Crystal Field Effects Research Articles

The critical behaviors of the mixed spin-1/2 and spin-2 model in a random crystal field

The effects of a random crystal field on the critical behavior of the mixed spin-1/2 and spin-2 Blume-Capel ferrimagnetic model are investigated on the Bethe lattice (BL). The sublattice dipole and quadrupole moments of the model are obtained in terms of exact recursion relations under the effect of the randomly distributed crystal field. It is assumed to be active randomly with probability p or inactive with probability 1-p for given lattice sites. In order to determine the nature of phase transitions, the thermal behaviors of the sublattice moments are investigated for given system parameters. The phase diagrams are obtained on the (D,T) planes for given values of p with q=3,4 and 6. It is found that the model exhibits only the second-order phase transitions and reentrant behavior for 0<p<1. The compensation temperatures, Tcomp, are also observed and its lines are demonstrated on the (D,Tcomp) planes.

Competition between magnetic order and charge localization in Na2IrO3 thin crystal devices

Spin orbit assisted Mott insulators such as sodium iridate (Na$_2$IrO$_3$) have been an important subject of study in the recent years. In these materials, the interplay of electronic correlations, spin-orbit coupling, crystal field effects and a honeycomb arrangement of ions bring exciting ground states, predicted in the frame of the Kitaev model. The insulating character of Na$_2$IrO$_3$ has hampered its integration to an electronic device, desirable for applications, such as the manipulation of quasiparticles interesting for topological quantum computing. Here we show through electronic transport measurements supported by Angle Resolved Photoemission Spectroscopy (ARPES) experiments, that electronic transport in Na$_2$IrO$_3$ is ruled by variable range hopping and it is strongly dependent on the magnetic ordering transition known for bulk Na$_2$IrO$_3$, as well as on external electric fields. Electronic transport measurements allow us to deduce a value for the localization length and the density of states in our Na$_2$IrO$_3$ thin crystals devices, offering an alternative approach to study insulating layered materials.

Crystal field effects on the photoemission spectra in Cr2O3 thin films: From multiplet splitting features to the local structure

Changes in the shape of X-ray photoemission (XPS) spectra can be related to changes in the local structure of a transition metal. By combining Crystal Field Multiplet calculations and well-controlled molecular beam epitaxy growth of α-Cr2O3(0001) thin films on α-Al2O3(0001) substrates, we prove that it is possible to link the features of Cr 2p XPS spectra with local distortions of CrO6 octahedra and d-orbitals reorganization. Hence, we show that the splitting of the Cr 2p3/2 envelope is related to the degeneracy of the t2g orbital triplet, which corresponds to a fully relaxed structure. Conversely, the broad unstructured Cr 2p3/2 envelope relies on splitting of t2g orbitals and it is the fingerprint of large trigonal distortions of CrO6 octahedra. Then, using the Cr 2p XPS as a structural tool for α-Cr2O3, we show that the Cr2O3 protective layer formed by oxidation of polycrystalline Ni30Cr alloy exhibits in-plane strains at early oxidation stages and grows preferentially along the c-axis.

Corrigendum: Entrapment of a Pseudo-Tetrahedral CoII Center by Thioether Sulfur Bound {Co2 (μ-L)} Fragments: Synthesis, Field-Induced Single-Ion Magnetism and Catechol Oxidase Mimicking Activity.

