Crystal Field Research Articles

Neodymium(III) Aqua Ion as a Model System in Ab Initio Crystal Field Analysis Beyond Point Charges and Crystal Field Theory.

Correlating the molecular structure with the electronic structure of lanthanide(III) solvates is a challenging task. Neodymium(III) in aqueous solution serves as an appealing and straightforward model to address this issue. Herein, the experimentally determined electronic structure of neodymium(III) in water is compared with ab initio calculated electronic structures based on various models of its molecular structure. This comparison enables the determination of the most reliable molecular structure. The findings reveal that the molecular structure of the neodymium(III) aqua ion that best aligns with its electronic structure corresponds to a nine coordinated neodymium(III) complex, surrounded by 17 water molecules in the second coordination sphere. The role of second-sphere water molecules was investigated by calculating the crystal field splitting of the five Kramers doublets within the 4I9/2 low-energy multiplet for several calculated molecular structures with coordination numbers of eight, nine, and ten. The results demonstrated that the shape of the donor molecular orbitals plays a critical role in the crystal field splitting of the neodymium(III) ion. Furthermore, the findings confirmed that the orientation of the donating orbitals, specifically the orientation of the O-H bonds in water, is essential for accurately describing the electronic structure. Finally, manual alteration of the Nd-O bond lengths revealed that CAS(3,7)CF calculations tend to underestimate the crystal field strength.

Probing the magnetic ground state of stretched diamond lattices NdTaO4 and NdNbO4: impact of spin–orbit coupling and crystal electric field

Magnetic systems, wherein competing degree of freedoms arising from spin orbit coupling and crystal electric field lead to non-trivial magnetic ground states, remains in the forefront of research in condensed matter physics. Here, we present a comprehensive investigation on three-dimensional rare-earth based spin systems NdTaO4and NdNbO4, where the Nd ions sit on a stretched diamond lattice. No signatures of long-range ordering and spin freezing are observed down to 1.8 K, in both cases. The low temperature Curie-Weiss analysis indicate towards the dominance of antiferromagnetic interactions between Nd spins. A three-level CEF model clearly explain the nature of susceptibility curve. At low temperatures, heat capacity data exhibit two-level Schottky anomaly associated with ground state Kramer's doublet. Additionally, the low temperature magnetic behaviour is found reliable to effective spin (Jeff) = ½ ground state, suggesting the presence of quantum fluctuations in both cases. First-principle calculations reveal a significant value of orbital moment with inclusion of spin orbit coupling and reinforce theJeff= ½ nature of the ground state.

Voltage-Gated 90° Switching of Bulk Perpendicular Magnetic Anisotropy in Ferrimagnets.

Unraveling the mechanism behind bulk perpendicular magnetic anisotropy (PMA) in amorphous rare earth-transition metal films has proven challenging. This is largely due to the inherent complexity of the amorphous structure and the entangled potential origins arising from microstructure and atomic structure factors. Here, we present an approach wherein the magneto-electric effect is harnessed to induce 90° switching of bulk PMA in Tb-Co films to in-plane directions by applying voltages of only -1.2 V. This manipulation is achieved by voltage-driven insertion of hydrogen atoms into interstitial sites between Tb and Co atoms, which serves as a perturbation to the local atomic structure. Using angle-dependent X-ray magnetic circular dichroism, we find that the anisotropy switching originates from the distortion of the crystal field around Tb, which reorients the alignment of Tb orbital moments. Initially aligned along Tb-Co bonding directions, the easy magnetization axis undergoes reorientation and switches by 90°, as substantiated by ab initio calculations. Our study not only concludes the atomic origin of Tb-Co atom bonding configuration in shaping bulk PMA but also establishes the groundwork for electrically programmable ferrimagnetic spintronics, such as controlling domain wall motion and programming artificial spin textures.

Monte Carlo Investigation of the Critical Behavior of a Core–Shell Nanotube

Monte Carlo simulation is employed to examine a ferrimagnetic Ising‐type hexagonal nanotube, featuring a spin‐ core embedded in a spin‐1 shell. The impacts of specific exchange interactions and the crystal field on the magnetization and phase diagrams of the system are analyzed and discussed. Under certain physical conditions, the nanotube exhibits first‐ and second‐order phase transitions and compensation temperatures. The characteristics highlighted are significant for the technological applications of these systems, such as magnetic storage devices, sensors, and drug delivery systems.

Theoretical Studies on the EPR Parameters and Local Structures for the Orthorhombic and Tetragonal Cu2+ Centers in Cu1-xHxZr2(PO4)3.

