Neutron diffraction and isotropic volume expansion caused by deuterium absorption into La(Fe0.88Si0.12)13

The crystal structures and magnetic properties of La(Fe0.88Si0.12)(13)D-1.5 and La(Fe0.88Si0.12)(13)H-1.6 have been investigated. The powder neutron diffraction declared that the compound La(Fe0.88Si0.12)(13)D-1.5 has a cubic NaZn13-type structure of the Fm (3) over barc space group in analogy with La(Fe0.88Si0.12)(13). The deuterium atoms in the compound La(Fe0.88Si0.12)(13)D-1.5 occupy the interstitial 24d site surrounded by two La atoms and four icosahedral clusters. The x-ray diffraction data of La(Fe0.88Si0.12)(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(Fe0.88Si0.12)(13)D-1.5 shows that the magnetic structure is ferromagnetic and the average magnetic moment per Fe is about 2.2 mu(B), almost the same as that of La(Fe0.88Si0.12)(13). Such behaviors can be explained as characteristic magnetovolume effects in itinerant-electron metamagnets. That is, when the hydrogen or deuterium atom at the 24d 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 TC without a pronounced increase of the magnetic moment.