Novel catalytic properties of quadruple perovskites

ABSTRACT Quadruple perovskite oxides AA′3 B 4O12 demonstrate a rich variety of structural and electronic properties. A large number of constituent elements for A/A′/B-site cations can be introduced using the ultra-high-pressure synthesis method. Development of novel functional materials consisting of earth-abundant elements plays a crucial role in current materials science. In this paper, functional properties, especially oxygen reaction catalysis, for quadruple perovskite oxides CaCu3Fe4O12 and AMn7O12 (A = Ca, La) composed of earth-abundant elements are reviewed.


Introduction
Perovskite oxides ABO 3 (A = alkaline, alkaline-earth and rare-earth metal ions; B = d-block transition metals) have been extensively investigated as promising functional materials in the past decades [1][2][3]. A large number of perovskite and related materials were synthesized and their properties were exhaustively illustrated. Partial chemical substitutions of A-and/or B-sites for ABO 3 -type perovskite and related structures drive structural and electronic transformations, leading to functional properties such as large magnetoresistance and high-temperature superconductivity [4,5]. A couple of different metal ions occupying the identical crystallographic sites make spatial ordering of atoms, crystallizing in ordered perovskite structures with multiplied chemical formulae such as A 2 BB′O 6 , AA′B 2 O 6 and AA′ 3 B 4 O 12 . Sr 2 FeMoO 6 is a double perovskite consisting of two B-site ions, Fe 3+ and Mo 5+ , in which a significant difference in valence states results in rock-salt type ordering of these two ions [6]. YBaMn 2 O 6 is another type of double perovskites with layered ordering of A-site ions Y 3+ and Ba 2+ [7]. The quadruple perovskite oxide family AA′ 3 B 4 O 12 is derived from 1:3-type A-site cation ordering; one quarter (A) is occupied by conventional A-site ions in icosahedral coordination, the remaining three quarters by transition metal ions in pseudosquare-planar coordination ( Figure 1). The quadruple perovskite oxide family was discovered by Bochu et al. in the 1970s [8,9], and has now been intensively investigated because of a lot of syntheses of new compounds using ultra high pressures above 10 GPa. Combinations of A-, A′-and B-site ions provide unexpected properties, some of which are functional. In this paper, recent advances on the catalytic properties of the quadruple perovskite oxides, especially those consisting of earth-abundant elements, are reviewed.

Electronic properties
Electronic properties of quadruple perovskite oxides have been widely investigated. CaCu 3 Ti 4 O 12 is the best-known quadruple perovskite and is investigated as a dielectric material owing to its large permittivity [38,39]. (Ca/La)Cu 3 Mn 4 O 12 exhibits a room-temperature magnetoresistance in low magnetic fields [40,41]. CaCu 3 Ru 4 O 12 and CaCu 3 Ir 4 O 12 show enhanced electron mass, like a heavy fermion [42,43]. NaMn 7 O 12 displays sequential charge/spin/orbital orderings [44]. These properties are summarized in the review [45].
The above-mentioned properties are mainly attributed to electronic interactions between A′-and B-site metals. A striking feature of A′-B site electronic interactions is intermetallic charge transfer reported for the ACu 3 Fe 4 O 12 system. A temperature-induced A′-to-B electron charge transfer from Cu to Fe (3Cu 2+ + 4Fe 3.75+ → 3Cu 3+ +4Fe 3+ ) occurs at ~400 K for LaCu 3 Fe 4 O 12 [13], although a charge disproportionation (2Fe 4+ → Fe 3+ +Fe 5+ ) below 210 K was reported for Ca-analogue CaCu 3 Fe 4 O 12 previously [17]. Later, an inverse B-to-A′ electron charge transfer from Fe to Cu simultaneously with the charge disproportionation was demonstrated for CaCu 3 Fe 4 O 12 [46], ensuring the bidirectionality of A′-B intersite charge transfer. The charge-disproportionation/transfer transitions are abruptly switched by bond strains on R-O (R: rare-earth metals) and Fe-O bonds for RCu 3 Fe 4 O 12 [47,48], evidencing multiple competing electronic states in ACu 3 Fe 4 O 12 series [49]. SrCu 3 Fe 4 O 12 displays a second-order continuous Cu-to-Fe electron charge transfer associated with a negative thermal expansion in the temperature range between ~200 and ~270 K [50][51][52]. More recently, adjustment of thermal expansion properties, that is, relaxations from first-to second-order charge transfer resulting in negative or near-zero thermal expansions in moderate temperature ranges were found for Mn-doped    [53] with permission from aiP Publishing llc.

