We applied magnesiothermic reduction to silica glass substrates at various conditions including solid state or solid-Mg liquid reaction, and solid-Mg vapor reaction. Magnesium silicide with highly oriented to the 110 direction against the substrate surface was obtained in the solid state reaction at temperatures from 600 • C to 700 • C using Mg grains while Si crystallites were obtained in the reaction with Mg film deposited on the glass substrate at 560 • C. On the other hand, in the reduction with Mg vapor at 575 • C, brown and amorphous layer was formed on the surface of the silica glass substrate. The layer was transparent in the visible and near-infrared regions, and showed an interference pattern in the transmission spectra, indicating the homogeneity of the layer. The thickness and refractive index were estimated as 770 nm and 1.94, respectively. As the reaction with Mg vapor proceeded further, Mg 2 Si and MgO crystallites were formed. Oxidation states and their depth profiles of silicon atoms in the layers were investigated by X-ray photoelectron spectroscopy. The silicon atoms in the brown and amorphous layer existed in intermediate oxidation states, −2 and +3. The reaction proceeding, the formal charge of the silicon atoms varied to −4 corresponding to Mg 2 Si and +2.

Magnesiothermic reduction has received attention as a method to produce silicon retaining the morphology of the silicon dioxides that serve as the starting materials of the reduction. We performed the reduction of silica glass substrates with magnesium deposited on them under various conditions, including changing the reaction temperature from 500 to 700°C, changing the reaction time from 15 to 420 minutes, and changing the thickness of the deposited magnesium film from 6.6 to 12.1 μm. Crystalline products and their amounts were investigated using X‐ray diffraction. In these reactions, the silica glass substrates were reduced to silicon and/or magnesium silicide depending on the conditions. Under the conditions where the reaction temperature is higher, the reaction time is longer, and the thickness of the magnesium is thinner, silicon was generated more predominantly than magnesium silicide, and vice versa. Under the intermediate conditions, both crystalline silicon and magnesium silicide were generated. Furthermore, it was revealed that the magnesium silicide was rapidly generated within 15 minutes at temperatures higher than 600°C. Then, as the reaction continued, the amount of magnesium silicide gradually decreased, and that of silicon increased. From these results, a two‐step reaction model was proposed: At first, silicon dioxide is reduced to magnesium silicide with the complete consumption of the deposited magnesium; and subsequently, the magnesium silicide reacts with unreacted silicon dioxide and silicon in the intermediate oxidation states to produce silicon. Based on this model, silicon and magnesium silicide were selectively prepared under well‐controlled conditions.