Hydrogen Absorption in Transition Metal Silicides: La₃Pd₅Si-Hydrogen System

Pressure-composition isotherm measurements show that the ternary lanthanum palladium silicide phase La3Pd5Si absorbs reversibly up to 5 hydrogen atoms per formula unit at 550 K and 14 bar hydrogen pressure. In-situ synchrotron and neutron powder diffraction reveals three phases, an α-phase having the limiting composition La3Pd5SiD∼1.6 at low deuterium pressure (at up to 9.5 bar D2 and 550 K), a β-phase La3Pd5SiD∼2.30−4 at intermediate deuterium pressure (2 and 550 K), and a relatively unstable γ-phase La3Pd5SiD∼5 at high deuterium pressure (obtained at 75 bar D2 and 293 K). While the α and β phases retain the symmetry of the H-free La3Pd5Si (space group Imma), the γ-phase undergoes a symmetry lowering (aγ∼aβ, bγ∼3bβ and cγ∼cβ, Vγ∼3Vβ, space group Pmnb). The structure of the α-phase contains isolated [Pd−D−Pd] fragments, which are joined into polymeric (−Pd−D−Pd−)n zig-zag chains in the β-phase. In the γ-phase some D sites depopulate, while new D sites are occupied, thus leading to a partial interruption of the zig-zag chains and the formation of isolated [D−Pd−D−Pd] and [D−Pd−D−Pd−D] fragments. This unexpected behavior can be attributed to the onset of repulsive Si−D and D−D interactions (Si−D > 3.0 Å, D−D > 2.1 Å) that divide the structure into Si-poor slabs that absorb hydrogen and Si-rich slabs that do not. The competition between silicon and deuterium which act as a transition metal ligand is further underlined by the fact that Pd atoms having one Si ligand are capable of forming Pd−D bonds, whereas Pd atoms having two Si ligands are not

Synthesis and Crystal Structure of Lithium Beryllium Deuteride Li₂BeD₄

Single-phase ternary deuteride Li2BeD4 was synthesized by a high-pressure high-temperature technique from LiD and BeD2. The crystal structure of Li2BeD4 was solved from X-ray and neutron powder diffraction data. The compound crystallizes in the monoclinic space group P21/c with lattice parameters a = 7.06228(9) Å, b = 8.3378(1) Å, c = 8.3465(1) Å, β =93.577(1)°, and Z = 8. Its structure contains isolated BeD4 tetrahedra and Li atoms that are located in the structure interstices. Li2BeD4 does not undergo any structural phase transitions at temperatures down to 8 K

Layered Oxychlorides [PbBiO₂]A_<i>n</i>+1B_<i>n</i>O_{3<i>n</i>–1}Cl₂ (A = Pb/Bi, B = Fe/Ti): Intergrowth of the Hematophanite and Sillen Phases

New layered structures corresponding to the general formula [PbBiO₂]­A_<i>n</i>+1B_<i>n</i>O_{3<i>n</i>–1}Cl₂ were prepared. Pb₅BiFe₃O₁₀Cl₂ (<i>n</i> = 3) and Pb₅Bi₂Fe₄O₁₃Cl₂ (<i>n</i> = 4) are built as a stacking of truncated A_<i>n</i>+1B_<i>n</i>O_{3<i>n</i>–1} perovskite blocks and α-PbO-type [A₂O₂]²⁺ (A = Pb, Bi) blocks combined with chlorine sheets. The alternation of these structural blocks can be represented as an intergrowth between the hematophanite and Sillen-type structural blocks. The crystal and magnetic structures of Pb₅BiFe₃O₁₀Cl₂ and Pb₅Bi₂Fe₄O₁₃Cl₂ were investigated in the temperature range of 1.5–700 K using X-ray and neutron powder diffraction, transmission electron microscopy and ⁵⁷Fe Mössbauer spectroscopy. Both compounds crystallize in the <i>I</i>4/<i>mmm</i> space group with the unit cell parameters <i>a</i> ≈ <i>a</i>_p ≈ 3.92 Å (a unit-cell parameter of the perovskite structure), <i>c</i> ≈ 43.0 Å for the <i>n</i> = 3 member and <i>c</i> ≈ 53.5 Å for the <i>n</i> = 4 member. Despite the large separation between the slabs containing the Fe³⁺ ions (nearly 14 Å), long-range antiferromagnetic order sets in below ∼600 K with the G-type arrangement of the Fe magnetic moments aligned along the <i>c</i>-axis. The possibility of mixing d⁰ and dⁿ cations at the B sublattice of these structures was also demonstrated by preparing the Ti-substituted <i>n</i> = 4 member Pb₆BiFe₃TiO₁₃Cl₂

