Structural evolution of Nd2NiO4.23 with temperature, indicating several 1st order phase transitions with sub-mesoscopic oxygen ordering and instant ordering kinetics below 800 K.

Hole doping in Nd2NiO4.00 can be achieved either by substituting the trivalent Nd atoms by bivalent alkaline-earth metals or by oxygen doping, yielding Nd2NiO4+delta. While the alkaline-earth-metal atoms are statistically distributed on the rare-earth sites, the extra oxygen atoms in the interstitial lattice remain mobile down to ambient temperature and allow complex ordering scenarios depending on delta and T. Thereby the oxygen ordering, usually setting in far above room temperature, adds an additional degree of freedom on top of charge, spin, and orbital ordering, which appear at much lower temperatures. In this study, we investigated the interplay between oxygen and spin ordering for a low oxygen doping concentration, i.e., Nd2NiO4.10. Although the extra oxygen doping level remains rather modest with only 1 out of 20 possible interstitial tetrahedral lattice sites occupied, we observed by single-crystal neutron diffraction the presence of a complex three-dimensional (3D) modulated structure related to oxygen ordering already at ambient, the modulation vectors being +/- 2/13a*+/- 3/13b*, +/- 3/13b*+/- 2/13b*, and +/- 1 /5a*+/- 1 /2c *, and satellite reflections up to fourth order. Temperature-dependent neutron-diffraction studies indicate the coexistence of oxygen and magnetic ordering below T-N similar or equal to 48 K, the wave vector of the Ni sublattice being k = (100). In addition, magnetic satellite reflections adapt exactly the same modulation vectors as found for the oxygen ordering, evidencing a unique coexistence of 3D modulated ordering for spin and oxygen ordering in Nd2NiO4.10. Temperature-dependent measurements of magnetic intensities suggest two magnetic phase transitions below 48 and 20 K, indicating two distinct onsets of magnetic ordering for the Ni and Nd sublattices, respectively.

To study the relationship between the properties of low-dimensional spin systems with weakly coupled Cu-triangles and their crystal structure, single crystals of Cs_3Cu_3Cl_8OH (1) and the new Cs_3Cu_3Cl_7.6Br_0.4OH (2) were grown. Both compounds are isostructural and crystallize in a monoclinic structure with space group P2_1/c. The magnetic susceptibility of (1) shows a maximum at 2.23 K and of (2) at 2.70 K, which are attributed to antiferromagnetic phase transitions. Furthermore, the magnetization along the b-axis at 1.9 K for both compounds shows a spin-flop transition into a new antiferromagnetic phase. This transition occurs at 0.61 T for (1) and at 2.0 T for (2). The antiferromagnetic order can be suppressed by a magnetic field B_C1= 1.1 T for (1) and B_C2= 1.2 T for (2). First single crystal neutron diffraction measured on (1) at different temperatures reveals the magnetic signal on the top of the nuclear reflection at (-1 0 0). Its magnetic ordering temperature was found to be at T_N1= 2.12(3) K.

