High Resolution Neutron Diffraction Techniques Research Articles

Large in-plane deformation of RuO6 octahedron and ferromagnetism of bulk SrRuO3

SrRuO3 is a ferromagnetic metal with several unusual physical properties such as zero thermal expansion below Tc, so-called Invar behavior. Another anomalous feature is that the a-axis lattice constant is larger than the b-axis lattice constant, a clear deviation from the predictions of the Glazer structural description with rigid RuO6 octahedron motion. Using high resolution neutron diffraction techniques, we show how these two structural anomalies arise from the irregular in-plane deformation, i.e. plastic behavior of the RuO6 octahedron, a weak band Jahn–Teller distortion. We further demonstrate that the ferromagnetic instability of SrRuO3 is related to the temperature-induced localization of Ru 4d bands.

On the evolution of the DyNiO3 perovskite across the metal–insulator transition though neutron diffraction and Mössbauer spectroscopy studies

The structural changes of polycrystalline DyNiO3 perovskite across the metal-insulator transition (TMI = 564 K) have been studied by high resolution neutron diffraction techniques together with Mössbauer spectroscopy, in a sample doped with 1.5 at.% 57Fe. In the insulating (semi-conducting) regime, below T(MI), the perovskite is monoclinic, space group (SG) P21/n, and the crystal structure contains two chemically different Ni1 and Ni2 cations, as a result of the charge disproportionation of Ni3+ cations. The beta parameter, characterizing the low-temperature monoclinic distortion, is smaller than 90.04 degrees for T < TMI, indicating a strongly pseudo-orthorhombic symmetry, although the internal monoclinic symmetry, implying the splitting and shifts of oxygen positions around the two Ni sites is perfectly detected by neutrons. Above TMI, DyNiO3 becomes orthorhombic, SG Pbnm. Upon heating across TMI, there is an abrupt convergence of the two sets (Ni1 and Ni2) of three Ni-O bond lengths, in the monoclinic-insulating phase, to three unique Ni-O distances in the orthorhombic-metallic phase upon entering the metallic region. The 57Fe Mössbauer spectra of an iron-doped (1.5 at.%) DyNiO3 sample recorded in the insulating, paramagnetic temperature range (TN < T < TMI) are discussed by supposing that the Fe3+ probe cations replace nickel in the two octahedral Ni1 and Ni2 sites. Electric field gradient calculations have shown that the 57Fe hyperfine parameters of Fe1 and Fe2 subspectra reflect a specificity of local structure corresponding to large (Ni1O6) and small (Ni2O6) octahedra. At T > TMI, the 57Fe spectrum gives clear evidence for the formation of an unique state for iron probe atoms and could, therefore, imply that the charge disproportionation in the (NiO6) subarray completely vanishes at the insulator-->metal transition.

R-Phase Phenomena in Neutron Diffraction Investigations of Thermomechanically Loaded NiTi Polycrystals

In situ high resolution neutron diffraction techniques were used to investigate martensitic transformation between the cubic austenite phase and hexagonal R-phase in the NiTi shape memory alloy. Thermally induced transformation and reorientation of the R-phase variants upon loading in tension and compression were studied. Rhombohedral distortion of the R-phase, the asymmetry in R-phase variant selection under stress, macroscopic stress-strain behavior and values of diffraction elastic constants in tension and compression are discussed.

The lattice stability and structure of delta -UZr2 at elevated temperatures

The lattice stability and structure of delta -UZr2 were studied by thermal analysis and high-resolution neutron diffraction techniques. The delta structure was confirmed to be the modified C32 (AlB2-type) structure as in the omega phase of Zr. The temperature dependence of the lattice parameters in the delta phase was determined between room temperature and 880 K. The heat effects at the phase transition from delta to BCC solid solutions were determined to be 5.6-4.4 kJ g-atom-1 for XZr=0.67-0.81. From the composition dependence, the transition enthalpy from omega to beta (BCC) phase in pure Zr was estimated to be 2.8 kJ g-atom-1.

High resolution neutron diffraction techniques for strain/stress measurements at a steady state reactor

Two high-resolution neutron diffraction techniques: three-axis nondispersive setup and two-axis energy dispersive transmission arrangement for nondestructive strain/stress measurements are described. Both techniques, mutually complementary provide sensitivity in relative difference between lattice spacings Δd/d down to 10−5 and FWHM of 10−3 rad permitting profile broadening analysis for reasonable sample volumes and counting times even at a medium power research reactor.

A re-examination of the incommensurate phase of RbD3(SeO3)2 by neutron diffraction

The incommensurate phase of RbD3(SeO3)2 has been re-examined by high resolution neutron diffraction techniques. The positions and widths of the superlattice peaks have been determined with high precision. In comparison with the earlier measurements the transition temperatures in the present sample are 7°K higher and the maximum incommensurability is a factor of 2 lower. These differences suggest that defects play an important role in the incommensurate phase.

Deuterium site occupation and magnetism in Ho 6Fe 23D x, compounds

High resolution neutron diffraction techniques were used to establish the hydrogen (deuterium) site occupation and sublattice magnetization of a series of Ho 6Fe 23D x compounds ( x = 1.5, 8.2, 12.1 and 15.7). The f.c.c. structure of the parent compound was maintained except for the D 12.1 sample which exhibited a tetragonal distortion. The f.c.c. lattice was reestablished for the higher D 15.7 composition. Diffraction and inelastic hydrogen vibration spectra data demonstrate conclusively that the deuterium atoms in the D 1.5 material fully occupy the octahedral a site with the residual in the tetrahedral f site. At the D 8.2 and higher concentration level, the a site is empty and the deuterium atoms partially occupy only the tetrahedral sites. The 4 K holmium magnetic moment was near the free-ion value of 10 μ B for all compounds and was oriented antiparallel to all four iron site moments (b, d, f 1 and f 2). For the cubic materials, the magnetization on the iron sites varied with deuteration in the range 0.8–2.4 μ B , with the D 15.7 compound showing the highest iron moment for all sites. A spin reorientation is observed in the tetragonal material between room temperature and 4 K with the holmium and iron b and n 1 moments rotating out of the basal plane.