High-pressure synthesis and physical properties of perovskite and post-perovskite Ca${}_{1-x}$Sr${}_x$Ir$O_3$

The post-perovskite (pPv) is the high-pressure phase of some highly distorted perovskites. The pPv phase of MgSi$O_3$ stabilized under 125 GPa and 2000 K cannot be quenched to ambient pressure. In contrast, the pPv CaIr$O_3$ can be synthesized under a modest pressure or even at ambient pressure. However, the pPv CaIr$O_3$ has not been fully characterized. We report here systematic structural studies, measurements of transport and magnetic properties including critical phenomena, specific heat, and thermal conductivity in a series of samples Ca${}_{1-x}$Sr${}_x$Ir$O_3$ synthesized under high pressure. The Ca${}_{1-x}$Sr${}_x$Ir$O_3$ samples exhibit an evolution from the pPv phase to the perovskite phase. We have also prepared the perovskite (Pv phase) CaIr$O_3$ with the wet chemical method. Rietveld refinements of the pPv and Pv phase CaIr$O_3$ have been made based on high-resolution synchrotron diffraction. In comparison with effects of the chemical substitution on the crystal structure and physical properties, we have studied the structure and magnetic properties of the pPv CaIr$O_3$ under hydrostatic pressure. Results have been discussed in the context of orbital ordering biased on the intrinsic structural distortion and the strong spin-orbit coupling that is much enhanced in these 5$d$ oxides with the pPv structure.

Figure 1

Crystal structures of (a) Pv and (b) pPv CaIrO${}_3$.

Figure 2

Observed (cross) and calculated (solid line) high-resolution XRD profiles of the Pv and pPv CaIrO${}_3$ measured at 295 K with synchrotron radiation (λ$=$0.412 207 Å). Some impurity phases, 0.2% IrO${}_2$ and 1.3% Ca${}_2$IrO${}_4$, in the sample of Pv CaIrO${}_3$ have been determined from the refinement.

Figure 3

Schematic diagram of octahedral-site distortion and orbital levels for the low-spin $d^5$ configuration without the spin-orbit coupling.

Figure 4

XRD patterns of the high-pressure phase of Ca${}_{1-x}$Sr${}_x$IrO${}_3$.

Figure 5

Lattice parameters as a function of $x$ in the (a) pPv and (b) Pv Ca${}_{1-x}$Sr${}_x$IrO${}_3$.

Figure 6

Temperature dependence of resistivity in the pPv and the Pv Ca${}_{1-x}$Sr${}_x$IrO${}_3$.

Figure 7

Temperature dependence of resistance of the pPv CaIrO${}_3$ under different pressures.

Figure 8

Temperature dependence of thermoelectric power for the Pv and the pPv Ca${}_{1-x}$Sr${}_x$IrO${}_3$.

Figure 9

Temperature dependence of magnetic susceptibility and inverse magnetic susceptibility for (a), (b) the Pv phase and (c), (d) the pPv phase of Ca${}_{1-x}$Sr${}_x$IrO${}_3$.

Figure 10

Schematic drawing of the [010] layer and major pathways for the superexchange interactions in the pPv CaIrO${}_3$. Arrows inside octahedra represent the orbital moment at room temperature.

Figure 11

Lattice parameters and magnetic transition temperature as a function of either $x$ in the pPv Ca${}_{1-x}$Sr${}_x$IrO${}_3$ or hydrostatic pressure in the pPv CaIrO${}_3$. Dashed lines inside the plot show schematically the behavior of $T_c$ ($T_N$) as a function of either the lattice expansion or high pressure for magnetic Pv's. The structural data of pPv CaIrO${}_3$ under pressure are from Ref. 22. A similar pressure effect on $T_c$ of pPv CaIrO${}_3$ has also been reported by Ohgushi et al. in Ref. 36.

Figure 12

The magnetization loop $M$ vs $H$ of the pPv CaIrO${}_3$ at different temperatures with ZFC through $T_c$.

Figure 13

The magnetization loop $M$ vs $H$ at 5 K for the pPv CaIrO${}_3$ cooled down through $T_c$ under different magnetic fields. Most curves have been obtained with a MPMS except one (±9 T) with a PPMS.

Figure 14

(a) Field dependence of the isothermal magnetization curves $M$($H$) from 102 to 115 K; (b) the Arrott plot, i.e., $M^2$ vs $H$/$M$, of the isotherms and the polynomial fitting curves to the high-field data of the pPv CaIrO${}_3$.

Figure 15

Temperature dependence of the spontaneous magnetization $M_s$ and the inverse initial susceptibility ${\chi }_0^{-1}$ together with the power-law fitting curves. (b) The log-log plots of $M_s$ and ${\chi }_0^{-1}$ vs reduced temperature $\left|t\right|\equiv |(T-T_c)/T_c|$ with $T_c$$=$108.2 K. (c) The log-log plots of the isotherms at $T$$=$108 and 109 K of the pPv CaIrO${}_3$; an interpolation of these two slopes was used to obtained the slope of the critical isotherm and thus the exponent δ. Other critical exponents have also been given inside the figures.

Figure 16

A scaling plot of $M/{\left|t\right|}^{\beta }$ vs $H/{\left|t\right|}^{\gamma +\beta }$ below and above $T_c$ with the critical exponents $\beta$$=$0.444, $\gamma$$=$1.043, and $T_c$$=$108.2 K for the pPv CaIrO${}_3$. The inset shows the log-log plot of the same data. Isotherms at different temperatures are distinguished by different symbols.

Figure 17

(a) Temperature dependence of thermal conductivity κ and (b) κ(300 K) as a function of $x$ in the pPv and the Pv Ca${}_{1-x}$Sr${}_x$IrO${}_3$.

Figure 18

Temperature dependence of specific heat $C_p$ for the pPv and the Pv Ca${}_{1-x}$Sr${}_x$IrO${}_3$.

Figure 19

Temperature dependence of thermal conductivity κ and curve fitting (solid lines) with the Debye model for the pPv and the Pv Ca${}_{1-x}$Sr${}_x$IrO${}_3$ and the pPv CaPtO${}_3$.