The high-pressure phase of alumina and implications for Earth's D″ layer

Abstract

Using ab initio simulations and high-pressure experiments in a diamond anvil cell, we show that alumina (Al2O3) adopts the CaIrO 3-type structure above 130 GPa. This finding substantially changes the picture of high-pressure behavior of alumina; in particular, we find that perovskite structure is never stable for Al 2O3 at zero Kelvin. The CaIrO3-type phase suggests a reinterpretation of previous shock-wave experiments and has important implications for the use of alumina as a window material in shock-wave experiments. In particular, the conditions of the stability of this phase correspond to those at which shock-wave experiments indicated an increase of the electrical conductivity. If this increase is caused by high ionic mobility in the CaIrO 3-type phase of Al 2O3, similar effect can be expected in the isostructural postperovskite phase of MgSiO 3 (which is the dominant mineral phase in the Earth's D؆ layer). The effect of the incorporation of Al on the perovskite͞postperovskite transition of MgSiO 3 is discussed.is an important ceramic material and a major chemical component of the Earth. High-pressure behavior of alumina is an issue of paramount importance for three reasons: (i) static compression experiments often rely on the ruby pressure scale (1), (ii) alumina is used as an optical window material in shock-wave experiments, and its properties (e.g., the electrical conductivity) will affect measurements, and (iii) alumina, although unlikely as an individual mineral phase in the Earth's mantle, is incorporated into mantle minerals and significantly affects their physical properties (e.g., refs. 2 and 3).According to theoretical (e.g., refs. 4 -6) and experimental (7-9) studies, at high pressures corundum transforms into the Rh 2 O 3 (II)-type structure. This well established transition, occurring at 80 -100 GPa (5-9), implies that the ruby pressure scale should not be used above Ϸ80 GPa in experiments that involve heating to temperatures where this phase transition will be kinetically feasible. Less clear evidence exists for a further transition. Lin et al. (9) reported that Rh 2 O 3 (II) structure is stable at least up to 130 GPa and 2,000 K, but above 130 GPa (at temperatures Ϸ1,500 K) shock-wave experiments of Weir et al. (10) indicated onset of an increase in the electrical conductivity that could be caused by a phase transition. Shock-wave Hugoniots of Al 2 O 3 compiled by Hama and Suito (11) also give evidence for a possible phase transition at 130 GPa with a small increase of density. Thomson et al. (6) predicted that an orthorhombic (Pbnm) perovskite structure becomes more stable than Rh 2 O 3 (II), but only above 200 GPa. The discovery of postperovskite CaIrO 3 -type polymorphs of Fe 2 O 3 (12) and MgSiO 3 (13,14) prompts the question of whether a similar phase exists for Al 2 O 3 .In our ab initio calculations, we consider four Al 2 O 3 polymorphs: corundum (space group R3 c), Rh 2 O 3 (II) (Pbcn), perovskite (Pbnm), and CaIrO 3 -type (Cmcm). Our calculations are based on the plane wave pseud...

Assessing thermochemical properties of materials through ab initio quantum-mechanical methods: the case of α-Al2O3

“…29,30,42 Mousavi has recently reported some quasi-harmonic thermal properties of corundum, as computed with a simplified Debye model (where the lattice dynamics of the crystal is not explicitly solved at the ab initio level of theory), which are found to largely disagree with available experimental data. 43 To the best of our knowledge, the present investigation constitutes the first complete ab initio description of thermodynamic and thermal structural and elastic properties of α-Al 2 O 3 .…”

“…The exceptional performance of LDA in reproducing equilibrium structural features of α-Al 2 O 3 has already been discussed in several ab initio studies. 30,[64][65][66][67][68][69][70][71] The good description of the temperature dependence of V (T ) and K(T ) up to T M , without strong deviations which are commonly seen in other systems (particularly on K(T )) well below T M , 32 62 are reported as full circles. Despite an overall good description of the thermal expansion of corundum by all functionals, this figure allows for a finer discussion.…”

“…Several computational studies on aluminum oxide polymorphs have been reported in recent years, which mainly apply standard DFT methods. [22][23][24][25][26][27][28][29][30] In this paper, we apply a recently developed technique based on the so-called quasi-harmonic approximation (QHA): a simple and effective method that is able to address the aforementioned issue in the case of corundum. 31,32 The thermal expansion coefficient, α(T ), heat capacities, C V (T ) and C P (T ), entropy, S(T ) and bulk modulus, K(T ) of corundum are calculated in its entire range of thermal stability (i.e.…”

Prediction of an U 2 S 3 -type polymorph of Al 2 O 3 at 3.7 Mbar

“…Two pressure-induced transitions have been predicted theoretically (10)(11)(12)(16)(17)(18) and confirmed experimentally (13)(14)(15)19): from corundum (space group R3 c), the stable ambient form, to the Rh 2 O 3 (II)-type structure (space group Pbcn) at Ϸ80-100 GPa and then to the CaIrO 3 -type PPV structure (space group Cmcm) at Ϸ130 GPa. These pressures are realized in the Earth's mantle.…”

Postperovskite phase equilibria in the MgSiO 3 –Al 2 O 3 system

“…If, however, comparing the transition pressures of end-member compositions, the pPv transition in MgSiO 3 is expected to occur at much lower pressure than in Al 2 O 3 at low temperatures, but it is comparable or rather higher in MgSiO 3 than in Al 2 O 3 at temperatures Ͼ3,000 K, because the Pv-pPv phase boundary in MgSiO 3 has a largely positive Clapeyron slope (2, 3), whereas the Rh 2 O 3 (II) (Rh)-pPv boundary in Al 2 O 3 has a largely negative one (12,20). This might affect the solubility of Al to Mg-Pv, and thus the stability of aluminous Pv and pPv, which were not considered in the previous studies on the phase equilibria in the MgSiO 3 -Al 2 O 3 system.…”