Neutron Diffraction and Isotropic Volume Expansion Caused by Deuterium Absorption into La(Fe0.88Si0.12)13

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The crystal structures and magnetic properties of La(Fe 0.88 Si 0.12 ) 13 D 1.5 and La(Fe 0.88 Si 0.12 ) 13 H 1.6 have been investigated. The powder neutron diffraction declared that the compound La(Fe 0.88 Si 0.12 ) 13 D 1.5 has a cubic NaZn 13 -type structure of the F m 3 c space group in analogy with La(Fe 0.88 Si 0.12 ) 13 . The deuterium atoms in the compound La(Fe 0.88 Si 0.12 ) 13 D 1.5 occupy the interstitial 24 d site surrounded by two La atoms and four icosahedral clusters. The x-ray diffraction data of La(Fe 0.88 Si 0.12 ) 13 H 1.6 are also consistently explained by the same site of hydrogen atom. The deuterium or hydrogen absorption brings out a notable increase of the Curie temperature T C . The neutron diffraction of the compound La(Fe 0.88 Si 0.12 ) 13 D 1.5 shows that the magnetic structure is ferromagnetic and the average magnetic moment per Fe is about 2.2 µ B , almost the same as that of La(Fe 0.88 Si 0.12 ) 13 . Such behaviors can be explained as characteristic magnetovolume effects in itinerant-electron metamagnets. That is, when the hydrogen or deuterium atom at the 24 d site causes the isotropic volume expansion, the magnetic free energy curve with a double minimum structure renormalized by spin fluctuations induces a marked increase of T C without a pronounced increase of the magnetic moment.