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Phase transitions in Al2O3. between corundum Rh2O3(II)-type and CaIrO3-type (post-perovskite-type) phases were examined at high pressure and high temperature in a laser-heated diamond-anvil cell (DAC) based on in situ X-ray diffraction measurements. The locations of corundum-Rh2O3(II) and Rh2O3(II)-CaIrO3 boundaries were precisely determined by conducting both forward and backward reaction experiments. The results demonstrate that corundum undergoes a phase transition toRh2O3(II)-type structure above 106 GPa at 1800 K with a negative Clapeyron slope of -6.5 ± 1.5 MPa/K, generally consistent with earlier experimental and theoretical works. The Rh2O3(II)-type phase further transforms into CaIrO3-type above 170 GPa at 2300 K, indicating the transition pressure much higher than earlier experimental work but in agreement with reported GGA calculations. The Clapeyron slope of this phase transition was found to be high negative (-20 ± 5 MPa/K). [ABSTRACT FROM AUTHOR]

Phase transitions in Al2O3. between corundum Rh2O3(II)-type and CaIrO3-type (post-perovskite-type) phases were examined at high pressure and high temperature in a laser-heated diamond-anvil cell (DAC) based on in situ X-ray diffraction measurements. The locations of corundum-Rh2O3(II) and Rh2O3(II)-CaIrO3 boundaries were precisely determined by conducting both forward and backward reaction experiments. The results demonstrate that corundum undergoes a phase transition toRh2O3(II)-type structure above 106 GPa at 1800 K with a negative Clapeyron slope of -6.5 ± 1.5 MPa/K, generally consistent with earlier experimental and theoretical works. The Rh2O3(II)-type phase further transforms into CaIrO3-type above 170 GPa at 2300 K, indicating the transition pressure much higher than earlier experimental work but in agreement with reported GGA calculations. The Clapeyron slope of this phase transition was found to be high negative (-20 ± 5 MPa/K). [ABSTRACT FROM AUTHOR]

Abstract: The thermal diffusivity (κ) of perovskite and post-perovskite CaIrO3 has been measured, at elevated pressure and temperatures up to 600°C, using the X-radiographic Ångström method. At high temperatures we find that the thermal diffusivity of post-perovskite is slightly below twice that of isochemical perovskite over the temperature range investigated. Assuming a similar effect occurs in MgSiO3 post-perovskite, the effect of the contrasting thermal transport properties between perovskite and post-perovskite on mantle dynamics has been investigated using simple two-dimensional convection models. These show a reduction in extent and increase in depth of post-perovskite lenses, as well as increased core–mantle-boundary heat-flux, broader upwellings and more vigorous downwellings when compared to the reference, constant a, case. [Copyright &y& Elsevier]

Compression behaviors of CaIrO with perovskite (Pv) and post-perovskite (pPv) structures have been investigated up to 31.0(1.0) and 35.3(1) GPa at room temperature, respectively, in a diamond-anvil cell with hydrostatic pressure media. CaIrO Pv and pPv phases were compressed with the axial compressibility of β > β > β and β > β > β, respectively and no phase transition was observed in both phases up to the highest pressure in the present study. The order of axial compressibility for pPv phase is consistent with the crystallographic consideration for layer structured materials and previous experimental results. On the other hand, Pv phase shows anomalous compression behavior in b axis, which exhibit constant or slightly expanded above 13 GPa, although the applied pressure remained hydrostatic. Volume difference between Pv and pPv phases was gradually decreased with increasing pressure and this is consistent with the results of theoretical study based on the ab initio calculation. Present results, combined with theoretical study, suggest that these complicate compression behaviors in CaIrO under high pressure might be caused by the partially filled electron of Ir. Special attention must be paid in case of using CaIrO as analog materials to MgSiO, although CaIrO exhibits interesting physical properties under high pressure. [ABSTRACT FROM AUTHOR]

The post-corundum phase transition has been investigated in Ti2O3 on the basis of synchrotron X-ray diffraction in a diamond anvil cell and transmission electron microscopy. The new polymorph of Ti2O3 was found at about 19 GPa and 1850 K, and this phase was stable even at about 40 GPa. A new polymorph of Ti2O3 can be indexed on a Pnma orthorhombic cell, and the unit-cell parameters are a=7.6965 (19) Å, b=2.8009 (9) Å, c=7.9300 (23) Å, V=170.95 (15) Å3 at 19 GPa, and a=7.8240 (2) Å, b=2.8502 (1) Å, c=8.1209 (3) Å, V=181.10 (1) Å3 at ambient conditions. The Birch-Murnaghan equation of state yields K0=206 (3) GPa and K'0=4 (fixed) for corundum phase, and K0=296 (4) GPa and K'0=4 (fixed) for the post-corundum phase. The molar volume decreases by 12% across the phase transition at around 20 GPa. The structural identification was carried out on a recovered sample by the Rietveld method, and a new polymorph of Ti2O3 can be identified as Th2S3-type rather than U2S3-type structure. The transition from corundum-type to Th2S3-type structure accompanies the drastic change of the form of polyhedron: from TiO6 octahedron in the corundum-type to TiO7 polyhedron in the Th2S3-type structures. [ABSTRACT FROM AUTHOR]

