Stable anisotropic single-layer of ReTe2: a first principles prediction

In order to investigate the structural, vibrational, electronic, and mechanical features of single-layer ReTe2first-principles calculations are performed. Dynamical stability analyses reveal that single-layer ReTe2crystallize in adistorted phase while its 1H and 1T phases are dynamically unstable. Raman spectrum calculations show that single-layer distorted phase of ReTe2exhibits 18 Raman peaks similar to those of ReS2and ReSe2. Electronically, single-layerReTe2is shown to be an indirect gap semiconductor with a suitable band gap for optoelectronic applications. In addition,it is found that the formation of Re-units in the crystal induces anisotropic mechanical parameters. The in-plane stiffnessand Poisson ratio are shown to be significantly dependent on the lattice orientation. Our findings indicate that single-layer form of ReTe2can only crystallize in a dynamically stable distorted phase formed by the Re-units. Single-layer ofdistorted ReTe2can be a potential in-plane anisotropic material for various nanotechnology applications.

Толеранс-фактор для соединений класса хантитов

An analysis was made of 85 compounds with a huntite type structure RM3(BO3)4, where R = rare-earth element (Y, La - Lu), M = Al, Sc, Cr, Fe, Ga. Crystal structure analysis revealed the critical displacements of atoms at phase transition R32 ↔ P3121 and their dependence from ionic radii. As a result, the tolerance factor was derived and its threshold, below which the structure is stable in the R32 phase, and above which the structure is stable in the distorted phase P3121. The formula has been tested on over 30 compounds with the huntite type structure and gave good agreement. Therefore, it can be used with confidence to predict new compounds. At this work, the tolerance factor revealed consistent patterns in huntites that were previously unknown.

Optically induced transient enhancement of a structural order parameter

We photoexcite SrTiO3 and EuTiO3 in their purely soft-mode-driven structurally distorted phase and trace the structural order parameter via ultra-short x-rays. We observe a rapid decay for SrTiO3 and an intriguing transient enhancement for EuTiO3.

Mode parameterization of structures with very low symmetry: PZT and magnetite

The parameterization of distorted structures in terms of symmetry modes is an effective and efficient method for both their description and refinement [1]. A basis of symmetry-adapted modes transforming according to irreducible representations not only provides a hierarchical division of the degrees of freedom consistent with the mechanism at the origin of the distorted phase, but it allows the avoidance of false refinement minima, typical of highly pseudo-symmetric phases. A reduction of the number of free parameters by setting to zero negligible marginal modes is also possible. The mode description is nowadays easily applicable through freely available programs [2,3], while direct single crystal and powder diffraction refinements under this parameterization are possible combining these programs with some of the most popular refinement codes. The mode description is especially effective when dealing with distorted structures of very low symmetry compared with that of the parent phase. In these cases, the hierarchy between strong primary modes and weak marginal ones is specially pronounced, minimizing the role of many secondary modes. The physical origin of each primary distortion is usually a set of unstable degenerate normal modes. This introduces correlations among the different phases in the phase diagram that become patent in a mode description and can be used both to characterize the evolution of the relevant order parameters and as a stringent test of proposed structural models. Furthermore, the fact that each of the primary mode distortions is basically associated with the activity (instability) of a single normal mode can yield a "single mode" signature in the mode decomposition, which represents a set of subtle additional structural constrains beyond conventional crystallography. We will illustrate these considerations using the examples of the monoclinic phases of ferroelectric PbZr1-xTixO3 (PZT) and the Verwey phase of magnetite.

STRUCTURAL PHASE DIAGRAM FOR THE THERMAL CONTROLLED DISTORTION BY Gd IONS IN THE (Nd1-yGdy)1.85Ce0.15CuO4 HTS SYSTEM

The influence of partial substitution of Nd by Gd on the temperature dependence of the lattice parameters of the optimal doped Nd 1.85 Ce 0.15 CuO 4n-type superconductor (maximum value of Tc) was studied by using X-ray diffraction measurements function of temperature. The transition from the structural normal phase (I4/mmm) to the distortion phase (Acam) was evidenced above y = 0.65 Gd substitution level in ( Nd 1-y Gd y)1.85 Ce 0.15 CuO 4. The structural phase transition temperature TI-A from undistorted phase to the distorted phase changes function of Gd concentration. The structural phase diagram for thermal controlled distortion by Gd ions, TI-A = f(y) was obtained, and compared by the superconducting phase diagram Tc(y).