The observation of topological insulating (TI) properties in A₂X₃ binary compounds with layered tetradymite structure [1,2] has triggered the exploration of ternary BA₂X₄ compounds based on these binary compounds [3]. In fact, recent electronic structure calculations have predicted many of them to be 3D-TIs, like rhombohedral SnSb₂Te₄ [4]. Moreover, the recent discovery of a new kind of chemical interaction, so-called metavalent bonding [5], which occurs in A₂X₃ binary compounds with layered tetradymite structure, has enhanced the investigation on ternary compounds composed by them.

Here, we present a joint experimental and theoretical study of the structural, vibrational and electrical properties of rhombohedral SnBi₂Te₄ [6] and SnSb₂Te₄ [7] at high pressure, which unveils the internal mechanisms developed during their compression. These properties are compared with their parent binary compounds (Bi₂Te₃ and SnTe in SnBi₂Te₄ and Sb₂Te₃ and SnTe in SnSb₂Te₄) and with related natural van der Waals materials.

In SnSb₂Te₄, the structural study reveals an isostructural phase transition occurring at 2.5 GPa, while a pressure-induced decomposition into their parent binary compounds is observed in both compounds above 7 GPa. These results are coherent with the high pressure electrical behavior and is supported by the analysis of their formation enthalpies. Furthermore, the analysis of the frequency of the Raman-active modes and the evolution under pressure of the ternary compounds reveal a strong correlation with the vibrational modes of its parent binary materials, leading to the detection of a Raman-inactive LO-mode of c‑SnTe. Thus, Raman spectroscopy of ternary compounds can be used as a useful tool to unveil Raman-inactive modes of their binary constituents.

Finally, we performed an electronic topology analysis to unveil the interactions given in both van der Waals heterostructures. The analysis of the electron localization function (ELF) reveal that in a single material (SnSb₂Te₄) we are able to distinguish three type of interactions: i) a van der Waals interaction between Te_external-Te_external atoms, which becomes progressively more ionic with compression; ii) a polar covalent bond between Sb-Te_external atoms, which remains unalterable under compression; and iii) a metavalent bonding between Sb-Te_internal and Sn-Te_internal atoms. The evaluation of the electronic densities and its Laplacians at the bond critical point of the last two bonds provides a new criterion to identify metavalent interactions when the material is very complex with coexistence of different kind of bonds.

References:

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5. M. Wuttig, V. L. Deringer, X. Gonze, C. Bichara et al. Advanced Materials 30, 1803777 (2018)
6. R. Vilaplana, J. A. Sans, F. J. Manjón, A. Andrada-Chacón, et al. Journal of Alloys and Compounds 685, 962 (2016)
7. J. A. Sans, R. Vilaplana, E. L. Da Silva, C. Popescu et al. Under revision.