Ab initio investigation of order-disorder transition in δ-AlOOH
Abstract
δ-AlOOH is a high-pressure polymorph of boehmite (α-AlOOH) with a wide pressure and temperature stability field. Previous mineral physics studies indicate that this mineral plays a vital role in the transport of water into Earth's deep interior, possibly down to the core-mantle boundary (CMB) at ~ 136 GPa. Attempts to resolve the structure of this mineral under pressure revealed that hydrogen bonds (H-bond) "symmetrize" under compression, a phenomenon similar to that observed in the ice VII- or ice VIII-X transition in H2O. Like H2O-ice, δ-AlOOH comes with its own "ice-disorder rules" and disorder is also observed experimentally prior to H-bond symmetrization [1]. H-bond disorder in δ-AlOOH had been suggested by an ab initio calculation as a possibility based on the intensities of Rahman active OH- stretching modes [2].
In this ab initio study, we address the order-disorder transition phenomenon that precedes H-bond symmetrization from several perspectives. We address: a) the accuracy of standard DFT functions to address the structural properties of this problem, b) energy barriers for proton-hopping, c) vibrational stability and the pressure-temperature range of validity of the quasiharmonic approximation (QHA), and d) order-disorder transition using a multi-configuration QHA calculation [3,4]. With these results, we can reproduce the structural behavior and some peculiarities in the neutron diffraction pattern under pressure consistently. [1] A. Sano-Furukawa et al., DOI: 10.1038/s41598-018-33598-2 [2] J. Tsuchiya et al., DOI: 10.2138/am.2008.2627 [3] K. Umemoto et al., DOI: 10.1016/j.cplett.2010.09.065 [4] T. Qin et al., DOI: 10.1016/j.cpc.2018.11.003 Research supported by DOE award DE-SC0019759.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMMR23B0092L
- Keywords:
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- 3904 Defects;
- MINERAL PHYSICS;
- 3909 Elasticity and anelasticity;
- MINERAL PHYSICS;
- 3919 Equations of state;
- MINERAL PHYSICS;
- 3924 High-pressure behavior;
- MINERAL PHYSICS;
- 3619 Magma genesis and partial melting;
- MINERALOGY AND PETROLOGY;
- 3621 Mantle processes;
- MINERALOGY AND PETROLOGY