Post-perovskite Structure Research Articles

Metallization of quasi-two-dimensional Mott insulator CaIrO 3 withS=12spins

Abstract We report on three attempts to metallize a S = 1 2 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- T c cuprates. The interplay between spin, charge, and orbital degrees of freedom is discussed.

High-pressure structural behavior and equation of state of NaZnF 3

We report the results of density functional theory ab-initio calculations and monochromatic synchrotron X-ray diffraction study carried out for orthorhombic NaZnF 3 in the pressure range 0–40 GPa. Perovskite-to-postperovskite phase transition was anticipated by first-principles computations and then observed in high-pressure diamond anvil cell synchrotron diffraction experiment between 14 and 22 GPa. Above 25 GPa postperovskite structure (CaIrO 3 type, space group Cmcm) coexists with another phase, yet unidentified. On decompression, pure postperovskite-type structure was found to be stable down to 4 GPa; below this pressure sample contained both perovskite and postperovskite modifications. Fit of experimental P–V data to the third-order Birch–Murnaghan equation of state gave bulk moduli, K P, 0 64.98±2.67 and 69.88±3.69 GPa for perovskite and postperovskite modifications, respectively. Both phases demonstrated strong anisotropy of compressibility. For postperovskite NaZnF 3, the highest compression was observed along the direction perpendicular to the planes of ZnF 6 octahedra arrangement.

High-pressure phase transitions of CaRhO3 perovskite

High-pressure phase transitions of CaRhO3 perovskite were examined at pressures of 6–27 GPa and temperatures of 1,000–1,930°C, using a multi-anvil apparatus. The results indicate that CaRhO3 perovskite successively transforms to two new high-pressure phases with increasing pressure. Rietveld analysis of powder X-ray diffraction data indicated that, in the two new phases, the phase stable at higher pressure possesses the CaIrO3-type post-perovskite structure (space group Cmcm) with lattice parameters: a = 3.1013(1) A, b = 9.8555(2) A, c = 7.2643(1) A, V m = 33.43(1) cm3/mol. The Rietveld analysis also indicated that CaRhO3 perovskite has the GdFeO3-type structure (space group Pnma) with lattice parameters: a = 5.5631(1) A, b = 7.6308(1) A, c = 5.3267(1) A, V m = 34.04(1) cm3/mol. The third phase stable in the intermediate P, T conditions between perovskite and post-perovskite has monoclinic symmetry with the cell parameters: a = 12.490(3) A, b = 3.1233(3) A, c = 8.8630(7) A, β = 103.96(1)°, V m = 33.66(1) cm3/mol (Z = 6). Molar volume changes from perovskite to the intermediate phase and from the intermediate phase to post-perovskite are –1.1 and –0.7%, respectively. The equilibrium phase relations determined indicate that the boundary slopes are large positive values: 29 ± 2 MPa/K for the perovskite—intermediate phase transition and 62 ± 6 MPa/K for the intermediate phase—post-perovskite transition. The structural features of the CaRhO3 intermediate phase suggest that the phase has edge-sharing RhO6 octahedra and may have an intermediate structure between perovskite and post-perovskite.

Density Functional Study of Calcium Nitride: Refined Geometries and Prediction of High-Pressure Phases

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.

Thermoelectricity of CaIrO3 ceramics prepared by spark plasma sintering

Polycrystalline bulk CaIrO3 as a main phase with minor phases of IrO2 and/or Ca2IrO4 was prepared by spark plasma sintering (SPS) at 1273 K. Metastable CaIrO3 with a perovskite-type structure (pv-CaIrO3) formed by heat-treatment at 1273 K for 43.2 ks and transformed into a post-perovskite structure (ppv-CaIrO3) by heat-treatment at 1273 K for 259.2 ks. The electrical conductivity (σ) of the pv-CaIrO3 specimen decreased from 1.74 × 104 to 1.45 × 104 Sm-1, while the σ of the ppv-CaIrO3 specimen increased from 5.0 × 101 to 9.8 × 102 Sm-1 with increasing temperature from 298 to 1023 K. The Seebeck coefficient (S) of the pv-CaIrO3 specimen was around -44 μVK-1 at room temperature and increase to the highest value of 40 μVK-1 at 1023 K. The S of the ppv-CaIrO3 specimen decreased with increasing temperature and had an almost constant value of about 60 μVK-1 above 600 K. The thermal conductivity (κ) of the ppv-CaIrO3 specimen decreased from 2.7 to 1.2 Wm-1K-1 and that of the pv-CaIrO3 specimen decreased from 1.5 to 1.2 Wm-1K-1 with increasing temperature from 298 to 1023 K. The highest dimensionless figure of merit values (ZT) of the pv-CaIrO3 and the ppv-CaIrO3 specimens were 0.02 and 0.003 at 1023 K, respectively.

