Layered Hydride CaNiGeH with a ZrCuSiAs-type Structure: Crystal Structure, Chemical Bonding, and Magnetism Induced by Mn Doping

Stimulated by the discovery of the iron oxypnictide superconductor with ZrCuSiAs-type structure in 2008, extensive exploration of its isostructural and isoelectronic compounds has started. These compounds, including oxides, fluorides, and hydrides, can all be simply recognized as valence compounds for which the octet rule is valid. We report herein the first example of a ZrCuSiAs-type hydride, CaNiGeH, which violates the octet rule. This hydride was synthesized by hydrogenation of the CeFeSi-type compound CaNiGe under pressurized hydrogen. Powder diffraction and theoretical simulation confirm that H enters into the interstitial position of the Ca₄ tetrahedron, leading to notable anisotropic expansion of the unit cell along the <i>c</i> axis. Density functional theory calculations indicate the modification of the chemical bonding and formation of ionic Ca–H bond as a result of hydrogen insertion. Furthermore, CaNiGeH shows Pauli paramagnetism and metallic conduction similar to that of CaNiGe, but its carrier type changes to hole and the carrier density is drastically reduced as compared to CaNiGe. Mn-doping at the Ni site introduces magnetism to both the parent compound and the hydride. The measurement demonstrates that hydrogenation of CaNi_1–<i>x</i>Mn_<i>x</i>Ge reduces ferromagnetic ordering of Mn ions and induces huge magnetic hysteresis, whereas the spin glass state observed for the parent compound is preserved in the hydride. The hydrogenation-induced changes in the electric and magnetic properties are interpreted in terms of development of two-dimensionality in crystal structure as well as electronic state

Layered Hydride CaNiGeH with a ZrCuSiAs-type Structure: Crystal Structure, Chemical Bonding, and Magnetism Induced by Mn Doping

Stimulated by the discovery of the iron oxypnictide superconductor with ZrCuSiAs-type structure in 2008, extensive exploration of its isostructural and isoelectronic compounds has started. These compounds, including oxides, fluorides, and hydrides, can all be simply recognized as valence compounds for which the octet rule is valid. We report herein the first example of a ZrCuSiAs-type hydride, CaNiGeH, which violates the octet rule. This hydride was synthesized by hydrogenation of the CeFeSi-type compound CaNiGe under pressurized hydrogen. Powder diffraction and theoretical simulation confirm that H enters into the interstitial position of the Ca₄ tetrahedron, leading to notable anisotropic expansion of the unit cell along the <i>c</i> axis. Density functional theory calculations indicate the modification of the chemical bonding and formation of ionic Ca–H bond as a result of hydrogen insertion. Furthermore, CaNiGeH shows Pauli paramagnetism and metallic conduction similar to that of CaNiGe, but its carrier type changes to hole and the carrier density is drastically reduced as compared to CaNiGe. Mn-doping at the Ni site introduces magnetism to both the parent compound and the hydride. The measurement demonstrates that hydrogenation of CaNi_1–<i>x</i>Mn_<i>x</i>Ge reduces ferromagnetic ordering of Mn ions and induces huge magnetic hysteresis, whereas the spin glass state observed for the parent compound is preserved in the hydride. The hydrogenation-induced changes in the electric and magnetic properties are interpreted in terms of development of two-dimensionality in crystal structure as well as electronic state

High-Temperature Neutron and X‑ray Diffraction Study of Fast Sodium Transport in Alluaudite-type Sodium Iron Sulfate

Sodium-ion battery is a potential alternative to replace lithium-ion battery, the present main actor in electrical energy storage technologies. A recently discovered cathode material Na_2.5Fe_1.75(SO₄)₃ (NFS) derives not only high energy density with very high voltage generation over 3.8 V, but also high-rate capability of reversible Na insertion as a result of large tunnels in the alluaudite structure. Here we applied high-temperature X-ray/neutron diffraction to unveil characteristic structural features related to major Na transport pathways. Thermal activation and nuclear density distribution of Na demonstrate one-dimensional Na diffusion channels parallel to [001] direction in full consistence with computational predictions. This feature would be common for the related (sulfo-)­alluaudite system, forming emerging functional materials group for electrochemical applications

