Crystal Field Research Articles

Theory of Magnetoacoustic Resonance to Probe Multipole Effects Due to a Crystal Field Quartet

Theory of Magnetoacoustic Resonance to Probe Multipole Effects Due to a Crystal Field Quartet

Enhancement strategy of thermal stability and photoluminescent intensity of Cr3+ doped Li3+xZn2–2xGaxSbO6 phosphor in a strong crystal field

Near-infrared phosphor-converted light-emitting diodes (pc-LEDs) have shown enormous influence in various applications such as night vision, non-destructive detection，especially in the realm of plant cultivation. However, the role of phosphors presently employed in plant cultivation is typically limited, serving primarily for the purpose of illumination. In this work, a series of near-infrared phosphors Li3+xZn2–2xGaxSbO6: 0.02Cr3+(LZGS: 0.02Cr3+) under 450 nm excitation was successfully synthesized by high-temperature solid-state method. Through precise modulation of Ga3+ ions, a blue shift in the emission spectrum was observed, demonstrating excellent overlap with the absorption spectrum of Phytochrome. To explain two phenomena of spectral blue shift and the enhancement in luminescence intensity, density functional theory calculations have been performed. Additionally, a crystal field strength Dq/B of 3.48 was achieved, which represents the highest value currently reported for Cr3+-doped phosphors. The results offer potential applications of LZGS: 0.02Cr3+ in temperature sensing, enabling precise detection of the environmental temperature for plant growth. Finally, pc-LED devices were made by combining LZGS: 0.02Cr3+ with 450 nm blue light chips and applied to plant lighting, temperature measurement, and anti-counterfeiting, demonstrating the potential of LZGS: 0.02Cr3+ in multifaceted applications.

Ferromagnetic ferroelectricity due to the Kugel-Khomskii mechanism of orbital ordering assisted by atomic Hund's second rule effects

The exchange interactions in insulators depend on the orbital state of magnetic ions, obeying certain phenomenological principles, known as Goodenough-Kanamori-Anderson rules. Particularly, the ferro order of alike orbitals tends to stabilize antiferromagnetic interactions, while the antiferro order of unlike orbitals favors ferromagnetic interactions. The Kugel-Khomskii theory provides a universal view on such coupling between spin and orbital degrees of freedom, based on the superexchange processes: namely, for a given magnetic order, the occupied orbitals tend to arrange in a way to further minimize the exchange energy. Then, if two magnetic sites are connected by the spatial inversion, the antiferro orbital order should lead to the ferromagnetic coupling break the inversion symmetry. This constitutes the basic idea of our work, which provides a pathway for designing ferromagnetic ferroelectrics: the rare but fundamentally and practically important multiferroic materials. After illustrating the basic idea on toy-model examples, we propose that such behavior can be indeed realized in the van der Waals ferromagnet VI3, employing for this analysis the realistic model derived from first-principles calculations for magnetic 3d bands. We argue that the intra-atomic interactions responsible for Hund's second rule, acting against the crystal field, tend to restore the orbital degeneracy of the ionic d2 state in VI3 and, thus, provide a necessary flexibility for activating the Kugel-Khomskii mechanism of the orbital ordering. In the honeycomb lattice, this orbital ordering breaks the inversion symmetry, stabilizing the ferromagnetic-ferroelectric ground state. The symmetry breaking leads to the canting of magnetization, which can be further controlled by the magnetic field, producing a huge change of electric polarization. Published by the American Physical Society 2024