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.

Some recent advances in understanding the mineralogy of Earth's deep mantle

Understanding planetary structure and evolution requires a detailed knowledge of the properties of geological materials under the conditions of deep planetary interiors. Experiments under the extreme pressure-temperature conditions of the deep mantle are challenging, and many fundamental properties remain poorly constrained or are inferred only through uncertain extrapolations from lower pressure-temperature states. Nevertheless, the last several years have witnessed a number of new developments in this area, and a broad overview of the current understanding of the Earth's lower mantle is presented here. Some recent experimental and theoretical advances related to the lowermost mantle are highlighted. Measurements of the equation of state and deformation behaviour of (Mg,Fe)SiO3 in the CaIrO3-type (post-perovskite) structure yield insights into the nature of the core-mantle boundary region. Theoretical studies of the behaviour of MgSiO3 liquids under high pressure-temperature conditions provide constraints on melt volumes, diffusivities and viscosities that are relevant to understanding both the early Earth (e.g. deep magma oceans) and seismic structure observed in the present Earth (e.g. ultra-low-velocity zones).

The local post-perovskite structure and its temperature dependence: atom-pair distances in CaIrO3revealed through analysis of the total X-ray scattering at high temperatures

The temperature-dependent post-perovskite structure model of MgSiO3is reinvestigated through analysis of the atom-pair distances observed experimentallyviaFourier transformation of X-ray diffraction and diffuse scattering, the total X-ray scattering, from CaIrO3. In contrast to the results of a previous Rietveld structure refinement, which shows a negative or null thermal expansion of Ir—O and Ca—O bond lengths within the average long-range structure, visual inspection of these atom-pair distances in the pair-distribution function, in addition to structure models fitted through least-squares refinement to this local-structure data, strongly suggests that these distances between atom pairs increase with temperature. The average long-range structure of CaIrO3, visible from Rietveld structure refinement, is distinct from the short-range structure (≤ 18 Å) at all of the temperatures examined in this study (325–1114 K) and is reproduced in structure models fitted to the pair-distribution function extending to sufficiently long atom-pair distances (≥ 50 Å). While previous data obtained with Rietveld structure refinement show the iridium coordination octahedra to distort with increasing temperature, models of the short-range structure demonstrate that these polyhedra instead reduce distortion and rotate in a manner similar to that occurring in the perovskite structure.

Numerical modelling of dislocations and deformation mechanisms in CaIrO 3 and MgGeO 3 post-perovskites—Comparison with MgSiO 3 post-perovskite

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: MgSiO 3, MgGeO 3 and CaIrO 3. 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 MgSiO 3 and CaIrO 3 lead to similar crystal-preferred orientation of the polycrystal aggregates.

Possible structural polymorphism in Al-bearing magnesiumsilicate post-perovskite

aB s T rac T In the present study, we summarize indications for the existence of kinked post-perovskite structures in the MAS system. X-ray diffraction data and Raman spectra of aluminous magnesium metasilicate post-perovskite are inconsistent with the CaIrO3 structure. Instead the observations are consistent with structures intermediate between the perovskite and the CaIrO3 structure. Ab initio calculations show that the enthalpies of the kinked structures are slightly higher than the CaIrO3 structure at 0 K. Finite temperature, minor element chemistry, kinetics of phase transformation, and actual stress regime are plausible reasons for the observed differences between the present and the previously reported postperovskite phases.

Gasketing optimized for large sample volume in the diamond anvil cell: first application to MgGeO3and implications for structural systematics of the perovskite to post-perovskite transition

Structure models of MgGeO3post-perovskite (Cmcm) are presented, along with a structure survey, demonstrating that all perovskite, post-perovskite and CaIrO3-type structures (ABX3) have specific ranges of the volume ratio between cation-centered polyhedra (VA:VB). The quality of the reported diffraction data and MgGeO3structure models is enhancedviaimplementation of a new graphite gasket for the diamond anvil cell, which stabilizes a larger sample volume, improving powder statistics during X-ray diffraction, andviathe thermal insulation required to achieve ultra-high temperatures while laser-heating samples at pressures near 100 GPa. The structure survey supports the theory that the pressure–temperature conditions under which the perovskite/post-perovskite phase transition occurs can be estimated by extrapolating the change inVA:VBto a value of 4, which corresponds to a maximum tilt ofBX6octahedra in the perovskite structure (Pbnm) where inter-octahedral anion–anion distances match the average intra-octahedral anion–anion distance. Once these short inter-octahedral distances between anions are reached in the perovskite structure, further tilting of octahedra and decrease of theVA:VBratio does not occur, driving the transition to post-perovskite structure as pressure is increased.