Layered Hydride CaNiGeH with a ZrCuSiAs-type Structure: Crystal Structure, Chemical Bonding, and Magnetism Induced by Mn Doping

Stimulated by the discovery of the iron oxypnictide superconductor with ZrCuSiAs-type structure in 2008, extensive exploration of its isostructural and isoelectronic compounds has started. These compounds, including oxides, fluorides, and hydrides, can all be simply recognized as valence compounds for which the octet rule is valid. We report herein the first example of a ZrCuSiAs-type hydride, CaNiGeH, which violates the octet rule. This hydride was synthesized by hydrogenation of the CeFeSi-type compound CaNiGe under pressurized hydrogen. Powder diffraction and theoretical simulation confirm that H enters into the interstitial position of the Ca₄ tetrahedron, leading to notable anisotropic expansion of the unit cell along the <i>c</i> axis. Density functional theory calculations indicate the modification of the chemical bonding and formation of ionic Ca–H bond as a result of hydrogen insertion. Furthermore, CaNiGeH shows Pauli paramagnetism and metallic conduction similar to that of CaNiGe, but its carrier type changes to hole and the carrier density is drastically reduced as compared to CaNiGe. Mn-doping at the Ni site introduces magnetism to both the parent compound and the hydride. The measurement demonstrates that hydrogenation of CaNi_1–<i>x</i>Mn_<i>x</i>Ge reduces ferromagnetic ordering of Mn ions and induces huge magnetic hysteresis, whereas the spin glass state observed for the parent compound is preserved in the hydride. The hydrogenation-induced changes in the electric and magnetic properties are interpreted in terms of development of two-dimensionality in crystal structure as well as electronic state

Layered Hydride CaNiGeH with a ZrCuSiAs-type Structure: Crystal Structure, Chemical Bonding, and Magnetism Induced by Mn Doping

Stimulated by the discovery of the iron oxypnictide superconductor with ZrCuSiAs-type structure in 2008, extensive exploration of its isostructural and isoelectronic compounds has started. These compounds, including oxides, fluorides, and hydrides, can all be simply recognized as valence compounds for which the octet rule is valid. We report herein the first example of a ZrCuSiAs-type hydride, CaNiGeH, which violates the octet rule. This hydride was synthesized by hydrogenation of the CeFeSi-type compound CaNiGe under pressurized hydrogen. Powder diffraction and theoretical simulation confirm that H enters into the interstitial position of the Ca₄ tetrahedron, leading to notable anisotropic expansion of the unit cell along the <i>c</i> axis. Density functional theory calculations indicate the modification of the chemical bonding and formation of ionic Ca–H bond as a result of hydrogen insertion. Furthermore, CaNiGeH shows Pauli paramagnetism and metallic conduction similar to that of CaNiGe, but its carrier type changes to hole and the carrier density is drastically reduced as compared to CaNiGe. Mn-doping at the Ni site introduces magnetism to both the parent compound and the hydride. The measurement demonstrates that hydrogenation of CaNi_1–<i>x</i>Mn_<i>x</i>Ge reduces ferromagnetic ordering of Mn ions and induces huge magnetic hysteresis, whereas the spin glass state observed for the parent compound is preserved in the hydride. The hydrogenation-induced changes in the electric and magnetic properties are interpreted in terms of development of two-dimensionality in crystal structure as well as electronic state

Layered Hydride CaNiGeH with a ZrCuSiAs-type Structure: Crystal Structure, Chemical Bonding, and Magnetism Induced by Mn Doping

