[en] 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 Fm3-bar c 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. (author)