ポストペロブスカイトアナログ物質の探索および物性研究

Two post-perovskite oxides were discussed. One is CaPtO3, which is a new analog to the isostructural MgSiO3. By analyzing structural parameters, we elucidate the role of covalency in the post-perovskite compounds. The other is a new solid solution Ca1-xNaxIrO3 (0 ≤ x ≤ 0.37). This system exhibits a filling-control metal-insulator transition on a quasi-two-dimensional lattice. These studies indicate that the post-perovskite structure is an excellent playground to search novel electronic phases of strongly correlated electrons.

Equations of state of CaIrO3 perovskite and post-perovskite phases

Unit-cell lattice parameters have been measured to ~8 GPa in a diamond anvil cell for two single crystals of CaIrO3: one with the perovskite (Pbnm) and the other with the post-perovskite (Cmcm) structure. The CaIrO3 post-perovskite structure is more compressible than the perovskite. A third-order Birch Murnaghan equation of state has been used to fit the measured P-V data with the following refined parameters: V0 = 229.463(8) Å3, K0 = 198(3) GPa, K’ = 1.2(8); and V0 = 226.38(1) Å3, K0 = 181(3) GPa, K’ = 2.3(8) for CaIrO3 perovskite and post-perovskite, respectively. The compressibility of the unit-cell axes of the perovskite structure is highly anisotropic with βa >> βc >> βb. In contrast, the b axis is the most compressible in the post-perovskite structure, whereas the a and c axes have similar compressibilities (with c slightly less compressible than a) and are much stiffer. A comparison between the compressibility of CaIrO3 perovskite and post-perovskite with the isostructural MgSiO3 phases, reveals a similar general behavior, although in detail CaIrO3 perovskite is more and the postperovskite less anisotropic than the corresponding MgSiO3 compounds.

High-pressure–high-temperature phase relations ofMgGeO3: First-principles calculations

The high-pressure--high-temperature behavior of $\mathrm{Mg}\mathrm{Ge}{\mathrm{O}}_{3}$ has been investigated by first-principles computations. It is found that at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the first transformation from ilmenite to orthorhombic perovskite at $24--38\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ is followed by the second one to the $\mathrm{Ca}\mathrm{Ir}{\mathrm{O}}_{3}$ structure at $51--56\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. Quasiharmonic free energy calculations suggest that the first transformation has a negative Clapeyron slope ($\ensuremath{-}9.3\phantom{\rule{0.3em}{0ex}}\mathrm{MPa}∕\mathrm{K}$ at $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$), whereas the second one has a less temperature-sensitive positive Clapeyron slope ($+7.8\phantom{\rule{0.3em}{0ex}}\mathrm{MPa}∕\mathrm{K}$ at $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$). It is also confirmed that the $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ structure does not have its own stability $P,T$ conditions. Pressure dependence of the Goldschmidt tolerance factor indicates a clear correlation between distortion of the perovskite structure and relative stability of perovskite and postperovskite structures.

Lattice preferred orientation in CaIrO3 perovskite and post-perovskite formed by plastic deformation under pressure

Lattice preferred orientations (LPO) developed in perovskite and post-perovskite structured CaIrO3 were studied using the radial X-ray diffraction technique combined with a diamond anvil cell. Starting materials of each phase were deformed from 0.1 MPa to 6 GPa at room temperature. Only weak LPO was formed in the perovskite phase, whereas strong LPO was formed in the post-perovskite phase with an alignment of the (010) plane perpendicular to the compression axis. The present result suggests that the (010) is a dominant slip plane in the post-perovskite phase and it is in good agreement with the crystallographic prediction, dislocation observations via transmission electron microscopy, and a recent result of simple shear deformation experiment at 1 GPa–1,173 K. However, the present result contrasts markedly from the results on MgGeO3 and (Mg,Fe)SiO3, which suggested that the (100) or (110) is a dominant slip plane with respect to the post-perovskite structure. Therefore it is difficult to discuss the behavior of the post-perovskite phase in the Earth’s deep interior based on existing data of MgGeO3, (Mg,Fe)SiO3 and CaIrO3. The possible sources of the differences between MgGeO3, (Mg,Fe)SiO3 and CaIrO3 are discussed.

