Fumitaka Takeiri

Oxyhydrides of (Ca,Sr,Ba)TiO3 perovskite solid solutions.

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.

Synthesis and Physical Properties of the New Oxybismuthides BaTi2Bi2O and (SrF)2Ti2Bi2O with a d^{1} Square Net

We have recently reported the \(d^{1}\) square-lattice compound BaTi2Sb2O, which shows superconductivity at \(T_{\text{c}} = 1.2\) K coexisting with a charge- or spin-density wave (CDW/SDW) state. Here, we successfully prepared two new oxybismuthides, BaTi2Bi2O and (SrF)2Ti2Bi2O, as the first Pn = Bi compounds in the \(A\)Ti2 Pn 2O family. The CDW/SDW state disappeared for both compounds, presumably owing to the considerable interaction between the Ti-\(3d\) and Bi-\(6s\) orbitals. The complete suppression of the CDW/SDW instability resulted in an enhanced \(T_{\text{c}}\) of 4.6 K for BaTi2Bi2O. However, (SrF)2Ti2Bi2O exhibits no superconductivity, suggesting the importance of the interlayer interaction for superconductivity.

Two Superconducting Phases in the Isovalent Solid Solutions BaTi2Pn2O (Pn = As, Sb, and Bi)

We have recently reported two superconductors with the \(d^{1}\) square lattice, BaTi2Sb2O (\(T_{\text{c}}=1.2\) K) and BaTi2Bi2O (\(T_{\text{c}}=4.6\) K). In order to find a clue behind the superc...

Topochemical Nitridation with Anion Vacancy-Assisted N3-/O2- Exchange

We present how the introduction of anion vacancies in oxyhydrides enables a route to access new oxynitrides, by conducting ammonolysis of perovskite oxyhydride EuTiO3-xHx (x ∼ 0.18). At 400 °C, similar to our studies on BaTiO3-xHx, hydride lability enables a low temperature direct ammonolysis of EuTi(3.82+)O2.82H0.18, leading to the N(3-)/H(-)-exchanged product EuTi(4+)O2.82N0.12□0.06. When the ammonolysis temperature was increased up to 800 °C, we observed a further nitridation involving N(3-)/O(2-) exchange, yielding a fully oxidized Eu(3+)Ti(4+)O2N with the GdFeO3-type distortion (Pnma) as a metastable phase, instead of pyrochlore structure. Interestingly, the same reactions using the oxide EuTiO3 proceeded through a 1:1 exchange of N(3-) with O(2-) only above 600 °C and resulted in incomplete nitridation to EuTiO2.25N0.75, indicating that anion vacancies created during the initial nitridation process of EuTiO2.82H0.18 play a crucial role in promoting anion (N(3-)/O(2-)) exchange at high temperatures. Hence, by using (hydride-induced) anion-deficient precursors, we should be able to expand the accessible anion composition of perovskite oxynitrides.

Tc Enhancement by Aliovalent Anionic Substitution in Superconducting BaTi2(Sb1-xSnx)2O

BaTi2Sb2O is a \(T_{\text{c}}=1.2\) K superconductor with a \(d^{1}\) square lattice, and isovalent Bi substitution for Sb can increase its \(T_{\text{c}}\) to 4.6 K (BaTi2Bi2O), accompanied by the complete suppression of charge density wave (CDW) or spin density wave (SDW) transition. In the present study, we demonstrate that aliovalent Sn substitution (hole doping) also increases \(T_{\text{c}}\) up to 2.5 K for BaTi2(Sb0.7Sn0.3)2O, while suppressing CDW/SDW transition only slightly. The overall electronic phase diagram of BaTi2(Sb,Sn)2O is qualitatively similar to that of cation-substituted (hole-doped) (Ba,Na)Ti2Sb2O, but quantitative differences such as in \(T_{\text{c}}\) are observed, which is discussed in terms of Ti–Pn hybridization and chemical disorder.