It has come to the attention of the authors that Figure 6, the values of effective magnetic moments for complexes 1 and 2, giso, E/D values in Abstract section and in the Magnetic properties sections, giso and E/D values in Table 1, and D value for complex 2 in Conclusions section were changed mistakenly in the final stage of manuscript preparation. The data published in Figure 6 reveal the results obtained for older measurement on different batches of the complexes. The correct values, corresponding to the results obtained for the same batches of complexes for which the DC and AC magnetic measurements were done, are presented below. These mistakes do not affect the integrity of the research and overall conclusions of the published paper. The corrected Figure 6 and corresponding text are given below. Magnetic properties. Investigation of the magnetic features of complexes 1 and 2 proceeded using a PPMS device on which the temperature (1.9–300 K) and field (0–9 T) dependent magnetic data were obtained (Figure 6). Figure 6 Temperature dependence of the effective magnetic moment (inset: temperature dependence of the molar magnetic susceptibility) for complexes 1 and 2 (Left). The isothermal magnetizations measured in the range of B=0–9 T and at T=2, 5 and 10 K (Right). The magnetic properties of the complexes are primarily due to the presence of central tetrahedral CoII ion since octahedral LS-CoIII ions are diamagnetic. The value of effective magnetic moment (μeff) of complex 1 at 300 K is 3.5 μB, slightly lower than the spin-only value of 3.9 μB for S=3/2, g=2.0. Thereafter upon cooling the μeff value raises slightly up to ca 75 K and then, it drops sharply to a minimum at 1.9 K, where it adopts the value of 3.2 μB. In the case of complex 2, the μeff value at 300 K is 5.0 μB which is again much higher that the calculated value for isotropic S=3/2 spin. It decreases gradually upon cooling up to ca 50 K and then, it declines sharply to a minimum at 4.4 K reaching the value of 4.2 μB. The high μeff value at 300 K indicates considerable contribution of a spin-orbit coupling to the ground state in the complex 2. Isothermal magnetization experiments (Figure 6) for 1 and 2 in the applied field range of B=0–9 T at different constant temperatures T=2, 5 and 10 K reveal that the magnetization does not show saturation suggesting the presence of magnetic anisotropy. To extract the parameters affected by magnetic anisotropy arising from crystal field effects (g, D), both the susceptibility and magnetization data were fitted simultaneously. The best fit parameters were calculated using the PHI program package[26] for S=3/2 and they are as follows: giso=2.05, D=−23.6 cm−1, E/D=0, zJ/hc=0 cm−1, χTIP=−13.0×10−4 cm3 mol−1 for 1 and giso=2.61, D=−24.3 cm−1, E/D=0.01, zJ/hc=0.05 cm−1, χTIP=2.0×10−4 cm3 mol−1 for 2. The presence of temperature-independent paramagnetism (TIP) term accounts for the four surrounding CoIII ions. The observed D values for 1 and 2 are relatively close compared to those (−31.31 cm−1 and −21.88 cm−1) obtained in our previous investigations with complexes having similar molecular structures.[10] The authors wish to emphasize that with these corrections, the reported ZFS parameters relevant for the discussion of single molecule magnetism (i.e. the D values) did not change at all and the overall conclusions published in the original version of the article are not affected.

Structural and morphological characterization of photoluminescent cerium-doped near UV-blue sodium ortho-phosphate phosphors

Cerium (Ce3+) doped sodium ortho-phosphate (NaMPO4) (M = Mg, Ca, Sr or Ba) were synthesized using solution combustion method. The phosphors were analyzed using X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), Energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV–vis spectroscopy and photoluminescence (PL) spectroscopy. As confirmed from the XRD data, NaCaPO4 and NaBaPO4 crystallized in an orthorhombic phase, while NaMgPO4 and NaSrPO4 crystallized in a monoclinic phase. The SEM data showed that the particle morphology was influenced by alkaline earth metal ions. The PL data exhibited broad emission bands with different emission maxima in the energy range of 2.7–4.2 eV, i.e. in the near UV-blue region of the electromagnetic wave spectrum. The difference in the PL emission spectra that is associated with crystal field effect and other factors is discussed.

Magnetic and thermodynamic characteristics of a rectangle Ising nanoribbon

Using Monte Carlo simulation, we investigated magnetic and thermodynamic characteristics of the rectangle core–shell ferrimagnetic mixed-spin (3/2, 2) Ising nanoribbon under the effects of sublattice crystal field, exchange coupling and temperature. Typical temperature dependence of the total magnetization, the core and shell magnetization, the susceptibility and the internal energy for several fixed values of Hamiltonian parameters were discussed in detail. The phase diagrams of the critical and compensation temperatures were presented in various Hamiltonian parameter planes. In addition, particular emphasis was given to the interesting multi-cycle hysteresis behaviors of the total system and sublattices. Our results show reasonable consistency compared with many previous theoretical and experimental studies.

Positional-Fe-doping-induced spin polarization effects on magnetoelectric properties and spin texture of 2D-SiC

Abstract The electronic structure and magnetic properties of positional-Fe-doped 2D-SiC were systematically studied using the first-principles plane wave pseudopotential based on the generalized gradient approximation(GGA) +U scheme. We found that d electrons with small-polaron behavior acted as itinerate conductive particles in the electron band structure of Fe-substituted Si-doped system, leading to its magnetic anisotropy. Additionally, the spin-up of Fe-d electrons was split as a result of the crystal field effect. The t2g and eg electron levels were risen, while the dz2 levels were lowered, as a deep impurity level was observed at −0.24 eV lower than the nearest d-orbital level. The spin texture of the doped matrix demonstrated that both Rashba–Dresselhaus-type SOC coexist in the polar system. Rashba–Dresselhaus SOC mainly induced the interaction between spin and charge, motivated the spin pumping in the polar matrix. These results confirmed that Fe-doped 2D-SiC is a potential next-generation spintronics material.

