M. Kassem

Mercury thioarsenate glasses: a hybrid chain/pyramidal network

Very little is known about mercury chalcogenide glasses. Using Raman spectroscopy and DFT modelling, we show that the (HgS)x(As2S3)1−x glasses, 0.0 ≤ x ≤ 0.5, form a hybrid Hg–S chain/As–S pyramidal network, highly unusual for metal chalcogenide glasses. This network is evidenced by Hg–S stretching modes at 300 and 370 cm−1 and an As–S spectral envelope centred at 340 cm−1. The decreasing glass transition temperature is consistent with a gradual substitution of more rigid corner-sharing CS-AsS3/2 pyramids by flexible (HgS2/2)n chain fragments. Nevertheless we cannot exclude completely the presence of a small fraction of HgS4/4 tetrahedral units. A non-monotonic change in electronic transport properties and metacinnabar β-HgS traces detected using neutron diffraction in large x = 0.5 samples support the dual structural role of mercury.

Ionic-to-Electronic Conductivity Crossover in CdTe–AgI–As2Te3 Glasses: An 110mAg Tracer Diffusion Study

Conductivity isotherms of (CdTe) x(AgI)0.5- x/2(As2Te3)0.5- x/2 glasses (0.0 ≤ x ≤ 0.15) reveal a nonmonotonic behavior with increasing CdTe content reminiscent of mixed cation effect in oxide and chalcogenide glasses. Nevertheless, the apparent similarity appears to be partly incorrect. Using 110mAg tracer diffusion measurements, we show that semiconducting CdTe additions produce a dual effect: (i) decreasing the Ag+ ion transport by a factor of ≈200 with a simultaneous increase of the diffusion activation energy and (ii) increasing the electronic conductivity by 1.5 orders of magnitude. Consequently, the conductivity minimum at x = 0.05 reflects an ionic-to-electronic transport crossover; the silver-ion transport number decreases by 3 orders of magnitude with increasing x.

Mercury Sulfide Dimorphism in Thioarsenate Glasses

Crystalline mercury sulfide exists in two drastically different polymorphic forms in different domains of the P,T-diagram: red chain-like insulator α-HgS, stable below 344 °C, and black tetrahedral narrow-band semiconductor β-HgS, stable at higher temperatures. Using pulsed neutron and high-energy X-ray diffraction, we show that these two mercury bonding patterns are present simultaneously in mercury thioarsenate glasses HgS-As2S3. The population and interconnectivity of chain-like and tetrahedral dimorphous forms determine both the structural features and fundamental glass properties (thermal, electronic, etc.). DFT simulations of mercury species and RMC modeling of high-resolution diffraction data provide additional details on local Hg environment and connectivity implying the (HgS2/2)m oligomeric chains (1 ≤ m ≤ 6) are acting as a network former while the HgS4/4-related mixed agglomerated units behave as a modifier.