Post-perovskite Research Articles

Comment on “Effect of particle size and strain on phase stability of (Li0.06Na0.94)NbO3” [J. Appl. Phys. 115, 174104 (2014)

It is shown that the powder XRD pattern attributed to the new “post perovskite” phase of the title niobate prepared by ball-milling actually corresponds to the cubic yttria stabilized zirconia, material of the milling balls.

Effect of particle size and strain on phase stability of (Li0.06 Na0.94) NbO3

Alkaline niobates are most suitable and excellent candidates for lead free piezoceramics, as they exhibit morphotropic phase boundary and have ultra-large piezoresponse similar to them. We provide direct experimental evidence of ferroelectric to paraelectric phase transition in (Li0.06 Na0.94)NbO3 with reduction of particle size using a combination of x-ray and neutron powder diffraction techniques at room temperature. Detailed Rietveld analyses of x-ray data show variation of particle sizes from micrometer to nanometer for sintered, calcined, and ball milled powders. The ferroelectric orthorhombic phase for micron sized powder (∼1.17 μm) is found to transform to paraelectric phase by reducing particle size to ∼10.8 nm. The crystal structure of paraelectric phase has been identified with tetragonal symmetry (P42/mmc) and is found to be a post perovskite phase. The low temperature neutron diffraction studies on the powders with different particle sizes reveal that orthorhombic to rhombohedral phase transition gets suppressed with reducing particle size.

Deformation microtextures in CaIrO3 post-perovskite under high stress conditions using a laser-heated diamond anvil cell

Deformation microstructures of CaIrO3 post perovskite (PPv) in a laser heated diamond anvil cell (DAC) are examined using transmission electron microscopy. Under a high stress and strain rate in the deformation condition, a high density of the {110} twin domains were at first time observed in post perovskite structure, as well as [100] and [001] dislocations on the (010) plane. The exchange of the a-axis for the b-axes in the twin mechanism can reduce a stress in a single crystal at the beginning of the uniaxial compression. The {110} plane in PPv corresponds with that of the crystallographic preferred orientation that was previously reported in deformed MgSiO3 and MgGeO3 PPv in DACs. This result implies that the formation of the twin domains can play an important role in CaIrO3 PPv deformations under high stress and strain rate conditions (e.g., Deformation by DAC).

Transformation textures in post‐perovskite: Understanding mantle flow in the D″ layer of the Earth

Deformation and texture formation in (Mg, Fe)SiO3 post perovskite (ppv) is a potential explanation for the strong seismic anisotropy that is found in the D″ layer of the Earth. However, different experimental approaches have resulted in different lattice preferred orientations (LPO) in deformed ppv that have led to ambiguity in the interpretation of deformation in the lowermost mantle. Here, we show that deformation of the analogue substance CaIrO3 during a phase transformation from perovskite to ppv leads to a transformation texture that differs from the CaIrO3 ppv deformation texture but resembles the results from ppv deformation experiments in diamond anvil cells. Assuming material spreading parallel to the core‐mantle boundary, our results predict a widespread shear wave splitting with fast horizontal S‐waves, which is compatible with seismic studies. Downwelling material that undergoes a phase transformation may develop a transformation texture that would locally result in vertically polarized fast S‐waves.

Density Functional Study of Calcium Nitride

The high-pressure behavior of Ca3N2 is studied up 100 GPa using density functional theory. Evaluation of many hypothetical polymorphs of composition A3X2 leads us to propose four high-pressure polymorphs for both α- and β-Ca3N2: (1) an anti-Rh2O3−II structure at 5 GPa, (2) an anti-B-sesquioxide structure at 10 GPa, (3) an anti-A-sesquioxide structure at 27 GPa, and (4) a hitherto unknown hexagonal structure (P63/mmc), derived from the post-perovskite structure of CaIrO3, at 38 GPa. The development of the density and bulk modulus under pressure has been examined.

Single-crystal X-ray diffraction study of CaIrO3

Single crystals of CaIrO3 were prepared via flux growth method. Crystal structure parameters, including the anisotropic displacement parameters, are determined based on a single-crystal X-ray diffraction experiment. The unit-cell dimensions are a = 3.147(2), b = 9.866(6), and c = 7.302(5) Å. The structure is a three-dimensional dense structure with small vacant spaces. The CaIrO3 structure can be described as a pseudo-one-dimensional oxide and is compared with Ca4IrO6 structure. The IrO6 octahedra are significantly distorted, in contrast to other octahedral Ir4+ compounds. The O-O distances for faces and edges shared between polyhedra are shorter than other non-shared edge distances. These effects are explained by Pauling’s rules and occur to decrease the repulsion between the cations. Thermal vibrations of Ca and Ir atoms are significantly anisotropic. Thermal vibrations of Ca and Ir atoms are restricted in orientation toward the shared face, shared edges, and shortest cation-cation directions. The single-crystal experiment shows that CaIrO3 crystals grow fastest along the a axis and that they assume a prism or needle shape. Strongly preferred orientation of such prism shaped CaIrO3-type post perovskite MgSiO3 crystals may develop under the shear flow in the Earth’s mantle.

Fe–Mg partitioning between post-perovskite and magnesiowustite

Fe–Mg partitioning between post-perovskite and magnesiowustite

High-Pressure Melting ofMgSiO3

The melting curve of MgSiO(3) perovskite has been determined by means of ab initio molecular dynamics complemented by effective pair potentials, and a new phenomenological model of melting. Using first principles ground state calculations, we find that the MgSiO(3) perovskite phase transforms into post perovskite at pressures above 100 GPa, in agreement with recent theoretical and experimental studies. We find that the melting curve of MgSiO(3), being very steep at pressures below 60 GPa, rapidly flattens on increasing pressure. The experimental controversy on the melting of the MgSiO(3) perovskite at high pressures is resolved, confirming the data by Zerr and Boehler.