Effects of crystal chemistry and local random fields on relaxor and piezoelectric behavior of lead-oxide perovskites

Random electric fields caused by the disordered distribution of heterovalent cations over the equivalent crystallographic positions are often considered to be the main reason of the relaxor behavior and the associated giant electromechanical response. Some models relate the development of the relaxor state to the crystal chemical properties of the constituent ions. In this work the functions of random fields and ferroactive cations are compared in perovskite solid solutions in the desirable for technological applications composition range of morphotropic phase boundary. The crystal structure, dielectric and ferroelectric properties, piezoelectric effect and electrostriction are investigated in technologically important ceramic solid solutions PbzBa1−z(Mg1/3Nb2/3)m(Zn1/3Nb2/3)y(Ni1/3Nb2/3)nTixO3 with substitutions in A (Pb2+ by Ba2+)- and B (Ti4+ by [B2+1/3Nb5+2/3]4+) perovskite sublattices. Two composition sections of the system are designed; in both of them the concentration of ferroactive ions, Pb2+ and Ti4+, respectively, varies within the same range of 15%, but the strength of quenched random electric fields changes significantly only in B-substituted ceramics. Similar behavior is found in both sections, including the same sequence of structural phase transitions and transformations from normal ferroelectric to relaxor state and similar evolution of properties with concentration. The results clearly demonstrate that the role of crystal chemical factors in the development of relaxor behavior and electromechanical performance of lead-oxide materials is not less important than that of quenched random fields. The mechanisms of the observed behavior are discussed in the frame of existing theoretical models.

Effect of Crystal Field on the Electronic Structure of the Two-Band Hubbard Model with Spin Crossover

We consider the change in the electronic structure of the two-band Hubbard model in the regime of strong electron correlations with spin crossover upon the passage through the crossover point depending on the crystal field growth. An abrupt semimetal–insulator–semimetal transition is detected during the passage through the spin crossover point in the absence of the spin–orbit interaction, which is accompanied by a jumpwise redistribution of the partial spectral weight between the poles of the Green function of Fermi quasiparticles. The role of the spin–orbit interaction and the change in the surface topology of surface of one-particle Green function zeros are considered.

Near infrared to visible upconversion photoluminescence from Er/Yb co-doped MoS2 nanosheets with tunable thickness

Er/Yb co-doped MoS2 nanosheets exhibit efficient upconversion photoluminescence emission in the visible wavelength region at 524 nm, 541 nm, 650 nm and downconversion emission at 1124 nm under the excitation of 980 nm laser. These upconversion and downconversion emissions are proposed to be due to the 2H11/2 - 4I15/2 (524 nm), 4S3/2 - 4I15/2 (541 nm), 4F9/2 - 4I15/2 (650 nm) transitions of Er3+ and 2F5/2 → 2F7/2 (1124 nm) transition of Yb3+ ions respectively. It is also found that the Yb3+ ions play a significant role as a sensitizer for the generation of visible upconversion emission of Er3+ ions via excited state energy transfer mechanism. Further, the effects of the layer numbers in co-doped MoS2 on the upconversion photoluminescence properties are also evaluated. The MoS2 nanosheet possessing more number of layers fascinates stronger emission intensities due to the predominant crystal field effect.

Oxygen K-edge X-ray Absorption Spectra.

We review oxygen K-edge X-ray absorption spectra of both molecules and solids. We start with an overview of the main experimental aspects of oxygen K-edge X-ray absorption measurements including X-ray sources, monochromators, and detection schemes. Many recent oxygen K-edge studies combine X-ray absorption with time and spatially resolved measurements and/or operando conditions. The main theoretical and conceptual approximations for the simulation of oxygen K-edges are discussed in the Theory section. We subsequently discuss oxygen atoms and ions, binary molecules, water, and larger molecules containing oxygen, including biomolecular systems. The largest part of the review deals with the experimental results for solid oxides, starting from s- and p-electron oxides. Examples of theoretical simulations for these oxides are introduced in order to show how accurate a DFT description can be in the case of s and p electron overlap. We discuss the general analysis of the 3d transition metal oxides including discussions of the crystal field effect and the effects and trends in oxidation state and covalency. In addition to the general concepts, we give a systematic overview of the oxygen K-edges element by element, for the s-, p-, d-, and f-electron systems.