Better understanding of oxygen diffusion in non-stoichiometric oxides becomes essential for further development of intermediate temperature solid oxide fuel cells. In this prospect, rare earth nickelates (R2NiO4+d) emerged as promising materials in which oxygen transport is driven by oxygen disorder and non-stoichiometry. Their crystal structure consists of RNiO3 perovskite layers sandwiched between RO-type rock salt layers. Extra oxygens are intercalated in tetrahedral sites inside the rock salt layer and diffusion is described by an interstitialcy mechanism [1] in which excess oxygens diffuse via apical oxygens as indicated in Fig. 1 (a). Therefore close to room temperature, oxygen diffusion is not Arrhenius-type (only at high temperature) rather a lattice activated process. In this talk, I will address structural studies which are important to get experimental evidences on oxygen migration mechanism in these oxides. I will present our single crystal neutron diffraction results on Nd2NiO4.25 compound in the temperature range between 2-450 K and I will highlight, especially, the influence of non-stoichiometry on oxygen transport and magnetic properties in this compound. Our neutron diffraction studies show that the average crystal structure of Nd2NiO4.25 compound is monoclinic (B2/m space group) at room temperature. No structural transition has been observed in the temperature range between 2-450 K. Reciprocal space plane mapping (on oriented single crystal) shows the evidence of 3d-ordering of excess oxygens in the whole temperature range. This result confirms the pinning of excess oxygens to the crystal lattice which makes the real structure incommensurate. However, these incommensurate oxygen superstructure reflections start to lose intensity around 400 K confirming the onset of oxygen diffusion through interstitials around this temperature. In our recent work [2] from scattering density studies (at room temperature) of average structure using the Maximum Entropy Method, we observed unusual high displacement factors both for equatorial and apical oxygen atoms showing large displacement amplitudes towards [001] and [110] with respect to the F-symmetry cell, respectively. This confirms the proposed interstitialcy diffusion mechanism and supports our previously obtained molecular dynamically simulated results [3] on Nd2NiO4.25 compound. Moreover, diffuse type incommensurate magnetic reflections have been observed below 100 K in the reciprocal space mapping. This confirms the magnetic ordering temperature for this compound which is about 115 K as identified from our specific heat data. Fig. 1 (b) shows the reconstructed (H0L)-reciprocal plane at 2 K for Nd2NiO4.25. Also, we have followed temperature evolution of incommensurate magnetic reflections in between 2-100 K through elastic q-scans. No positional shifts of these magnetic reflections have been observed in this temperature range. In conclusion, we have studied the influence of excess oxygens on crystal and magnetic structure as well as on oxygen transport for the Nd2NiO4.25 compound using single crystal neutron diffraction in between 2-450 K. We show that these excess oxygens are 3d-ordered below 450 K and are essential to have high oxygen mobility in the intermediate temperature range. Finally, the presence of diffuse-type magnetic reflections at low temperature confirms the weak inter-plane magnetic correlation among Ni2+ magnetic moments through oxygen interstitials.

The structures of new compound [CuX(pyz) 2 ](BF 4 ) with X = Cl -and Br -and pyz = pyrazine were determined by single crystal X-ray diffraction.These tetragonal compounds crystallize in space group P4/nbm.They are built from [Cu(pyz) 2 ] 2+ layers which are connected by X - ions along the c-axis.Charge is compensated by BF 4 -ions in the voids of the 3D coordination compound.The antiferromagnetic interactions between the Cu 2+ ions are mainly two-dimensional (2D) located within the [Cu(pyz) 2 ] 2+ layers.This results in a broad maximum of the magnetic susceptibility around 9 K. Towards lower temperature a kink is observed at 4 K which indicates long-range 3D magnetic order.The magnetic unit cell is doubled along the c-axis (k = 0,0,1/2) and the ordered magnetic moment amounts to μ x = 0.76(8) μ B /Cu 2+ at 1.5 K.The moments are antiferromagnetically coupled along the b-and c-axes.Long-range 3D magnetic order is observed below T N = 3.9(1) K.A fit of a 2D Heisenberg model to the magnetic susceptibility data results in J || = 9.6 K.

The existing controversy about the symmetry of the crystal structure of the ground state of the critical doped La1.95Sr0.05CuO4 has been resolved by analyzing the single crystal neutron diffraction data collected between 5 and 730 K. We observed small but significant intensities for “forbidden” reflections given by extinction rules of the orthorhombic Bmab space group at low temperatures. A careful investigation of neutron diffraction data reveals that the crystal structure of La1.95Sr0.05CuO4 at 5 K is monoclinic with B2/m (2/m 1 1) space group. The monoclinic structure emerges from the orthorhombic structure in a continuous way; however, the structure is stable below ∼120 K which agrees with other observed phenomena. Our results on symmetry changes are crucial for the interpretation of physical properties also in other high temperature superconductors with similar structures.

