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CrI$_3$ represents one of the most important van der Waals systems on the route to understanding two-dimensional magnetic phenomena. Being arranged in a specific layered structure it also provides a unique opportunity to investigate structural transformations in dimension-confined systems. CrI$_3$ is dimorphic and possesses a higher symmetry low-temperature phase, which is quite uncommon. It contrasts with vanadium trihalides which show a higher symmetry high-temperature. An explanation of this distinct behavior together with a large cycle-dependent transition hysteresis is still an open question. Our low-temperature X-ray diffraction study conducted on CrI$_3$ single crystals complemented by magnetization and specific heat measurements was focused mainly on specific features of the structural transition during cooling. Our results manifest that the structural transition during cooling relates to the formation of structural domains despite the lower symmetry structure transforming to a higher symmetry one. We propose that these domains could control the transition temperature and also the size of thermal hysteresis.

Most transition-metal trihalides are dimorphic. The representative chromium-based triad, CrCl3, CrBr3, CrI3, is characterized by the low-temperature phase adopting the trigonal BiI3 type while the structure of the high-temperature phase is monoclinic of AlCl3 type (C2/m). The structural transition between the two crystallographic phases is of the first-order type with large thermal hysteresis in CrCl3 and CrI3. We studied crystal structures and structural phase transitions of vanadium-based counterparts VCl3, VBr3, and VI3 by measuring specific heat, magnetization, and X-ray diffraction as functions of temperature and observed an inverse situation. In these cases, the high-temperature phase has a higher symmetry while the low-temperature structure reveals a lower symmetry. The structural phase transition between them shows no measurable hysteresis in contrast to CrX3. Possible relations of the evolution of the ratio c/a of the unit cell parameters, types of crystal structures, and nature of the structural transitions in V and Cr trihalides are discussed., Comment: 11 pages, 6 figures, 1 table. J. Phys.: Condens. Matter (2022)

We report on temperature-dependent soft X-ray absorption spectroscopy (XAS) measurements utilizing linearly polarized synchrotron radiation to probe magnetic phase transitions in iron-rich Fe1+yTe. X-ray magnetic linear dichroism (XMLD) signals, which sense magnetic ordering processes at surfaces, start to increase monotonically below the N\'eel temperature TN = 57 K. This increase is due to a progressive bicollinear antiferromagnetic (AFM) alignment of Fe spins of the monoclinic Fe1+yTe parent phase. This AFM alignment was achieved by a [100]-oriented biasing field favoring a single-domain state during cooling across TN. Our specific heat and magnetization measurements confirm the bulk character of this AFM phase transition. On longer time scales, however, we observe that the field-biased AFM state is highly unstable even at the lowest temperature of T = 3 K. After switching off the biasing field, the XMLD signal decays exponentially with a time constant {\tau} = 1506 s. The initial XMLD signal is restored only upon repeating a cycle consisting of heating and field-cooling through TN. We explain the time effect by a gradual formation of a multi-domain state with 90 deg rotated AFM domains, promoted by structural disorder, facilitating the motion of twin-domains. Significant disorder in our Fe1+yTe sample is evident from our X-ray diffraction and specific heat data. The stability of magnetic phases in Fe-chalcogenides is an important material property, since the Fe(Te1-xSex) phase diagram shows magnetism intimately connected with superconductivity., Comment: Conference proceeding for JEMS 2020

The results of a single-crystal X-ray-diffraction study of the evolution of crystal structures of VI3 with temperature with emphasis on phase transitions are presented. Some related specific-heat and magnetization data are included. The existence of the room-temperature trigonal crystal structure R-3 (148) has been confirmed. Upon cooling, VI3 undergoes a structural phase transition to a monoclinic phase at Ts ~ 79 K. Ts is reduced in magnetic fields applied along the trigonal c-axis. When VI3 becomes ferromagnetic at TFM1 ~ 50 K, magnetostriction-induced changes of the monoclinic-structure parameters are observed. Upon further cooling, the monoclinic structure transforms into a triclinic variant at 32 K which is most likely occurring in conjunction with the previously reported transformation of the ferromagnetic structure. The observed phenomena are preliminarily attributed to strong magnetoelastic interactions.

