Equations of state and phase boundary for stishovite and CaCl 2-type SiO 2 2 3

30 Silica is thought to be present in the Earth’s lower mantle in subducting plates, in 31 addition to being a prototypical solid whose physical properties are of broad interest. It is known 32 to undergo a phase transition from stishovite to the CaCl2-type structure at ~50–80 GPa, but the 33 exact location and slope of the phase boundary in pressure-temperature space is unresolved. 34 There have been many previous studies on the equation of state of stishovite, but they span a 35 limited range of pressures and temperatures, and there has been no thermal equation of state of 36 CaCl2-type SiO2 measured under static conditions. We have investigated the phase diagram and 37 equations of state of silica at 21–89 GPa and up to ~3300 K using synchrotron X-ray diffraction 38 in a laser-heated diamond anvil cell. The phase boundary between stishovite and CaCl2-type 39 SiO2 can be approximately described as T = 64.6(49)*P – 2830(350), with temperature T in 40 Kelvin and pressure P in GPa. The stishovite data imply K0' = 5.24(9) and a quasi-anharmonic T 41 dependence of –6.0(4)  10 GPa*cm/mol/K for a fixed q = 1, ɣ0 = 1.71, and K0 = 302 GPa, 42 while for the CaCl2-type phase K0 = 341(4) GPa, K0' = 3.20(16), and ɣ0 = 2.14(4) with other 43 parameters equal to their values for stishovite. The behaviors of the a and c axes of stishovite 44 with pressure and temperature were also fit, indicating a much more compressible c axis with a 45 lower thermal expansion as compared to the a axis. The phase transition between stishovite and 46

[1]  R. Hemley.  Pressure Dependence of Raman Spectra of SiO2 Polymorphs: α‐Quartz, Coesite, and Stishovite , 2013 .

[2]  D. Rubie,et al.  Simultaneous partitioning of silicon and oxygen into the Earth's core during early Earth differentiation , 2013 .

[3]  K. Fischer,et al.  he global range of subduction zone thermal models , 2010 .

[4]  Y. Fei,et al.  Internally consistent thermodynamic database for iron to the Earth's core conditions , 2010 .

[5]  S. Batsanov.  High pressure crystal chemistry , 2009 .

[6]  S. Sutton,et al.  Advanced flat top laser heating system for high pressure research at GSECARS: application to the melting behavior of germanium , 2008 .

[7]  S. Sinogeikin,et al.  The post-stishovite phase transition in hydrous alumina-bearing SiO2 in the lower mantle of the earth , 2007, Proceedings of the National Academy of Sciences.

[8]  A. Oganov,et al.  Ruby, metals, and MgO as alternative pressure scales: A semiempirical description of shock-wave, ultrasonic, x-ray, and thermochemical data at high temperatures and pressures , 2007 .

[9]  T. I. Dyuzheva,et al.  Elastic constants of stishovite up to its amorphization temperature , 2005 .

[10]  M. Gillan,et al.  Structural stability of silica at high pressures and temperatures , 2005 .

[11]  A. Oganov,et al.  Intrinsic anharmonicity in equations of state and thermodynamics of solids , 2004 .

[12]  S. Ono,et al.  Stability of CaCl2‐type and α‐PbO2‐type SiO2 at high pressure and temperature determined by in‐situ X‐ray measurements , 2003 .

[13]  T. Yamanaka,et al.  Bonding character of SiO2 stishovite under high pressures up to 30 Gpa , 2002 .

[14]  T. Ahrens,et al.  Dynamic compression of SiO2: A new interpretation , 2002 .

[15]  Wiest,et al.  Ab initio studies of , 2000, The Journal of organic chemistry.

[16]  J. Crain,et al.  Ab initio elasticity of three high‐pressure polymorphs of silica , 1997 .

[17]  R. Liebermann,et al.  Elasticity of stishovite at high pressure , 1996 .

[18]  X. Gonze,et al.  The Pressure-induced Ferroelastic Phase-transition of Sio2 Stishovite , 1995 .

[19]  R. Cohen.  Bonding and elasticity of stishovite SiO2 at high pressure: Linearized augmented plane wave calculations , 1991 .

[20]  T. Yagi,et al.  New pressure-induced transformations of silica at room temperature , 1990, Nature.

[21]  T. Yagi,et al.  A new, post-stishovite highpressure polymorph of silica , 1989, Nature.

[22]  Peter M. Bell,et al.  Calibration of the ruby pressure gauge to 800 kbar under quasi‐hydrostatic conditions , 1986 .

[23]  Gregory A. Lyzenga,et al.  Shock temperatures of SiO2 and their geophysical implications , 1983 .

[24]  J. M. Brown,et al.  Thermodynamic parameters in the Earth as determined from seismic profiles , 1981 .

[25]  D. L. Anderson,et al.  Preliminary reference earth model , 1981 .

[26]  P. Keyes,et al.  THE NEMATIC-ISOTROPIC TRANSITION AT HIGH PRESSURES I : THE P-V-T EQUATION OF STATE , 1979 .

[27]  S. P. Marsh,et al.  On the equation of state of stishovite , 1963 .

[28]  F. Birch.  Elasticity and Constitution of the Earth's Interior , 1952 .

[29]  I. Brevik,et al.  THERMODYNAMIC PROPERTIES OF THE , 1998 .