Abstract: In this study, we propose a theoretical approach to test the validity of the isomechanical analogues for post-perovskite structures. Intrinsic plastic properties are evaluated for three materials exhibiting a post-perovskite phase: MgSiO3, MgGeO3 and CaIrO3. Dislocation properties of each structure are determined using the Peierls–Nabarro model based on first-principles calculations of generalised stacking fault and the plastic properties are extended to crystal-preferred orientations using a visco-plastic self-consistent method. This study provides intrinsic parameters of plastic deformation such as dislocation structures and Peierls stresses that can be directly compared between the three materials. It appears that it is very difficult to draw any simple conclusions on polycrystalline deformation simply by comparing single crystal properties. In particular, contrasting single crystal properties of MgSiO3 and CaIrO3 lead to similar crystal-preferred orientation of the polycrystal aggregates. [Copyright &y& Elsevier]

We have measured Raman spectra of the postperovskite (PPv) phase in CaIrO3 up to 30 GPa to constrain the Grüneisen parameter (γ). We identified a total of 4 strong modes between 200 and 650 cm-1, which is in contrast with the Raman spectra of Mn2O3 and MgGeO3-PPv where at least nine different modes have been detected. We found no sign of a phase transition in the Raman spectra of PPv CaIrO3, which supports the stability of the PPv phase up to 30 GPa and room temperature in CaIrO3. The spectroscopic Grüneisen parameter, γsp,0 = 1.66~1.72, constrained from our Raman data, is in excellent agreement with the thermodynamic Grüneisen parameter, γth,0 = 1.75 ± 0.05, calculated from recent XRD measurements (Martin et al. 2007) on CaIrO3-PPv synthesized at high pressure and temperature similar to our starting material. Our result suggests that γsp constrained by Raman measurements provides a reasonable estimate on the γth of the PPv phase in CaIrO3. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: At near ambient conditions, CaIrO3 is isostructural with the high-pressure polymorph MgSiO3 “post-perovskite” (pPv). MgSiO3 pPv is thought to be a major phase in the earth''s lowermost mantle. CaIrO3 can thus serve as an analog for studying deformation of the pPv phase under conditions achievable with a multi-anvil deformation apparatus. Here we study the rheologic behavior of CaIrO3 pPv at a variety of pressure and temperature conditions from 2 GPa to 6 GPa and 300 K to 1300 K and various strain rates. Sintered, polycrystalline CaIrO3 pPv, cylindrical in shape, was deformed in the D-DIA multi-anvil press in several shortening cycles up to 20% axial strain at each temperature and pressure. Shortening cycles were followed by lengthening back to 0% strain. Quantitative texture information was obtained using in-situ synchrotron X-ray diffraction and the Rietveld method to analyze images. In all cases we find that (010) lattice planes align perpendicular to the compression direction upon shortening, and that there is little change in texture with temperature or pressure. This texture pattern is consistent with slip on (010)[100]. The texture observed here is different from that produced in room temperature diamond anvil cell (DAC) experiments on MgGeO3 and MgSiO3 pPv which both display textures of (100) and {110} lattice planes at high angles to the compression direction. This implies that CaIrO3 pPv may not be a good analog for the plastic behavior of MgSiO3 pPv. [Copyright &y& Elsevier]

The structure of CaIrO3 (Cmcm) has been refined at high pressure and at low temperature using time-of-flight neutron powder diffraction data. Evidence supporting deviation from space group Cmcm to Cmc21 is inconclusive. As CaIrO3 (Cmcm) unit-cell volume changes, refinements indicate deformation of cation-centered coordination polyhedra, rather than tilting. Structure models demonstrate Ca2+-centered polyhedra are an order of magnitude more compressible than Ir4+-centered octahedra. Bond valence sums show significant chemical strain (over-bonding) of calcium and oxygen at ambient conditions. Implications for structure change in MgSiO3 post-perovskite are discussed and a method for predicting the Clapeyron slope between perovskite and post-perovskite phases is proposed based on extrapolation of the volume-ratio between cation-centered polyhedra. [ABSTRACT FROM AUTHOR]

Abstract: The thermal expansion of CaIrO3, the archetype phase of the “post-perovskite” polymorph of MgSiO3, has been determined by X-ray powder diffraction from 113 to 1173K. For temperatures above 298K, the volumetric coefficient of thermal expansion, α(T), can be represented by α(T)= a 0 + a 1(T), with a 0 =2.42(3)×10−5 K−1 and a 1 =0.73(4)×10−8 K−2. The data for the full range of temperature investigated can be fitted well by a second-order Grüneisen approximation to the zero-pressure volumetric equation of state, with the internal energy calculated via a Debye model, and parameters: θ D =703(11)K, V 0 =225.666(6)Å3, and (V 0 K 0/γ′)=2.77(2)×10−17 J, where θ D is the Debye temperature, V 0 the unit-cell volume at T = 0K, the first derivative with respect to pressure of the incompressibility, K 0, and γ′ is a Grüneisen parameter. The results of similar procedures, applied to the individual lattice parameters, suggest that the degree of elastic anisotropy shown by CaIrO3 at ambient pressure may be much greater than that calculated for the high-pressure post-perovskite polymorph of MgSiO3. [Copyright &y& Elsevier]

