Influence of interstitial and substitutional atoms on the crystal structure of La(FeSi)13

The influence of interstitial hydrogen or carbon on the crystal structure of LaFe11.5Si1.5Hδ (δ = 0, 1.2, and 2) and LaFe11.5Si1.5Cδ (δ = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) has been investigated based on the Rietveld analyses of powder X-ray diffraction spectra. Effects of Ce substitution for La are also studied for comparison. The incorporation of interstitial atoms causes a lattice expansion of the compounds while leaves the structural symmetry unchanged. Accompanying the lattice expansion, Fe–Fe bond exhibits a concomitant variation. Four of the five Fe–Fe bonds show a tendency to expansion. The largest elongation occurs for the shortest inter-cluster bond, and the relative change is as large as ∼2.53% as δ increases from 0 to 2 for LaFe11.5Si1.5Hδ. In contrast, the longest Fe–Fe bond shrinks considerably (−0.97%). Similarly effects on Fe–Fe bonds are produced by the paramagnetic to ferromagnetic transition, though the bond variation is smaller. Increase in Ce content produces, fascinatingly, essentially the same effects as the decrease of interstitial content, though Ce occupies different crystallographic sites than those occupied by interstitial atoms. Influence of interstitial atoms on magnetic behaviors may be dominated by the change of the shortest Fe–Fe bond.