Crystal and magnetic structure transitions in bimno3+δ ceramics driven by cation vacancies and temperature

The crystal structure of BiMnO$_{3+δ}$ ceramics has been studied as a function of nominal oxygen excess and temperature using synchrotron and neutron powder diffraction, magnetometry and differential scanning calorimetry. Increase in oxygen excess leads to the structural transformations from the monoclinic structure (C2/c) to another monoclinic (P2$_{1}$/c), and then to the orthorhombic (Pnma) structure through the two-phase regions. The sequence of the structural transformations is accompanied by a modification of the orbital ordering followed by its disruption. Modification of the orbital order leads to a rearrangement of the magnetic structure of the compounds from the long-range ferromagnetic to a mixed magnetic state with antiferromagnetic clusters coexistent in a ferromagnetic matrix followed by a frustration of the long-range magnetic order. Temperature increase causes the structural transition to the nonpolar orthorhombic phase regardless of the structural state at room temperature; the orbital order is destroyed in compounds BiMnO$_{3+δ}$ (δ ≤ 0.14) at temperatures above 470 °C

Valence, exchange interaction, and location of Mn ions in polycrystalline MnxGa1−xN (x = 0.04)

We present an experimental study for polycrystalline samples of the diluted magnetic semiconductor MnxGa1−xN (x 0.04) in order to address some of the existing controversial issues. X-ray and neutron diffraction, x-ray absorption near-edge structure, and electron paramagnetic resonance experiments were used to characterize the structural, electronic, and magnetic properties of the samples, and inelastic neutron scattering was employed to determine the magnetic excitations associated with Mn monomers and dimers. Our main conclusions are as follows: (i) The valence of the Mn ions is 2+. (ii) The Mn2+ ions experience a substantial single-ion axial anisotropy with parameter D = 0.027(3) meV. (iii) Nearest-neighbor Mn2+ ions are coupled antiferromagnetically. The exchange parameter J = −0.140(7) meV is independent of the Mn content x; i.e., there is no evidence for hole-induced modifications of J towards a potentially high Curie temperature postulated in the literature

Crystal and magnetic structure of the Ca 3 Mn 2 O 7 Ruddlesden-Popper phase: neutron and synchrotron x-ray diffraction study

Abstract The crystallographic and magnetic structures of Ca 3 Mn 2 O 7 RuddlesdenPopper phase have been determined by a combination of neutron and synchrotron x-ray diffraction. Two-phase behaviour observed at room temperature is attributed to an incomplete structural phase transition. The magnetic structure was solved in the Cm c2 1 Shubnikov group with dominant G-type antiferromagnetic order in the perovskite bilayers. The temperature evolution of the structural and magnetic parameters is presented

Crystal and Magnetic Structure Transitions in BiMnO3+δ Ceramics Driven by Cation Vacancies and Temperature

The crystal structure of BiMnO3+δ ceramics has been studied as a function of nominal oxygen excess and temperature using synchrotron and neutron powder diffraction, magnetometry and differential scanning calorimetry. Increase in oxygen excess leads to the structural transformations from the monoclinic structure (C2/c) to another monoclinic (P21/c), and then to the orthorhombic (Pnma) structure through the two-phase regions. The sequence of the structural transformations is accompanied by a modification of the orbital ordering followed by its disruption. Modification of the orbital order leads to a rearrangement of the magnetic structure of the compounds from the long-range ferromagnetic to a mixed magnetic state with antiferromagnetic clusters coexistent in a ferromagnetic matrix followed by a frustration of the long-range magnetic order. Temperature increase causes the structural transition to the nonpolar orthorhombic phase regardless of the structural state at room temperature; the orbital order is destroyed in compounds BiMnO3+δ (δ ≤ 0.14) at temperatures above 470 °C

Mechanistic and Kinetic Study of the Electrochemical Charge and Discharge of La₂MgNi₉ by in Situ Powder Neutron Diffraction

The intermetallic La₂MgNi₉ has been investigated as negative electrode material for NiMH battery by means of in situ neutron powder diffraction. This hydride-forming compound exhibits suitable plateau pressures ranging within the practical electrochemical window and leads to significant reversible electrochemical capacities. Charge and discharge of the composite electrode have been performed in beam following various current rates and galvanostatic intermittent titration. From the diffraction data analysis, phase amounts and cell volumes have been extracted, allowing the interpretation of the hydride formation and decomposition. From the evolution of the diffraction line widths, differences are observed between charge and discharge with the possible formation of an intermediate γ phase on charge. The electrode readily responds to current rate variations and does not show any kinetic limitation in the range C/10 and C/5 (C/<i>n</i>: full capacity C in <i>n</i> hours). This material shows excellent properties regarding electrochemical storage of energy