The effect of the anisotropic connectivity within the layers of the post-perovskite, CaIrO3, structure are examined using a Rigid Unit Mode and group theory approach, and possible commensurate tilt structures are given. It is shown that the anisotropic bonding of the sheet may give rise to preferred directions of buckling for such structures and anisotropy in the speed of sound. [ABSTRACT FROM AUTHOR]

Abstract: We report on three attempts to metallize a antiferromagnetic Mott insulator CaIrO3 with the quasi-two-dimensional post-perovskite structure. By controlling the dimensionality and the electron filling, we can successfully drive a metal–insulator transition, while an insulating state is robust against the applied pressure. These results are compared with related experiments for the post-perovskite Mg0.9Fe0.1SiO4, the layered perovskite Ca2RuO4, and high- cuprates. The interplay between spin, charge, and orbital degrees of freedom is discussed. [Copyright &y& Elsevier]

Exploring exotic interface magnetism due to charge transfer and strong spin-orbit coupling has profound application in the future development of spintronic memory. Here, the emergence and tuning of topological Hall effect (THE) from a CaMnO 3 /CaIrO 3 /CaMnO 3 trilayer structure are studied in detail, which suggests the presence of magnetic Skyrmion-like bubbles. First, by tilting the magnetic field direction, the evolution of the Hall signal suggests a transformation of Skyrmions into topologically-trivial stripe domains, consistent with behaviors predicted by micromagnetic simulations. Second, by varying the thickness of CaMnO 3 , the optimal thicknesses for the THE signal emergence are found, which allow identification of the source of Dzyaloshinskii-Moriya interaction (DMI) and its competition with antiferromagnetic superexchange. Employing high-resolution transmission electron microscopy, randomly distributed stacking faults are identified only at the bottom interface and may avoid mutual cancellation of DMI. Last, a spin-transfer torque experiment also reveals a low threshold current density of ≈10 9 A m -2 for initiating the bubbles' motion. This discovery sheds light on a possible strategy for integrating Skyrmions with antiferromagnetic spintronics., (© 2020 Wiley-VCH GmbH.)

We report the control of the interplane magnetic exchange coupling in CaIrO 3 perovskite thin films and superlattices with SrTiO 3 . By analyzing the anisotropic magneto-transport data, we demonstrate that a semimetallic paramagnetic CaIrO 3 turns into a canted antiferromagnetic Mott insulator at reduced dimensions. The emergence of a biaxial magneto-crystalline anisotropy indicates the canted moment responding to the cubic symmetry. Extending to superlattices and probing oxygen octahedral rotation by half-integer X-ray Braggs diffraction, a more complete picture about the canted moment evolution with interplane coupling can be understood. Remarkably, a rotation of the canted moments' easy axes by 45° is also observed by a sign reversal of the in-plane strain. These results demonstrate the robustness of anisotropic magnetoresistance in revealing quasi two-dimensional canted antiferromagnets, as well as valuable insights about quadrupolar magnetoelastic coupling, relevant for designing future antiferromagnetic spintronic devices.

Abstract: We have successfully synthesized Ca1−x Na x IrO3 with the post-perovskite structure and elucidated their electronic properties. CaIrO3 is a S =1/2 antiferromagnetic Mott insulator with the Néel temperature (T N) of 115K. A gradual decrease in T N upon carrier doping culminates in a paramagnetic state at x ∼0.3 with the simultaneous change from an insulator to a metal. The metallic state for x ⩾0.31 shows the non-Fermi liquid behavior originating from antiferromagnetic fluctuations. [Copyright &y& Elsevier]

Analysis of pressure-temperature dependent monochromatic X-ray powder diffraction data yield the bulk modulus [KT = 180.2(28) GPa] and thermal expansion coefficients [α0 = 2.841(34) x 105 K-1; α1 = 3.37(48) x 10-9 K-2] of CaIrO3, the structure model for post-perovskite MgSiO3. CaIrO3 is orthorhombic (Cmcm, space group 63, Z = 4) with best-fit unit-cell parameters, a = 3.14147(5) Å, b = 9.87515(19), c = 7.29711(11), and V = 226.3754(78) ų at 1 bar and 300 K. The c-axis of CaIrO3 has a small compressibility and a large thermal expansion when compared to the other principal axes. Rietveld structure refinement reveals changes in CaIrO3 as a function of temperature in terms of IrO6 octahedra distortion. Dissociation of CaIrO3 at high temperature has possible implications for the post-perovskite MgSiO3 structure, Earth's lower mantle, and D" layer. [ABSTRACT FROM AUTHOR]