Optical study of Te8 ring clusters: comparison with density functional theory and a step towards materials design using nanoporous zeolite space

Abstract

The Te₈ ring molecule (cluster) is poorly investigated due to the lack of experimental data. Here, we report an experimental and theoretical study of a regular array of oriented Te₈ rings formed in the ∼1.14 nm diameter cavities of zeolite LTA, which are arranged in a cubic lattice with a spacing of ∼1.2 nm. Single crystals of LTA with encapsulated tellurium (LTA-Te) were studied using Raman spectroscopy (RS) and optical absorption spectroscopy (OAS). The experimental LTA-Te spectra were found to be in agreement with those calculated using density functional theory (PBE0 hybrid functional and def2-TZVP basis sets) for the crown-shaped Te₈ ring molecule with D_4d symmetry. Using polarization–orientation RS, we show that the Te₈ rings are oriented by their major axes along the 4-fold axes of cubic LTA. We also show that the site symmetry of Te₈ in LTA-Te is lower than D_4d. Te₈ bond-bending modes are well described in the harmonic approximation, while bond-stretching modes are mixed due to the reduced ring symmetry and, probably, anharmonicity. Importantly, OAS data of LTA-Te display dependence on the Te₈ concentration, implying the interaction of the rings from neighbouring LTA cavities with the generation of the valence and conduction electron bands of such a cluster crystal.