Crystal Field Effects Research Articles

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 a-Cr2O3(0001) thin films on a-Al2O3(0001) substrates, we prove that it 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 a-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.

Effect of nitrogen substitution on luminescence tuning in garnets.

Anionic substitution is attracting research interest as a property modulation strategy. Although the effect of nitrogen incorporation on luminescence tuning has been widely reported, the correlation between dodecahedral expansion on Si4+-N3- co-substitution and crystal-field splitting of Ce3+ in garnets is rarely discussed. This work is devoted to unraveling the structure-property relationship between anionic substitution and spectroscopy tuning. Ligand movement patterns of a dodecahedron and an octahedron are investigated for tetragonal distortion and inter-facial distance, both of which indicate an energy level shift originating from the crystal-field effect. The quantitative crystal-field calculation is performed on the basis of ligand coordinates to derive the analytical expression for further confirmation. This work complements the substitution effects of both cationic and anionic chemical species on spectral tuning in garnets, and will be helpful in material design and property modulation of garnet-based luminescent materials.

Changing the magnetic states of an Fe/BaTiO3 interface through crystal field effects controlled by strain.

The search for better and inexpensive magnetoelectric materials is now commonplace in solid state physics. Intense coupling between technologically viable electric and magnetic properties, embedded in a single material, is still an attribute greatly pursued by the scientific community. Following this line of thought, using DFT, the study of a specific interface between the TiO2 layer of BaTiO3 and a monolayer of Fe atoms is presented, probing different uni-axial strain effects of the considered supercell. Depending on the strain, several different metastable magnetic states are predicted: a perfectly balanced antiferromagnetic state, an unbalanced ferrimagnetic state, a ferromagnetic state, and a non-magnetic state where each atom has its total magnetic moment quenched. Since these multiple magnetic states can be reversibly controlled by strain, under optimized conditions, this interface can switch from the ferromagnetic state (μ≈ 2.2 μB per Featom) to the non-magnetic state (μ = 0 μB per Featom), enabling enticing prospects for technological applications.

Terahertz spectroscopic characterization of spin mode and crystal-field transition in high-throughput grown <inline-formula><tex-math id="Z-20201027023844-1">\begin{document}$ {\bf Sm}_{ x}{\bf Pr}_{ 1– x}{\bf FeO_3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20200732_Z-20201027023844-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20200732_Z-20201027023844-1.png"/></alternatives></inline-formula> crystals

Terahertz (THz) transient has become an effective method to study the optical and electronic spin characteristics of the rare earth orthoferrites <i>R</i>FeO<sub>3</sub>. High-throughput grown crystal sample is sliced at different locations, then the continuously tunable rare earth elements co-doped single crystal Sm<sub><i>x</i></sub>Pr<sub>1–<i>x</i></sub>FeO<sub>3</sub> is studied with antiferromagnetic spin mode (qAFM) and crystal field transitions of rare earth ions under zero magnetic fields. Using THz time-domain spectroscopy, the qAFM resonance frequencies of Sm<sub>0.2</sub>Pr<sub>0.8</sub>FeO<sub>3</sub> and Sm<sub>0.4</sub>Pr<sub>0.6</sub>FeO<sub>3</sub> single crystals are located on the connection line of the qAFM frequencies of PrFeO<sub>3</sub> (0.57 THz) and SmFeO<sub>3</sub> (0.42 THz), therefore the frequency of qAFM increases linearly with doping concentration of Sm<sup>3+</sup> ion increasing. The Sm<sub>0.4</sub>Pr<sub>0.6</sub>FeO<sub>3</sub> crystal undergoes a temperature-induced spin reorientation phase transition at about 160 K. When the crystal temperature is lower than 80 K, a wide band absorption peak of about 0.5 THz appears in the absorption spectrum of Sm<sub>0.2</sub>Pr<sub>0.8</sub>FeO<sub>3</sub> due to the crystal field effect. Our results show that THz spectral data not only allow us to monitor the quality of rare earth orthoferrite crystals prepared by high throughput and analyze the rare earth elements of the sample, but also improve the ability to analyze the physical properties of the co-doped <i>R</i>FeO<sub>3</sub>.