We present news from the Swiss Spallation Neutron Source SINQ and its instruments for neutron diffraction. The neutron diffraction group operates four instruments: the single-crystal diffractometer ZEBRA, the two powder diffractometers HRPT and DMC, and the strain scanner POLDI as shown in Fig. 1. The new neutron single-crystal diffractometer ZEBRA, which replaces the TriCS instrument, is now in commissioning phase. It will allow crystallographic and parametric studies (temperature, magnetic and electric fields, pressure). ZEBRA is designed to achieve high peak-to-background ratio and to operate in high magnetic fields up to 11 Tesla. The instrument is aimed at resolving challenges emerging in systems that are available as small crystals only and requiring extreme sample environments. The applications of the thermal high-resolution multi-detector powder diffractometer HRPT are high-resolution refinements of chemical and magnetic structures as well as phase analysis of novel materials. Recently HRPT was equipped with computer controlled sample changers of eight (RT) and five (1.7-310 K) samples with sample rotation, which significantly increases the efficiency of operation. The exclusive stroboscopic mode allows measurements of crystal structures as a function of time with time resolution down to 10ms. The cold neutron powder diffractometer DMC is complementary to HRPT and ZEBRA and is designed for high-intensity measurements of weak magnetic intensities. Its recent success in mapping the reciprocal space of single crystals prompts the development of a new high-efficiency two-dimensional neutron detector, which will be available in the near future. A new non-magnetic sample table allows fields up to 6 Tesla. Figure 1. Diffraction instruments at the Swiss Spallation Neutron Source SINQ: a) ZEBRA, b) HRPT, c) DMC and d) POLDI.

$BaCuSi_2O_6$ is a quasi-two dimensional spin dimer system and a model material for studying Bose-Einstein condensation (BEC) of magnons in high magnetic fields. The new $Ba_{1-x}Sr_xCuSi_2O_6$ mixed system, which can be grown with x < 0.3, and $BaCuSi_2O_6$, both grown by using a crystal growth method with enhanced oxygen partial pressure, have the same tetragonal structure ($I4_1/acd$) at room temperature. The mixed system shows no structural phase transition, so that the tetragonal structure is stable down to low temperatures. The oxygen partial pressure acts as control parameter for the growth process. A detailed understanding of the crystal structure depending on the oxygen content will enable the study of the spin dynamics of field-induced order states in this model magnetic compound of high current interest with only one type of dimer layers, which shows the same distance between the Cu atoms, in the structure.

The alpha-MnS-based FeXMn1-XS (0< x < 0.3) solid solutions are synthesized and shown to be new Mott materials with the rock salt structure. Neutron diffraction data show that the chemical-pressure-induced Neel temperature shift from 150 (x = 0) to 200 K (x = 0.29) observed in these materials is accompanied by a decrease in the NaCl-type cubic lattice parameters. It is established that at the symmetry transformation in the compositions with x = 0.25 and 0.29 the structural transition occurs, which is followed by the magnetic transition. These features make the FeXMn1-XS solid solutions interesting for both fundamental study of the interrelation between the magnetic, electrical, and structural properties in MnO-type strongly correlated electron systems and application. (C) 2015 Elsevier B.V. All rights reserved.

The α-MnS-based Fe X Mn 1− X S (0 x x = 0) to 200 K ( x = 0.29) observed in these materials is accompanied by a decrease in the NaCl-type cubic lattice parameters. It is established that at the symmetry transformation in the compositions with x = 0.25 and 0.29 the structural transition occurs, which is followed by the magnetic transition. These features make the Fe X Mn 1− X S solid solutions interesting for both fundamental study of the interrelation between the magnetic, electrical, and structural properties in MnO-type strongly correlated electron systems and application.

