L Journal of Alloys and Compounds 327 (2001) 253–262 www.elsevier.com / locate / jallcom Volumetric properties and phase relations of silica — thermodynamic assessment a, a b b * Huahai Mao , B. Sundman , Zhongwu Wang , S.K. Saxena a Department of Materials Science and Engineering, Royal Institute of Technology, 100 44 Stockholm, Sweden b Florida International University, Center for Study of Matter at Extreme Conditions, Miami, FL 33199, USA Received 11 January 2001; received in revised form 24 April 2001; accepted 24 April 2001 Abstract Taking into account various available experimental data, an improved internally consistent data set for the SiO system is obtained by 2 thermodynamic assessment. The volumetric properties for SiO polymorphs are reassessed using the CALPHAD method. A two- 2 coefficient expression is used to describe the thermal expansion and bulk modulus. For liquid phase, one equation is achieved in the temperature range of 298–4000 K either for thermal expansion or for bulk modulus. Phase diagrams have been calculated up to 40 GPa. It has been concluded that, in the pressure range lower than 20 GPa, there is no significant difference between the phase diagrams calculated by the Birch–Murnaghan equation of state (EOS) and by Murnaghan EOS.  2001 Elsevier Science B.V. All rights reserved. Keywords: Thermodynamic assessment; Silica; Phase diagram; Volumetric properties 1. Introduction were presented. However, melting relations were not considered in that database. In the following year, an The phase diagram for Si–O plays an important role in a internally consistent data set on the thermodynamic prop- variety of fields such as metallurgy, ceramics, and petrolo- erties of the silica polymorphs up to 15 GPa ( a-quartz, gy. The polymorphism of silica (SiO ) has a broad range b-quartz, tridymite, cristobalite, coesite, stishovite and 2 of pressures and temperatures, and it occurs widely in liquid phase) was obtained by Swamy et al. [4]. Saxena [5] mineral assemblages in the Earth’s crust and mantle. modified the thermodynamic parameters (heat capacity Although free SiO most likely does not exist in substan- coefficients, enthalpy and entropy at 1 bar) for coesite and 2 tial quantities, it is a constituent of the high-pressure stishovite. Fabrichnaya and Sundman [6] assessed the phases of pyroxene, olivine, and garnet. The high pressure thermodynamic parameters in the Fe–Si–O system. The transformations of crystalline silica are often used as equation-of-state (EOS) parameters for liquid phase were calibration standards in high-pressure piston-cylinder and reassessed, while thermochemical parameters for coesite diamond anvil experiments. and stishovite at 1 bar were taken from Saxena [5] and Many different kinds of experimental data, e.g. other parameters from Swamy et al. [4]. However, in the calorimetric data, volumetric data, and phase equilibrium previous studies, two different equations were used, below data, etc., for silica phases are available. In addition, and above 2700 K, to describe the volumetric properties of general computer programs for optimization of thermo- SiO liquid. Additionally, the volumetric properties for 2 dynamic data have been developed, which make it possible solid phases in the SiO system (such as thermal expansion 2 to take into account all available experimental information of b-quartz and stishovite, isothermal compressibility of on a system simultaneously (e.g. THERMO-CALC set of cristobalite, and so on) can be improved. In the present Sundman et al. [1]; ChemSage program of Eriksson and study, the volumetric properties of SiO polymorphs are 2 Hack [2]). Saxena et al. [3] created a geochemical data- reassessed taking into account all kinds of available base, where thermodynamic data for high-pressure phases experimental data. One single equation is used for thermal expansion and for bulk modulus of liquid SiO in the 2 temperature range of 298–4000 K. Finally, an improved *Corresponding author. Tel.: 146-8-790-9638. E-mail address: huahai@met.kth.se (H. Mao). internally consistent data set on the thermodynamic prop- 0925-8388 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388(01)01465-7