A summary of the “113” Alkaline-earth iridates AIrO3 (A=Ca, Sr, Ba)
Perovskite iridates have emerged as a new paradigm for studying the strongly correlated electron physics with strong spin-orbit coupling. The “113” alkaline-earth iridates AIrO3 (A = Ca, Sr, Ba) display a rich variety of crystallographic and electronic states and are now attracting growing research interest. This chapter aims to provide an overview for these “113” iridates, including the materials’ synthesis, crystal structure, major physical properties, and other interesting results such as the effects of pressure and chemical substitutions, as well as theoretical perspectives.
- Perovskite iridates
- Spin-orbit coupling
The discoveries of high-transition-temperature superconductivity in cuprates and the colossal magnetoresistance in manganites made the first-row (3d) transition-metal oxides (TMOs) with perovskite-related structures the central topics of condensed matter physics over the past four decades. The strong electron–electron correlations intrinsic for these narrow-band 3d-electron systems are believed to be at the heart of rich physics. Following the general wisdom based on the 3d TMOs, the third-row (5d) counterparts having a spatially much extended 5d orbitals were expected to have much reduced electron–electron correlations, U, and broaden bandwidth, W, i.e. U << W, leading to a Pauli paramagnetic metallic ground state, Figure 1(a). Such an expectation, however, was recently found to be violated in many 5d-electron iridium oxides (iridates), such as Sr2IrO4 , in which an antiferromagnetic insulating ground state was instead observed. Recent studies have revealed that such discrepancy originates from the inherently strong spin-orbit coupling (SOC) for these heavy 5d elements, which have a typical value of SOC, ζSO ≈ 0.3–0.5 eV, comparable with the magnitude of U and W, and thus cannot be treated as a negligible perturbation as in the 3d TMOs.
Since an unrealistically large U is required to open a Mott gap in Sr2IrO4, Figure 1(b), Kim et al.  proposed that the strong SOC splits the otherwise broad t2g band of the octahedral-site, low-spin Ir4+(5d 5) array into a filled, low-energy Jeff = 3/2 quartet band and a half-filled, high-energy Jeff = 1/2 doublet band, Figure 1(c, e). A moderate Hubbard U can then open a Mott gap, leading to the SOC-driven Jeff = 1/2 Mott insulating state, Figure 1(d). Subsequent experimental  and theoretical  investigations have confirmed such a novel Jeff = 1/2 state in the strong SOC limit. Since then, the 5d TMOs have emerged as a new paradigm for studying the strongly correlated electron physics with strong SOC. In particular, the iridates have attracted special attention in that the combination of relativistic SOC and electron–electron correlations has been proposed to generate more exotic, unprecedented quantum states of matters, such as the strong topological insulators, Weyl semimetal, quantum spin liquids, and even unconventional superconductors .
Since the importance of SOC was first recognized in Sr2IrO4, which is the
Although there are many publications dealing with an individual compound, a monograph that provides a comprehensive overview for these “113” alkaline-earth iridates is still lacking to our knowledge. Taking into account the growing research interests on these iridates, it is imminent to summarize the currently available knowledge in a single chapter. Thus, this chapter aims to bring together the available information in literature for these “113” iridates. In the following, we will give a comprehensive literature survey for each AIrO3, covering the materials’ synthesis, crystal structure, and major physical properties, as well as other interesting results such as the effects of chemical substitutions and theoretical investigations. Finally, we will give a brief concluding remark on the current research status and provide an outlook on the future research directions on these iridates.
|CaIrO3||pPv||AF insulator with TN = 110 K, stripe-type AF order with spin canting;|
|Pv||PM semimetal with possible Dirac node protected by reflection symmetry|
|SrIrO3||6H||Exchange enhanced PM metal with nFL behaviors due to proximity to a FM QCP|
|Pv||PM semimetal with possible Dirac node protected by reflection symmetry|
|BaIrO3||9R||Weak FM insulator with a simultaneous CDW formation below Tc ≈ 180 K|
|5H||Weak FM metal with Tc ≈ 50 K|
|6H||Exchange enhanced PM metal with nFL behaviors due to proximity to a FM QCP|
|3C||FL PM metal|
CaIrO3 has two different orthorhombic polymorphs, i.e. the layered pPv structure with space group
There are some discrepancies in literature regarding the synthesis of pPv CaIrO3 at ambient pressure. In the earlier studies [12, 22], it was reported that single-phase pPv phase cannot be obtained at ambient pressure through a solid-state reaction from CaCO3 and IrO2 in air. Recently, Harai et al.  reported that pure pPv CaIrO3 can be prepared by heating the stoichiometric mixture of CaO and IrO2 powders sealed in an evacuated silica tube at 1000°C over 20 h. On the other hand, since the pPv structure is a high-pressure phase, pPv CaIrO3 can be readily obtained by utilizing HPHT synthesis. For example, Ohgushi et al.  reported the synthesis of single-phase pPv CaIrO3 at 4 GPa and 1150°C.