The defect structures of the orthorhombical and tetragonal Cu2+ centers in Cu1-xHxZr2(PO4)3 are theoretically studied by analyzing their experimental electron paramagnetic resonance (EPR) parameters, based on the perturbation formulas of these parameters for a 3d9 ion in orthorhombically and tetragonally elongated octahedra, respectively. The above centers are attributed to the Cu2+ ions locating at M(1) site, and the crystal field parameters (CFPs) are quantitatively determined from the superposition model and the local structures of the Cu2+ sites. Based on the calculation, the parallel Cu-O bonds may undergo the relative elongations ΔZO (≈ 0.113 Å) and ΔZT (≈ 0.102 Å) for the orthorhombical and tetragonal Cu2+ centers in Cu1-xHxZr2(PO4)3 along z-axis, respectively. Meanwhile, the planar Cu-O bonds are found to experience the relative variation δr (≈ 0.056 Å) along the x- and y-axes for the orthorhombical Cu2+ center because of the Jahn-Teller (JT) effect. The theoretical EPR parameters based on the above local structures agree well with the observed values.

Impact of the Angular Alignment on the Crystal Field and Intrinsic Doping of Bilayer Graphene/BN Heterostructures.

The ability to tune the energy gap in bilayer graphene makes it the perfect playground for the study of the effects of internal electric fields, such as the crystalline field, which are developed when other layered materials are deposited on top of it. Here, we introduce a novel device architecture allowing simultaneous control over the applied displacement field and the crystalline alignment between two materials. Our experimental and numerical results confirm that the crystal field and electrostatic doping due to the interface reflect the 120° symmetry of the bilayer graphene/BN heterostructure and are highly affected by the commensurate state. These results provide unique insight into the role of twist angle in the development of internal crystal fields and intrinsic electrostatic doping in heterostructures. Our results highlight the importance of layer alignment, beyond the existence of a moiré superlattice, to understand the intrinsic properties of a heterostructure.

T2 Occupancy as an Effective and Predictive Descriptor for the Design of High‐Performance Spinel Oxide Peroxidase‐like Nanozymes

AbstractNanozymes are next generation of enzyme mimics. Due to the lack of activity descriptors, most nanozymes were discovered through trial‐and‐error strategies or by accident. While eg occupancy in an octahedral crystal field was proven as an effective descriptor, the t2 in a tetrahedral crystal field has rarely been explored. Here, we first identified t2 occupancy as an effective and predictive descriptor. Then, we predicted and demonstrated that spinel oxide nanozymes (AB2O4) with a t2 occupancy of around 4.4 at A site had the highest activity. Furthermore, we introduced Oβ content as a secondary descriptor. The dual descriptor strategy resulted in a three‐dimensional volcanic curve, converging at a vertex. To surpass the limitations of volcanic curves, a dual site optimizing strategy was proposed, guiding the optimization of both A and B sites as Cu and Co, respectively. The designed CuCo2O4 exhibited the highest activity, achieving around 100‐ and 2‐fold enhancement compared to initial material and the state‐of‐the‐art spinel oxide nanozyme LiCo2O4, respectively. Density functional theory calculations provided a theoretical basis for the catalytic process. This work provides a new strategy for the rational design of nanozymes, and t2 occupancy may also be applicable to the design of other catalysts.

Dynamic Covalent Bonds-Mediated Color-Switchable Circularly Polarized Luminescence in Helical Assemblies of Achiral Liquid Crystal Block Copolymer Films.

Circularly polarized luminescence (CPL) film attracted considerable attention in information storage and encryption, three-dimensional display, and chiral recognition. However, due to the limited molecular mobility within thin film, achieving a high asymmetry factor and non-contact modulation of CPL remain challenging. In this work, color-switchable homochiral CPL films with high luminescence asymmetry factor (glum~0.11) were constructed based on the co-assembly of a liquid crystal block copolymer (poly (ethylene oxide)-b-poly (methyl methacrylate) bearing cyanostilbene group) with axial chiral dinaphthalene diamine derivatives (R/S-DINBPA). Mechanistic investigation revealed that the efficient stacking of R/S-DINBPA with cyanostilbene groups and the liquid crystal field provide the driving force for chiral transfer and amplification. The dynamic covalent bonds of the cyanostilbene groups endow the film with chiral fixation ability and enable precise modulation of CPL luminescence bands under UV light irradiation, making it suitable for chiral patterned anti-counterfeit encryption. This facile strategy provides a general platform for designing intelligent chiroptical materials.