Magnetic susceptibility of pyrochlores R2Ti2O7: R = Gd, Dy, Tb

We present the study of magnetic susceptibility of pyrochlores Gd2Ti2O7, Dy2Ti2O7 and Tb2Ti2O7 which belong to frustrated magnets. The temperature dependence of the susceptibility χ(T) of powder samples was measured in the temperature range from 4.2 to 300 K and analyzed in the framework of mean field approximation taking into account both crystal field effects and anisotropy of the exchange interaction. We developed a simple approach allowing us to estimate interaction energies of magnetic ions from χ(T) dependences and data on wave functions of the crystal-field states of the ions using a minimum number of fitting parameters. We revealed a strong anisotropy of the exchange interaction in the paramagnetic state of Gd2Ti2O7 and obtained nearest-neighbor effective bond energies of the magnetic ions in Tb2Ti2O7 and Dy2Ti2O7 which are in good agreement with the temperatures of formation of the spin-ice state in these compounds.

Luminescence of uranium(VI) after liquid-liquid extraction from HCl by Aliquat® 336 in n-dodecane:1-decanol by time-resolved laser-induced luminescence spectroscopy

Abstract To investigate the extraction of uranium(VI) in HCl media by Aliquat® 336 in 1:99 (v:v) 1-decanol:n-dodecane mixture, our objective is to identify the complexe(s) in the organic phase by time-resolved laser-induced luminescence spectroscopy (TRLS). The extraction mechanism is supposed to involve the formation of [ U O 2 C l 4 2 − ⋅ ( R 4 N + ) 2 ] $[U{O_2}Cl_4^{2 - } \cdot {({R_4}{N^ + })_2}]$ in the organic phase. The occurrence of such a species leads to the presence of the UO 2 Cl 4 2 − ${\rm{U}}{{\rm{O}}_2}{\rm{Cl}}_4^{2 - }$ species in the organic solution, which luminescence shows particular features. The luminescence spectra and decay time evolutions are obtained in the organic phase as a function of HCl concentration in the aqueous phase (0.5–6 M). The extraction of UO 2 Cl 4 2 − ${\rm{U}}{{\rm{O}}_2}{\rm{Cl}}_4^{2 - }$ is confirmed by the particular spectrum of uranium(VI) in the organic phase, and the typical splitting of the luminescence bands, due to the crystal field effect, is clearly evidenced. The stoichiometry is verified using luminescence intensity variation as a function of the activity of Cl−, and extraction constants are calculated both using the specific interaction theory and Pitzer model. A decomposition of the spectrum of the extracted complex in the organic phase is also proposed. The decay time variation as a function of temperature allows estimating the activation energy of the luminescence process of the extracted complex.

Understanding high ordering temperature in Gd6FeBi2 magnet: Critical behavior, electronic structure and crystal-field analysis

Gd6FeBi2 is reported as the only one room-temperature magnet among more than hundreds of compounds with its structural type, which makes it more attractive in potential applications. To reveal the origin of such high ordering temperature, critical behaviors, electronic structure and crystal-field effects of Gd6FeBi2 are investigated in this work. The short-range Gd-Fe ferrimagnetic interaction is supported by the crystal and electronic structure analyses. Unlike the strong Fe-Fe exchange interactions, the Gd-Fe exchange interaction shows limited influence on the critical exponents determined by long-range exchange interactions, which seems a common feature in rare earth – transition metal based alloys without exchange interactions between transition-metal atoms. However, the strong Gd-Fe hybridization reduces the influence of vibronic couplings on the short-range exchange interaction and thus allows a high Tc. The broadening or splitting mechanism of Gd 4f-electron bands is addressed based on crystal-field analysis and likely another factor for elevated Tc in Gd6FeBi2 and Gd-based compounds with non-magnetic elements. Different magnetic behaviors among isostructural compounds, and the relationship between the band splitting and crystal-field effects is also discussed.

Magnetic properties of binary axb1−x hexagonal monolayers: a Monte Carlo study

ABSTRACTThe magnetic properties of binary Ax(S = 3/2)B1−x(σ=2) hexagonal monolayers (A and B are the magnetic ions with spins S and σ, respectively) are studied via Monte Carlo simulations. The partial, total magnetizations and magnetic susceptibilities of binary Ax(S)B1−x(σ) hexagonal monolayers have been found for one proportion of spin-σ (x = 0.3). Magnetic phase diagrams of spin-σ (tC−x) of our system were deduced. The effect of crystal field on the total magnetizations has been studied for different proportions of spin-σ and for different values of exchange interactions between half-integer spins. The coercive field and remanent magnetization have been established for different proportions, x, exchange interactions R, exchange interactions between spins(σ–σ) and temperatures values.

