Crystal Structure of (NH4)4H2(SeO4)3 and (ND4)4D2(SeO4)3 below 180 K

The behavior of fullerenes C60 under hydrothermal conditions between 200 and 800°C, and under 100 MPa pressure, in the absence and in the presence of nickel is reported. The highest temperature of fullerene stability in water was 400°C after a 48-h-long treatment. Increasing the temperature and/or time of the hydrothermal treatment resulted in transformation of fullerene to amorphous carbon. At 700°C, high-quality open-ended multiwalled carbon nanotubes were formed in the vicinity of nickel particles. These nanotubes typically had an outer diameter of 30-40 nm and a wall thickness of 5 nm, with a graphitization level similar to that of carbon nanotubes prepared by chemical vapor deposition. The potential for large-scale synthesis of the carbon nanotubes by the hydrothermal technique is discussed. Since the present study was conducted under conditions which are common in the geological environment, our results imply that carbon nanotubes may form in natural hydrothermal systems