3d Ions in Solids and Microscopic Crystal Field Effects: Theoretical Analysis and Relations with Experimental Spectroscopic Data

The transition metal ions with unfilled 3d electron shell are important activator ions for solid-state lasing and solid-state lighting. Since their spectroscopic properties depend significantly on the nearest environment, studies of the microscopic crystal field effects (influence of variation of the impurity center geometry) acquire additional importance. In the present paper, methods of calculating the crystal field strength 10Dq and its dependence on the interionic distances R are described. Relation between the 10Dq(R) functions and experimentally observed quantities such the Stokes shift is highlighted. The results of performed 10Dq(R) calculations for several crystals doped with various 3d ions are summarized and discussed.

Crystal-field effects in Er3Ru4Al12 with a distorted kagome lattice

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Local Structure in U(IV) and U(V) Environments: The Case of U3O7.

A comprehensive analysis of X-ray absorption data obtained at the U L3-edge for a systematic series of single-valence (UO2, KUO3, UO3) and mixed-valence uranium compounds (U4O9, U3O7, U3O8) is reported. High-energy resolution fluorescence detection (HERFD) X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) methods were applied to evaluate U(IV) and U(V) environments, and in particular, to investigate the U3O7 local structure. We find that the valence state distribution in mixed-valence uranium compounds cannot be confidently quantified from a principal component analysis of the U L3-edge XANES data. The spectral line broadening, even when applying the HERFD-XANES method, is sensibly higher (∼3.9 eV) than the observed chemical shifts (∼2.4 eV). Additionally, the white line shape and position are affected not only by the chemical state, but also by crystal field effects, which appear well-resolved in KUO3. The EXAFS of a phase-pure U3O7 sample was assessed based on an average representation of the expanded U60O140 structure. Interatomic U–O distances are found mainly to occur at 2.18 (2), 2.33 (1), and 3.33 (5) Å, and can be seen to correspond to the spatial arrangement of cuboctahedral oxygen clusters. The interatomic distances derived from the EXAFS investigation support a mixed U(IV)–U(V) valence character in U3O7.

Analysis of crystal field effects and interactions using X-ray restrained ELMOs

The purpose of this study is to assess the capability of X-ray restrained molecular wavefunction methods to reproduce the perturbation due to the crystal field.The effect of the crystal environment on the molecular charge densities has been thoroughly investigated experimentally and theoretically. However, the same analysis in terms of molecular orbitals is unprecedented. We specifically analysed the extremely localized molecular orbitals (ELMOs) because they allow for a valence bond like interpretation that is closer to the typical chemical reasoning.For a set of test molecules, we calculated first principles wavefunctions (called primary wavefunctions) and the corresponding scattered intensities (primary structure factors), which we afterwards used for X-ray restrained Hartree-Fock and ELMO calculations. This mimics the typical procedure of modelling wavefunctions from experimental X-ray diffraction, having the primary wavefunction as benchmark.This study follows a previous investigation concerning the effects of electron correlation, where it was shown that those subtle features could be retrieved only to a minor extent. The electric field generated by surrounding molecules is instead more substantial. Therefore, retrieving this information should be more successful and, in case, extremely useful for supramolecular chemistry and crystal engineering. The work also provides useful indications to experimentalists who want to exploit the potentialities offered by the X-ray restrained wavefunction methods.

Antiferromagnetic Spin-3/2 Ising Model Under the Influence of Random Crystal Field

The critical behaviors of the antiferromagnetic spin-3/2 Ising model are studied by using the exact recursion relations (ERR) in the presence of a random crystal field on the Bethe lattice (BL). The sublattice order parameters, magnetizations, and quadrupolar moments are obtained in terms of ERRs under the effect of random crystal field which was either turned on or off with probability p and 1 − p, respectively, in a bimodal form. The nature of phase transitions, thus, the phase diagrams of the model are calculated on the (H, T) planes for the randomly changing crystal field values with probability p for the coordination numbers q = 3 and 4. It is found that the model exhibits both second- and first-order phase transitions in addition to some critical points, i.e., tricritical point (TCP), critical end point, double critical point, and isolated end point. We have also observed the reentrant behavior for some values of our system parameters.

