Goldanskii-Karyagin asymmetry in Fe3(CO)12

The Mössbauer effect spectra of Fe₃(CO)₁₂ have been measured from 4.2 to 295 K and reveal an unusual temperature dependence in the relative area of the two components of the quadrupole doublet associated with the two equivalent iron sites and in the relative area of the two observed quadrupole doublets. The asymmetry in the outer quadrupole doublet is attributed to the Goldanskii-Karyagin effect. The strong temperature dependence of the relative areas of the two quadrupole doublets results from a differing temperature dependence of the recoil free fractions for the two chemically different sites.     

Goldanskii-Karyagin effect versus preferred orientations (texture)

There are two main contributions influencing the relative line intensities in a hyperfine pattern of a polycrystalline material: 1) Lattice vibrational anisotropy-Goldanskii-Karyagin effect (GKE) and 2) preferred orientation of the crystallites (texture). We present the results of experiments and computer calculations of the line-intensity ratios for various orientation distributions. The two competing effects are hard to distinguish from an experimental point of view, and this evaluation should enable one to appreciate the magnitude of their respective contributions. A variety of textures is selected and discussed: For instance, the interesting case where the GKE and the texture produce the same effect on the relative line intensities. Generally speaking, it seems that in many qualitative discussions of quadrupole line asymmetries the influence of texture is underestimated, especially for small deviations from randomness. Conversely, the asymmetry in the spectra is often explained by making the ad hoc assumption of a GKE without realizing that this would require an unrealistic lattice vibration anisotropy. This work was done in partial fulfillment of the requirement for the degree “Dr. rer. nat.”.     

A density matrix approach to the Goldanskii-Karyagin effect

A density matrix formalism has been introduced to describe the perturbation of the Mössbauer line intensities caused by the anisotropic recoilless fraction in random absorbers. General properties of the matrix elements are discussed and relationships between thermal tensors and density matrices are investigated with the attention focused on the possibility to extract as much information about the thermal tensors as possible from the observed line intensities. It is shown, that quartic (anharmonic) anisotropy may be observable in quadrupolarly dominated transitions under favourable conditions.     

The Orientation of the Antiferromagnetic Order in FePS3

Hyperfine Interactions - Mössbauer spectroscopy was performed on FePS3. Magnetic moments of a single crystal were found to be parallel to the z-axis at temperatures below 96 K. The internal...     

Study of the valence bands of FePS3 and NiPS3 by resonant-photoemission spectroscopy

Summary We have investigated the valence bands of the tiophosphates FEPS₃ and NiPS₃ with the resonant-photoemission technique. Some structures of the energy distribution curves show rapid intensity changes when the photon energy is scanned across the Fe or Ni 3p absorption thresholds. Instead, other structures show smooth excitation spectra. We have related the former to the transition metal ion 3d states (direct photoemission and satellites) and the latter to the (P₂S₆)⁴⁻ states. These results support also a strongly ionic picture of these compounds.

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Riassunto Le bande di valenza dei tiofosfati FePS₃ e NiPS₃ sono state studiate utilizzando la tecnica della fotoemissione risonante. Brusche variazioni dell'intensità di alcune strutture degli spettri di distribuzione dell'energia dei fotoelettroni sono state osservate quando l'energia dei fotoni era variata intorno alle soglie di assorbimento 3p del Fe e del Ni. Invece altre strutture mostravano degli spettri di eccitazione lisci. Le prime sono state associate agli stati 3d del metallo di transizione (fotoemissione diretta e satelliti), le seconde agli stati dei gruppi (P₂S₆)⁴⁻. Infine, i risultati ottenuti confermano il modello fortemente ionico di questi composti.

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Резюме Мы исследовали валентные зоны тифосфатов FePS₃ и NiPS₃ с помощью техники резонансной фотоэмиссии. Некоторые структуры кривых энергетичеких распределений обнаруживают быстрые изменения интенсивности, когда энергия фотона проходит через пороги 3p поглощения Fe или Ni. Напротив, другие структуры обнаруживают плавные спектры возбуждения. Первые структуры связаны с переходом ионов металла в состоянии 3d (прямая фотоэмиссии и сателлиты), вторые структуры связаны с (P₂S₆)⁴⁻ состояниями. Полученные результаты подтверждают сильно ионную модель этих соединений.

     

Goldanskii-Karyagin effect in α-SnF2: A neutron diffraction and Mössbauer absorption study

Single crystal neutron diffraction data have been obtained at several temperatures for α-SnF₂. These data were used to evaluate the Goldanskii-Karyagin effect expected for the¹¹⁹Sn transition, and compared with the effect measured directly by means of the Mössbauer absorption in powdered samples, free of preferential orientation. These results provide further support for Karyagin's theory. The values of the anisotropy parameterg ₁₁ obtained from the neutron diffraction data are somewhat lower than those obtained from the Mössbauer absorption.     