Needle-shaped pPv CaIrO3 single crystals have been reported to grow out of the CaCl2 flux. By adopting a tenfold flux and a relatively low soaking temperature of 836 and 950°C, respectively, Sugahara et al.  and Hirai et al.  obtained tiny single crystals for the purpose of crystal-structure refinements. On the other hand, Ohgushi et al.  seems to grow sizable pPv CaIrO3 single crystals for anisotropic magnetic property measurements by employing a higher flux molar ratio (16:1) and a higher soaking temperature of 1200°C. However, our attempts by using the latter approach ended up with Ca2IrO4 rather than the pPv CaIrO3.
Because Pv CaIrO3 is a metastable phase, it cannot be synthesized via a solid-state reaction route at ambient pressure. Alternatively, Sarkozy et al.  reported the preparation of pure Pv phase by thermal decomposition at 650–700°C in air of the hydroxide intermediate CaIr(OH)6, which can be obtained according to the following wet-chemical reaction scheme:
By following this approach, we obtained nearly single-phase Pv CaIrO3 with a trace amount of IrO2 (0.2 wt.%) and Ca2IrO4 (1.3 wt.%) . Recently, Kojitani et al.  determined a large positive Clapeyron slope for the pPv/Pv transition of CaIrO3, i.e. Pv structure is the high-temperature phase of pPv. Thus, Pv CaIrO3 can be obtained by transforming pPv phase at higher temperature under given pressures. For example, Ohgushi et al.  have reported the synthesis of single-phase Pv CaIrO3 at 1 GPa and 1450°C. In addition, thin films of Pv CaIrO3 have recently been epitaxially stabilized on various substrates [26, 32].
2.2. Crystal structure
2.3. Physical properties
The Mott insulating nature of quasi-2D pPv CaIrO3 have motivated Ohgushi et al.  to metallize it via the carrier doping. They successfully prepared a series of hole-doped Ca1–
In addition to the interest in fundamental physics, the CaIrO3 ceramics have also been investigated by Keawprak et al.  for the potential thermoelectric applications. They prepared both phases of CaIrO3 with spark plasma sintering technique and evaluated their thermoelectric properties from room temperature up to 1023 K. The highest dimensionless figure of merit (ZT) reaches 0.02 and 0.003 for Pv and pPv phase, respectively.
Depending on the synthesis conditions, SrIrO3 can form in two different structures, i.e. the monoclinically distorted 6H polytype and the orthorhombic GdFeO3-type Pv structure . The former is a rare stoichiometric oxide exhibiting non-Fermi-liquid behaviours near a ferromagnetic quantum critical point . The latter was recently found to be an exotic narrow-band semimetal that may harvest many topological and magnetic insulating phases [10, 41, 42].
