SODIUM NIOBATE AS A POSSIBLE NEW PRESSURE CALIBRANT IN HYDROTHERMAL DIAMOND ANVIL CELL STUDIES

In this study, we propose sodium niobate (NaNbO₃) as a new pressure calibration material in hydrothermal diamond anvil cell (HDAC) studies. Empirically determined transition temperatures at high pressure in NaNbO₃ and BaTiO₃ (barium titanate) and modifications made to the HDAC experimental setup at the University of Tennessee, Knoxville are also presented.

NaNbO₃ is one in a group of well-studied minerals with perovskite-type structure. It has six known structural transitions between -100 °C and 641 °C at atmospheric pressure. In this study, the monoclinic to orthorhombic transition occurring at 373 °C and 1 atm was found to be most applicable in HDAC studies because it is rapid, reversible and easily identified by an abrupt change in birefringence. At high pressure this transition was found to have a positive Clapeyron slope (~6.0 MPa/°C) that intersects density isochores of pure water.

An evaluation of high-pressure transition temperatures determined in HDAC studies for BaTiO₃ and NaNbO₃ shows that, in contrast to Shen et al. (1996), our data results in more accurate transition temperatures on the heating cycle rather than the cooling cycle of each experiment. This may result from fluid loss reduction techniques, which were implemented in the experimental setup. These modifications include a decrease in gasket diameter, a reduction in the amount of gasket polishing, and heating/cooling of the sample chamber in small (~300 °C) intervals.