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, Ag 3 AsS 3 , has been studied in darkness above 300 K and compared with its natural counterpart pyrargyrite, Ag 3 SbS 3. The behavior of proustite is characterized by silver ion mobility within the structure. Proustite and ͑to a lesser extent͒ pyrargyrite were investigated as a function of temperature by x-ray and neutron powder diffraction, dielectric spectroscopy, and dynamic mechanical analysis. At 305 K ͑280 K for pyrargyrite͒ proustite undergoes a second-order phase transition, exhibiting a positive nonsymmetry breaking spontaneous strain of the unit cell, with thermal expansion along ͓001͔ 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 T c ͑305 K͒ with an activation energy of 0.42 eV ͑0.40 eV for pyrargarite͒. 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 pyrargyrite͒ 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.

On features of crystal structure of semiconductor-ferroelectric Ag3AsS3

“…At the same time, there was observed a weak change in the slope of curve a(T) at the temperatures T = 170…200 K on both figures, so the slope angle is smaller at T > 200 K than T < 170 K. It should be noted that dependences obtained during heating almost do not differ from the above. It should be noted that in the paper [1], the parameters a and c were determined with poor experimental accuracy, so the changing of angle slope c(T) was not observed within the temperature range 170 to 220 K. It is important that the character of the dependence c(T) is practically the same for both samples in the dark mode and under laser irradiation. However, there observed was an important difference between them for temperatures Т < 170 K. Namely, parameter c monotonically increases with temperature decreasing for samples within the temperature range 100 to 170 K in the dark.…”

“…Because of these properties, proustite belongs to promising materials of nonlinear optics. That is why its physical properties are actively investigated within the temperature range 4.2 to 600 K [1,2]. The crystals of proustite belong to the noncentro-symmetrical class of trigonal symmetry at room temperature (space group 6 3v C ) [2][3][4].…”

Impedance spectroscopy of synthetic proustite at high pressures

“…The unique physical properties of crystalline proustite (Ag 3 AsS 3 ) (e.g., its ionic conductivity, varia tions in heat capacity at low temperatures, phase tran sitions, optical and acoustic characteristics, and ferro electric properties) allow us to apply it in acoustoopti cal, optoelectrical, electroacoustic, and other devices [1][2][3][4]. The external effect on proustite properties, i.e., the material functioning with variation in external parameters, is of theoretical and practical interest.…”

Photoelectrochemical Study of Pyrargyrite in Acid Media

“…We propose that this material is of interest for water splitting because the conduction band is aligned for H + reduction to H 2 . Pyrargyrite is a naturally occurring silver ore. Its crystal structure has been studied from mined samples; 12 natural crystals present high conductivity due to Ag ion mobility 13 with relatively low activation energy (ca. 0.5 eV).…”

“…0.5 eV). 12,13 This is one example of simple Ag-containing sulfosalts 12 that have received increasing attention because of their ionic conductive properties. 14 Pyrargyrite synthesized by a solid state route and its solid solutions of intermediate compositions covering Ag 2 S, Sb 2 S 3 and Ag 3 SbS 3 were studied electrochemically in aqueous cyanide solutions in the context of Ag extraction from Ag 3 SbS 3 and Ag 3 AsS 3 .…”