Neutron Diffraction and EXAFS Studies of K_2<i>x</i>/3Cu[Fe(CN)₆]_2/3·<i>n</i>H₂O

The crystal structure of copper hexacyanoferrate (CuHCF), K_2<i>x/</i>3Cu­[Fe­(CN)₆]_2/3·<i>n</i>H₂O, with nominal compositions <i>x</i> = 0.0 and <i>x</i> = 1.0 was studied by neutron powder diffraction (NPD) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The compound crystallizes in space group <i>Fm</i>3̅<i>m</i>, with <i>a</i> = 10.1036(11) Å and <i>a</i> = 10.0588(5) Å for <i>x</i> = 0.0 and <i>x</i> = 1.0, respectively. Difference Fourier maps for <i>x</i> = 0.0 show that the coordinated water molecules are positioned at a site 192l close to vacant N positions in the −Fe–C–N–Cu– framework, while additional zeolitic water molecules are distributed over three sites (8c, 32f, and 48g) in the −Fe–C–N–Cu– framework cavities. The refined water content for <i>x</i> = 0.0 is 16.8(8) per unit cell, in agreement with the ideal 16 (<i>n</i> = 4). For <i>x</i> = 1.0, the refinement suggests that 2.6 K atoms per unit cell (<i>x</i> = 0.98) are distributed only over the sites 8c and 32f in the cavities, and 13.9(7) water per unit cell are distributed over all the four positions. The EXAFS data for Fe, Cu, and K K-edges are in agreement with the NPD data, supporting a structure model with a linear −Fe–C–N–Cu– framework and K⁺ ions in the cavities

Anomalous Magnetic Properties of Nanoparticles Arising from Defect Structures: Topotaxial Oxidation of Fe_1–<i>x</i>O|Fe_3−δO₄ Core|Shell Nanocubes to Single-Phase Particles

Here we demonstrate that the anomalous magnetic properties of iron oxide nanoparticles are correlated with defects in their interior. We studied the evolution of microstructure and magnetic properties of biphasic core|shell Fe_1–<i>x</i>O|Fe_3−δO₄ nanoparticles synthesized by thermal decomposition during their topotaxial oxidation to single-phase nanoparticles. Geometric phase analysis of high-resolution electron microscopy images reveals a large interfacial strain at the core|shell interface and the development of antiphase boundaries. Dark-field transmission electron microscopy and powder X-ray diffraction concur that, as the oxidation proceeds, the interfacial strain is released as the Fe_1–<i>x</i>O core is removed but that the antiphase boundaries remain. The antiphase boundaries result in anomalous magnetic behavior, that is, a reduced saturation magnetization and exchange bias effects in single-phase nanoparticles. Our results indicate that internal defects play an important role in dictating the magnetic properties of iron oxide nanoparticles

Hydrogenous Zintl Phase Ba₃Si₄H_<i>x</i> (<i>x</i> = 1–2): Transforming Si₄ “Butterfly” Anions into Tetrahedral Moieties

The hydride Ba₃Si₄H_<i>x</i> (<i>x</i> = 1–2) was prepared by sintering the Zintl phase Ba₃Si₄, which contains Si₄^6– butterfly-shaped polyanions, in a hydrogen atmosphere at pressures of 10–20 bar and temperatures of around 300 °C. Initial structural analysis using powder neutron and X-ray diffraction data suggested that Ba₃Si₄H_<i>x</i> adopts the Ba₃Ge₄C₂ type [space group <i>I</i>4/<i>mcm</i> (No. 140), <i>a</i> ≈ 8.44 Å, <i>c</i> ≈ 11.95 Å, <i>Z</i> = 8] where Ba atoms form a three-dimensional array of corner-condensed octahedra, which are centered by H atoms. Tetrahedron-shaped Si₄ polyanions complete a perovskite-like arrangement. Thus, hydride formation is accompanied by oxidation of the butterfly polyanion, but the model with the composition Ba₃Si₄H is not charge-balanced. First-principles computations revealed an alternative structural scenario for Ba₃Si₄H_<i>x</i>, which is based on filling pyramidal Ba₅ interstices in Ba₃Si₄. The limiting composition is <i>x</i> = 2 [space group <i>P</i>4₂/<i>mmm</i> (No. 136), <i>a</i> ≈ 8.4066 Å, <i>c</i> ≈ 12.9186 Å, <i>Z</i> = 8], and for <i>x</i> > 1, Si atoms also adopt tetrahedron-shaped polyanions. Transmission electron microscopy investigations showed that Ba₃Si₄H_<i>x</i> is heavily disordered in the <i>c</i> direction. Most plausible is to assume that Ba₃Si₄H_<i>x</i> has a variable H content (<i>x</i> = 1–2) and corresponds to a random intergrowth of <i>P</i>- and <i>I</i>-type structure blocks. In either form, Ba₃Si₄H_<i>x</i> is classified as an interstitial hydride. Polyanionic hydrides in which H is covalently attached to Si remain elusive

