d¹ Square Lattice Superconductors

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Titanium pnictide oxides BaTi₂Pn₂O (Pn = As, Sb, Bi) have a layered structure comprising Ti₂Pn₂O layers separated by Ba cations, as shown in Figure 1.⁹ The Ti³⁺ cations with heteroleptic octahedral coordination of TiO₂Pn₄ forms a Ti₂O square lattice, which is an anticonfiguration to the CuO₂ square lattice in high-T_c cuprates. The electronic configuration of 3d¹ is complementary to Cu²⁺ (3d⁹).

Figure 1. (a) Crystal structures of BaTi₂Pn₂O (Pn = As, Sb, Bi). (b) Ti₂O square lattice (c) TiO₂Pn₄ octahedron.

BaTi₂As₂O shows metallic behavior without any trace of superconductivity, but with anomalies in magnetic susceptibility and electrical resistivity at 200 K, which was ascribed to a charge-density-wave (CDW) transition.¹⁰ We synthesized new antimonide and bismuthide, BaTi₂Sb₂O and BaTi₂Bi₂O,^11,12 and observed a significant reduction of CDW transition to T_CDW = 50 K for BaTi₂Sb₂O, while a CDW instability completely disappears in BaTi₂Bi₂O. The CDW suppression/elimination led to superconductivity with T_c = 1.2 K (BaTi₂Sb₂O) and 4.6 K (BaTi₂Bi₂O). Specific heat, ^121/123Sb-nuclear quadrupole resonance (NQR), and muon spin relaxation measurements on BaTi₂Sb₂O revealed a full-gap s-wave state, with a novel microscopic coexistence of superconductivity and CDW instability.^13–15

The isovalent anionic solid solution of BaTi₂(As_xSb_1−x)₂O and BaTi₂(Sb_1−yBi_y)₂O displays an unusual electronic phase diagram,^16,17 as shown in Figure 2. The Sb-for-As substitution (x = 1 → 0) continuously decreases T_CDW and the superconducting phase appears with a T_c maximum at around y = 0.2. With a further increase of y, the T_c exhibits a sudden jump to 4.5 K at y = 0.60 and remains nearly constant to the terminal composition of BaTi₂Bi₂O (y = 1). It is thus clear that there are two distinct superconducting phases, SC1 for 0 ≤ x ≤ 0.1 and 0 ≤ y ≤ 0.55, and SC2 for 0.6 ≤ y ≤ 1. A similar two-dome structure in T_c has also been reported in the iron pnictide superconductor LaFeAs(O_1−xH_x), which was ascribed to the multiband structures by Fe-3d orbitals.¹⁸ Likewise, the two-dome structure of our titanium pnictide oxides appears to link with a multiband character at the Fermi level, which is indeed supported by theoretical studies, where Ti 3d_xy, 3d_x2−y2, and 3d_z2 orbitals are occupied at the Fermi level E_F.^19–22

Figure 2. Electronic phase diagram of BaTi₂(As_xSb_1−x)₂O and BaTi₂(Sb_1−yBi_y)₂O. Blue circle and red circle show the T_CDW and T_c, respectively. SC denotes superconducting phase.

The overall electronic phase diagram (Figure 2) strongly suggests that the superconductivity in the SC1 phase is driven by the suppression of CDW phase, as often seen in other superconductors. Theoretically, a lattice instability associated with the formation of \(\sqrt{2} \) × \(\sqrt{2} \) × 1 superstructure is predicted in BaTi₂Sb₂O.^22,23 Experimentally, however, no superlattice has been observed below T_CDW, implying that the CDW state in BaTi₂Sb₂O is unconventional. Instead, the NQR/NMR spectra of BaTi₂Sb₂O provided clear evidence of broken in-plane four-fold symmetry,¹⁴ whereas high-resolution neutron diffraction study detected a small lattice distortion to the orthorhombic symmetry with the orthorhombicity, η = 2 × (a − b)/(a + b), of 0.22% and 0.05% (at 20 K) for BaTi₂As₂O and BaTi₂Sb₂O, respectively.²⁴ The observed distortions are much smaller than typically observed in Jahn-Teller compounds and iron pnictides. It means that the electronic instability drives the lattice distortion. One possible scenario to account for the experimental results is an intra-unit-cell nematic charge order or an orbital order.^21,24