Compression, thermal expansion, structure, and instability of CaIrO3, the structure model of MgSiO3 post-perovskite

Analysis of pressure-temperature dependent monochromatic X-ray powder diffraction data yield the bulk modulus [KT = 180.2(28) GPa] and thermal expansion coeficients [α0 = 2.841(34) × 10-5 K-1; α1 = 3.37(48) × 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) Å3 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.

On the polyhedral volume ratios VA /VB in perovskites ABX 3

This paper presents analytical expressions for the calculation of ratios of cation coordination polyhedra volumes (V(A)/V(B)) for perovskites ABX(3) of the Stokes-Howard diagram directly from atomic coordinates. We show the advantages of quantifying perovskite structure distortion with polyhedral volume ratios rather than with tilting angles, and discuss why space groups with multiple crystallographically inequivalent A or B sites (I4/mmm, Immm, P4(2)/nmc etc.) are much less common than those with a single A and B site (I4/mcm, R\bar 3c, Pnma etc.). Analysis of crystallographic data for approximately 1300 perovskite structures of oxides, halides and chalcogenides from the Inorganic Crystal Structure Database revealed that the most highly distorted perovskites belong to the space group Pnma and formally lower-symmetry perovskites (I2/m, I2/a) are less distorted geometrically. Critical values of the V(A)/V(B) ratios for the most common phase transitions Pnma <--> I4/mcm and Pnma<--> R\bar 3c are estimated to be approximately 4.85 with the possible intermediate space group Imma stable in the very narrow range of V(A)/V(B) approximately 4.8-4.9. Transitions to post-perovskite CaIrO(3)-type structures may be expected for V(A)/V(B) < 3.8.

Ca1−xNaxIrO3 as a novel quasi-two-dimensional Mott transition system

We have successfully synthesized Ca1−xNaxIrO3 with the post-perovskite structure and elucidated their electronic properties. CaIrO3 is a S=1/2 antiferromagnetic Mott insulator with the Néel temperature (TN) of 115K. A gradual decrease in TN 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.

Rietveld structure refinement of perovskite and post-perovskite phases of NaMgF3 (Neighborite) at high pressures

Neighborite (NaMgF 3 ) with the perovskite structure, transforms to a post-perovskite (ppv) phase between 27 and 30 GPa. The ppv phase is observed to the highest pressures achieved (56 GPa) at room temperature and transforms to an as yet unknown phase upon heating. Rietveld structure refinement using monochromatic synchrotron X-ray diffraction data provide models for the perovskite and post-perovskite structures at high pressure. The refined models at 27(1) GPa indicate some inter-octahedral F-F distances rival the average intra-octahedral distance, which may cause instability in the perovskite structure and drive the transformation to the post-perovskite phase. The ratio of A-site to B-site volume ( V A / V B ) in perovskite structured NaMgF 3 (ABX 3 ), spans from 5 in the zero-pressure high-temperature cubic perovskite phase to 4 in this high-pressure perovskite phase at 27(1) GPa, matching the V A / V B value in post-perovskite NaMgF 3 . Using Rietveld refinement on post-perovskite structure models, we observe discrepancies in pattern fitting, which may be described in terms of development of sample texture in the diamond-anvil cell, recrystallization, or a change of space group to Cmc 2 1 , a non-isomorphic subgroup of Cmcm —the space group describing the structure of CaIrO 3 .

Stability and compressibility of the high-pressure phases of Al 2O 3 up to 200 GPa: Implications for the electrical conductivity of the base of the lower mantle

We have used a laser-heated diamond anvil cell to investigate the stability and compressibility of Cmcm CaIrO 3-type (post-perovskite structure) Al 2O 3 at pressures up to 200 GPa. A phase transformation from the Pbcn Rh 2O 3(II)-type to the CaIrO 3-type structure was observed at 130 GPa, which is consistent with previous theoretical studies. The observed CaIrO 3-type structure in Al 2O 3 is the same as that in MgSiO 3 post-perovskite, the main mineral of Earth’s lowermost mantle. We also calculated the Raman shifts of CaIrO 3-type Al 2O 3 and MgSiO 3 using density-functional perturbation theory. The similarity of the crystal structures and Raman spectra of CaIrO 3-type Al 2O 3 and MgSiO 3 suggests that the other physical properties of the two phases could be similar as well. Based on the high electrical conductivity of CaIrO 3-type Al 2O 3, we predicted a profile of electrical conductivity at the bottom of the lower mantle, which can explain Earth’s rotation period changes of a few milliseconds in Earth’s length of day on decadal timescales, if the exchange of angular momentum between the solid mantle and fluid core occurs by an electromagnetic coupling between the conducting core and mantle.