The methodology of single crystal growth of metastable $\beta$-Ti alloy TIMETAL LCB in an optical floating zone furnace is presented in this paper. Chemical compositions of both precursor material and single crystals were checked. It was found that the concentration of base alloying elements did not change significantly during the growth process, while the concentrations of interstitial elements O and N increased. DSC measurement determined that this concentration shift has a slight impact on ongoing phase transformations, as in the single-crystalline material peak associated with $\alpha$ phase precipitation moves by a few degrees to a lower temperature and peak attributed to diffusion controlled growth of $\omega$ particles shifts to a higher temperature. X-ray reciprocal space maps were measured and their simulation showed that the single crystal has a mosaic structure with mean size of mosaic blocks of approximately 60 nm.

Nanosized particles of $\omega$ phase in a $\beta$-Ti alloy were investigated by small-angle X-ray scattering using synchrotron radiation. We demonstrated that the particles are spontaneously weakly ordered in a three-dimensional cubic array along the $\langle 100\rangle$-directions in the $\beta$-Ti matrix. The small-angle scattering data fit well to a three-dimensional short-range-order model; from the fit we determined the evolution of the mean particle size and mean distance between particles during ageing. The self-ordering of the particles is explained by elastic interaction between the particles, since the relative positions of the particles coincide with local minima of the interaction energy. We performed numerical Monte Carlo simulation of the particle ordering and we obtained a good agreement with the experimental data.

Layered iridates have been the subject of intense scrutiny on account of their unusually strong spin-orbit coupling, which opens up a narrow gap in a material that would otherwise be a metal. This insulating state is very sensitive to external perturbations. Here, we show that vertical compression at the nanoscale, delivered using the tip of a standard scanning probe microscope, is capable of inducing a five orders of magnitude change in the room temperature resistivity of Sr2IrO4. The extreme sensitivity of the electronic structure to anisotropic deformations opens up a new angle of interest on this material, and the giant and fully reversible perpendicular piezoresistance makes iridates a promising material for room temperature piezotronic devices.

A common way of speeding up powder diffraction measurements is the use of one or two dimensional detectors. This usually goes along with worse resolution and asymmetric peak profiles. In this work the influence of a straight linear detector on the resolution function in the Bragg-Brentano focusing geometry is discussed. Due to the straight nature of most modern detectors geometrical defocusing occurs which heavily influences the line shape of diffraction lines at low angles. An easy approach to limit the resolution degrading effects is presented. The presented algorithm selects an adaptive range of channels of the linear detector at low angles, resulting in increased resolution. At higher angles still the whole linear detector is used and the data collection remains fast. Using this algorithm a well-behaved resolution function is obtained in the full angular range, whereas using the full linear detector the resolution function varies within one pattern which hinders line shape and Rietveld analysis., Comment: 20 pages, 5 figures

We present a comprehensive study of graphene grown by chemical vapor deposition on copper single crystals with exposed (100), (110) and (111) faces. Direct examination of the as-grown graphene by Raman spectroscopy using a range of visible excitation energies and microRaman mapping shows distinct strain and doping levels for individual Cu surfaces. Comparison of results from Raman mapping with X-ray diffraction techniques and Atomic Force Microscopy shows it is neither the crystal quality nor the surface topography responsible for the specific strain and doping values, but it is the Cu lattice orientation itself. We also report an exceptionally narrow Raman 2D band width caused by the interaction between graphene and metallic substrate. The appearance of this extremely narrow 2D band with full-width-at-half maximum (FWHM) as low as 16 cm-1 is correlated with flat and undoped regions on the Cu(100) and (110) surfaces. The generally compressed (~ 0.3% of strain) and n-doped (Fermi level shift of ~250 meV) graphene on Cu(111) shows the 2D band FWHM minimum of ~20 cm-1. In contrast, graphene grown on Cu foil under the same conditions reflects the heterogeneity of the polycrystalline surface and its 2D band is accordingly broader with FWHM > 24 cm-1., Comment: Manuscript 34 pages, 6 Figures; Supporting Information 12 pages, 14 Figures