Layered Oxychlorides [PbBiO₂]A_<i>n</i>+1B_<i>n</i>O_{3<i>n</i>–1}Cl₂ (A = Pb/Bi, B = Fe/Ti): Intergrowth of the Hematophanite and Sillen Phases

New layered structures corresponding to the general formula [PbBiO₂]­A_<i>n</i>+1B_<i>n</i>O_{3<i>n</i>–1}Cl₂ were prepared. Pb₅BiFe₃O₁₀Cl₂ (<i>n</i> = 3) and Pb₅Bi₂Fe₄O₁₃Cl₂ (<i>n</i> = 4) are built as a stacking of truncated A_<i>n</i>+1B_<i>n</i>O_{3<i>n</i>–1} perovskite blocks and α-PbO-type [A₂O₂]²⁺ (A = Pb, Bi) blocks combined with chlorine sheets. The alternation of these structural blocks can be represented as an intergrowth between the hematophanite and Sillen-type structural blocks. The crystal and magnetic structures of Pb₅BiFe₃O₁₀Cl₂ and Pb₅Bi₂Fe₄O₁₃Cl₂ were investigated in the temperature range of 1.5–700 K using X-ray and neutron powder diffraction, transmission electron microscopy and ⁵⁷Fe Mössbauer spectroscopy. Both compounds crystallize in the <i>I</i>4/<i>mmm</i> space group with the unit cell parameters <i>a</i> ≈ <i>a</i>_p ≈ 3.92 Å (a unit-cell parameter of the perovskite structure), <i>c</i> ≈ 43.0 Å for the <i>n</i> = 3 member and <i>c</i> ≈ 53.5 Å for the <i>n</i> = 4 member. Despite the large separation between the slabs containing the Fe³⁺ ions (nearly 14 Å), long-range antiferromagnetic order sets in below ∼600 K with the G-type arrangement of the Fe magnetic moments aligned along the <i>c</i>-axis. The possibility of mixing d⁰ and dⁿ cations at the B sublattice of these structures was also demonstrated by preparing the Ti-substituted <i>n</i> = 4 member Pb₆BiFe₃TiO₁₃Cl₂

High Oxide Ion Conductivity in Al-Doped Germanium Oxyapatite

The apatite La10-x□x(Ge5.5Al0.5O24)O2.75-1.5x (10 − x = 9.80, 9.75, 9.67, 9.60, 9.50, and 9.40) series has been prepared and the single phase existence range has been established, 9.75 ≥ 10 − x ≥ 9.45. The hexagonal crystal structures of La9.5□0.5(Ge5.5Al0.5O24)O2 have been determined at room temperature, 500 °C, and 900 °C from neutron powder diffraction data using the Rietveld method. The room-temperature unit cell parameters were a = 9.9206(4) Å, c = 7.2893(3) Å, V = 621.29(6) Å3, and Z = 1, and this refinement converged to RWP = 3.03 and RF = 1.30%. The most important structural result is the presence of interstitial oxygen ion associated with vacancies at the apatite oxide anions channels. Oxide ion conductivities have been measured by impedance spectroscopy. La9.5□0.5(Ge5.5Al0.5O24)O2 shows very high oxide conductivity, 0.16(1) S·cm-1 at 800 °C, with negligible electronic contribution. The ionic transport number, obtained by combination of impedance and ion-blocking data, is higher than 0.99 in the studied oxygen partial pressure range, 0.21 to 10-20 atm

Antiferroelectric (Pb,Bi)_1−<i>x</i>Fe_1+<i>x</i>O_3−<i>y</i> Perovskites Modulated by Crystallographic Shear Planes