Investigation of the Order–Disorder Rotator Phase Transition in KSiH₃ and RbSiH₃

The β–α (order–disorder) transition in the silanides ASiH₃ (A = K, Rb) was investigated by multiple techniques, including neutron powder diffraction (NPD, on the corresponding deuterides), Raman spectroscopy, heat capacity (<i>C</i>_<i>p</i>), solid-state ²H NMR spectroscopy, and quasi-elastic neutron scattering (QENS). The crystal structure of α-ASiH₃ corresponds to a NaCl-type arrangement of alkali metal ions and randomly oriented, pyramidal, SiH₃^– moieties. At temperatures below 200 K ASiH₃ exist as hydrogen-ordered (β) forms. Upon heating the transition occurs at 279(3) and 300(3) K for RbSiH₃ and KSiH₃, respectively. The transition is accompanied by a large molar volume increase of about 14%. The <i>C</i>_<i>p</i>(<i>T</i>) behavior is characteristic of a rotator phase transition by increasing anomalously above 120 K and displaying a discontinuous drop at the transition temperature. Pronounced anharmonicity above 200 K, mirroring the breakdown of constraints on SiH₃^– rotation, is also seen in the evolution of atomic displacement parameters and the broadening and eventual disappearance of libration modes in the Raman spectra. In α-ASiH₃, the SiH₃^– anions undergo rotational diffusion with average relaxation times of 0.2–0.3 ps between successive H jumps. The first-order reconstructive phase transition is characterized by a large hysteresis (20–40 K). ²H NMR revealed that the α-form can coexist, presumably as 2–4 nm (sub-Bragg) sized domains, with the β-phase below the phase transition temperatures established from <i>C</i>_<i>p</i> measurements. The reorientational mobility of H atoms in undercooled α-phase is reduced, with relaxation times on the order of picoseconds. The occurrence of rotator phases α-ASiH₃ near room temperature and the presence of dynamical disorder even in the low-temperature β-phases imply that SiH₃^– ions are only weakly coordinated in an environment of A⁺ cations. The orientational flexibility of SiH₃^– can be attributed to the simultaneous presence of a lone pair and (weakly) hydridic hydrogen ligands, leading to an ambidentate coordination behavior toward metal cations

Sr₂GaScO₅, Sr₁₀Ga₆Sc₄O₂₅, and SrGa_0.75Sc_0.25O_2.5: a Play in the Octahedra to Tetrahedra Ratio in Oxygen-Deficient Perovskites

Three different perovskite-related phases were isolated in the SrGa_1–<i>x</i>Sc_<i>x</i>O_2.5 system: Sr₂GaScO₅, Sr₁₀Ga₆Sc₄O₂₅, and SrGa_0.75Sc_0.25O_2.5. Sr₂GaScO₅ (<i>x</i> = 0.5) crystallizes in a brownmillerite-type structure [space group (S.G.) <i>Icmm</i>, <i>a</i> = 5.91048(5) Å, <i>b</i> = 15.1594(1) Å, and <i>c</i> = 5.70926(4) Å] with complete ordering of Sc³⁺ and Ga³⁺ over octahedral and tetrahedral positions, respectively. The crystal structure of Sr₁₀Ga₆Sc₄O₂₅ (<i>x</i> = 0.4) was determined by the Monte Carlo method and refined using a combination of X-ray, neutron, and electron diffraction data [S.G. <i>I</i>4₁/<i>a</i>, <i>a</i> = 17.517(1) Å, <i>c</i> = 32.830(3) Å]. It represents a novel type of ordering of the B cations and oxygen vacancies in perovskites. The crystal structure of Sr₁₀Ga₆Sc₄O₂₅ can be described as a stacking of eight perovskite layers along the <i>c</i> axis ...[−(Sc/Ga)­O_1.6–SrO_0.8–(Sc/Ga)­O_1.8–SrO_0.8−]₂.... Similar to Sr₂GaScO₅, this structure features a complete ordering of the Sc³⁺ and Ga³⁺ cations over octahedral and tetrahedral positions, respectively, within each layer. A specific feature of the crystal structure of Sr₁₀Ga₆Sc₄O₂₅ is that one-third of the tetrahedra have one vertex not connected with other Sc/Ga cations. Further partial replacement of Sc³⁺ by Ga³⁺ leads to the formation of the cubic perovskite phase SrGa_0.75Sc_0.25O_2.5 (<i>x</i> = 0.25) with <i>a</i> = 3.9817(4) Å. This compound incorporates water molecules in the structure forming SrGa_0.75Sc_0.25O_2.5·<i>x</i>H₂O hydrate, which exhibits a proton conductivity of ∼2.0 × 10^–6 S/cm at 673 K