Structural ordering and antisite defect formation in double perovskites

We formulate an effective model for B-B′ site ordering in double perovskite materials A₂BB′O₆. Even within the simple framework of lattice-gas type models, we are able to address several experimentally observed issues including nonmonotonic dependence of the degree of order on annealing temperature, and the rapid decrease of order upon overdoping with either B or B′ species. We also study ordering in the “ternary” compounds A₂BB′_1−yB″_yO₆. Although our emphasis is on the double perovskites, our results are easily generalizable to a wide variety of binary and ternary alloys.     

Order-disorder transitions: structural and magnetic ordering in double perovskites

Using an effective lattice gas-like model for B-B′ site ordering we study order-disorder transitions in double perovskite materials A₂BB′O₆. We motivate this effective model from a microscopic hamiltonian. Using this model, we are able to address several experimentally observed issues including nonmonotonic dependence of the degree of order on annealing temperature, and the rapid decrease of order upon overdoping with either B or B′ species. We also study ordering in the ‘ternary’ compounds A₂BB′_1−yB″_yO₆, using a variant of the Blume-Emery-Griffiths model. Several issues related to structural and magnetic ordering are discussed.     

Ferroelastic phase transitions induced by the orbital and rotation ordering in perovskites

The phases formed in cubic crystals upon ferrodistortion and antiferrodistortion orientational ordering of anion polyhedra distorted as a result of the Jahn-Teller effect are established in terms of the statistical model in the mean-field approximation. For the four-sublattice three-minimum model, there are four types of ordered phases, and one of them is the intrinsic ferroelastic (ferrodistortion) phase. The conditions providing the appearance of ordering of a particular type are determined. The effect of orbital ordering on the rotational structural distortions is discussed. The conditions of the formation of ordered phases (D _4h ⁵ , D _4h ¹⁸ ) that are most frequently encountered in perovskites are found within the eight-sublattice three-minimum model. It turns out that, under certain conditions, these phases can coexist in the same crystal and transform into each other. The variants of their mutual transformations are considered.     

Vacancy ordering and superstructure formation in dry and hydrated strontium tantalate perovskites: A TEM perspective

Crystal structures of Sr₄(Sr₂Ta₂)O₁₁ and Sr₄(Sr_1.92Ta_2.08)O_11.12, synthesized by solid state reaction technique in dry and hydrated state have been studied mainly using Transmission Electron Microscopy. Due to the lesser ability of X-rays to probe details in oxygen sublattice, the change in crystal symmetry due to ordering of oxygen vacancies could be detected better using Transmission Electron Microscopy. After detailed analysis through TEM, it was observed that no major change occurs in the cation sublattice. The TEM observations are compared with XRD data and discussed. The crystal symmetries and corresponding unit cells of all the perovskites based on the ordering of oxygen vacancies is deduced. Crystal unit cells based on the observations are proposed with ideal atomic coordinates. Finally an attempt is made to explain the water uptake behaviour of these perovskites based on the proposed crystal structure.     

Magnetic ordering in the perovskites Eu1−x CaxMnO3 (0⩻x⩻0.5)

The magnetic properties of Eu_1−x Ca_xMnO₃ have been investigated. As the calcium content increased up to x=0.2, the magnetization and the blocking temperature of the magnetic moments of clusters increased and the magnetic anisotropy decreased. As the calcium content increased further, the magnetization decreased, while the “freezing” temperature of the magnetic moments increased. Anomalies of the magnetic properties were observed in compositions with x=0.4 and 0.5 at T=40 K; these anomalies are attributed to a transition to the antiferromagnetic state in the charge-ordered phase. Fiz. Tverd. Tela (St. Petersburg) 39, 117–120 (January 1997)     

Ferromagnetic ordering of iron impurities in the valence-fluctuating semiconductor SmB6

The electron paramagnetic resonance (EPR) of the valence-fluctuating semiconductor SmB₆ doped by 1 at % Fe is studied. The EPR measurements are performed on a SmB₆ single crystal in a temperature range of 1.6–300.0 K. A number of resonance lines whose g factors indicate the presence of iron ions in the Fe⁰, Fe⁺, Fe²⁺, and Fe³⁺ states have been detected. The iron ions are ferromagnetically ordered below a Curie temperature T = 100 K, and this ordering can be caused by the exchange interaction of impurity ions due to matrix polarization (a similar mechanism is observed in PdFe alloys). This exchange interaction is estimated to be significantly higher than that in PdFe; this fact can result from a very high density of states in the narrow f band, which is characteristic of a valence-fluctuating material.     

Magnetic ordering in double perovskites R2CoMnO6 (R = Y, Tb) investigated by high resolution neutron spectroscopy

We have investigated low energy nuclear spin excitations in double perovskite compounds R(2)CoMnO(6) (R=Y, Tb) by inelastic neutron scattering with a high resolution back-scattering spectrometer. We observed inelastic signals at about 2.1 μeV for Y(2)CoMnO(6) and also for Tb(2)CoMnO(6) at T = 2 K in both energy-loss and energy-gain sides. We interpret these inelastic peaks to be due to the transitions between the hyperfine split nuclear levels of the (59)Co nucleus. The inelastic peaks move towards the central elastic peak and finally merge with it at the magnetic ordering temperature T(C). The energy of the low energy excitations decreases continuously and becomes zero at T(C) ≈ 75 K for Y(2)CoMnO(6) and T(C) ≈ 100 K for Tb(2)CoMnO(6). For Tb(2)CoMnO(6), which contains magnetic rare earth ions, additional quasielastic scattering due presumably to the fluctuations of large Tb magnetic moments was observed. The present study reveals the magnetic ordering of the Co sublattice. The results of this investigation along with that obtained by us for other compounds indicate the presence of unquenched orbital moments in some of the Co compounds.     

