Evolution of nontrivial Fermi surface features in the band structures of the homologous members Pb5Bi6Se14 and Pb5Bi12Se23

High-quality single crystals of ${(\mathrm{PbSe})}₅{({\mathrm{Bi}}₂{\mathrm{Se}}₃)}_3m$ were grown and analyzed by nuclear magnetic resonance (NMR) spectroscopy. We report on $⁷⁷\mathrm{Se}$ and $²⁰⁷\mathrm{Pb}$ NMR shifts and nuclear spin-lattice relaxation measurements in the naturally formed heterostructure homology ${(\mathrm{PbSe})}₅{({\mathrm{Bi}}₂{\mathrm{Se}}₃)}_3m$ with $m=1 ({\mathrm{Pb}}₅{\mathrm{Bi}}₆{\mathrm{Se}}₁₄)$ and $m=2 ({\mathrm{Pb}}₅{\mathrm{Bi}}₁₂{\mathrm{Se}}₂₃)$. A distinct site-specific contribution has been detected for both nuclei as a function of temperature, which reveals an electronic changeover from a semiconducting ${\mathrm{Pb}}₅{\mathrm{Bi}}₆{\mathrm{Se}}₁₄$ to a semimetalliclike ${\mathrm{Pb}}₅{\mathrm{Bi}}₁₂{\mathrm{Se}}₂₃$ system with nontrivial band structure features near the Fermi level. The temperature dependences of the relaxation rates are dominated by significant changes in the topology of energy dispersions accompanied with band edges and crossings in the region of the Fermi surface. These results, which interrogate nuclear spin interactions from selected atomic sites, clearly expose the effects of the added ${\mathrm{Bi}}₂{\mathrm{Se}}₃$ layer on the crystal and electronic structure of ${\mathrm{Pb}}₅{\mathrm{Bi}}₁₂{\mathrm{Se}}₂₃$. These findings provide direct microscopic insight into the unconventional and dual nature of the electronic structure of these homologous thermoelectric and topologically nontrivial compounds.