We studied the synthesis reaction of Mg ₂NiH ₄ hydride films when a nickel plate and magnesium hydride powder interacted in the presence of hydrogen at 450 °C. The hydrogen pressure (>6 MPa) was higher than the synthesis equilibrium pressure for both Mg ₂NiH ₄ and MgH ₂ hydrides. Film examination with scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and thermal desorption spectroscopy methods revealed that a dense polycrystalline film of Mg ₂NiH ₄ hydride grows on a thin (0.9 μm) underlayer of intermetallic alloy MgNi ₂. The synthesis time variation made it possible to create Mg ₂NiH ₄ films with a width of 0.2 to 4 μm. Based on analysis of the growth kinetics, the crystal structure of the material, and energies of chemical bonds between atoms, we show that the interface between MgNi ₂ and Mg ₂NiH ₄ is the most probable location of the growth reaction. We have obtained cathode luminescence spectra of the Mg ₂NiH ₄ hydride film. Mg ₂NiH ₄ is found to be an indirect gap semiconductor with a gap value of ∼1.6 eV. However, luminescence with quanta energy higher than the band gap is registered. We associate this fact with Van Hove singularities in the Mg ₂NiH ₄ density of states.