Ion-Exchange Reactions to Construct 2D Quantum Magnets

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Low-temperature topochemical reactions such as intercalation and ion-exchange reactions enable a rational design of new types of structures which cannot be obtained by conventional high-temperature solid state reactions.²⁵ A wide range of oxide materials have been investigated as precursors for such reactions.²⁶ In particular, the Dion-Jacobson (DJ) type layered perovskite has manifested as a platform of various topochemical reactions.^25,27 The DJ phase has a general chemical formula given by A′A_n−1B_nO_3n+1, where A′ is an alkali metal, A is an alkaline earth or rare earth metal, B is a d⁰ transition metal, and n is the number of perovskite layers. The interlayer A′ cations are highly reactive and subject to ion-exchange reactions, while the perovskite blocks are chemically inert. Most notably, Wiley et al. demonstrated a co-exchange of cations and anions within the interlayer space;²⁸ a reaction of the non-magnetic oxide RbLaNb₂O₇ (n = 2) with CuCl₂ results in a magnetic compound (CuCl)LaNb₂O₇ (Figure 3). This reaction can be extended to n = 3 and 4 phases.^29–34 In (CuX)A_n−1B_nO_3n+1 (X = Cl, Br), the Cu²⁺ cations are trans-coordinated with two apical oxide ions from the perovskite blocks and four halide ions within the ab plane. As a result, the CuO₂X₄ octahedra share edges to form a square-lattice network, as in the high-T_c cuprates. The co-insertion of halide anions allows super-exchange pathways within the CuX layer. These CuX layers are spatially separated by the non-magnetic perovskite blocks, with the interlayer distance of ∼12 Å for n = 2, ∼16 Å for n = 3, and ∼20 Å for n = 4. These features make (CuX)A_n−1B_nO_3n+1 a promising candidate for 2D quantum magnetic phenomena, or even superconductivity if holes/electron can be donated.

Figure 3. Topochemical ion-exchange reaction to obtain 2D magnetic compounds starting from non-magnetic Dion-Jacobson type perovskite oxides. The ion-exchanged copper halide array forms a square lattice network.

We have found novel quantum magnetic states in (CuX)LaNb₂O₇.²⁸ The magnetic susceptibilities (Figure 4) exhibit a broad maximum centred at 16.5 K (X = Cl) and 36 K (X = Br),^35,36 a characteristic feature of low-dimensional antiferromagnets. While the bromide undergoes a magnetic transition at T_N = 30 K with a stripe spin texture,³⁶ the chloride does not show any anomaly associated with magnetic transition. Inelastic neutron scattering experiments on (CuCl)LaNb₂O₇ revealed a spin-singlet ground state and a triplet excitation gap of 2.3 meV.³⁵ The triplet state is destabilized by applying a magnetic field and when it reaches the spin-singlet ground state at 11 T, a field-induced transition involving Bose-Einstein condensation of magnons occurs.^37,38 Cationic and anionic substitution lead to the observation of novel phenomena including a quantum phase separation.^39–41 The observed properties were initially interpreted in terms of a frustrated square-lattice, or the J₁-J₂ model with an antiferromagnetic nearest-neighbour interaction J₁ and a ferromagnetic next-nearest neighbour interaction J₂ (see Figure 3).⁴² However, the in-depth structural analysis revealed the presence of cooperative tilting of the NbO₆ octahedra in the perovskite block, which lead to a large displacement of Cu and X atoms,^43–45 giving a 2a_p × 2a_p × c_p superlattice (space group Pbam). To our surprise, first principles calculations show that the fourth-nearest Cu–Cl–Cl–Cu pathway, J₄, is responsible for the spin dimer formation (Figure 5), and together with a ferromagnetic component, the magnetism of (CuCl)LaNb₂O₇ can be best described by a ferromagnetic version of the Shastry-Sutherland model.^46–48