Catalytic properties
Recently, catalytic properties of quadruple perovskite oxides synthesized under high pressure were reported. Fe 4+ -containing perovskites, CaFeO 3 , SrFeO 3 and CaCu 3 Fe 4 O 12 display catalytic activities for the oxygen evolution reaction (OER; 4OH -→ O 2 +2H 2 O+4ein alkaline conditions). These are comparable to or exceed the activities of state-of-the-art OER catalysts such as Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) [55] and RuO 2 (Figure 4). OER plays a crucial role in energy conversions such as water splitting and recharge of metal-air batteries, as well as the importance of the oxygen reduction reaction (ORR; O 2 +2H 2 O+4e -→ 4OHin alkaline conditions) for fuel cell operations [56][57][58]. CaCu 3 Fe 4 O 12 is a promising OER catalyst because of the earth-abundant constituent elements (Ca, Cu and Fe).
The origin of highly active OER catalysis of Fe 4+perovskites could be explained by two factors: metaloxygen covalency and electronic states. Suntivich et al. [59] exhibited a volcano-shaped plot of OER catalytic activity for ABO 3 -type perovskites, in which the activity is highest in the vicinity of the e g~1 .2 orbital occupancy of B-site ions. Furthermore, Grimaud et al. [60] reported that OER activities of Co-containing double perovskites LnBaCo 2 O 6-δ (Ln = lanthanide metals) are related to the energy levels of oxygen 2p band center; the OER activity is enhanced when oxygen 2p band center is moderately close to the Fermi level. These tendencies are explained by the OER reaction model in which charge transfer from adsorbed reactants and intermediates to transition metal ions (active sites) of a catalyst is facilitated to decrease the energy barrier of a rate-determining step when the metal-oxygen covalency becomes strong [61]. The unusual high valence ions (Fe 4+ ) may form strong covalency with oxygen ions [62], thus the high OER activity is expected for Fe 4+ -perovskites.
More interestingly, CaCu 3 Fe 4 O 12 is more active for OER than simple perovskite counterparts. Since high OER activity is not known for cupric oxides, the Cu ions incorporated into A′-sites are not expected to contribute to the OER activity. In addition, synergistic effects between Cu and Fe do not seem to be achieved because another Cu-Fe complex oxide (spinel-type CuFe 2 O 4 ) has a lower activity. Crystal structure analysis suggests that heavily tilted Fe-O-Fe bonds make the neighboring adsorbates close enough to interact, possibly enabling two activesite reaction mechanisms ( Figure 5). This mechanism is different from conventional single-active-site reactions   ions with electronic isolation from coordinating oxygen ions are easily dissolved in the electrolyte for SrFeO 3 . On the other hand, CaCu 3 Fe 4 O 12 has a widespread covalent bonding network in which pseudosquare-planar CuO 4 and octahedral FeO 6 units are electronically connected. The degradation of perovskite oxide OER catalysts is an essential issue [60,63,64] but efficient strategies have not been established yet. This finding provides a new design principle for highly active and robust catalysts for OER.
To elucidate A′-site cation contributions to a reaction mechanism, comparison of simple and quadruple perovskite catalysts with identical elements/ions was conducted [65]. Since Mn-based perovskites AMnO 3 and for simple perovskite catalysts and is expected to avoid rate-determining steps of conventional mechanisms [59].
Another remarkable feature of CaCu 3 Fe 4 O 12 catalyst is durability in OER conditions. The simple perovskites CaFeO 3 and SrFeO 3 were readily degraded during 100 sequential OER measurements because of progressive surface amorphization ( Figure 6). In contrast, CaCu 3 Fe 4 O 12 retained the initial OER activity after 100 OER measurements. The enhancement of stability for CaCu 3 Fe 4 O 12 is attributed to the widespread covalent bonding network consisting of Cu-O and Fe-O bonds. Figure 7 illustrates the electron density distributions of SrFeO 3 and CaCu 3 Fe 4 O 12 obtained from Rietveld refinement and maximum entropy method analysis of synchrotron X-ray diffraction data. It is expected that highly ionic Sr  compositional differences are suppressed, compared with the above-described couple (CaFeO 3 and CaCu 3 Fe 4 O 12 ). Figure 8 shows linear sweep voltammograms of Mn-perovskites in OER conditions. There are clear differences in catalytic activity between AMnO 3 and AMn 7 O 12 series, namely, overpotentials are lower by 0.1 V and specific activities at 1.7 V versus reversible hydrogen electrode (RHE) are about thirtyfold for AMn 7 O 12 . This observation implies higher catalytic activities of quadruple perovskite oxides are derived not from electronic factors but structural effects altering reaction mechanisms. Indeed, first-principle calculations do not explain the origin of increased OER activity for quadruple manganese perovskites. In contrast, structure analysis based on Rietveld refinement of synchrotron X-ray diffraction data suggests a structure-activity    relationship for Mn-oxide catalysts. Figure 9 shows the specific activity versus average Mn-Mn distances for various manganese oxide catalysts including corner/ edge-shared Mn-O polyhedral units, in analogy with the Mn-Mn distance dependence of photocatalytic turnover frequency for various manganese oxides [66]. The specific activity is enhanced as the Mn-Mn distance increases from ~3 to ~3.2 Å, although poorly active for longer Mn-Mn distances (~3.8 Å). This feature is explained by possible O-O bond formation for a two-site reaction mechanism. Figure 10 illustrates the proposed OER mechanism for quadruple perovskite catalysts, in which the two neighboring active sites (A′-and B-site Mn atoms) are close enough to connect their adsorbates. Rate-determining steps including O-O bond formation are avoided in this mechanism [59]. AMn 7 O 12 catalysts also catalytically active for ORR, as well as most manganese oxides ( Figure 11). Thus, AMn 7 O 12 perovskites are bifunctional catalysts for oxygen reactions (OER and ORR).

Conclusions
Recent advances on novel functional properties for quadruple perovskite oxides are reviewed. Highly active oxygen reaction catalysis for quadruple perovskite oxides consisting of earth-abundant elements demonstrates that utilization of ultra-high-pressure synthesis facilitates development of novel functional materials. A great number of still-unexplored compounds synthesized under high pressure also remain promising candidates for functional materials. Thus, further investigation of high-pressure-synthesized materials can give new impetus to materials science.