Radiation effects of electrons on multilayer FePS3 studied with laser plasma accelerator

Space radiation with inherently broadband spectral flux poses a huge danger to astronauts and electronics on aircraft, but it is hard to simulate such feature with conventional radiation sources. Using a tabletop laser-plasma accelerator, we can reproduce exponential energy particle beams as similar as possible to these in space radiation. We used such an electron beam to study the electron radiation effects on the surface structure and performance of two-dimensional material (FePS₃). Energetic electron beam led to bulk sample cleavage and damage between areas of uneven thickness. For the FePS₃ sheet sample, electron radiation transformed it from crystalline state to amorphous state, causing the sample surface to rough. The full widths at the half maximum of characteristic Raman peaks became larger, and the intensities of characteristic Raman peaks became weak or even disappeared dramatically under electron radiation. This trend became more obvious for thinner samples, and this phenomenon was attributed to the cleavage of P-P and P-S bonds, destabilizing the bipyramid structure of [P₂S₆]^4- unit. The results are of great significance for testing the maximum allowable radiation dose for the two-dimensional material, implying that FePS₃ cannot withstand such energetic electron radiation without an essential shield.     

M2, 3 absorption spectra of transition metal ion in MnPS3, FePS3 and NiPS3

Room temperature absorption spectra resulting from the 3p → 3d excitation of transtion metal ions in Mn, Fe and Ni thiophosphates are measured in the photon energy region between 45 and 80 eV. The observed spectral features are well explained by the localized model for the final excited states.     

Magnetism,exchange and crystal field parameters in the orbitally unquenched Ising antiferromagnet FePS3

FePS₃ is a layered antiferromagnet (T _N=123 K) with a marked Ising anisotropy in magnetic properties. The anisotropy arises from the combined effect of the trigonal distortion from octahedral symmetry and spin-orbit coupling on the orbitally degenerate⁵ T _2g ground state of the Fe²⁺ ion. The anisotropic paramagnetic susceptibilities are interpreted in terms of the zero field Hamiltonian, ℋ=Σ_i [δ(L _iz ² −2)+|λ|L _i .S _i ]−Σ _ij J _ij S _i .S _j . The crystal field trigonal distortion parameter Δ, the spin-orbit coupling λ and the isotropic Heisenberg exchange,J _ij, were evaluated from an analysis of the high temperature paramagnetic susceptibility data using the Correlated Effective Field (CEF) theory for many-body magnetism developed by Lines. Good agreement with experiment were obtained for Δ/k=215.5 K; λ/k=166.5 K;J _nn k=27.7 K; andJ _nnn k=−2.3 K. Using these values of the crystal field and exchange parameters the CEF predicts aT _N=122 K for FePS₃, which is remarkably close to the observed value of theT _N. The accuracy of the CEF approximation was also ascertained by comparing the calculated susceptibilities in the CEF with the experimental susceptibility for the isotropic Heisenberg layered antiferromagnet MnPS₃, for which the high temperature series expansion susceptibility is available.     

Raman spectroscopy investigation on the pressure-induced structural and magnetic phase transition in two-dimensional antiferromagnet FePS3

The layered van der Waals antiferromagnetic FePS₃ has received considerable attention because long range magnetic ordering can remain with single atoms layer, which offers potential applications in future ultrathin devices. Here, we perform Raman spectroscopy to systematically explore the variations of lattice vibration and crystal structure under pressure up to 18.9 GPa. We observe two structural phase transitions at approximately 4 GPa and 13 GPa, respectively. Moreover, by monitoring spin-related Raman modes, we demonstrate a pressure-induced magnetic structure transition above 2 GPa. These modes disappear accompanying the second structural phase transition and insulator-to-metal transition (IMT), indicating the suppression of long-range magnetic ordering, in agreement with earlier neutron powder diffraction experiments.     

The Goldanskii-Karyagin effect in α-SnF2: A comparison of Mössbauer and neutron diffraction results

A single crystal of α-SnF₂ has been subjected to analysis by neutron diffraction at four different temperatures in order to evaluate the Goldanskii-Karyagin effect. The values of the anisotropy parameter, g₁₁, obtained from the neutron diffraction data are lower than those obtained by Mössbauer spectroscopy.     

Goldanskii-Karyagin effect and external magnetic field method as tools to measure anisotropy of the recoilless fraction in amorphous materials

Diffraction of X-rays or neutrons can not be used to obtain details about the atomic vibrational anisotropy in the case of amorphous materials due to the lack of well-defined Bragg reflections. Mössbauer spectroscopy can yield some information in such cases, either via the Goldanskii-Karyagin effect or by applying a magnetic field, preferably along the beam axis. The latter method can be applied to the (preferably diamagnetic) samples exhibiting an electric quadrupole interaction (preferably non-axial) and the magnetic field should be chosen in such a way as to produce significant off-diagonal elements in the hyperfine hamiltonian. The external magnetic field method is capable of yielding much more information than the Goldanskii-Karyagin effect in most cases, provided sufficiently strong magnetic fields are available. Some examples of the¹²⁹I Mössbauer spectra have been calculated to show the usefulness and sensitivity of the external magnetic field method.     