Sensitive dependence of pairing symmetry on Ni-eg crystal field splitting in the nickelate superconductor La3Ni2O7

The discovery of high-temperature superconductivity in La3Ni2O7 under pressure has drawn great attention. However, consensus has not been reached on its pairing symmetry in theory. By combining density-functional-theory (DFT), maximally-localized-Wannier-function, and linearized gap equation with random-phase-approximation, we find that the pairing symmetry of La3Ni2O7 is dxy, if its DFT band structure is accurately reproduced by a downfolded bilayer two-orbital model. More importantly, we reveal that the pairing symmetry of La3Ni2O7 sensitively depends on the crystal field splitting between two Ni-eg orbitals. A slight increase in Ni-eg crystal field splitting alters the pairing symmetry from dxy to s±. Such a transition is associated with the change in inverse Fermi velocity and susceptibility, while the shape of Fermi surface remains almost unchanged. Our work highlights the sensitive dependence of pairing symmetry on low-energy electronic structures in multi-orbital superconductors, which calls for care in the downfolding procedure when one calculates their pairing symmetry.

Mn-Mn Dimers Induced Multimode Emitters in Mn2+-Activated AZn4(PO4)3 (A = K, Rb, and Cs) with Unique [Zn4PO12] Chains and [ZnOn] Groups.

Mn2+-doped luminescent materials play a significant role in a variety of fields, including modern lighting, displays, and imaging. Mn2+ exhibits a broad and adjustable emission, hinging on the local environment of the crystal field and the interaction of the 3d5 electrons. However, it is still a challenge to realize the precise control of the emission of Mn2+ ions due to site-prior occupation in a specific lattice. Here, the formation of Mn-Mn dimers is proposed to be an effective strategy to design a novel red emission. Multimode emitters in Mn2+-activated AZn4(PO4)3 (A = K, Rb, and Cs) with unique [Zn4PO12] chains and [ZnOn] groups are observed to achieve regular green and unusual red emissions. KZn4(PO4)3:Mn2+ shows broad dual emissions at 542 and 608 nm, attributed to [MnO4] and [Mn2O7] dimers, respectively. While K is replaced with Rb and Cs, the Zn ions form [ZnO4] tetrahedra and [ZnO5] octahedra. RbZn4(PO4)3:Mn2+ exhibits a broad red emission at 618 nm, ascribing to [Mn2O7] and [Mn2O8] dimers. CsZn4(PO4)3:Mn2+ also displays a broad orange-red emission at 608-620 nm with increasing doping levels, deriving from energy transfer from [MnO5] to [Mn2O7] and [Mn2O8] dimers. This work provides a framework for creating novel red emissions from Mn2+-doped luminescent materials.

Relaxation dynamics of a mixed ferrimagnetic Ising system with random anisotropy

The relaxation dynamics of a mixed spin-1/2 and spin-1 ferrimagnetic Ising system with random anisotropy has been investigated using Onsager’s theory of irreversible thermodynamics. The magnetic Gibbs energy production, arising due to irreversible processes, is computed using the equilibrium mean-field Gibbs energy, based on the variational principle and the Gibbs–Bogoliubov inequality. In the framework of linear response theory, the time derivatives of the sublattice magnetizations are treated as fluxes conjugate to their corresponding generalized forces. Two relaxation times are computed, and their dependence on temperature and crystal field variances is examined near phase transition points for four distinct topologies, each corresponding to different phase diagrams. These phase diagrams emerge from random anisotropy drawn from a bimodal probability distribution: P(Δi)=12δ(Δi-Δ(1+α))+δ(Δi-Δ(1-α)). One of the relaxation times, denoted as τ1, increases rapidly and diverges near the critical and tricritical points separating the ferrimagnetic and paramagnetic phases. For α≥0, critical slowing down, characterized by the divergence of τ1, is observed near the isolated ordered critical points between the ferrimagnetic and disorder-induced ferrimagnetic phases. Finally, the variance of the relaxation times is analyzed across regions of crystal field and temperature, as well as the values of α at which re-entrant phenomena occur, due to the competing interactions in the random system.

Understanding the microscopic origin of the magnetic interactions in CoNb2O6

Motivated by the on-going discussion on the nature of magnetism in the quantum Ising chain CoNb2O6, we present a first-principles-based analysis of its exchange interactions with additional modeling, addressing drawbacks of a purely density functional theory ansatz. This method allows us to extract and understand the origin of the magnetic couplings—including all symmetry-allowed terms - and resolve conflicting model descriptions in CoNb2O6. We find that the twisted Kitaev chain and transverse-field ferromagnetic Ising chain views are mutually compatible, although additional off-diagonal exchanges are required for a complete picture. We show that the dominant exchange interaction is a ligand-centered process—involving eg electrons -, rendered anisotropic by low-symmetry crystal fields in CoNb2O6, resulting in dominant Ising exchange. Smaller bond-dependent anisotropies are found to originate from d − d kinetic exchange processes involving t2g electrons. We demonstrate the validity of our low-energy model by comparing its predictions to measured THz and INS spectra.