We demonstrate for the first time a possibility to vary the anion content in perovskites over a wide range through a long-range-ordered arrangement of crystallographic shear (CS) planes. Anion-deficient perovskites (Pb,Bi)1−xFe1+xO3−y with incommensurately modulated structures were prepared as single phases in the compositional range from Pb0.857Bi0.094Fe1.049O2.572 to Pb0.409Bi0.567Fe1.025O2.796. Using a combination of electron diffraction and high-resolution scanning transmission electron microscopy, we constructed a superspace model describing a periodic arrangement of the CS planes. The model was verified by refinement of the Pb0.64Bi0.32Fe1.04O2.675 crystal structure from neutron powder diffraction data ((3 + 1)D S.G. X2/m(α0γ), X = [1/2,1/2,1/2,1/2], a = 3.9082(1) Å, b = 3.90333(8) Å, c = 4.0900(1) Å, β = 91.936(2)°, q = 0.05013(4)a* + 0.09170(3)c* at T = 700 K, RP = 0.036, RwP = 0.048). The (Pb,Bi)1−xFe1+xO3−y structures consist of perovskite blocks separated by CS planes confined to nearly the (509)p perovskite plane. Along the CS planes, the perovskite blocks are shifted with respect to each other over the 1/2[110]p vector that transforms the corner-sharing connectivity of the FeO6 octahedra in the perovskite framework to an edge-sharing connectivity of the FeO5 pyramids at the CS plane, thus reducing the oxygen content. Variation of the chemical composition in the (Pb,Bi)1−xFe1+xO3−y series occurs mainly because of a changing thickness of the perovskite block between the interfaces, that can be expressed through the components of the q vector as Pb6γ+2αBi1−7γ−αFe1+γ−αO3−3γ−α. The Pb, Bi, and Fe atoms are subjected to strong displacements occurring in antiparallel directions on both sides of the perovskite blocks, resulting in an antiferroelectric-type structure. This is corroborated by the temperature-, frequency-, and field-dependent complex permittivity measurements. Pb0.64Bi0.32Fe1.04O2.675 demonstrates a remarkably high resistivity >0.1 T Ω cm at room temperature and orders antiferromagnetically below TN = 608(10) K

Antiferroelectric (Pb,Bi)_1−<i>x</i>Fe_1+<i>x</i>O_3−<i>y</i> Perovskites Modulated by Crystallographic Shear Planes

We demonstrate for the first time a possibility to vary the anion content in perovskites over a wide range through a long-range-ordered arrangement of crystallographic shear (CS) planes. Anion-deficient perovskites (Pb,Bi)1−xFe1+xO3−y with incommensurately modulated structures were prepared as single phases in the compositional range from Pb0.857Bi0.094Fe1.049O2.572 to Pb0.409Bi0.567Fe1.025O2.796. Using a combination of electron diffraction and high-resolution scanning transmission electron microscopy, we constructed a superspace model describing a periodic arrangement of the CS planes. The model was verified by refinement of the Pb0.64Bi0.32Fe1.04O2.675 crystal structure from neutron powder diffraction data ((3 + 1)D S.G. X2/m(α0γ), X = [1/2,1/2,1/2,1/2], a = 3.9082(1) Å, b = 3.90333(8) Å, c = 4.0900(1) Å, β = 91.936(2)°, q = 0.05013(4)a* + 0.09170(3)c* at T = 700 K, RP = 0.036, RwP = 0.048). The (Pb,Bi)1−xFe1+xO3−y structures consist of perovskite blocks separated by CS planes confined to nearly the (509)p perovskite plane. Along the CS planes, the perovskite blocks are shifted with respect to each other over the 1/2[110]p vector that transforms the corner-sharing connectivity of the FeO6 octahedra in the perovskite framework to an edge-sharing connectivity of the FeO5 pyramids at the CS plane, thus reducing the oxygen content. Variation of the chemical composition in the (Pb,Bi)1−xFe1+xO3−y series occurs mainly because of a changing thickness of the perovskite block between the interfaces, that can be expressed through the components of the q vector as Pb6γ+2αBi1−7γ−αFe1+γ−αO3−3γ−α. The Pb, Bi, and Fe atoms are subjected to strong displacements occurring in antiparallel directions on both sides of the perovskite blocks, resulting in an antiferroelectric-type structure. This is corroborated by the temperature-, frequency-, and field-dependent complex permittivity measurements. Pb0.64Bi0.32Fe1.04O2.675 demonstrates a remarkably high resistivity >0.1 T Ω cm at room temperature and orders antiferromagnetically below TN = 608(10) K