High-Pressure Synthesis of Manganese Oxyhydride with Partial Anion Order.

The high-pressure synthesis of a manganese oxyhydride LaSrMnO3.3 H0.7 is reported. Neutron and X-ray Rietveld analyses showed that this compound adopts the K2 NiF4 structure with hydride ions positioned exclusively at the equatorial site. This result makes a striking contrast to topochemical reductions of LaSrMnO4 that result in only oxygen-deficient phases down to LaSrMnO3.5 . This suggests that high H2 pressure plays a key role in stabilizing the oxyhydride phase, offering an opportunity to synthesize other transition-metal oxyhydrides. Magnetic susceptibility revealed a spin-glass transition at 24 K that is due to competing ferromagnetic (Mn(2+) -Mn(3+) ) and antiferromagnetic (Mn(2+) -Mn(2) , Mn(3+) -Mn(3+) ) interactions.

Superconductivity in the Hypervalent Compound Ba2Bi(Sb1−xBix)2 with a Square-Honeycomb Lattice

We studied the structural and physical properties of the hypervalent system Ba2Bi(Sb1−xBix)2 (0 ≤ x ≤ 1) with an anisotropic “square–honeycomb” layer. We found that the orthorhombic Ba2BiSb2 (x = 0) shows a charge density wave (CDW) transition at approximately 230 K accompanied by a significant elongation of the b-axis, indicating the quasi-one-dimensional nature along the b-axis in its electronic state, as supported by first-principles calculations. This CDW transition is rapidly suppressed with increasing x, leading to the appearance of superconductivity for 0.375 ≤ x ≤ 1. The superconducting transition temperature Tc increases slightly with x and the maximum Tc was 4.4 K for Ba2Bi3 (x = 1).

Hypervalent Bismuthides La3MBi5 (M = Ti, Zr, Hf) and Related Antimonides: Absence of Superconductivity

We successfully synthesized the ternary bismuthides La3MBi5 (M = Ti, Zr, Hf). These compounds crystallize in the hexagonal Hf5Sn3Cu-anti type structure (space group: P63/mcm) consisting of face-sharing MBi6 octahedral chains and hypervalent Bi linear chains, both separated by La atoms. Magnetic susceptibility and electrical resistivity measurements revealed that all of the compounds, including the solid solution La3Ti(Bi1-xSbx)5, exhibit a Pauli paramagnetic behavior without any trace of superconductivity down to 1.85 K, as opposed to a recently reported 4 K superconductivity in La3TiSb5. The absence of superconductivity is supported by first-principles band calculations of La3TiBi5 and La3TiSb5 that demonstrate similar electronic structures with three-dimensional Fermi surfaces.

AgFeOF2: A Fluorine-Rich Perovskite Oxyfluoride

We synthesized a silver iron oxyfluoride AgFeOF2 by using a high-pressure reaction. Synchrotron X-ray and neutron diffraction, X-ray absorption, and 57Fe Mössbauer spectroscopy indicate that AgFeOF2 crystallizes in the ideal perovskite structure with iron in a trivalent state, although electron microscopy revealed weak super-reflections. A possible partial ordering in the FeO2F4 octahedron is inferred from Mössbauer spectroscopy. The synthesis of the fluorine-rich sample offers an opportunity to study a composition-property relation in AFeIIIO3- nF n ( n = 0, 1, and 2). AgFeOF2 exhibits a G-type antiferromagnetic ordering below TN ≈ 480 K, which is much lower than the n = 0 and 1 cases, suggesting a weaker superexchange interaction between Fe moments via F 2p orbitals (vs O 2p orbitals).

Chemical Pressure-Induced Anion Order–Disorder Transition in LnHO Enabled by Hydride Size Flexibility

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 Ln3+ ions (La-Nd) to a disordered arrangement ( Fm3̅ m) for smaller Ln3+ (Sm-Er). Structural analysis reveals that with the increase of Ln3+ 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 OLn4 and larger HLn4 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.