Electrical transport of chalcogenides from Ag–As–Se–Te system: revealing the role of silver doping

This article deliberates the analysis of some optical, electrical, and structural parameters of Ag_x(As_0.33Se_0.335Te_0.335)_100−x glasses synthesized from cascade heating method. Optical measurements have been performed to determine optical band gap using Tauc’s plots and to discuss the obtained values in the context of Ag doping influence. AC conductivity measurements were made over the frequency domain of 30 Hz to 20 kHz with the temperature interval from 293 to 353 K. The optical band gap decreases from ~ 0.65 eV (for x = 0) to ~ 0.55 eV at 7 at.% Ag, confirming band-gap narrowing due to Ag-induced defect states. However, at higher Ag contents (9–13%), the band-gap trend deviates, with E_g saturating around ~ 0.5 eV. This indicates significant structural modifications at high doping, while AC conductivity follows the correlated barrier hopping model. Frequency response of AC conductivity was investigated to identify the credible conductivity mechanism and to explore the potential of different roles of Ag dopant in amorphous matrix depending on various concentrations. Activation energy exhibits a non-monotonic dependence on Ag concentration, decreasing for highly doped samples (x ≥ 7 at.%) and suggesting a transition in the dominant conduction mechanism. Impedance spectra were recorded aiming to provide more details about the conduction mechanism and to highlight the role of silver doping. The obtained results revealed single depressed semicircular Nyquist arcs for 0–5% Ag glasses, consistent with one predominant relaxation process, whereas glasses with ≥ 7% Ag exhibited an additional low-frequency tail and second arc. The EIS spectrum analysis showed requirement of a second R||CPE element branch for the Ag-rich compositions, reflecting the emergence of Ag⁺ ion diffusion pathways at high doping. The use of more complex equivalent circuits in description of impedance response of the samples with x ≥ 7 at.% corresponds with assumption of Ag-rich microdomains formation. The EDS analysis was conducted to explore the spatial distribution of silver across the sample surfaces and confirmed a transition from heterogeneous to uniform Ag distribution with increase in silver content (x ≥ 7 at.%).