LaNi5−xPtx: NMR investigation of structural and electronic properties

Pt-site Knight shifts $\kappa$, relaxation times, and crystallographic data have been obtained for the $\mathrm{La}{\mathrm{Ni}}_{5-x}{\mathrm{Pt}}_x$ ($0\le x\le 5$) system. There are two transition-metal sites: one, $A$, in a La layer and one, $B$, between La layers; different $\kappa \text{'}\mathrm{s}$ are observed for each site. The system is found to be completely miscible and NMR intensities and crystallographic results show that Pt prefers the $B$ site and that considerable ordering occurs for $x\lesssim 4$. The ${}_{}{}^{195}{}_{}{}^{}\mathrm{Pt}$ $\kappa \text{'}\mathrm{s}$ are ∼1% and ∼0% for the $A$ and $B$ sites, respectively, in La${\mathrm{Pt}}_5$; the $A$-site $\kappa$ is approximately 0 at La${\mathrm{Ni}}_4$Pt and the $B$-site $\kappa$ reaches -2% in La${\mathrm{Ni}}_{4.75}$ ${\mathrm{Pt}}_{0.25}$. The shifts, taken together with relaxation times, indicate that $s$-band effects dominate, particularly for the $A$ site, in La${\mathrm{Pt}}_5$ and that the Pt atoms on both sites become more transition-metal-like with increasing $x$. Exchange enhancement is shown to be an essential factor at the Ni-rich end. Variations in chemical activity, as in hydride formation, are discussed in light of these electronic differences between La${\mathrm{Ni}}_5$ and La${\mathrm{Pt}}_5$.