Structural and NMR properties of niobium dichalcogenides intercalated with post transition metals

The intercalation phase of the transition-metal dichalcogenide Nb${\mathrm{Se}}_2$, intercalated with Ga, In, Tl, Sn, and Pb, as well as Nb${\mathrm{S}}_2$, intercalated with In, have been prepared and studied by a variety of techniques. For most intercalates, single-intercalation-phase samples of the formula $M_{\frac{2}{3}}\mathrm{Nb}{X}_2$ were obtained, where $M$ is the intercalate and $X$ a chalcogen. The changes in the lattice parameters occurring upon intercalation have been determined from x-ray powder patterns. No superconducting transition was observed in any of the intercalated samples above 1.5 K. The microscopic properties were studied from 1.5 to 4.2 K using nuclear-magnetic-resonance (NMR) techniques, with the prime emphasis on the determination of the electric field gradients (EFG) at the Nb sites. For each intercalated sample the EFG was reduced by (35-60)% from that found in Nb${\mathrm{S}}_2$ and Nb${\mathrm{Se}}_2$. For the ${\mathrm{Tl}}_{\frac{2}{3}}\mathrm{Nb}{\mathrm{Se}}_2$ sample, the Nb line shape was fit by a single EFG while for all other intercalates at least two EFGs were needed to fit the Nb line shape, indicating two sites with inequivalent NMR properties. This change in the EFG at the Nb sites upon intercalation is attributed to charge transfer from the intercalated atoms into the conduction band of the layered compound. Knight-shift and spin-lattice relaxation-time measurements on the intercalate resonance indicate conduction-band behavior at the intercalate site but with a low density of states at the Fermi energy. The Sn Mössbauer line shape was studied in ${\mathrm{Sn}}_{\frac{2}{3}}\mathrm{Nb}{\mathrm{Se}}_2$. Only a single resonance was observed indicating a lack of any large quadrupole splitting as opposed to the two-line spectrum which has been observed for Sn in ${\mathrm{Sn}}_1$Ta${\mathrm{S}}_2$.