T2 Occupancy as an Effective and Predictive Descriptor for the Design of High-Performance Spinel Oxide Peroxidase-like Nanozymes.

Nanozymes are next generation of enzyme mimics. Due to the lack of activity descriptors, most nanozymes were discovered through trial-and-error strategies or by accident. While eg occupancy in an octahedral crystal field was proven as an effective descriptor, the t2 in a tetrahedral crystal field has rarely been explored. Here, we first identified t2 occupancy as an effective and predictive descriptor. Then, we predicted and demonstrated that spinel oxide nanozymes (AB2O4) with a t2 occupancy of around 4.4 at A site had the highest activity. Furthermore, we introduced Oβ content as a secondary descriptor. The dual descriptor strategy resulted in a three-dimensional volcanic curve, converging at a vertex. To surpass the limitations of volcanic curves, a dual site optimizing strategy was proposed, guiding the optimization of both A and B sites as Cu and Co, respectively. The designed CuCo2O4 exhibited the highest activity, achieving around 100- and 2-fold enhancement compared to initial material and the state-of-the-art spinel oxide nanozyme LiCo2O4, respectively. Density functional theory calculations provided a theoretical basis for the catalytic process. This work provides a new strategy for the rational design of nanozymes, and t2 occupancy may also be applicable to the design of other catalysts.

A rare case of a zero-field single-ion magnet in a cerium(III) pseudo-icosahedral complex.

The synthesis and structural characterisation of [Ln(Tp2-Fu)2]I (1-Ln; Ln = La, Ce, Pr, Nd) (Tp2-Fu = hydrotris(3-(2'-furyl)-pyrazol-1-yl)borate) have been reported as an isomorphous series adopting pseudo-icosahedral ligand field geometries. Continuous shape measurement (CShM) analyses on the crystal field environments of 1-Ln show the smallest values yet reported for complexes employing two hexadentate ligands (e.g. bis-scorpionate environments), with the smallest belonging to 1-La. Single-ion magnetism for 1-Ce, 1-Pr and 1-Nd was probed with ac magnetic susceptibility studies revealing slow magnetic relaxation for 1-Nd in applied magnetic fields and in zero-applied field for 1-Ce, which is a rare observation for Ce(III)-based single-ion magnets. The energy barrier to magnetic relaxation for 1-Ce was experimentally determined to be Ueff = 30(5) cm-1, which is comparable to that of other cerium-based single-molecule magnets in the literature, where these systems stablise the mJ = ±5/2 state and possess large energy gaps between the ground and first excited state that do not agree with the experimentally determined barrier.

Structural and luminescent properties of a Cr3+/Sm3+ doped GdAlO3 orthorhombic perovskite for solid-state lighting applications.

The Cr3+ and Sm3+ doped GdAlO3 perovskite with formula Gd0.995Sm0.005Al0.995Cr0.005O3, was synthesized via a solid-state reaction method, and its structure, morphology, and photoluminescence properties were thoroughly investigated. The compound crystallizes in the orthorhombic Pbnm space group, with Cr3+ transition-metal ions substituting Al3+ in the octahedral symmetry site, and Sm3+ lanthanide (rare-earth) ions occupying the tetrahedral site. The material's morphology and chemical composition homogeneity were evaluated through Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis. Photoluminescence excitation (PLE) and emission spectra (PL) were used to shed light on the electronic structure of the Cr3+ cations, through crystal field analysis in the O h symmetry site. Theoretical studies enabled the precise assignment of the Cr3+ 3d-3d transitions. The intra-configurational 4f-4f transitions of Sm3+ resulted in a variety of excitation bands appearing in the high-wavelength range of the PLE spectrum. The photoluminescence studies supported the occurrence of energy transfer in the doped GdAlO3 perovskite between Gd3+, Sm3+ and Cr3+ ions. The obtained results suggest the high potential of the synthesized material for solid state lighting applications.

