Neutron scattering study of the magnetoelectric compound GaFeO 3

We present a neutron scattering study carried out on single-crystals of the magnetoelectric compound GaFeO₃, prepared by the floating-zone (FZ) method. The refinement of the crystal structure indicates the existence of a large amount of iron on the gallium site, corresponding to a strong internal substitution disorder. We show that the Ga/Fe disorder and the magnetic transition temperature are weakly dependent on the growth conditions. The disorder also affects the magnetic excitations; our measurements evidence damped spin waves, well defined only close to the Brillouin zone centre.     

Spin densities in a ferromagnetic bimetallic chain compound: polarized neutron diffraction and DFT calculations

The spin population distribution in the ferromagnetically coupled hetero-bimetallic chain compound [MnNi(NO(2))(4)(en)(2)] (en = 1,2-ethanediamine) has been investigated by means of polarized neutron diffraction experiments, and the results compared with those from theoretical estimates obtained via calculations based on density functional theory on dinuclear molecular models of the chain. The spin distributions obtained from experiment and from theory are consistent and reflect a larger spin delocalization from the Ni atom due to the more covalent character of the Ni-N bonds compared to the Mn-O ones. Also a nearly isotropic spin distribution is observed for the more ionic d(5) Mn(2+) ion and a clearly anisotropic distribution for the d(8) Ni(2+) ion. The use of dinuclear molecular models for the calculation of the exchange coupling constant between Ni and Mn provide upper and lower limits (+17.6 and -4.2 cm(-)(1)) for the experimentally determined value (+1.3 cm(-)(1)), depending on how the missing part of the chain is simulated, but yield essentially the same spin distribution. The Mn(II)-Ni(II) weak ferromagnetic coupling in the chain is interpreted in a spin delocalization mechanism as resulting from the weakness of the overlap between the magnetic orbitals centered on nickel and those centered on manganese which are only weakly delocalized on the ligands.     

Systematic trends in the237neptunium Mössbauer isomer shift: Overlap of IV,V and VI neptunium oxidation states and correlation between isomer shift and crystal structure

The influence of the neptunium ion environment on the²³⁷Np Mössbauer isomer shifts has been studied in various metal coordination complexes: fluorides, oxides, oxide fluorides and polycarboxylates. A linear dependence between the isomer shift and the mean neptunium-ligand distance in a series of hexavalent Np compounds has been evidenced and the feasibility of overlapped isomer shift areas, namely Np(IV), Np(V) and Np(VI) has been established.     

A neutron scattering study of strong-symmetric hydrogen bonds in potassium and cesium hydrogen bistrifluoroacetates: Determination of the crystal structures and of the single-well potentials for protons

The crystal structures of potassium and cesium bistrifluoroacetates, KH(CF(3)COO)(2) and CsH(CF(3)COO)(2), respectively, were determined at room and cryogenic temperatures with the single crystal neutron diffraction technique. The crystals belong to the monoclinic space groups, I2a and A2a, respectively, and there is no evidence of any structural phase transition. In both crystals, trifluoroacetate entities in centrosymmetric dimers are linked by very short hydrogen bonds lying across a center of inversion. The thermal parameters provide no evidence of any double minimum potential for hydrogen bond protons. Single-minimum potentials were determined via best fitting to the inelastic neutron scattering spectral profiles of the stretching vibrations. They comprise a narrow well for the ground state and a very broad quasiharmonic well for excited states. The spread out of the wave functions of these states shows that protons are no longer confined between the oxygens. Presumably, they are attracted by the lone pairs of oxygen atoms. These potentials emphasize the covalent nature of the OO bond and the ionic character of the hydrogen bond proton.     

Neutron scattering studies of K(3)H(SO(4))(2) and K(3)D(SO(4))(2): The particle-in-a-box model for the quantum phase transition

In the crystal of K(3)H(SO(4))(2) or K(3)D(SO(4))(2), dimers SO(4)???H???SO(4) or SO(4)???D???SO(4) are linked by strong centrosymmetric hydrogen or deuterium bonds whose O???O length is ≈2.50 A?. We address two open questions. (i) Are H or D sites split or not? (ii) Is there any structural counterpart to the phase transition observed for K(3)D(SO(4))(2) at T(c) ≈ 85.5 K, which does not exist for K(3)H(SO(4))(2)? Neutron diffraction by single-crystals at cryogenic or room temperature reveals no structural transition and no resolvable splitting of H or D sites. However, the width of the probability densities suggest unresolved splitting of the wavefunctions suggesting rigid entities H(L1∕2) -H(R1∕2) or D(L1∕2) -D(R1∕2) whose separation lengths are l(H) ≈ 0.16 A? or l(D) ≈ 0.25 A?. The vibrational eigenstates for the center of mass of H(L1∕2) -H(R1∕2) revealed by inelastic neutron scattering are amenable to a square-well and we suppose the same potential holds for D(L1∕2) -D(R1∕2). In order to explain dielectric and calorimetric measurements of mixed crystals K(3)D((1 - ρ))H(ρ)(SO(4))(2) (0 ≤ ρ ≤ 1), we replace the classical notion of order-disorder by the quantum notion of discernible (e.g., D(L1∕2) -D(R1∕2)) or indiscernible (e.g., H(L1∕2) -H(R1∕2)) components depending on the separation length of the split wavefunction. The discernible-indiscernible isostructural transition at finite temperatures is induced by a thermal pure quantum state or at 0 K by ρ.