We report the cobalt doping dependencies of the ferroelectric polarization and underlying incommensurate spiral magnetic ordering in multiferroic Mn1-xCoxWO4 (0 <= x < 0.18). The spiral ordering changed its rotational plane upon cobalt doping (x similar to 0.1), followed by heliconical ordering at higher doping (x > 0.15). These consecutive changes were accompanied by simultaneous 90 degrees flops of ferroelectric polarizations. Such doping dependencies are driven by the competitions between the anisotropy fields of MnWO4 and CoWO4, the spatial distributions of which are nearly orthogonal to each other. We propose a model that can consistently describe the two consecutive flops based on a combined picture of magnetic anisotropy and biquadratic exchange.

We report a combined synchrotron X-ray and neutron diffraction study on as-grown La(2)CoO(4.14) single-crystal from 10 to 470 K. Unprecedented structural features in terms of a (3 + 2)D incommensurate modulation have been detected and characterized in the Low Temperature Orthorhombic (LTO) phase already at room temperature despite the complex twinning that was unravelled. A new intermediate phase between the LTO and High Temperature Tetragonal (HTT) phases has been observed for the first time (in the range of 413-433 K). The transformation from LTO to this so-called HTLO (High Temperature Less Orthorhombic) phase is associated to a lowering of orthorhombicity and a loss of one modulation vector, yielding a (3 + 1)D incommensurate modulation. Conversely, above 433 K the HTT phase appears as nonmodulated but exhibits a strong dynamic disorder of CoO(6) octahedra, which has been characterized in detail by reconstruction of nuclear densities via the Maximum Entropy Method (MEM).

Using magnetoelectric measurements and neutron diffraction, we investigated multiferroic properties of Co2Y hexaferrite Ba0.3Sr1.7Co2Fe12O22 in zero and finite magnetic field (H perpendicular to c). Upon zero-field cooling, a longitudinal heliconical magnetic structure was observed below 280 K, which involves incommensurate planar helical ordering. When the magnetic field was applied perpendicular to its c axis, electric polarization was observed and the incommensurate ordering was replaced by a commensurate one as commonly observed in other hexaferrites. Electric polarization remained at its maximum during field reversal at 10 K, which indicates that magnetic anisotropy within the basal planes stabilizes the field-induced electric polarization.

The single crystal structure of the title compound is studied over the temperature range 10—470 K.

We report a neutron diffraction and muon spin relaxation $\ensuremath{\mu}$SR study of static and dynamical magnetic properties of ${\mathrm{BaCo}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$, a quasi-one-dimensional spin-chain system. A proposed model for the antiferromagnetic structure includes: a propagation vector ${\mathrm{kP\vec}}_{\text{AF}}=(0,0,1)$, independent of external magnetic fields for fields below a critical value ${H}_{c}(T)$. The ordered moments of 2.18 ${\ensuremath{\mu}}_{B}$ per Co ion are aligned along the crystallographic $c$ axis. Within the screw chains, along the $c$ axis, the moments are arranged antiferromagnetically. In the basal planes the spins are arranged ferromagnetically (forming zigzag paths) along one of the axes and antiferromagnetically along the other. The temperature dependence of the sublattice magnetization is consistent with the expectations of the three-dimensional (3D) Ising model. A similar behavior is observed for the internal static fields at different muon stopping sites. Muon time spectra measured at weak longitudinal fields and temperatures much higher than ${T}_{N}$ can be well described using a single muon site with an exponential muon spin relaxation that gradually changes into an stretched exponential on approaching ${T}_{N}$. The temperature-induced changes of the relaxation suggest that the Co fluctuations dramatically slow down and the system becomes less homogeneous as it approaches the antiferromagnetic state.