Complex Permittivity Measurement Based on EMNZ Metamaterial Using Biased Magnetic Field for Liquid Crystals

Complex Permittivity Measurement Based on EMNZ Metamaterial Using Biased Magnetic Field for Liquid Crystals

A promising high-sensitive 1D photonic crystal magnetic field sensor based on the coupling of Fano\\Tamm resonance in far IR region

This paper presents a novel investigation of a magnetic sensor that employs Fano/Tamm resonance within the photonic band gap of a one-dimensional crystal structure. The design incorporates a thin layer of gold (Au) alongside a periodic arrangement of Tantalum pentoxide () and Cesium iodide () in the configuration . We utilized the transfer matrix method in conjunction with the Drude model to analyze the formation of Fano/Tamm states and the permittivity of the metallic layer, respectively. These states can be manipulated based on the left-handed and right-handed circular polarization of electromagnetic waves, along with an applied magnetic field. Several key parameters were optimized, including material selection, layer thickness, unit cell periodicity, and the angle of incidence, to enhance the sensor performance. Additionally, we investigated how variations in magnetic field strength influence the position of Fano/Tamm resonance in the reflectivity spectrum of the interacting electromagnetic waves within a specific wavelength range of 60 μm to 140 μm. The proposed sensor displays good performance investigated by calculating several parameters like, sensitivity, figure of merit, quality factor and resolution. One of them, it shows a maximum sensitivity of 57 nm/Tesla within a magnetic field strength of 20 to 140 Tesla, positioning it as a promising candidate for various applications in magnetic field measurement and telecommunications, particularly in the unique far-infrared region.

Scalable Synthesis of 2D ErOCl with Sub-meV Narrow Emissions at Telecom Band.

Van der Waals (vdWs) materials are promising candidates for hetero-integration with silicon photonics toward miniaturization and integration. VdWs materials like molybdenum telluride and black phosphorus, despite being prominent, exhibit air sensitivity, and their room temperature emissions can be significantly broadened by tens of meV. Here, a self-encapsulation strategy is developed to scalably synthesize robust 2D vdWs ErOCl with sub-meV narrow emissions at the telecom C-band. Diverse 2D rare earth materials are also grown via chemical vapor deposition (TmOCl, YbOCl, HoOCl, DyOCl, SmOCl, NdOCl, TbOCl, GdOCl, EuOCl, and PrOCl), demonstrating the strategy's generalizability. The as-grown ErOCl exhibits high crystalline quality and excellent ambientand thermal stability (300°C). Photoluminescence analysis reveals a series of narrow emissions across the visible to near-infrared spectrum. The ErOCl's emission at the telecom band is narrowest among 2D luminescent materials, and suitable for integrating with photonic chips. Temperature-dependent photoluminescence spectra facilitate the understanding of emission mechanisms, analyzed using a crystal field perturbation model. Moreover, these emissions can be tuned by external magnetic fields. This research not only pioneers a novel strategy for synthesizing 2D rare earth materials but also paves the way for innovative building blocks in the realm of on-chip optical communications.

Chloride, Alkoxide, or Silicon: The Bridging Ligand Dictates the Spin State in Dicobalt Expanded Pincer Complexes.

We report the synthesis and characterization of a series of high- and low-spin dicobalt complexes of the tBuPNNP expanded pincer ligand. Reacting this dinucleating ligand in its neutral form with two equiv of CoCl2(tetrahydrofuran)1.5 yields a high-spin dicobalt complex featuring one Co inside and one Co outside of the dinucleating pocket. Performing the same reaction in the presence of two equivalents of KOtBu provides access to a high-spin dicobalt complex wherein both Co centers are bound within the PNNP pocket, and this complex also features a bridging OtBu ligand. Reacting either of the high-spin complexes with excess diethyl silane affords a low-spin dicobalt complex containing two unusual bridging Si-based ligands. These complexes were investigated using NMR spectroscopy, XAS, single crystal X-ray structure determination, and computational methods, showing that the Si-based ligands are best described as base-stabilized silylenes.

Study of the boundary migration of recrystallized grains under inhomogeneous deformation field by crystal plasticity – phase field simulation

ABSTRACT In this study, a grain boundary (GB) migration model based on crystal plasticity (CP) and phase field (PF) was constructed for recrystallization of magnesium alloys. The model introduced the stored energy field obtained by CP into the PF simulations, aiming to analyze the effect of the inhomogeneous stored energy generated under the plastic deformation on the GB migration. It was found that the curvature plays a more significant role in the inhomogeneous stored energy field than in the uniform deformation field. Although grains with hard orientation have a lower deformation storage energy, they are still partially swallowed due to mutual influence between GBs near the triple junction. The high energies in the original GBs increase the migration rate at the triple junctions locally for a short period of time, but it is not enough to make a significant difference over extended periods at mesoscopic scales. The simulation results in this study are helpful in explaining the previous experimental observations.