Octahedral crystal-field splitting in non-conventional coordinate system

This work quantitatively analyzes the octahedral crystal-field effect in non-conventional coordinate setting, which is widely used in explaining the emission peak shift of phosphors. The detailed calculation process involves expansion of the crystal-field potential, the ϕ, θ integration, substitution of the radial part and solution of the secular determinant. It provides a deeper understanding of crystal-field effect.

Synthesis, magnetization, and heat capacity of triangular lattice materials NaErSe2 and KErSe2

In this paper we report the synthesis, magnetization and heat capacity of the frustrated magnets \emph{A}ErSe$_2$(\emph{A}=Na,K) which contain perfect triangular lattices of Er$^{3+}$. The magnetization data suggests no long-range magnetic order exists in \emph{A}ErSe$_2$(\emph{A}=Na,K), which is consistent with the heat capacity measurements. Large anisotropy is observed between the magnetization within the \emph{ab} plane and along the \emph{c} axis of both compounds. When the magnetic field is applied along \emph{ab} plane, anomalies are observed at 1.8 $\mu_B$ in NaErSe$_2$ at 0.2 T and 2.1 $\mu_B$ in KErSe$_2$ at 0.18 T. Unlike NaErSe$_2$, a plateau-like field-induced metamagnetic transition is observed for H$\|$\emph{c} below 1 K in KErSe$_2$. Two broad peaks are observed in the heat capacity below 10 K indicating possible crystal electric field(CEF) effects and magnetic entropy released under different magnetic fields. All results indicate that \emph{A}ErSe$_2$ are strongly anisotropic, frustrated magnets with field-induced transition at low temperature. The lack of signatures for long-range magnetic order implies that these materials are candidates for hosting a quantum spin liquid ground state.

In-situ tailoring upconversion processes from lanthanide ions doped ferroelectric films through piezoelectric strain

The ability to manipulate the upconversion processes in lanthanide doped phosphors is of great significance for both scientific research and practical applications. In this regards, we have demonstrated in-situ tailoring the upconversion processes in Yb3+, Er3+ and Tm3+ tri-doped BaTiO3 thin films through piezoelectric strain. In contrast to conventional chemical routes, the physical approach can eliminate the persisted sample-to-sample inherent inhomogeneities, and identify the lattice deformation impact on the intra-configurational 4f transitions of dopant ions, as well as inter-lanthanide ions energy transfers. Herein, the reduced photoluminescence intensities and the elongated lifetimes for the multi-color emissions mainly arise from strain-induced improved lattice symmetry, which contributes to the reduction of radiative probabilities of dopant Er3+ and Tm3+ ions. While the reduced distances between the dopant ions enhance the energy transfer. These two mechanisms are entangled and act simultaneously. The observed phenomena indicate that the influence of the variation of crystal-field effect on the upconversion emissions suppresses that of the interionic distance-dependent energy transfer under 0.2% biaxial strain imposed by PMN-PT substrates. Our work has unambiguously provided new opportunities to render the upconversion processes. More intriguingly, integrating lanthanide-doped ferroelectric films with giant piezoelectric actuators provide an in-situ and dynamic modulation of the upconversion emissions.

Low-temperature thermodynamic and magnetic properties of clathrate-like arsenide Eu7Cu44As23

Experimental investigation of heat capacity, CP, unit cell parameter, a, as well as the ESR absorption for a clathrate-like compound Eu7Cu44As23 has been performed in the temperature range of 2–300 K. The CP(T) function exhibits a sharp maximum at the temperature of ferromagnetic ordering, Tc ≈ 17 K, whereas the a(T) shows no anomaly at this temperature. This unusual behavior is governed by the clathrate-like structure of Eu7Cu44As23, where Eu2+ guest cations residing in oversized cavities of the host framework order ferromagnetically imposing no influence on the framework dynamics. The analysis of maximum of the Cp(T) function at 2–20 K allows determining the exchange integral, J, which is a characteristic of the magnetic interaction Eu2+ – Eu2+: J ≈ 0.27 K The analysis of the temperature dependencies of heat capacity, CP(T), and unit cell volume, V(T), in the frames of the Debye-Einstein model enabled determining characteristic temperatures of host and guest subsystems. Along with this, the contribution of two-level systems into thermodynamic properties of Eu7Cu44As23 was evaluated and related to the possibility of guest Eu2+ cations to occupy nonequivalent positions inside the cages of the host framework. The ESR of Eu2+ in Eu7Cu44As23 was studied at 5–300 K. The intensity of the ESR signal shows a sharp drop at low temperatures, which confirms the ferromagnetic character of the magnetic phase transition in Eu7Cu44As23. The experimental ESR spectrum was approximated by two Dyson lines, L1 and L2, which correspond to two g-factors: g1 = 2.081 ± 0,030 and g2 = 2,094 ± 0,016. The increase in L1 linewidth with temperature at 75–275 K is d(ΔH1)/dT = 0.027 G/K. These values of g-factors and d(ΔH1)/dT are derived the exchange integrals of the interaction of the Eu2+ local moment with conduction electrons J1* = 0.241 eV and J1** = 0.252 eV accordingly. For L2 J2* = 0.255 eV. Value J2** ≈ 0.26 eV is valid in 100–130 K interval only. The change of ΔH2(T) slope at higher temperatures may be due to crystal field effect.