NaCl crystals grown by the evaporation of an aqueous salt solution in microgravity on the International Space Station (ISS) were characterized and compared to salt crystals grown on earth. NaCl crystallized as thin wafers in a supersaturated film of 200–700μm thickness and 50mm diameter, or as hopper cubes in 10mm diameter supersaturated spheres. Neutron diffraction shows no change in crystal structure and in cell parameters compared to earth-grown crystals. However, the morphology can be different, frequently showing circular, disk-like shapes of single crystals with 〈111〉 perpendicular to the disks, an unusual morphology for salt crystals. In contrast to the growth on earth the lateral faces of the microgravity tabular hopper crystals are symmetrical because they are free floating during the crystallization process. Hopper cubes were produced without the need to suspend the growing crystals by an ongoing stirring. “Fleur de Sel” is shown as an example of two-dimensional growth of salt on earth and compared to the space grown crystals. It is shown that in microgravity conditions brine fluid inclusions form within the salt crystals.

Using single-crystal X-ray diffraction at 293, 200 and 100 K, and neutron diffraction at 50 K, we have refined the positions of all atoms, including hydrogen atoms (previously undetermined), in the structure of coquimbite (P (3) over bar 1c, a=10.924(2)/10.882(2) angstrom, c=17.086(3) / 17.154(3) angstrom, V=1765.8(3)/1759.2(5) angstrom(3), at 293 / 50 K, respectively). The use of neutron diffraction allowed us to determine precise and accurate hydrogen positions. The O-H distances in coquimbite at 50 K vary between 0.98 and 1.01 angstrom. In addition to H2O molecules coordinated to the Al3+ and Fe3+ ions, there are rings of six "free" H2O molecules in the coquimbite structure. These rings can be visualized as flattened octahedra with the distance between oxygen and the geometric center of the polyhedron of 2.46 angstrom. The hydrogen-bonding scheme undergoes no changes with decreasing temperature and the unit cell shrinks linearly from 293 to 100 K. A review of the available data on coquimbite and its "dimorph" paracoquimbite indicates that paracoquimbite may form in phases closer to the nominal composition of Fe-2(SO4)(3)center dot 9H(2)O. Coquimbite, on the other hand, has a composition approximating Fe1.5Al0.5(SO4)(3)center dot 9H(2)O. Hence, even a "simple" sulfate Fe2-xAlx(SO4)(3)center dot 9H(2)O may be structurally rather complex.

The structure of oxygen-intercalated La2CuO4.07 has been investigated at 20 and 300 K by neutron diffraction on an electrochemically oxidized single crystal. At 20 K, reconstruction of the nuclear density by maximum entropy method shows strong displacements of the apical oxygen atoms towards [100] with respect to the F-centred unit cell, whilst displacements towards [110] and [100] were both found to be present at ambient temperature. Combining structural studies with first-principles lattice dynamical calculations, we interpret the displacements of the apical oxygen atoms to be at least partially of dynamic origin already at ambient temperature. Strong displacements of the apical oxygen atoms of stoichiometric and oxygen-doped \( {\hbox{L}}{{\hbox{a}}_{{2}}}{\hbox{Cu}}{{\hbox{O}}_{{{4} + \delta }}} \) and corresponding associated lattice instabilities, i.e. low-energy phonon modes, are considered as a general prerequisite of low-temperature oxygen diffusion mechanisms. Lattice dynamical calculations on \( {\hbox{L}}{{\hbox{a}}_{{2}}}{\hbox{Cu}}{{\hbox{O}}_{{{4} + \delta }}} \) suggest that the oxygen species diffusing at low temperature are not the interstitial but, more prominently, the apical oxygen atoms. The presence of interstitial oxygen atoms is, however, important to amplify via specific, low-energy phonon modes, a dynamic exchange mechanism between apical and vacant interstitial oxygen sites, thus allowing a dynamically triggered, shallow potential oxygen diffusion pathway. The crucial role of lattice dynamics to enable low-temperature oxygen mobility in K2NiF4-type oxides is discussed on a microscopic scale and compared to similar low-temperature oxygen diffusion mechanisms, recently proposed for non-stoichiometric oxides with Brownmillerite-type structure.