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

Abstract

We predict by first principles a phase transition in alumina at Ϸ3.7 Mbar and room temperature from the CaIrO 3-type polymorph to another with the U 2S3-type structure. Because alumina is used as window material in shock-wave experiments, this transformation should be important for the analysis of shock data in this pressure range. Comparison of our results on all high-pressure phases of alumina with shock data suggests the presence of two phase transitions in shock experiments: the corundum to Rh 2O3(II)-type structure and the Rh 2O3(II)-type to CaIrO3-type structure. The transformation to the U 2S3-type polymorph is in the pressure range reached in the mantle of recently discovered terrestrial exoplanets and suggests that the multi-megabar crystal chemistry of planetforming minerals might be related to that of the rare-earth sulfides.alumina ͉ first-principles calculation ͉ high-pressure phase transition ͉ postperovskite ͉ rare-earth sulfide structure A lumina, Al 2 O 3 , is an important compound in high-pressure technology and geophysics. It is used as window material in shock-wave experiments (1). Ruby, Al 2 O 3 doped with chromium, serves as a pressure calibrant in diamond-anvil-cell experiments (2). In the Earth's mantle, Al 2 O 3 is a major chemical component in solid solution with MgSiO 3 garnet, perovskite (PV), and postperovskite (PPV). This formation of solid solutions with alumina changes the properties of the pure phases, such as their equations of state, thermoelastic properties, phase boundaries, electrical conductivity, oxidation, and spin states of iron impurities in MgSiO 3 , etc. (e.g., refs. 3-9).The experimental and theoretical literature on phase transitions in Al 2 O 3 is quite extensive (10-19). 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. The CaIrO 3 -type polymorph is the highest-pressure form identified experimentally so far for both Al 2 O 3 and MgSiO 3 . A phase transition in the CaIrO 3 -type phase in Al 2 O 3 or MgSiO 3 should change the properties of their solid solution, a key input in the modeling of planetary interiors. A first-principles study predicted the dissociation of CaIrO 3 -type MgSiO 3 into CsCl-type MgO and cotunnite-type SiO 2 at Ϸ1.1 TPa (20), a pressure relevant for the giant planets and exoplanets. The same type of dissociation was also predicted for NaMgF 3 , a low-pressure analog of MgSiO 3 (21). However, CaIrO 3 -type Al 2 O 3 is expected to undergo a nondissociative post-PPV transition because the equivalent dissociation products (AlO and AlO 2 ) are unlikely to form. Here, we report the first-principles prediction of a phase transition in Al 2 O 3 from the CaIrO 3 -type polymorph to another with the U 2 S 3 -type s...

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

“…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.…”

“…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 .…”

Possible magnetic fields of super earths generated by convecting, conducting oxides

“…Thermal pressures on the sapphire Hugoniot to 340 GPa [13] are small relative to the calculated 300-K isotherm [21]. Shock dissipation goes into damage and disorder of the strong sapphire lattice, as well as into thermal energy.…”

“…[8]: solid curve, fit to experimental Hugoniot data [13]; open and solid squares, Rh 2 O 3 II-type, and CaIrO 3 -type, respectively, in DAC [14]; dotted, dashed, and dasheddotted curves, theory for corundum, Rh 2 O 3 II-type, and CaIrO 3 -type, respectively [21]. Figure 1 is plot of pressure versus compression of sapphire (i) using the linear u s -u p fit to Hugoniot data [13], (ii) measured points of sapphire compressed in a DAC, laser-heated, and thermally quenched to 300 K [14], and (iii) theoretical 300-K isotherm [21]. Static compression of corundum (α-Al 2 O 3 ) in a DAC produces only disordered corundum at all pressures.…”

Metallic liquid hydrogen and likely Al2O3 metallic glass

“…It has generally been assumed since then that temperature is the mechanism of shock dissipation. The fact that entropy dominates dissipation up to 400 GPa on the sapphire Hugoniot is suggested by the fact that Hugoniot data of sapphire up to 340 GPa [19] are virtually coincident with the theoretical Hugoniot [20]. Above ~400 GPa Hugoniot pressure increases dramatically, which implies thermal pressure increases dramatically [21], which implies temperature increases dramatically.…”