Magnetic order in RCr\mathsf{_{2}}Si\mathsf{_{2}} intermetallics

The magnetic structure and ordering temperatures of three intermetallic compounds which crystallize in the tetragonal ThCr₂Si₂ structure, TbCr₂Si₂, HoCr₂Si₂ and ErCr₂Si₂, have been determined by neutron diffraction, differential scanning calorimetry and magnetization measurements. The Cr-sublattice orders anti-ferromagnetically with Néel temperatures of 758 K for TbCr₂Si₂, 718 K for HoCr₂Si₂ and 692 K for ErCr₂Si₂. Chromium atoms located at 4d crystallographic sites are aligned anti-parallel along the c-axis, with G_ZCr magnetic modes. In contrast with metallic bcc Cr, the refined room temperature value of the ordered Cr moment is anomalously large for all three compounds. No long range magnetic order of the R sublattice in TbCr₂Si₂ and HoCr₂Si₂ is observed, whilst the Er sublattice in ErCr₂Si₂ orders independently of the Cr sublattice below 2.4 K with moments ferromagnetically aligned in the basal plane.Received: 4 November 2003, Published online: 30 January 2004PACS: 75.25. + z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.) - 75.30.Cr Saturation moments and magnetic susceptibilities - 75.50.Ee Antiferromagnetics     

Structural studies of La \mathsf{_{1-x}}Sr\mathsf{_{x}}MnO \mathsf {_{3 + \delta}} (x = 0.1-1.0)

We report here for the first time (particularly for $x \ge 0.5$ ) a systematic structural study using Rietveld Profile Refinement of powder X-ray diffraction data on the series of polycrystalline compounds La_1-x Sr _x MnO $_{3 + \delta}$ (0.1 ≤ x ≤ 1.0). The iodometric redox titration results show that the compounds $0.1\le x\le 0.4$ and the end compound are oxygen excess and deficient respectively and the compounds in the compositional range 0.5 ≤ x ≤ 0.9 are oxygen stoichiometric within the experimental error. It is found that the structure remains hexagonal until x = 0.4 composition. On further doping, at x = 0.5 composition, a structural transition to orthorhombic phase is observed. Around this composition, very small variations in the Mn-O(2)-Mn and average Mn-Mn bond distances are observed. For above x = 0.5, until x = 0.8 composition, the structure remains orthorhombic with reduced orthorhombic distortion. For the next compound, x = 0.9, a mixed hexagonal and orthorhombic phase is observed where the hexagonal phase is 6 layered with stacking sequence of ABCACB type and the orthorhombic phase is more distorted than that of x = 0.8 composition. The end compound is a four layered hexagonal structure with stacking sequence ABAC type which is more distorted than ABCACB type. As one goes down the series, a decrease in the volume per formula unit and average Mn-O bond distance are observed except at x = 0.9 composition. The observed structural transitions from hexagonal to orthorhombic to layered hexagonal phase can be explained under the electrostatic limit.     

Interplay between structural and magnetic properties of Lu\mathsf{_{2}}Fe \mathsf{_{17}} at high pressure

A crystal structure of Lu₂Fe₁₇ was studied by means of neutron powder diffraction method in the pressure range up to 1.65 GPa and ambient temperature. Modification of the magnetic properties corresponding to the pressure-induced change of the crystal structure of Lu₂Fe₁₇ is analysed on the basis of local moment model in the mean field approximation. The experimentally observed suppression of FM state and stabilization of the non-collinear AFM state under high pressure in Lu₂Fe₁₇ can be explained qualitatively from model calculations. FM and non-collinear AFM states in Lu₂Fe₁₇ are found to be nearly degenerate at ambient pressure with the estimated difference between the total energy minima of mRyd and the application of high pressure removes this degeneracy in the favor of non-collinear AFM state.Received: 6 September 2004, Published online: 5 November 2004PACS: 61.50.Ks Crystallographic aspects of phase transformations; pressure effects - 75.25. + z Spin arrangements in magnetically ordered materials - 61.12.Ld Neutron diffraction     

A theoretical study of the Y$\mathsf{_{3}}$O clusters

Hybrid density functional calculations are performed to study the structural and electronic properties of neutral, anionic and cationic Y₃O clusters. The most stable structures of these clusters are found to be triply bridging oxygen atom structures with C_S symmetry. The ground states of Y₃O, Y₃O^- and Y₃O ⁺ are doublet (²A), triplet (³A) and singlet (¹A), respectively. The calculated electron affinities and ionization potentials are in good agreement with the available experimental data. Time-dependent density functional theory is used to calculate the low-lying excited states. A theoretical assignment for the features in the experimental photoelectron spectra is given.Received: 5 November 2003, Published online: 20 January 2004PACS: 36.40.Mr Spectroscopy and geometrical structure of clusters - 31.15.Ew Density-functional theory - 34.50.Gb Electronic excitation and ionization of molecules; intermediate molecular states (including lifetimes, state mixing, etc.)     

Elastic scattering of \sf{59.54}-keV \sf{\gamma}-rays in elements with \sf{22 \leq {Z} \leq {92}} at momentum transfer \sf{0.4 \leq {x} \leq 4.7~{\AA}^{\sf -1}}

Differential cross sections for elastic scattering of the 59.54 keV γ-rays in elements with 22 ≤Z ≤92 have been measured over the angular range 10^○–160^○corresponding to the momentum transfer 0.4 ≤x ≤4.7 Å^-1. The measurements at forward and backward angles were performed using the ²⁴¹Am radioactive point-source, target and the Ge detectors in the transmission and reflection arrangements, respectively. The measured differential scattering cross sections are compared with those based on the form-factor (FF) formalism and state-of-the-art S-matrix calculations to differentiate between their relative efficacies and to check angular-dependence of the anomalous scattering factors (ASF) incorporated as correction to the modified form-factor (MF). The S-matrix values exhibit agreement with the measured data at backward angles and differences ～10% at forward angles. The scattering cross sections based on the MF including ASF’s are in general lower at various angles by up to 20% for medium- and high-Z elements. The observed deviations being higher at the forward angles infer possibility of angular-dependence of ASF’s.     

Tetrahedron Equation and Quantum R Matrices for Spin Representations of {B^{(1)}_n}, {D^{(1)}_n} and {D^{(2)}_{n+1}}

It is known that a solution of the tetrahedron equation generates infinitely many solutions of the Yang-Baxter equation via suitable reductions. In this paper this scheme is applied to an oscillator solution of the tetrahedron equation involving bosons and fermions by using special 3d boundary conditions. The resulting solutions of the Yang-Baxter equation are identified with the quantum R matrices for the spin representations of ${B^{(1)}_n}$ , ${D^{(1)}_n}$ and ${D^{(2)}_{n+1}}$ .