The superlattice in iron antimonate: The detection of cationic ordering by electron diffraction

Iron antimonate, in which the cations have previously been envisaged as being randomly distributed over the cationic sites in the rutile-type lattice, has been found by electron diffraction to contain a suplattice composed of an ordered array of cations such that three rutile-type lattices are stacked along the c-direction.     

Magnetooptics and magnetic ordering in ferrite nanoparticles in glass doped with iron and rare-earth elements

Magnetic circular dichroism and X-ray diffraction were used to investigate the structure and magnetooptical properties of nanoparticles formed in potassium-aluminum-germanium-boron glass doped with iron and rare-earth elements. It is demonstrated that in thermally processed glass, the main magnetic phase of the formed nanoparticles is γ-Fe₂O₃ maghemite.     

Helicoidal distributed-feedback cavity action in a ferroelectric liquid crystal

Second-Harmonic (SH) generation was observed in homeotropically aligned ferroelectric liquid-crystal cells using fundamental light propagating along the helicoidal axis. Relatively strong SH light was observed in the helicoidal structure when the optical pitch was nearly the same as the SH-light wavelength, though, otherwise, the cancellation of the generated SH light by the helicoid results in negligibly weak SH intensity. Because the observed SH light is independent of the cell thickness, the SH light is attributed to the one generated from the surface region and the SH light generated inside the cell is confined and lost by the helicoid. This phenomenon indicates the helicoidal Distributed-FeedBack (DFB) cavity action and suggests the possibility of the helicoidal DFB laser using dye-doped ferroelectric liquid-crystal cells.     

Helicoidal fields and spin polarized currents in carbon nanotube-DNA hybrids

We report on theoretical studies of electronic transport in the archetypical molecular hybrid formed by DNA wrapped around single-walled carbon nanotubes (CNTs). Using a Green's function formalism in a π-orbital tight-binding representation, we investigate the role that spin-orbit interactions play on the CNT in the case of the helicoidal electric field induced by the polar nature of the adsorbed DNA molecule. We find that spin polarization of the current can take place in the absence of magnetic fields, depending strongly on the direction of the wrapping and length of the helicoidal field. These findings open new routes for using CNTs in spintronic devices.     

Phase transitions in manganese-containing perovskites

Solid solutions with a perovskite structure of the general formula A′_1?xA″_xMnO₃ (including the previously unknown compounds with A′=La or Pr and A″=Cd or Bi) are studied. It is found that the structure of the orthorhombic phases formed from rhombohedral phases with a decrease in the temperature is characterized by strong anisotropy of the Mn-O bond lengths. This anisotropy drastically decreases at lower temperatures.     

Hyperfine interactions in cubic perovskites

ABO₃ perovskites display several physical properties determined by the characteristics of A and B cations. These compounds have cubic structure at high temperature. Lower symmetry cells that are distorted cubes are found at low temperature. Defects modify the properties of these compounds. Under standard conditions oxygen vacancies are produced. Cation substitution also alters the characteristics of perovskites. These materials have been studied by Perturbed Angular Correlation (PAC) spectroscopy and other hyperfine techniques. In this way abundant information is available to determine charge distributions close to probes. In the cubic phase perturbations were detected that are produced by the interaction of probes with defects. To show up these effects we analyze the quadrupole interaction at ¹⁸¹Ta in several compounds: ABO₃ with A=Ca, Sr and Ba, BaTi_1−x Hf _x O₃ and PbZr_1−x Ti _x O₃ for 0⩽x⩽1. Three different quadrupole interactions were found and are interpreted in terms of distinct probe-oxygen vacancy configurations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Commensurate ordering of iron phthalocyanine on Ag(1 1 1) surface

Epitaxial growth of iron phthalocyanine on a silver (1 1 1) surface has been studied with scanning tunneling microscopy in ultrahigh vacuum. While several structures were observed, this report concentrates on what appeared to be the most stable phase. This phase has commensurate two-dimensional ordering with oblique unit vectors (b₁, b₂) where b₁ = 5a₁ + 2a₂ and b₂ = 5a₂, where a₁ and a₂ are unit vectors of Ag(1 1 1) surface. The rotation angle of iron phthalocyanine relative to the silver [1 0 ] direction was determined to be 16.63° from the high-resolution image analysis. The oblique structure is more commensurate than that of iron phthalocyanine on graphite and of copper phthalocyanine on the Ag(1 1 1) surface. The observed FePc on Ag(1 1 1) structure is similar to that of FePc on Au(1 1 1), but has significantly higher surface density. Bias dependence of the images was observed and is consistent with the expected small HOMO–LUMO gap.     

Electrostrictive effects in non-polar perovskites

Polarization-related electrostrictive coefficients Q_h have been determined for relaxation dielectrics 0.856Sr TiO₃-0.144 Bi_2/3 TiO₃ and (K_3/4Bi_1/4) (Zn_1/6Nb_5/6)O₃, and for a simple non-polar perovskite BaZrO₃ by measuring the hydrostatic pressure dependence of the dielectric permittivity. Anomalous variation in the electrostrictive coefficient with temperature and frequency were observed in (K_3/4Bi_1/4) (Zn_1/6Nb_5/6)O₃ in the relaxation-temperature region. This behavior can be explained qualitatively with a simple model based on potential barriers separating alternative cation sites. The empirical rule noted previously for ferroelectric perovskites, that the electrostrictive Q coefficient increases with cation order from disordered, through simple and then ordered perovskites is confirmed again in non-polar perovskites.