Non-centrosymmetric Y3FeGaSe7 and Y3Mn0·5SiSe7 with extremely low thermal conductivities

Two novel heterometallic selenides, Y₃FeGaSe₇ and Y₃Mn_0·5SiSe₇, have been successfully synthesized by high-temperature elemental reactions. The non-centrosymmetric structures of the selenides with the P6₃ space group were solved using the X-ray diffraction (single crystal) method. Although both compounds are isostructural, the Y₃FeGaSe₇ structure is stoichiometric, while the Y₃Mn_0·5SiSe₇ structure has a half-occupied Mn site, making it non-stoichiometric. Each structure has six independent crystallographic sites: one Y site, one Fe/Mn site, one Ga/Si site, and three Se sites. The YSe₆, Fe/MnSe₆, and Ga/SiSe₄ units are the primary motifs of title selenides that are connected to form three-dimensional (3D) frameworks. The Y₃FeGaSe₇ and Y₃Mn_0·5SiSe₇ are semiconductors that agree with the theoretical electronic structure studies. The experimental direct optical bandgap energies are 1.1(1) eV and 1.3(1) eV for the Fe and Mn-containing phases, respectively. The electrical resistivity (ρ) studies show an exponential decrease of ρ values at higher temperatures as expected for semiconducting samples. The Y₃FeGaSe₇ shows high positive values of the Seebeck coefficient with a maximum of 294.4 μVK⁻¹ at 773 K. Both selenides exhibit extremely low values of thermal conductivity at 773 K: ∼0.40 Wm⁻¹K⁻¹ (Y₃FeGaSe₇) and 0.43 Wm⁻¹K⁻¹ (Y₃Mn_0·5SiSe₇). The magnetic studies confirm a high spin divalent state of Mn in the Y₃Mn_0·5SiSe₇ phase. No long-range magnetic ordering was found down to 2 K from the magnetic susceptibility studies of the polycrystalline Y₃Mn_0·5SiSe₇. The ab-initio DFT calculations suggest that the strongest bonding exists between the Si/Ga and Se atoms as compared to transition metals and Se atoms in the title selenide structures.