Neutron diffraction and isotropic volume expansion caused by deuterium absorption into La(Fe_0:88Si_0:12)₁₃

The crystal structures and magnetic properties of La(Fe _0:88Si_0:12)₁₃D_1:5 and La(Fe _0:88Si_0:12)₁₃H_1:6 have been investigated. The powder neutron diffraction declared that the compound La(Fe_0:88Si_0:12)₁₃D_1:5 has a cubic NaZn₁₃-type structure of the Fm33c space group in analogy with La(Fe_0:88Si_0:12)₁₃. The deuterium atoms in the compound La(Fe_0:88Si_0:12)₁₃D_1:5 occupy the interstitial 24d site surrounded by two La atoms and four icosahedral clusters. The x-ray diffraction data of La(Fe_0:88Si _0:12)₁₃H_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_0:88Si_0:12) ₁₃D_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_0:88Si_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 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 T_C without a pronounced increase of the magnetic moment.