Monte Carlo study of magnetic and thermodynamic properties of a ferrimagnetic mixed-spin Ising nanotube with double (surface and core) walls

Magnetic properties of Ising nanotubes with double surface and core walls were studied using Monte Carlo study. Total and partial magnetizations and magnetic susceptibilities with walls surface and in the core walls have been given. The effect of crystal field and reduced exchange interactions at surface on total magnetization at surface and core walls were studied. Magnetic hysteresis cycles with double surface and core walls have been found.

Gap Opening in Twisted Double Bilayer Graphene by Crystal Fields.

Crystal fields occur due to a potential difference between chemically different atomic species. In van der Waals heterostructures such fields are naturally present perpendicular to the planes. It has been realized recently that twisted graphene multilayers provide powerful playgrounds to engineer electronic properties by the number of layers, the twist angle, applied electric biases, electronic interactions, and elastic relaxations, but crystal fields have not received the attention they deserve. Here, we show that the band structure of large-angle twisted double bilayer graphene is strongly modified by crystal fields. In particular, we experimentally demonstrate that twisted double bilayer graphene, encapsulated between hBN layers, exhibits an intrinsic band gap. By the application of an external field, the gaps in the individual bilayers can be closed, allowing to determine the crystal fields. We find that crystal fields point from the outer to the inner layers with strengths in the bottom/top bilayer [Formula: see text] = 0.13 V/nm ≈ [Formula: see text] = 0.12 V/nm. We show both by means of first-principles calculations and low energy models that crystal fields open a band gap in the ground state. Our results put forward a physical scenario in which a crystal field effect in carbon substantially impacts the low energy properties of twisted double bilayer graphene, suggesting that such contributions must be taken into account in other regimes to faithfully predict the electronic properties of twisted graphene multilayers.

Signs of anisotropic magnetoresistance in Co2MnGa Heusler alloy epitaxial thin films based on current direction

Anisotropic magnetoresistance (AMR) effects in Cox(Mn0.44Ga0.56)100−x epitaxial thin films were investigated in the temperature range of 5–300 K by changing the current (I) direction to the crystal axis and the Co content (x). The AMR ratios were mostly positive for I // Co2MnGa[100] and negative for I // Co2MnGa[110] and showed peak values at x = 49.7 at% for both current directions. AMR ratios calculated from s–d scattering theory, including crystal field effects with density of states information from first-principles calculations, well reproduced experimental features.

Ab initio molecular dynamics simulations of the ferroelectric-paraelectric phase transition in sodium nitrite

This paper reports on the first ab initio molecular dynamics study of the ferroelectric sodium nitrite, shedding light on its ferroelctric-paraelectric phase transition. The remnant polarization ${P}_{r}$ was calculated using a Mulliken population analysis and maximally localized Wannier functions. Especially the Wannier based model is in outstanding agreement with experimental findings and previous Berry phase calculations. The simulations predict a ferroelectric Curie temperature ${T}_{c}$ between 425 and $450\phantom{\rule{4pt}{0ex}}\mathrm{K}$ in excellent agreement with the experimental value of $437\phantom{\rule{4pt}{0ex}}\mathrm{K}$. In addition, the anomalous lattice behavior (shrinking of the $c$ axis) during the phase transition is reproduced. Furthermore, the analysis of the phase transition revealed a combined displacive and order-disorder mechanism. The crystal field effect in the material could be quantified by investigating the molecular dipoles based on the maximally localized Wannier functions and the intermolecular charge transfer by analyzing the Mulliken charges. In agreement with earlier experimental and theoretical findings, the polarization reversal mechanism was found to be dominated by a $c$-axis rotation of the nitrite ions. The molecular insight into such a simple and prototypical material serves as a basis for a further development of more complex crystalline ferroelectrics, using a design principle inspired by ${\mathrm{NaNO}}_{2}$.