Stimulated by the discovery of the iron oxypnictide superconductor with ZrCuSiAs-type structure in 2008, extensive exploration of its isostructural and isoelectronic compounds has started. These compounds, including oxides, fluorides, and hydrides, can all be simply recognized as valence compounds for which the octet rule is valid. We report herein the first example of a ZrCuSiAs-type hydride, CaNiGeH, which violates the octet rule. This hydride was synthesized by hydrogenation of the CeFeSi-type compound CaNiGe under pressurized hydrogen. Powder diffraction and theoretical simulation confirm that H enters into the interstitial position of the Ca₄ tetrahedron, leading to notable anisotropic expansion of the unit cell along the <i>c</i> axis. Density functional theory calculations indicate the modification of the chemical bonding and formation of ionic Ca–H bond as a result of hydrogen insertion. Furthermore, CaNiGeH shows Pauli paramagnetism and metallic conduction similar to that of CaNiGe, but its carrier type changes to hole and the carrier density is drastically reduced as compared to CaNiGe. Mn-doping at the Ni site introduces magnetism to both the parent compound and the hydride. The measurement demonstrates that hydrogenation of CaNi_1–<i>x</i>Mn_<i>x</i>Ge reduces ferromagnetic ordering of Mn ions and induces huge magnetic hysteresis, whereas the spin glass state observed for the parent compound is preserved in the hydride. The hydrogenation-induced changes in the electric and magnetic properties are interpreted in terms of development of two-dimensionality in crystal structure as well as electronic state

Layered Hydride CaNiGeH with a ZrCuSiAs-type Structure: Crystal Structure, Chemical Bonding, and Magnetism Induced by Mn Doping

Stimulated by the discovery of the iron oxypnictide superconductor with ZrCuSiAs-type structure in 2008, extensive exploration of its isostructural and isoelectronic compounds has started. These compounds, including oxides, fluorides, and hydrides, can all be simply recognized as valence compounds for which the octet rule is valid. We report herein the first example of a ZrCuSiAs-type hydride, CaNiGeH, which violates the octet rule. This hydride was synthesized by hydrogenation of the CeFeSi-type compound CaNiGe under pressurized hydrogen. Powder diffraction and theoretical simulation confirm that H enters into the interstitial position of the Ca₄ tetrahedron, leading to notable anisotropic expansion of the unit cell along the <i>c</i> axis. Density functional theory calculations indicate the modification of the chemical bonding and formation of ionic Ca–H bond as a result of hydrogen insertion. Furthermore, CaNiGeH shows Pauli paramagnetism and metallic conduction similar to that of CaNiGe, but its carrier type changes to hole and the carrier density is drastically reduced as compared to CaNiGe. Mn-doping at the Ni site introduces magnetism to both the parent compound and the hydride. The measurement demonstrates that hydrogenation of CaNi_1–<i>x</i>Mn_<i>x</i>Ge reduces ferromagnetic ordering of Mn ions and induces huge magnetic hysteresis, whereas the spin glass state observed for the parent compound is preserved in the hydride. The hydrogenation-induced changes in the electric and magnetic properties are interpreted in terms of development of two-dimensionality in crystal structure as well as electronic state

A Comparative Study of LiCoO₂ Polymorphs: Structural and Electrochemical Characterization of O2‑, O3‑, and O4-type Phases

O4-type LiCoO₂ as a third polymorph of LiCoO₂ is prepared by an ion-exchange method in aqueous media from OP4-[Li, Na]­CoO₂, which has an intergrowth structure of O3-LiCoO₂ and P2-Na_0.7CoO₂. O4-type LiCoO₂ is characterized by synchrotron X-ray diffraction, neutron diffraction, and X-ray absorption spectroscopy. Structural characterization reveals that O4-type LiCoO₂ has an intergrowth structure of O3- and O2-LiCoO₂ with stacking faulted domains. Three LiCoO₂ polymorphs are formed from the close-packed CoO₂ layers, which consist of edge-shared CoO₆ octahedra, whereas the oxide-ion stacking is different: cubic in the O3-phase, cubic/hexagonal in the O2-phase, and alternate O3 and O2 in the O4-phase. Structural analysis using the DIFFaX program suggests that the O4-phase consists of approximately 30% of O12-domains, while stacking faults are not evidenced for O2-phase. The results suggest that a nucleation process for Na/Li ion-exchange kinetically dominates a growth process of ideal O4-domains because the presence of CoO₂–Li–CoO₂ blocks as O3-domains could be expected to prevent through-plane interaction of Na layers. Electrochemical behavior and structural transition processes for three LiCoO₂ polymorphs are compared in Li cells. A new phase, OT^#4-type Li_0.5CoO₂, is first isolated as an intergrowth phase of O3- and T^#2-Li_0.5CoO₂. However, some deviations from ideal behavior as the O2/O3-intergrowth phase are also noted, presumably because of the existence of stacking faults

