An inverse Ruddlesden-Popper nitride Ca7(Li1−xFex)Te2N2 grown from Ca flux

Nitridoferrates containing monovalent iron ions are a class of materials of recent interest as potentially novel magnetic materials. Aiming at the exploration of nitridoferrates of calcium, we report the single crystal growth from Ca ?ux and crystal structure of the first member (n?=?2) of a series of inverse Ruddlesden-Popper nitrides with a general formula of A_n?1A'₂B_nX_3n+1, where A?=?Li/Fe, A'?=?Te, B?=?N, and X?=?Ca. Single crystal X-ray diffraction analyses indicate the crystal with a composition of Ca₇(Li_0.32(1)Fe_0.68(1))Te₂N₂ and the tetragonal space group I4/mmm (a?=?4.7884(1) Å, c?=?25.3723(4) Å, Z?=?2). The structure features alternately stacking NaCl-type A'X slabs and the perovskite-type ABX₃ slabs along the c axis. The Li/Fe atoms are located in cuboctahedral cavities surrounded by eight Ca₆N octahedra in the ABX₃ slab. This work demonstrates the viability of the Ca-rich ?ux as a suitable solvent for the exploration of new complex nitrides with interesting crystal structure and properties.     

Universality of the reversal transition in growth phenomena of molecular crystals

ABSTRACT

The fundamental origin of the reversal transition during crystal growth has been investigated by Monte Carlo simulations. Evidence is provided supporting the geometrical nature of the transition, arising from the broken translational invariance at the growth front and the interactions of multipoles of opposed parity. The range of interactions, the details of the force field and model are irrelevant to the occurrence and only determine the functional behaviour of the relevant physical quantities like the localisation of the critical point and the morphology of the phase diagram.     

Thermodynamic consistency and fast dynamics in phase-field crystal modeling

A general formulation is presented to derive the equation of motion and demonstrate thermodynamic consistency for several classes of phase-field (PF) and PF crystal (PFC) models. It can be applied to models with a conserved and non-conserved phase-field variable, describing either locally uniform or periodic stable states, and containing slow as well as fast thermodynamic variables. The approach is based on an entropy functional formalism previously developed in the context of PF models for locally uniform states [P. Galenko and D. Jou, Phys. Rev. E 71 (2005) p.046125] and thus allows to extend several properties of the latter to PF models for periodic states, i.e., PFC models.     

Distribution of Cr atoms in a strained and strain-relaxed Fe89.15Cr10.75 alloy: a Mössbauer effect study

An Fe_89.15Cr_10.75 alloy in a heavily strained (by cold rolling) state and in strain-relaxed states was studied by means of conversion electrons Mössbauer spectroscopy. Analysis of the spectra in terms of a two-shell model revealed significant differences between the studied samples, particularly in values of the hyperfine field and the distribution of Cr atoms within the first two neighbour shells. The latter is quantified in terms of short-range order parameters.     

Crystal growth and defect study of BiSbTe3

BiSbTe₃ single crystals have been grown by the Bridgman technique. Microscopic observations of the as-grown crystals reveal typical features, such as striations on the top free surface, which are attributed to the effect of growth conditions. A nitric-acid-based reagent capable of revealing dislocations has been developed and tested. Etch pits are produced at the dislocation sites, but some discrepancies have been observed on matched cleavage surfaces. The structural difference between the matched cleavage surfaces is discussed.     

Analysis of the relation between structure and the superconducting transition in YBa2 Cu3 O∼7

Abstract

It is shown that in the high-T_c YBa₂ Cu₃ O_～7 superconductor the critical temperature is a function of the orthorhombic distortion (b–a)/a of the unit cell. From the extrapolation of the (b - a)/a ratio against T_c , a maximum critical temperature of 66 K for the tetragonal phase of YBa₂ Cu₃ O_～7 was predicted. From the correlation between the transition width δT_c and the orthorhombic distortion there was deduced an upper limit for T_c , in the orthorhombic phase of YBa₂ Cu₃ O_～7 of 94·5K.     

Pressure-induced phase transition in nanocrystalline La0.9Mn0.8Fe0.2O3.15 perovskite studied by Mössbauer spectroscopy

We report here on a pressure-induced phase transition in nanocrystalline La-deficient perovskite oxide, synthesised through the sol–gel technique, using Mössbauer spectroscopy, electrical resistivity and X-ray diffraction measurements at room temperature. At ambient pressure, the high-spin Fe³⁺ ion is distributed in two different environments. Below 0.6 GPa, a sudden dip in the resistivity curve and a single-environment Fe³⁺ doublet in the Mössbauer pattern are indicative of lattice compaction. A high-spin to low-spin Fe³⁺ transition at 2.1 GPa, and an orthorhombic to monoclinic structural transformation at 4.9 GPa, as revealed through Mössbauer patterns, are also associated with sudden dips in the electrical resistivity. X-ray diffraction patterns support the results.     

Investigations of the optical spectra and EPR g-factors for the tetragonal Ce3+ centers in YPO4 and LuPO4 crystals

The optical spectral band positions and EPR g-factors (g_‖ , g _⊥) for the tetragonal Ce³⁺ centers in YPO₄ and LuPO₄ crystals with the zircon-structure are calculated using a complete diagonalization (of the energy matrix) method (CDM) related to 4f¹ ions in tetragonal symmetry. In this method, the Zeeman interaction term are added to the Hamiltonian in the conventional CDM and so no perturbation calculations are required to obtain the g factors. The crystal-field parameters used in the calculations are obtained from the superposition model in which the local lattice relaxation related to the bonding lengths is considered. The calculated results are in reasonable agreement with the experimental values. It is found that the four observed optical bands for both the systems can be attributed to Ce³⁺ ions in a tetragonal crystal field.