Main Alkaloids of Securinega suffruticosa. Nitrogen Atom Inversion in Securinine

The main alkaloids such as securinine (1), allosecurinine (2), and securitinine (3) are isolated from the Securinega suffruticosa plant and identified by spectral techniques. The spatial structure of 3 is determined by single crystal XRD. The inversion of the pyramidal-tetrahedral nitrogen atom of compound 1 in the crystal and the solution is analyzed based on single crystal XRD and NMR spectroscopy data. By the computational methods and NMR spectroscopy it is shown that the configuration of the nitrogen atom in securinine is related to the alkaloid salt formation rather than the crystal field effect (packing factor).

Copper(I)-Based Highly Emissive All-Inorganic Rare-Earth Halide Clusters

Summary The development of new environmentally friendly luminescent materials is crucial for future solid-state lighting, sensor, and display applications. Here, a Cu(I)-based all-inorganic rare-earth halide material, Rb 8 CuSc 3 Cl 18 , has been synthesized by a solid-state reaction method. In this compound, two Cu(I) ions are connected to three rare-earth halide octahedra to form a paddle-wheel-like cluster. The Cu(I) coordinated rare-earth halide clusters contribute to a strong blue photoluminescence emission. This Cu(I)-regulated emission can be extended to other isostructural compounds, such as Rb 8 CuY 3 Cl 18 . Moreover, the crucial role of Cu(I) has been illustrated by the isostructural non-emissive Rb 8 AgSc 3 Cl 18 . On the basis of comprehensive spectroscopy studies and density functional theory calculations, we found that Cu(I) photo-oxidation and correct orbital-energy-level alignment are crucial for the observed bright-blue emission through a proposed metal (Cu)-to-octahedra ([ScCl 6 ] 3− ) charge-transfer mechanism. The discovery of Cu(I)-based all-inorganic rare-earth halide clusters establishes a new strategy for constructing promising emissive halide materials.

Comprehensive study of the magnetic phase transitions in Tb3Co combining thermal, magnetic and neutron diffraction measurements

A comprehensive study of the magnetic phase transitions in Tb3Co has been undertaken combining different techniques. Using single crystal neutron diffraction in the paramagnetic state a weak crystal structure distortion from the room temperature orthorhombic structure of the Fe3C type described with the Pnma space group toward structure with lower symmetry has been observed with cooling below 100 K. At 81 K there is a second order phase transition to an antiferromagnetic incommensurate phase with the propagation vector k = (0.155, 0, 0). As derived from thermal diffusivity measurements, the critical exponents for this transition are very close to the 3D-Heisenberg universality class, proving that the magnetic interactions are short-range but with a deviation from perfect isotropy due to crystal field effects. At T2 ≈ 70 K there is another magnetic phase transition to a ferromagnetic state whose character is shown to be weakly first order. The low temperature magnetic state has a non-coplanar ferromagnetic structure with strong ferromagnetic components of Tb magnetic moments along the crystallographic c-axis. The application of an external magnetic field B = 2 T along the c crystallographic axis suppresses the incommensurate antiferromagnetic phase and gives rise to the ferromagnetic phase. The magnetic entropy peak change as well as the refrigerant capacity indicate that Tb3Co is a competitive magnetocaloric material in this temperature range.

A remarkably tunable emission from red to yellow to green in Mn4+-activated CaAl12O19 phosphor via co-doping Bi3+

In this study, Bi3+ and Mn4+ co-doped CaAl12O19 phosphors were prepared via high-temperature solid-state reaction method, and their crystal structure, particle morphologies, luminescence properties, charge compensation mechanism and crystal field effect were investigated in detail. A most remarkable tuning effect can be achieved when the Bi3+ concentration is 5% in CaAl12O19:Bi3+, 0.05% Mn4+ and the electric multipolar interaction is inferred to be the main mechanism for concentration quenching effect. It is also found that, as charge compensators rather than sensitizers, Bi3+ ions can effectively tailor the emissions in a wide range through partial reducing Mn4+ to Mn2+. The charge compensation physics presumably involves the shift of Fermi energy level (EF) and variations of valence states of manganese ions after Bi3+ introduction. Moreover, the authors calculated the splitting energy (Δ) and Racah parameters (B and C) for Mn4+ in octahedral sites and Mn2+ in tetrahedral sites by combining excitation spectra with Tanabe-Sugano diagram. The experimental results will be of certain helpfulness in developing phosphors used in white LED.