Figure 4. Magnetic susceptibility of (CuX)LaNb₂O₇ (X = Cl, Br). The raw data (closed) for X = Cl is corrected by subtracting impurity contribution.^34,35

Figure 5. (a) Major exchange interactions in the CuCl layer of (CuCl)LaNb₂O₇. Blue, gray, and red lines represent J₁, J₂, and J₄ respectively.⁴⁵ (b) Shastry-Sutherland spin model, where spin dimers are orthogonally arranged.⁴⁶

Triple-layered (CuCl)Ca₂Nb₃O₁₀ and (CuBr)Sr₂Nb₃O₁₀ have a non-magnetic and magnetic (helical) ground state, respectively.^49–51 (CuBr)Sr₂Nb₃O₁₀ has a 1/3 magnetization plateau for 2.0 < H < 8.1 T (Figure 6),⁵⁰ which is unprecedented since a 1/2 plateau is expected for the J₁-J₂ model.⁵² Neutron diffraction experiments up to 10 T show that a competition between the two magnetic phases with propagation vectors of k = (0, 0, 0) and (0, 1/3, 0.46) incidentally induces the 1/3 plateau.⁵³ Compounds with other 3d transition ions (thus having different spin degrees of freedom) were also studied. For example, (MCl)LaNb₂O₇ (M = V, Cr, Mn, Fe, Co) exhibit various spin ordered states including stripe order, G-type order, and partially antiferromagnetic state.^54–58

Figure 6. Magnetization curves of (CuBr)Sr₂Nb₃O₁₀ at 1.3 K, with a 1/3 plateau-like behaviour.^34,50

In this way, topochemical reactions offer extensive, rational and flexible design of 2D magnetic system, leading to discoveries of novel physical properties. This approach contrasts with the conventional search for materials via solid state reaction, in the sense that the systematic understanding and controlling is possible by varying constituent, M, X, A, B and n.

Square-Planar Coordinate Iron in Oxides

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Square planar coordination is often found in oxides containing d⁴ or d⁹ electronic configurations, most notably copper oxide high-T_c superconductors with the CuO₂ square-lattice layer. Iron oxides prepared by conventional synthetic techniques are usually found with iron in octahedra, pyramids and tetrahedra. In 2007, SrFeO₂ with iron in square planar coordination with the “infinite-layer” structure was prepared from a low-temperature (280 °C) topochemical reduction of SrFeO₃ perovskite,⁵⁹ using the CaH₂ reduction method developed by Hayward and Rosseinsky.⁶⁰ FeO₄ square planes share corners to construct the square lattice, as in cuprate high-T_c superconductors. The FeO₂ layers are separated by Sr²⁺ cations (Figure 1). The iron is in a divalent state with a high-spin configuration (S = 2) and has the electronic configuration of \((d_{z^{2}})^{2}(d_{xz},d_{yz})^{2}(d_{xy})^{1}(d_{x^{2} - y^{2}})^{1}\).^61,62 Only a handful (distorted) square planar coordinate iron oxides (e.g., gillespite BaFeSi₄O₁₀) were known previously, but with the FeO₄ units being isolated with respect to each other.^63,64 The applied topochemical hydride reduction has opened a pathway to square coordinate iron oxides. For example, hydride reduction of Ruddlesden-Popper perovskite oxides of Sr₂FeO₄ and Sr₃Fe₂O₇ resulted in Sr₂FeO₃ and Sr₃Fe₂O₅, which is, together with SrFeO₂, given by Sr_n+1Fe_nO_2n+1 (n = 1, 2, ∞; Figure 7).^32,65

Figure 7. Crystal Structure of SrFeO₂ (a), Sr₂FeO₃ (b), and Sr₃Fe₂O₅ (c). The red, orange and green spheres represent oxygen, iron and strontium respectively.

