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.
Oxynitrides have been explored extensively in the past decade because of their interesting properties, such as visible-light absorption, photocatalytic activity and high dielectric permittivity. Their synthesis typically requires high-temperature NH3 treatment (800-1,300 °C) of precursors, such as oxides, but the highly reducing conditions and the low mobility of N(3-) species in the lattice place significant constraints on the composition and structure-and hence the properties-of the resulting oxynitrides. Here we show a topochemical route that enables the preparation of an oxynitride at low temperatures (<500 °C), using a perovskite oxyhydride as a host. The lability of H(-) in BaTiO3-xHx (x ≤ 0.6) allows H(-)/N(3-) exchange to occur, and yields a room-temperature ferroelectric BaTiO3-xN2x/3. This anion exchange is accompanied by a metal-to-insulator crossover via mixed O-H-N intermediates. These findings suggest that this 'labile hydride' strategy can be used to explore various oxynitrides, and perhaps other mixed anionic compounds.
While cation order-disorder transitions have been achieved in a wide range of materials and provide crucial effects in various physical and chemical properties, anion analogues are scarce. Here we have expanded the number of known lanthanide oxyhydrides, LnHO (Ln = La, Ce, Pr, Nd), to include Ln = Sm, Gd, Tb, Dy, Ho, and Er, which has allowed the observation of an anion order-disorder transition from the anion-ordered fluorite structure ( P4/ nmm) for larger Ln ions (La-Nd) to a disordered arrangement ( Fm3̅ m) for smaller Ln (Sm-Er). Structural analysis reveals that with the increase of Ln radius (application of negative chemical pressure), the oxide anion in the disordered phase becomes too under-bonded, which drives a change to an anion-ordered structure, with smaller OLn and larger HLn tetrahedra, demonstrating that the size flexibility of hydride anions drives this transition. Such anion ordering control is crucial regarding applications that involve hydride diffusion such as catalysis and electrochemical solid devices.
We report on the hydride (H–) conductivity in fluorite-type LnHO oxyhydrides (Ln = lanthanide) using samples prepared under high pressure. It is found that, despite its “stoichiometric” composition, the anion-ordered phase (Ln = La, Nd) exhibits hydride conductivity (e.g., 2.3 × 10–5 S cm–1 for NdHO at 300 °C), while the anion-disordered one (Ln = Gd, Er) is an ionic insulator. The systematic structural analysis combined with computational calculations has revealed the indirect interstitial mechanism, where H– anions migrate between the tetrahedral and octahedral sites through a triangular Ln3 bottleneck expanded by the anion order, with a critical bottleneck radius of 1.18 Å. This study may offer a general guide for the design and control of suitable anion diffusion pathways for oxyhydrides and more generally mixed-anion compounds.
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