Synthesis and optical properties of Mn-doped CaWO4 nanoparticles

Pure and Mn-doped calcium tungstate (Mn:CaWO4) nanoparticles (NPs) were synthesized by hydrothermal method which lead to the formation of high quality and narrow sized particles. The concentration of Mn in CaWO4 NPs was varied from 1% to 4% by weight. A distinct peak of Mn (d–d) transition was observed which is actually spin-forbidden but may be allowed due to crystal field effects. Electron spin resonance measurements revealed inclusion of Mn2+ at the substitutional site.. Room temperature photoluminescence emission (PL) and excitation (PLE) spectroscopies were carried out to understand the interactions of sp-d levels. PL spectra revealed the presence of both band-edge (∼450 nm) and Mn2+ related orange luminescence (∼530 nm). Time dependent photoluminescence (TDPL) studies were also carried out where the life time value of 4.3 ms obtained was in accordance with measurement for Mn in other systems. Thus TDPL further strengths the presence of Mn in 2+ state in CaWO4 NPs.

Effect of crystal field on the formation and diffusion of oxygen vacancy at anatase (101) surface and sub-surface

The intrinsic oxygen vacancy (Ov) in TiO2 is a vital topic of semiconductor materials. Here, we use the first-principles calculations to explore the formation and diffusion properties of Ov at anatase (101) slab, combined with the crystal filed theory. The calculated result shows that Ov at subsurface is not only more stable than that at surface, but also prefers to cluster together at subsurface. The stability of Ov at subsurface is mainly attributed to the Ti3d orbitals in different crystal fields occupied by the excess electrons. Further calculation reveals that the Ov diffusion is greatly affected by the excess electrons. These results presented here show the importance of 3d orbitals in different crystal field, which should be helpful to understand the behavior of Ov at other transition metal oxide.

Influence of low-symmetry component of crystal field on gemstones colors: Cr3+ in ruby and emerald

The origin of difference in the colors of two gemstones – red ruby Al2O3:Cr3+ and green emerald Be3Al2(SiO3)6: Cr3+ is explained by analyzing the crystal field effects. It is shown that the stronger crystal field in ruby and the significantly greater low-symmetry component of crystal field in emerald are the main sources of the observed color difference. Calculated energy levels of the Cr3+ ions in both materials agree well with the experimental results.

Effect of Co2+ content on supercapacitance properties of hydrothermally synthesized Ni1-xCoxFe2O4 nanoparticles

Rich physical properties of ferrite nanoparticles, which exhibit a spinel structure, are sensitive to the distribution of cations in tetrahedral and octahedral sites as well as the crystal field effect, both of which can modify the electronic structure. Here, using a hydrothermal method, we incorporated Co+2 ions into nickel ferrite (NiFe2O4) nanoparticles to synthesize Ni1-xCoxFe2O4 (x = 0.0, 0.5, 1) nanoparticles and studied how their structural, electronic, and electrochemical properties are influenced by the Co+2 content. We found that the employed surfactant agent (here, CTAB) is critically important and affects the size and the specific surface area of the nanoparticles. X-ray diffractometry indicated that the obtained nanoparticles have an inverse spinel structure and field emission scanning electron microscopy showed that the nanoparticles' size is in the range 15–20 nm. Fourier-transform infrared spectroscopy and energy dispersive X-ray spectroscopy confirmed the chemical compositions of the samples. UV–visible spectrophotometry indicated that the optical bandgap of Ni1-xCoxFe2O4 decreases from 2.2 to 1.95 eV as the Co+2 content increases. Electrochemical properties of the samples were studied using cycling voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy techniques. The specific capacitances achieved by the supercapacitors constructed from the prepared NiFe2O4, Ni0.5Co0.5Fe2O4, and CoFe2O4 nanoparticles were calculated from cycling voltammetry curves at the scan rate of 5 mV/s as 534.5, 630, and 991 F/g, respectively, and from galvanostatic charge-discharge curves at the current density of 0.25 A/g as 467.43, 522.80 and 655.60 F/g, respectively. We discussed that the increase in the specific capacitances as the Co+2 content increases is because the Co+2 content increases the available ways for the electrode to react with the electrolyte. Finally, we performed a density functional theory study on the structures to provide more insight into the increased specific capacitance.