Despite the apparent 2D structure, SrFeO₂ undergoes an antiferromagnetic order at a high temperature of T_N = 473 K. However, the subsequent experimental and theoretical studies have revealed that Fe moments are indeed strongly coupled three dimensionally, via in-plane superexchange interactions and out-of-plane direct exchange interactions.^61,62 This contrasts with the isostructural SrCuO₂ with the quasi-2D magnetic correlations. The effect of dimensional reduction is observed in Sr₃Fe₂O₅ and Sr₂FeO₃ with T_N = 296 K and 179 K, respectively. A strong spin-lattice coupling SrFeO₂ giving a transverse displacement of Fe and O atoms is observed at temperatures close to T_N.⁶⁶

The isovalent Sr-site substitution in SrFeO₂ provides a substantial impact on the structure. CaFeO₂ crystallizes in a distorted infinite-layer structure with Ca²⁺ in a six-fold coordination (vs eight-fold coordination at Sr²⁺), which is enabled by distortion and coherent rotation of FeO₄ square-planar units (Figure 8a).^67,68 The barium substitution beyond 30% induces a transition to a new structural type related to LaNiO_2.5 (Figure 8b).⁶⁹ The FeO₄ square-planar coordination in CaFeO₂ is distorted toward tetrahedral coordination, while it is rhomboidally distorted in BaFeO₂. Aliovalent substitution of Sr with Nd³⁺, Sm³⁺ and Ho³⁺ results in a charge compensation by forming (Sr_1−xLn_x)Fe²⁺O_2+y/2. The inserted oxygen induces a buckling of the FeO₂ layers.⁷⁰ These observations show the flexible nature of FeO₄ square-planar coordination. Such a distortion is hardly found in cuprates, where only d_x2−y2 orbitals are active. In other words, the ability of square-planar Fe²⁺ to distort largely is related to the multi-orbital nature. Fe-site substitution brings about interesting magnetic behaviour. Mn²⁺ substitution gives rise to a “random fan-out state” induced by site-random interlayer couplings.⁷¹ Square planar iron can also assist other transition metals to adopt square planar coordination. Several 1:1 solid solutions are reported, including SrFe_0.5Ru_0.5O₂ with Ru²⁺ in an intermediate spin state (S = 1)⁷² and Sr₂FeIrO₄ with Ir²⁺ in a low-spin state (S = 1/2).⁷³

Figure 8. Crystal Structure of CaFeO₂ (a), BaFeO₂ (b). The Blue and dark green spheres represent calcium and barium respectively. In (b), the different iron sites are represented orange, blue and purple spheres.

Transition-Metal Oxyhydrides

Section:

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Among mixed-anion compounds, oxyhydrides (oxide-hydride) have attracted considerable attention in recent years. The Oxford group successfully synthesized the first oxyhydride with a transition metal, LaSrCoO₃H_0.7, using a topochemical reaction of Ruddlesden-Popper LaSrCoO₄ with CaH₂ (Figure 9b).⁷⁴ The replaced hydride anions are positioned at the equatorial site, in contrast to oxychlorides and oxybromides of this structural type.⁷⁵ The double-layered analogue, Sr₃Co₂O_4.33H_0.84, was also obtained from Sr₃Co₂O_7−δ.⁷⁶ The very low valence of Co^1.7+ in these compounds is understood given the presence of highly reductive H⁻ anions. Although mixed-valenced, the Co-based oxyhydrides are electronically insulating. We applied the synthetic strategy to BaTiO₃ perovskite and obtained BaTiO_3−xH_x (x ≤ 0.6) (Figure 9a) where H⁻ and O²⁻ are disordered at the anion site.⁷⁷ ATiO_3−xH_x (A = Sr, Ca, Eu) were also prepared from the corresponding oxides.^78,79 Unlike the cobalt-based oxyhydrides, Ti takes a ubiquitous valence of +3.4–+4,⁸⁰ as seen in Magnéli phases Ti_nO_2n−1, and ATiO_3−xH_x exhibit a metallic conductivity.

