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Computational Study of High Pressure Polymorphic Transformations in Monazite-type LaPO4

Abstract

Polymorphic transformations in LaPO4 are investigated as a function of pressure using density functional theory (DFT) based calculations under the generalized gradient approximation. Monazite-type (P21/n) → barite-type (Pbnm) structural transformation is identified at 16.2 GPa and experimentally no transformation is observed near this pressure. A discontinuity in the pressure-volume relation (of 4.16% volume discontinuity compared to monazite structure at the same pressure) and unitcell dimensions are observed around 28 GPa which is matching well with previous experimental results. The pressure of discontinuity matches with DFT calculated monazite-type (P21/n) → post barite-type (P212121) structural transformation pressure. The equation of state, single crystal elastic constants and phonon dispersion curves of the different polymorphs as a function of pressure are determined. Both barite-type (Pbnm) and post barite-type (P212121) structures are mechanically and dynamically stable at 27 GPa indicating monazite-type (P21/n) → barite-type (Pbnm) phase transformation may be hindered by kinetic barrier. The phase transformation in monazite-type LaPO4 is driven by softening of C25 single crystal elastic constant. Moreover, tilting in PO4 tetrahedra as a function of pressure leads to a change in La chemical environment and creating space for construction of LaO12 polyhedral from LaO9 due to phase transformation.

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Publication details

The article was received on 16 Aug 2017, accepted on 13 Feb 2018 and first published on 13 Feb 2018


Article type: Paper
DOI: 10.1039/C7CP05587K
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    Computational Study of High Pressure Polymorphic Transformations in Monazite-type LaPO4

    P. S. Ghosh, K. Ali and A. Arya, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C7CP05587K

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