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We investigated the carbon solubility into silica under high temperature condition. Mixtures of amorphous silica and graphite powder sealed in silica glass tubes were heated at 1300°C. The a lattice parameter and unit cell volume of α-cristobalite obtained are slightly increased compared with that heated without graphite. The c lattice parameter, on the other hand, almost unchanged. There is no clear dependency of the lattice parameter variations on carbon content mixed as the starting material. After re-heating the samples under atmospheric oxygen, the a lattice parameter and unit cell volume were reduced from the previous values. These changes indicate the possibility that carbon certainly incorporated into the α-cristobalite was oxidized with the atmospheric oxygen. The ab-initio calculation of disiloxane molecule showed that with carbon substitution for the bridging oxygen the Si-C bond distance increases whereas the Si-C-Si bond angle decreases compared with the Si-O bond distance and Si-O-Si bond angle. Consequently, the distance between Si atoms increases with the carbon substitution for the bridging oxygen. Since the expansion of Si-Si distance contributes twice as much to the a and b-axes than the c-axis in the unit cell of α-cristobalite, the ab-initio simulation result supports the observation that a lattice parameter increases with the carbon substitution relative to the c lattice parameter. The study strongly suggests that under reduced and high temperature conditions carbon is substituted not for silicon but for oxygen in α-cristobalite structure.

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