Figure 9. Structures of transition metal oxyhydrides, where gray, red, blue spheres represent A (= Alkali metals, La), O, and H atoms, respectively, and transition metal cations lie in the octahedral center. Perovskite structures with (a) anion disorder (e.g. ATiO_3−xH_x, SrCrO₂H) and (c) anion order (SrVO₂H). Ruddlesden-Popper-type perovskites with (b,d) single octahedral layers (e.g., (b) for Sr₂VO₃H and LaSrCoO₃H_0.7, and (d) for LaSr₃NiRuO₄H₄) and (f) double octahedral layers (e.g., Sr₃V₂O₅H₂). (e) A 6H-type hexagonal structure with corner- and face-shared octahedra (e.g., BaVO_3−xH_x).

SrVO₂H, Sr₂VO₃H, Sr₃V₂O₅H₂, and LaSr₃NiRuO₄H₄ have an anion-ordered perovskite-type structure with trans-MO₄H₂ octahedra (Figure 9).^81–83 The trans geometry lifts the triple degeneracy of vanadium t_2g orbitals and the on-site electron-electron repulsion further splits the low-lying half-filled d_xz/d_yz orbitals, making these materials a Mott insulator. DFT calculations on SrVO₂H show two-dimensional electronic structures.^83,84 The V–H–V interaction along the c axis is much weaker than the in-plane V–O–V interaction owing to the orthogonality between V t_2g and H 1s orbitals. The hydride anion can effectively block the out-of-plane interactions even under high pressure of ∼50 GPa.

High-pressure synthesis is also a useful approach to synthesize oxyhydrides.^85–88 A magnanese oxyhydride of LaSrMnO_3.3H_0.7 with the Ruddlesden-Popper perovskite structure is only accessible by taking a high-pressure route;⁸⁶ otherwise, oxygen deficient oxide LaSrMnO_4−δ is formed. A cubic perovskite oxyhydride SrCrO₂H (Figure 9a) can be synthesized at 1000 °C and 5 GPa. This phase shows G-type spin ordering at T_N ∼ 380 K,⁸⁵ which is higher than those of isoelectric perovskite oxides RCr³⁺O₃ (R = rare earth element) and any other Cr(III)-containing oxides. The enhancement of T_N in SrCrO₂H (vs RCrO₃) is reasonably explained by the increased tolerance factor (t = 1). Such tolerance-factor control via cation/anion co-exchange (Sr²⁺/H⁻ \( \Leftrightarrow \) La³⁺/O²⁻) could be a promising strategy for extensive tuning of structures and resultant magnetic and other properties. Although known transition-metal oxyhydrides had been limited to those having vertex-linked octahedral networks, BaVO_3−xH_x synthesized under 5 GPa adopted a hexagonal structure with corner- and face-shared octahedral coordination (Figure 9e).⁸⁸ For x = 0.3, H⁻ anions are located selectively at the face-shared sites, which appears to suppress electron hopping along the c axis (or confine electrons within the 2D sheet), leading to an increased electron-electron correlation as observed by NMR. A further pressurization to 7 GPa stabilizes a cubic perovskite structure (Figure 9a).

Anion Exchange Reactions and Catalysis

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Many well-known mixed anion systems such as oxynitrides or oxyfluorides are prepared from oxide precursors, which can sometimes be a topochemical anion exchange reaction. For this review, we rephrase anion exchange reactions as topochemical, in order to emphasize that only the essential aspects of the crystal structure change under a pure anion exchange reaction. Classic examples are the topochemical fluorination of layered copper oxides as superconductors, while more recent examples are the topochemical formation of oxynitrides from perovskite precursors with NH₃ treatment. For oxynitrides, NH₃ is the common nitriding reagent, while fluorination can be conducted with F₂, NH₄F, or solid fluoropolymer powders, and hydride-based reactions usually consist of introduction by metal hydrides as we will see below.