The 6H phase can be readily prepared in the polycrystalline form at ambient pressure by sintering the stoichiometric mixture of SrCO3 and IrO2 (or Ir) at 900–1,100°C in air . Single crystals of 6H phase with dimensions ~0.4 × 0.4 × 0.6 mm3 have been grown in Pt crucibles with the SrCl2 self-flux techniques . The Pv phase is a HP form of SrIrO3. Longo et al.  performed the first HPHT syntheses and established the temperature–pressure phase diagram for the 6H-Pv transformation of SrIrO3. It was found that the 6H phase transforms to the Pv structure above 1,650°C at 2 GPa and above 700°C at 5 GPa. Recent HPHT syntheses of Pv SrIrO3 were usually performed at 1,000–1,100°C and 5–6 GPa [43, 44]. For these samples, Rietveld refinements on the powder XRD patterns evidenced the presence of ~3–4 wt.% IrO2 impurity. Since the Pv phase is metastable, it remains a challenge to obtain sizable bulk single crystals under HP conditions. However, Pv SrIrO3 films and superlattices have been stabilized at ambient pressure via applying the epitaxial strain with various techniques, including the metalorganic chemical vapour deposition , pulsed laser deposition , and reactive oxide molecular-beam expitaxy . As discussed below, given the tolerance factor
3.2. Crystal structure
In the original work by Longo et al., the oxygen positional parameters were not refined due to the low scattering of oxygen relative to Ir and Sr. Based on the neutron diffraction data, Qasim et al.  recently provided a full refinement on the crystal structure of 6H SrIrO3 with
3.3. Physical properties
As the end member of the Ruddlesden–Popper series Sr
Recent angle-resolved photoemission spectroscopy on Pv SrIrO3 films by Nie et al.  has uncovered such an exotic semimetallic state with very narrow bands near the Fermi surface consisting of heavy hole-like pockets around (±π, 0) and (0, 0) and light electron-like pockets at (±π/2, ±π/2). Surprisingly, the bandwidth of Pv SrIrO3 is found to be narrower than that of Sr2IrO4, in contrary to the general expectations of broaden bandwidth with increasing dimensionality . Since the semimetallic ground state has been confirmed experimentally, it is of particular interest to achieve the proposed topological and/or magnetic states via tuning the SOC, U, and/or lattice symmetry. In this regard, Matsuno et al.  have made an important step towards these exotic phases; they tailored a spin-orbit magnetic insulator out of the semimetallic state via controlling the dimensionality of [(SrIrO3)m, SrTiO3] superlattices. By utilizing HPHT synthesis, we prepared a series of Sn-doped SrIr1–
At ambient pressure, BaIrO3 crystallizes in the nine-layer (9R) polytype. It is the first known ferromagnetic insulator with
The ambient-pressure 9R phase can be readily obtained by sintering the stoichiometric mixtures of BaCO3 and Ir at 1,000°C in air. The sample should be cooled down slowly for the last sintering in order to ensure an oxygen stoichiometry . Single crystals have been reported to grow out of the BaCl2 flux at a relatively low temperature of 1,000 K . HPHT synthesis is needed for all the other polytypes [19, 21, 55, 56]. For the HP syntheses around 1,000°C, the 9R polytype is stable up to 3 GPa, the 5H phase exists only in a narrow pressure range around 4 GPa, the 6H phase is stabilized in a wide pressure range from 5 to ~20 GPa, and the 3C phase was finally obtained at 25 GPa. We have employed the two-stage (Walker- or Kawai-type) multianvil systems for the HPHT syntheses. During the HPHT experiments, the sample was first compressed to the desired pressure by eight truncated tungsten carbide anvils, and then the temperature was increased to ~1,000°C and kept for 30 min before quenching to room temperature. The resultant samples were recovered after releasing pressure and then subjected to various characterizations at ambient pressure.
4.2. Crystal structure
The small tetragonal distortion of the 3C BaIrO3 phase is unexpected; we should have a cubic phase as found for BaRuO3 formed under high pressure. Such a distortion to tetragonal symmetry by cooperative rotations of the IrO6/2 octahedra about the
4.3. Physical properties
The observation of weak ferromagnetism and insulating ground state in the 9R BaIrO3 has attracted renewed interest in recent years in light of the SOC-driven Mott insulating state for iridates. As for the nature of the weak ferromagnetism, there also exist long-standing discrepancies. Experimentally, a tiny Ir moment of ~0.03 μB/Ir was observed below
Although the atomic-like nature of Ir local moment in 9R BaIrO3 was found to be extremely stable against temperature, pressure, and chemical substitutions [52, 61], these external stimuli can easily lead to a breakdown of the weak ferromagnetism and nonmetallic ground state. For example, Cao et al.  grown a series of Sr-doped Ba1-xSrxIrO3 single crystals and found that the chemical pressure applied via Sr doping drastically suppresses
We have summarized in this chapter the current knowledge on the materials’ synthesis, crystal structure, and physical properties of the “113” alkaline-earth iridates AIrO3 (A = Ca, Sr, Ba), which display a rich variety of crystallographic and electronic states that are of great current research interest. For CaIrO3, it can form in either the layered pPv or the orthorhombic Pv structure, and thus serves as an important analogue of MgSiO3 to investigate the Pv/pPv transformation in the Earth’s lowermost mantle in geosciences. Corresponding to different crystal structures, their electronic ground states differ sharply: the pPv phase is an antiferromagnetic Mott insulator with
We are grateful to J.-Z. Zhou, J. B. Goodenough, José Alonso, Y. Uwatoko, and M. Akaogi for collaborations on work related to this review. This work is supported by the National Basic Research Program of China (Grant No. 2014CB921500), the National Science Foundation of China (Grant Nos. 11304371, 51402019), and the strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB07020100).
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