargyrite, Ag₃SbS₃. The behavior of proustite is characterized by silver ion mobility within the structure. Proustite and (to a lesser extent) pyrargyrite were investigated as a function of temperature by x-ray and neutronpowderdiffraction, dielectric spectroscopy, and dynamic mechanical analysis. At 305 K (280 K for pyrargyrite) proustite undergoes a second-order phase transition, exhibiting a positive nonsymmetry breaking spontaneous strain of the unit cell, with thermal expansion along [001] changing from negative to positive. This strain results from the onset of thermally induced hopping of silver ions, as revealed by impedance spectroscopy. It may be described as an almost undamped Debye oscillator, which is not present below Tc (305 K) with an activation energy of 0.42 eV (0.40 eV for pyrargarite). Around 420 K the high-frequency conductivity of proustite begins to increase, accompanied by elastic softening. At this temperature increasing random disorder of silver within possible unoccupied sites in the structure leads to increased electrical conductivity and destabilizes the material. When almost all silver ions are disordered into a so-called “molten sublattice,” a transition to fast ion conduction (at 540 K in proustite and 490 K in pyrargyrite) is reached. The additional component to the thermal expansion disappears and a linear negative spontaneous strain suggests a second-order phase transition. Above the transition silver seems to be maximally disordered, the structure itself is weakened and the sample starts to decompose. Show more