Topochemical nitridation: While numerous oxynitrides have been reported, the compounds which are prepared by anion exchange, i.e., topochemically, are a minority. For example, the tantalate and niobate oxynitrides are typically prepared by alkali earth carbonates and oxides under flowing NH₃;^89,90 this may be due to the rather inflexible nature of niobate and tantalate in terms of redox, as replacing O²⁻ with H⁻ requires a change in cation valence. With other elements, such as Fe, this is not always a problem, as the topochemical nitridation of Sr₂FeMoO₆ to Sr₂FeMoO_4.9N_1.1 using NH₃ has been reported.⁹¹ Additional topochemical reactions to form oxynitrides, however, seem to be possible with oxyhydride precursors as shown below.

Topochemical fluorination: With fluorination reactions, typically there can be a number of variations. For example, a one-to-one exchange of O²⁻ and F⁻ results in reductive fluorination, often seen with low fluorine doping levels or the use of fluoropolymers under anaerobic conditions as for the synthesis of RbLaNb₂O₆F⁹² or superconducting WO_2.6F_0.4.⁹³ Simple anion exchange without any redox requires two fluorides to be inserted with the removal of one oxide anion; this appears to be the case of the fluorination for some brownmillerites LaACuGaO₅ (A = Sr, Ca) when using XeF₂ as a fluorinating agent.⁹⁴ Alternatively, oxidative fluorination can occur, where extra fluoride insertion accompanies (or even may solely occur) exchange with oxide. An early example is the use of F₂ gas for fluoride insertion/exchange to convert Sr₂CuO₃ to the superconducting Sr₂CuO₂F_2+δ,⁹⁵ and more recent examples are the preparation of the Sr₂TiO₃F₂ and LaSrFeO₃F₃ phases,^96,97 both from their oxide Ruddlesden-Popper phases. Fluorination can also be conducted after removing some lattice oxide in a preliminary separate reaction, as for the synthesis of Sr₇Mn₄O₁₃F₂ from Sr₇Mn₄O₁₅.⁹⁸ These distinctions between the various types of fluorination reactions have been summarized by Slater,⁹⁹ and the various oxyfluorides prepared by fluorine (exchange) reactions have been reviewed by McCabe,⁹⁷ as we turn to hydride-based anion reactions as this is where the new developments are.

Topochemical exchange reactions involving hydride: Many of the recent metal oxyhydrides, such as C12A7:H⁻,¹⁰⁰ LaSrCoO₃H_0.7,⁷⁵ LaSr₃NiRuO₄H₄,¹⁰¹ BaTiO_3−xH_x,⁷⁸ and Sr₂VO₃H⁸⁷ have been prepared by anion exchange reactions of oxide precursors. The hydrogen source in this case is often a metal hydride such as NaH or CaH₂, rather than H₂. The use of these strongly reducing hydride agents apparently permits lower reaction temperatures (typically 200–600 °C), making the synthesis of these materials possible. The scope of various oxyhydrides⁵ and some studies on the formation mechanisms^77,102 has been reported elsewhere, so we now turn to further reactions using oxyhydrides as a starting point.

