In oxides, the substitution of non-oxide anions (F(-),S(2-),N(3-) and so on) for oxide introduces many properties, but the least commonly encountered substitution is where the hydride anion (H(-)) replaces oxygen to form an oxyhydride. Only a handful of oxyhydrides have been reported, mainly with electropositive main group elements or as layered cobalt oxides with unusually low oxidation states. Here, we present an oxyhydride of the perhaps most well-known perovskite, BaTiO(3), as an O(2-)/H(-) solid solution with hydride concentrations up to 20% of the anion sites. BaTiO(3-x)H(x) is electronically conducting, and stable in air and water at ambient conditions. Furthermore, the hydride species is exchangeable with hydrogen gas at 400 °C. Such an exchange implies diffusion of hydride, and interesting diffusion mechanisms specific to hydrogen may be at play. Moreover, such a labile anion in an oxide framework should be useful in further expanding the mixed-anion chemistry of the solid state.

A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature (T c). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving non-magnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe have considerable overlap, how each order affects superconductivity remains perplexing. Here we construct the three-dimensional electronic phase diagram, temperature (T) against pressure (P) and isovalent S-substitution (x), for FeSe1−xSx. By simultaneously tuning chemical and physical pressures, against which the chalcogen height shows a contrasting variation, we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended non-magnetic tetragonal phase emerges, where T c shows a striking enhancement. The completed phase diagram uncovers that high-T c superconductivity lies near both ends of the dome-shaped antiferromagnetic phase, whereas T c remains low near the nematic critical point.

We prepared a new two-dimensional oxyantimonide, BaTi 2 Sb 2 O, which shows a superconducting transition at 1.2 K, representing the first superconductivity in a system with Ti 3þ (d 1 ) in a square lattice. The TiO 2 Sb 4 mixed anionic coordination stabilizes a unique half-filled Ti d xy orbital configuration in Ti 2 O plane, which is analogous to Cu 2þ (d 9 ) in the high-T c superconductors. A charge density wave (CDW)-or spin density wave (SDW)-like anomaly appears at 50 K, which is significantly reduced compared with 200 K for the isostructural and non-superconducting BaTi 2 As 2 O.Since the discovery of high-T c superconductivity in cuprates, 1) there has been a longstanding quest to find novel superconductors. Although several classes of materials such as MgB 2 , iron pnictides, and fullerides show high T c 's, 2-4) the cuprates still hold the highest T c record. Yet, the mechanism for its occurrence is unclear and still under debate despite intense investigation. To clarify the mechanism of high-T c superconductivity, there have been a variety of attempts to find a novel superconductor that is isostructural and isoelectric with the high-T c cuprates. One plausible approach is carrier doping into a perovskite oxide with a 3d 1 electron configuration, such as AE 2 V 4þ O 4 (AE = Sr, Ba). Here, the electronic configuration is complementary with respect to the 3d 9 cuprates; although La 2 Cu 2þ O 4 has one hole per Cu 2þ , AE 2 V 4þ O 4 has one electron per V 4þ . However, as will be discussed later, this view can be seen as an oversimplified description, since it neglects orbital degeneracy derived from the octahedral crystal field around the transition metal. In fact, experimentally, carrier-doped AE 2 V 4þ O 4 does not show superconductivity, but shows metallic conductivity. 5,6) As shown in Fig. 1, the titanium oxypnictides Na 2 Ti 3þ 2 Pn 2 O (Pn = As, Sb) 7) and La 2 CuO 4 are somewhat similar in structure. Na 2 Ti 2 Pn 2 O has a Ti 2 O square net that adopts the anticonfiguration to the CuO 2 square net in La 2 CuO 4 . In this net, Ti 3þ (3d 1 ) is coordinated octahedrally by two oxide anions and four pnictide anions, and these TiO 2 Pn 4 octahedra share edges to form the square lattice. BaTi 2 As 2 O [ Fig. 1(c)], where the two sodium cations have been replaced with one barium cation, has the same square lattice framework and has recently been reported. 8) The mixed anionic coordination of TiO 2 Pn 4 and the octahedral connectivity in the ab plane, as shown in Fig. 1(f ), provide a unique opportunity for the t 2g orbitals to split to a greater extent (relative to pure oxide coordination), owing to the anions having different valences, electronegativities, and ionic radii.Unfortunately, none of these compounds show superconductivity. 9,10) Interestingly, the susceptibility and resistivity showed an anomaly at T a ¼ 330 K for Na 2 Ti 2 As 2 O, 120 K for Na 2 Ti 2 Sb 2 O, and 200 K for BaTi 2 As 2 O, which is ascribed to a CDW or SDW (CDW/SDW) transition. Given that CDW/SDW instabilities are also commonly ...

The oxyhydride solid solutions (Ca,Sr)TiO(3-x)H(x) and (Sr,Ba)TiO(3-x)H(x) have been prepared by reducing the corresponding ATiO(3) oxides with calcium hydride. Under the reaction conditions examined, a hydride content of x = 0.1-0.3 was obtained for all compositions. Compared to our previous result with BaTiO(3-x)H(x), the larger particle size in this study (20-30 μm vs 170 nm) resulted in a somewhat lower hydride amount despite prolonged reaction times. We examined changes in cell volume, octahedral tilt angle, and site occupancy of different anion sites after conversion to oxyhydrides; it appears that these oxyhydrides fit the geometrical descriptions typical for regular ABO(3) perovskites quite well. The hydrogen release temperature, previously shown to be indicative of the hydride exchange temperature, however, does not scale linearly with the A-site composition, indicating a potential effect of chemical randomness.

Epitaxial thin films of titanium perovskite oxyhydride ATiO(3-x)H(x) (A = Ba, Sr, Ca) were prepared by CaH(2) reduction of epitaxial ATiO(3) thin films deposited on a (LaAlO(3))(0.3)(SrAl(0.5)Ta(0.5)O(3))(0.7) substrate. Secondary ion mass spectroscopy detected a substantial amount and uniform distribution of hydride within the film. SrTiO(3)/LSAT thin film hydridized at 530 °C for 1 day had hydride concentration of 4.0 × 10(21) atoms/cm(3) (i.e., SrTiO(2.75)H(0.25)). The electric resistivity of all the ATiO(3-x)H(x) films exhibited metallic (positive) temperature dependence, as opposed to negative as in BaTiO(3-x)H(x) powder, revealing that ATiO(3-x)H(x) are intrinsically metallic, with high conductivity of 10(2)-10(4) S/cm. Treatment with D(2) gas results in hydride/deuteride exchange of the films; these films should be valuable in further studies on hydride diffusion kinetics. Combined with the materials' inherent high electronic conductivity, new mixed electron/hydride ion conductors may also be possible.