Variations in the chemical nature of A and B cations, as well as changes in P and T, are accommodated in the framework structure of perovskite (general formula [XII]A[VI]BO3) by octahedral tilting and more distorted polyhedra. The structural answer to these internal and external stresses is a decreasing of the symmetry (Mitchell, 2002). From the cubic archetype, perovskites often become orthorhombic. This class of perovskites received considerable attention in Earth Sciences since it was discovered that the major minerals of the upper mantle (i.e. olivine, pyroxene, and garnet) transform into the denser (Mg,Fe)SiO3 perovskite-structured mineral, at P-T conditions of the upper/lower mantle interface (Navrotsky & Weidner, 1989). Considered as the dominant component of the lower mantle, this orthorhombic compound remarked on the importance to understand the relationship between external condition, structural properties, and chemistry of perovskite phases. For these reasons, perovskites are the subject of many studies devoted to establish their HP behaviour. Part of these studies (Andrault & Poirier, 1991; Thomas, 1998; Zhao et al., 2004) attempt to predict the evolution of these compounds under HP regime by means of semiempirical and theoretical models. As a common generalization, the structural evolution of orthorhombic perovskites with P can be rationalized in terms of relative compressibilities of the two polyhedra (AO12 and BO6): when the AO12 site is more compressible than the BO6 octahedron, the volume reduction will lead to an increasing of the octahedral tilting; conversely, when the AO12 site is less compressible than the BO6 octahedron, the structure will evolve by decreasing the octahedral tilting raising its symmetry towards the cubic archetype. The model proposed by Zhao et al. (2004) predicts that the polyhedral compressibility ratio can be devised as the ratio of the estimated variation of bond valence in the A and B polyhedral sites due to the change of the average metal–oxygen bond distance. In this contribution the structural evolution of perovskites upon pressure will be modeled through a new polyhedral bond valence approach. The obtained results will be compared with those derived from previous models, and it will be shown as transition metal ions at the B site act as an incompressibility factor.

Effect of Transition Metal Ions (TMI) on the compressibility of orthorhombic perovskites

ARDIT, Matteo;CRUCIANI, Giuseppe
2014

Abstract

Variations in the chemical nature of A and B cations, as well as changes in P and T, are accommodated in the framework structure of perovskite (general formula [XII]A[VI]BO3) by octahedral tilting and more distorted polyhedra. The structural answer to these internal and external stresses is a decreasing of the symmetry (Mitchell, 2002). From the cubic archetype, perovskites often become orthorhombic. This class of perovskites received considerable attention in Earth Sciences since it was discovered that the major minerals of the upper mantle (i.e. olivine, pyroxene, and garnet) transform into the denser (Mg,Fe)SiO3 perovskite-structured mineral, at P-T conditions of the upper/lower mantle interface (Navrotsky & Weidner, 1989). Considered as the dominant component of the lower mantle, this orthorhombic compound remarked on the importance to understand the relationship between external condition, structural properties, and chemistry of perovskite phases. For these reasons, perovskites are the subject of many studies devoted to establish their HP behaviour. Part of these studies (Andrault & Poirier, 1991; Thomas, 1998; Zhao et al., 2004) attempt to predict the evolution of these compounds under HP regime by means of semiempirical and theoretical models. As a common generalization, the structural evolution of orthorhombic perovskites with P can be rationalized in terms of relative compressibilities of the two polyhedra (AO12 and BO6): when the AO12 site is more compressible than the BO6 octahedron, the volume reduction will lead to an increasing of the octahedral tilting; conversely, when the AO12 site is less compressible than the BO6 octahedron, the structure will evolve by decreasing the octahedral tilting raising its symmetry towards the cubic archetype. The model proposed by Zhao et al. (2004) predicts that the polyhedral compressibility ratio can be devised as the ratio of the estimated variation of bond valence in the A and B polyhedral sites due to the change of the average metal–oxygen bond distance. In this contribution the structural evolution of perovskites upon pressure will be modeled through a new polyhedral bond valence approach. The obtained results will be compared with those derived from previous models, and it will be shown as transition metal ions at the B site act as an incompressibility factor.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/2091813
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