Structural Origin of the Anisotropic and Isotropic Thermal Expansion of K₂NiF₄‑Type LaSrAlO₄ and Sr₂TiO₄

K₂NiF₄<i>-</i>type LaSrAlO₄ and Sr₂TiO₄ exhibit anisotropic and isotropic thermal expansion, respectively; however, their structural origin is unknown. To address this unresolved issue, the crystal structure and thermal expansion of LaSrAlO₄ and Sr₂TiO₄ have been investigated through high-temperature neutron and synchrotron X-ray powder diffraction experiments and ab initio electronic calculations. The thermal expansion coefficient (TEC) along the <i>c</i>-axis (α_<i>c</i>) being higher than that along the <i>a</i>-axis (α_<i>a</i>) of LaSrAlO₄ [α_<i>c</i> = 1.882(4)­α_<i>a</i>] is mainly ascribed to the TEC of the interatomic distance between Al and apical oxygen O2 α­(Al–O2) being higher than that between Al and equatorial oxygen O1 α­(Al–O1) [α­(Al–O2) = 2.41(18)­α­(Al–O1)]. The higher α­(Al–O2) is attributed to the Al–O2 bond being longer and weaker than the Al–O1 bond. Thus, the minimum electron density and bond valence of the Al–O2 bond are lower than those of the Al–O1 bond. For Sr₂TiO₄, the Ti–O2 interatomic distance, <i>d</i>(Ti–O2), is equal to that of Ti–O1, <i>d</i>(Ti–O1) [<i>d</i>(Ti–O2) = 1.0194(15)<i>d</i>(Ti–O1)], relative to LaSrAlO₄ [<i>d</i>(Al–O2) = 1.0932(9)<i>d</i>(Al–O1)]. Therefore, the bond valence and minimum electron density of the Ti–O2 bond are nearly equal to those of the Ti–O1 bond, leading to isotropic thermal expansion of Sr₂TiO₄ than LaSrAlO₄. These results indicate that the anisotropic thermal expansion of K₂NiF₄-type oxides, <i>A</i>₂<i>B</i>O₄, is strongly influenced by the anisotropy of <i>B</i>–O chemical bonds. The present study suggests that due to the higher ratio of interatomic distance <i>d</i>(<i>B</i>–O2)/<i>d</i>(<i>B</i>–O1) of <i>A</i>₂^2.5+<i>B</i>³⁺O₄ compared with <i>A</i>₂²⁺<i>B</i>⁴⁺O₄, <i>A</i>₂^2.5+<i>B</i>³⁺O₄ compounds have higher α­(<i>B</i>–O2), and <i>A</i>₂²⁺<i>B</i>⁴⁺O₄ materials exhibit smaller α­(<i>B</i>–O2), leading to the anisotropic thermal expansion of <i>A</i>₂^2.5+<i>B</i>³⁺O₄ and isotropic thermal expansion of <i>A</i>₂²⁺<i>B</i>⁴⁺O₄. The “true” thermal expansion without the chemical expansion of <i>A</i>₂<i>B</i>O₄ is higher than that of <i>AB</i>O₃ with a similar composition

On the Structure of α‑BiFeO₃

Polycrystalline and monocrystalline α-BiFeO₃ crystals have been synthesized by solid state reaction and flux growth method, respectively. X-ray, neutron, and electron diffraction techniques are used to study the crystallographic and magnetic structure of α-BiFeO_3. The present data show that α-BiFeO₃ crystallizes in space group <i>P</i>1 with <i>a</i> = 0.563 17(1) nm, <i>b</i> = 0.563 84(1) nm, <i>c</i> = 0.563 70(1) nm, α = 59.33(1)°, β = 59.35(1)°, γ = 59.38(1)°, and the magnetic structure of α-BiFeO₃ can be described by space group <i>P</i>1 with magnetic modulation vector in reciprocal space <b>q</b> = 0.0045<b>a</b>* – 0.0045<b>b</b>*, which is the magnetic structure model proposed by I. Sosnowska applied to the new <i>P</i>1 crystal symmetry of α-BiFeO₃