Raman spectroscopy and DFT modelling of Tl2S-GeS2 crystals and glasses

•Tl2S-GeS2 crystals and glasses were studied by Raman spectroscopy and DFT modelling.•Structural motifs in glasses are similar to Tl4Ge4S10, Tl4Ge2S6, and Tl4GeS4 crystals.•A structural difference between semiconducting and ion-conducting glasses is revealed.•Thallium poor semiconducting glasses consist of Ge4S10 adamantane-like units.•Thallium rich ion-conducting glasses contain ES-Ge2S6, CS-Ge2S7 and CS-Ge3S10 units. Thallium thiogermanate glasses and crystals were studied by Raman spectroscopy accompanied by DFT modelling. The experimental Raman spectra of the known crystalline compounds in the Tl2S-GeS2 system are consistent with the crystal structure and DFT modelling of the respective Tl-Ge-S clusters, representing structural motifs in c-Tl4Ge4S10, c-Tl4Ge2S6, and c-Tl4GeS4. The Raman spectra of the (Tl2S)x(GeS2)1-x glasses can be divided into two groups: (1) semiconducting glasses at x ≤ 0.3, and (2) ion-conducting vitreous alloys at x > 0.3. At x ≤ 0.3, the continuous g-GeS2 network is transformed into a fragmented structure consisting of adamantane-like units Ge4S10 held together by thallium species. In the ion-conducting composition domain at x > 0.3 takes place further fragmentation of tetrahedral tetramers Ge4S10 into edge-sharing ES-Ge2S6 dimers and small corner-sharing CS-oligomers, i.e., CS-Ge2S7 dimers or CS-Ge3S10 trimers.