Mössbauer study of the transition metal phosphorus trichalcogenide FePS3 and spin glass Mn0.5Fe0.5PS3

The transition metal phosphorus trichalcogenides MnPS₃ and the FePS₃ are CdCl₂ type layered compounds, where the transition metal ions form a hexagonal lattice. While these compounds order anti-ferromagnetically at low temperature, the magnetic structures are different. We have reported that these mixtures Mn_0.5Fe_0.5PS₃ is a spin glass with a glass transition temperature T _g=33.7 K. Then, in this work, we report that the results of the temperature variation of the ⁵⁷Fe Mössbauer spectra of FePS₃ and Mn_0.5Fe_0.5PS₃, in detail. In the anti-ferromagnetic state of FePS₃, the hyperfine magnetic field H _int increases with decreasing temperature and the Isomer shift I. S. increases slightly with decreasing temperature. However in Mn_0.5Fe_0.5PS₃, the two broadened peaks are observed and the two peaks became a single peak with decreasing temperature at about 50.0 K, which is higher than T _g=33.7 K. In the spin glass Mn_0.5Fe_0.5PS₃, the Mössbauer spectra suggest that the magnetic interactions exist far above T _g.     

PTCR effect in BaBiO3-doped BaTiO3 ceramics

BaBiO₃ shows, at room temperature, a distorted perovskite lattice with monoclinic unit cell (I2/m) characterized by two different B sites, occupied respectively by Bi³⁺ and Bi⁵⁺. The influences of BaBiO₃ content on the electrical property and the microstructure of BaTiO₃-based materials have been studied. At a low dopant concentration the grain size is not influenced significantly by the donor concentration. With an increase in BaBiO₃ content the grain size decreases rapidly. All the prepared BaBiO₃ doping BaTiO₃-based thermistors show typical PTC effect. As the amount of BaBiO₃ added in BaTiO₃-based ceramics increases, resistivity appears to exhibit a minimum value. At high BaBiO₃ content (≥ 0.3), the resistivity increased again with increasing BaBiO₃ content. At a given content of BaBiO₃, the influence of sintering temperature on the electrical properties of samples has been investigated. A maximum of ρ_max/ρ_min ratio was obtained at the sintering temperature equal to 1290 °C at a given content of BaBiO₃.     

Stark effect in KCrO3Cl

The Stark effect has been measured on four lines in the crystal spectrum of KCrO₃Cl. The two lines found at 5476 Å and 5379 Å represent 0-0 transitions to a split ¹ E(C _3n ) state. From the directions of the permanent dipoles in the two excited states it is concluded that both states are distorted relative to the ground-state geometry. No indication of a resonance interaction in the unit cell was found.     

Electrolyte gate-controlled Kondo effect in SrTiO3

We report low-temperature, high-field magnetotransport measurements of SrTiO(3) gated by an ionic gel electrolyte. A saturating resistance upturn and negative magnetoresistance that signal the emergence of the Kondo effect appear for higher applied gate voltages. This observation, enabled by the wide tunability of the ionic gel-applied electric field, promotes the interpretation of the electric field-effect-induced 2D electron system in SrTiO(3) as an admixture of magnetic Ti(3+) ions, i.e., localized and unpaired electrons, and delocalized electrons that partially fill the Ti 3d conduction band.     

Size effect in isometric PbTiO3 crystals

The phase transitions Pm3m⇄P4mm and domain-structure formation in 5–500 μm isometric PbTiO₃ crystals were investigated. The phase transition is characterized by a high rate and by the formation of a single flat interphase boundary {023}. A size effect was observed: In crystals smaller than a critical size (about 20 μm), formation of 90° domains stops and, in agreement with a phenomenological theory, the temperature hysteresis of the phase transition doubles. Fiz. Tverd. Tela (St. Petersburg) 40, 1546–1547 (August 1998)     

Jahn-Teller effect in low-spin Ni3+ in the LaAlO3 ceramic

This paper reports on an EPR study of LaAl_1?x Ni_xO₃ solid solutions with x≤0.12 made in the 4.2-to 300-K temperature range. In the X range, the broadening of the single EPR line with g _eff=2.148 was observed at temperatures below ～40 K. In the Q range, a slightly anisotropic EPR line with g _⊥ ^′ =2.145±0.002 and g _‖ ^′ =2.154±0.002 transforms to a rhombic-symmetry spectrum with g ₁=2.183±0.002, g ₂=2.143±0.002, and g ₃=2.118±0.002. It is shown that the observed low-temperature effects are due to the lowered symmetry of the complex under the combined action of the tetragonal Jahn-Teller distortions and of the trigonal component of the crystal field.     

Resistance switching effect in LaAlO3/Nb-doped SrTiO3 heterostructure

The authors report on the fabrication and electronic transport property of LaAlO₃/Nb-doped SrTiO₃ heterostructure. The current–voltage curves of this heterostructure show hysteresis and a remarkable resistance switching behavior, which increase dramatically with decreasing temperature. Multiresistance states were realized by voltage pulses with different amplitudes and polarities and the ratio of the electrical pulse induced resistance change is larger than 10⁴. More interestingly, the relaxation of junction current after switching follows the Curie–von Schweidler law J∝t ⁻ⁿ with an exponential increase of n with temperature. The results were discussed in terms of the trap-controlled space charge limited conduction process via defects near the interface of the heterostructure.