Compared to the anions such as oxide, fluoride, or nitride, systems with hydride offer unique possibilities. Certain metals or alloys containing Mg, Ni, etc., reversibly hydrogenate/dehydrogenate to very large extents at relatively moderate temperatures compared to equivalent materials for oxygen. While this effect is not observed at the same magnitude in oxyhydrides, the hydride anion within an oxide framework is still rather easily exchangeable by most standards. Work by Kobayashi et al.,⁷⁸ Yajima et al.,¹⁰³ and Masuda et al.¹⁰⁴ explicitly examine this, starting with the oxyhydride BaTiO_3−xH_x. When heated under inert atmospheres, BaTiO_3−xH_x releases the lattice hydride as H₂ gas at temperatures around 400 °C,⁷⁸ reminiscent of TiH₂ decomposition. Heating instead under a D₂ atmosphere leads to H/D exchange at similar temperatures yielding BaTiO_3−xD_x.⁷⁸ The precise exchange temperature and activation energies involved, together with its dependence on composition have been determined by Ya et al.¹⁰⁵ Isotope exchange of ¹⁶O/¹⁸O of course has been demonstrated,¹⁰⁶ but usually occurs at higher temperatures, so it is evident that hydride is more readily exchanged. This is probably related to the lighter mass and smaller charge of hydride in comparison to oxide.

This facile exchange can lead to chemical transformations otherwise difficult to achieve. For example, oxynitrides are typically made by treating under NH₃ at strenuous temperatures; 900 °C is necessary to prepare BaTiO₂N_0.1.^103,104 In contrast, treating the oxyhydride BaTiO_2.5H_0.5 with NH₃, or even N₂ at 400 °C results in higher N contents (BaTiO_2.5N_0.3).^103,104 Other than direct H⁻/N³⁻ exchange, the vacancies created by the leaving hydride species can also result in enhanced nitrogen content during thermal processing; this was demonstrated by Mikita et al. during the synthesis of EuTiO₂N from EuTiO_3−xH_x.¹⁰⁷ The anion-vacancy enhanced H/O exchange is also shown by mimicking the well-known strategy to improve ionic conductivity.¹⁰⁸ Other than being thermolabile, hydride is also susceptible to attack from acidic groups as such as HF; and this has been utilized to prepared BaTiO_2.5H_0.25F_0.25 from the oxyhydride BaTiO_2.5H_0.5 at fairly moderate conditions.¹⁰⁴ This product is closely related to the reported BaTiO_2.9F_0.1, which we note is accessible only by high pressure/temperature reaction (3 GPa, 1300 °C).¹⁰⁹

Catalysis with mixed anion systems: While many of the studies have focused on the electronic and magnetic properties of these mixed anion systems, interesting chemical applications are also possible. Controlling the oxide/nitride, or oxide/halide ratio results in a varied optical band gap, and thus providing a means for optimizing photocatalytic activity.¹¹⁰ Apart from these catalysts, however, one can envision the constituent anions actually being part of the catalytic cycle. For pure oxides, this is already known as they often function as good oxidation catalysts.¹¹¹ This naturally leads one to suspect oxyhydrides, and oxysulfides, etc. as having activity for new chemical transformations involving hydrogen or sulfur. This has been recently discovered for BaTiO_2.5H_0.5 and TiH₂, which have been found to be active catalysts for the Haber-Bosch synthesis of ammonia from N₂ and H₂.¹¹² The active site here is a Ti center on the surface, which contrasts conventional catalysts, where a supported metal such as Fe, Ru, or Co on a support acts as the active site. Other titanium oxides are shown to be inactive towards NH₃ synthesis, so it is the presence of hydride in the lattice which makes this reaction possible.¹¹² The oxyhydride can also be used as a catalyst support for Ru, Fe, Co, or Ni particles, leading to enhanced activities for NH₃ synthesis¹¹³ and CO₂ methanation reactions.¹¹⁴ This results are also mirrored by other catalytic supports such as LiH,¹¹⁵ electrides (which reversible convert in-situ between hydrides)¹¹⁶ and LnHO;¹¹⁷ in all of these cases, it is noteworthy that all of the reactions involved can be viewed as hydrogenation reactions. It is hence conceivable that in the future, other catalytic reactions involving H, N, S, and O will become possible or enhanced through the use of mixed anion systems.