Slicing the Perovskite Structure with Crystallographic Shear Planes: The A_<i>n</i>B_<i>n</i>O_{3<i>n</i>−2} Homologous Series

A new AnBnO3n−2 homologous series of anion-deficient perovskites has been evidenced by preparation of the members with n = 5 (Pb2.9Ba2.1Fe4TiO13) and n = 6 (Pb3.8Bi0.2Ba2Fe4.2Ti1.8O16) in a single phase form. The crystal structures of these compounds were determined using a combination of transmission electron microscopy and X-ray and neutron powder diffraction (S.G. Ammm, a = 5.74313(7), b = 3.98402(4), c = 26.8378(4) Å, RI = 0.035, RP = 0.042 for Pb2.9Ba2.1Fe4TiO13 and S.G. Imma, a = 5.7199(1), b = 3.97066(7), c = 32.5245(8) Å, RI = 0.032, RP = 0.037 for Pb3.8Bi0.2Ba2Fe4.2Ti1.8O16). The crystal structures of the AnBnO3n−2 homologues are formed by slicing the perovskite structure with (1̅01)p crystallographic shear (CS) planes. The shear planes remove a layer of oxygen atoms and displace the perovskite blocks with respect to each other by the 1/2[110]p vector. The CS planes introduce edge-sharing connections of the transition metal−oxygen polyhedra at the interface between the perovskite blocks. This results in intrinsically frustrated magnetic couplings between the perovskite blocks due to a competition of the exchange interactions between the edge- and the corner-sharing metal−oxygen polyhedra. Despite the magnetic frustration, neutron powder diffraction and Mössbauer spectroscopy reveal that Pb2.9Ba2.1Fe4TiO13 and Pb3.8Bi0.2Ba2Fe4.2Ti1.8O16 are antiferromagnetically ordered below TN = 407 and 343 K, respectively. The Pb2.9Ba2.1Fe4TiO13 and Pb3.8Bi0.2Ba2Fe4.2Ti1.8O16 compounds are in a paraelectric state in the 5−300 K temperature range

Slicing the Perovskite Structure with Crystallographic Shear Planes: The A_<i>n</i>B_<i>n</i>O_{3<i>n</i>−2} Homologous Series

A new AnBnO3n−2 homologous series of anion-deficient perovskites has been evidenced by preparation of the members with n = 5 (Pb2.9Ba2.1Fe4TiO13) and n = 6 (Pb3.8Bi0.2Ba2Fe4.2Ti1.8O16) in a single phase form. The crystal structures of these compounds were determined using a combination of transmission electron microscopy and X-ray and neutron powder diffraction (S.G. Ammm, a = 5.74313(7), b = 3.98402(4), c = 26.8378(4) Å, RI = 0.035, RP = 0.042 for Pb2.9Ba2.1Fe4TiO13 and S.G. Imma, a = 5.7199(1), b = 3.97066(7), c = 32.5245(8) Å, RI = 0.032, RP = 0.037 for Pb3.8Bi0.2Ba2Fe4.2Ti1.8O16). The crystal structures of the AnBnO3n−2 homologues are formed by slicing the perovskite structure with (1̅01)p crystallographic shear (CS) planes. The shear planes remove a layer of oxygen atoms and displace the perovskite blocks with respect to each other by the 1/2[110]p vector. The CS planes introduce edge-sharing connections of the transition metal−oxygen polyhedra at the interface between the perovskite blocks. This results in intrinsically frustrated magnetic couplings between the perovskite blocks due to a competition of the exchange interactions between the edge- and the corner-sharing metal−oxygen polyhedra. Despite the magnetic frustration, neutron powder diffraction and Mössbauer spectroscopy reveal that Pb2.9Ba2.1Fe4TiO13 and Pb3.8Bi0.2Ba2Fe4.2Ti1.8O16 are antiferromagnetically ordered below TN = 407 and 343 K, respectively. The Pb2.9Ba2.1Fe4TiO13 and Pb3.8Bi0.2Ba2Fe4.2Ti1.8O16 compounds are in a paraelectric state in the 5−300 K temperature range