Singlet Oxygen Photocatalytic Generation by Silanized TiO2 Nanoparticles

AbstractA commercial TiO2 sample, used as received or hydrothermally treated to increase surface hydroxylation, has been functionalized by surface modification with hexadecyltrimethoxysilane. The anchoring of the silane has been characterized by means of FTIR and solid‐state NMR spectroscopies, and the grafting density was determined by thermogravimetric and N2 physisorption analyses. The silane moieties induce a partial decrease of the shielding of the valence electrons of the Ti ions at the surface, and a local modification of their crystal field, as demonstrated by XPS and UV/Vis spectroscopy, respectively. The changes in coordination and the produced oxygen vacancies result in the formation of Ti3+ defects localized in the sub‐surface region, as revealed by EPR spectroscopy. These paramagnetic centers are stabilized in the silanized samples, as the electron transfer to O2 is efficiently inhibited even under UV irradiation. However, the amount of Ti3+ centers appears to be correlated with the singlet oxygen (1O2) formation rate. Accordingly, epoxidation of limonene under UV light, chosen as a model photocatalytic reaction triggered by 1O2, occurred with higher selectivity when TiO2 was silanized and upon simultaneous NIR irradiation. These evidences suggest that in the silanized sample 1O2 may be generated through Förster‐type energy transfer from excited sub‐surface Ti3+ centers.

Low-Energy Magnetic States of Tb Adatom on Graphene.

Electronic structure and magnetic interactions of a Tb adatom on graphene are investigated from first principles using combination of density functional theory and multiconfigurational quantum chemistry techniques including spin-orbit coupling. We determine that the six-fold symmetry hollow site is the preferred adsorption site and we investigate electronic spectrum for different adatom oxidation states including Tb3+, Tb2+,Tb1+, and Tb0. For all charge states, the Tb 4f8configuration is retained with other adatom valence electrons being distributed over 5dxy, 5dx2+y2, and 6s/5d0single-electron orbitals. We find strong intra-site adatom exchange coupling that ensures that the 5d6sspins are parallel to the 4fspin. For Tb3+, the energy levels can be described by theJ= 6 multiplet split by the graphene crystal field. For other oxidation states, the interaction of 4felectrons with spin and orbital degrees of freedom of 6s5delectrons in the presence of spin-orbit coupling results in the low-energy spectrum composed closely lying effective multiplets that are split by the graphene crystal field. Stable magnetic moment is predicted for Tb3+and Tb2+adatoms due to uniaxial magnetic anisotropy and effective anisotropy barrier around 440 cm-1controlled by the temperature assisted quantum tunneling of magnetization through the third excited doublet. On the other hand, in-plane magnetic anisotropy is found for Tb1+and Tb0adatoms. Our results indicate that the occupation of the 6s5dorbitals can dramatically affect the magnetic anisotropy and magnetic moment stability of rare earth adatoms.

Halogen Impact on the Supramolecular Organization of Chiral Phthalimide Emitters Displaying Room Temperature Phosphorescence.

Room temperature phosphorescence from organic materials has attracted an increasing attention in the recent years due to their potential application in various advancing technologies, notably in bioimaging and displays. In this context, heavy atoms such as halogen ones revealed useful tools to enhance the spin-orbit coupling (SOC) of molecular organic phosphors. However, the effect of halogen at the supramolecular level remains less understood, especially in the field of molecular crystals where additional factors can impact the phosphorescence emission. Here, we investigate external effect of halogens on the phosphorescence of chiral phthalimides molecular crystals. The results show that changing the nature of the halogen atom onto the phthalimide core leads to an evolution of the photophysical properties of the materials which does not necessarily follow the classical trend imposed by the expected internal heavy atom effect. Beyond this aspect, we showed that the halogen atom has a profound impact on the packing between the chromophores at the supramolecular level which is of paramount importance towards the optical properties (PLQY and lifetimes) of the different phosphors examined.

The Effect of Hydrostatic Pressure on Structure, Crystal‐Field Strength, and Emission Properties of Neat and Ni2+‐Activated KMgF3

AbstractTo understand excellent emission and sensitivity for hydrostatic pressure luminescent ions host material, the first principles calculations carried out within density functional theory (DFT) framework are performed to clarify the electronic structure of neat and doped with Ni2+ ions KMgF3 single crystals. The results of band structure calculations show that F2p states are the principal contributors to the KMgF3 valence band, mainly in its upper and central parts, while in the energy band gap of the KMgF3:Ni2+ phosphor, new electronic states associated with the Ni2+ 3d‐orbitals are formed. Furthermore, the zero phonon line (ZPL) spin‐forbidden transition emission energies, (3A2⇄1E) ZPL, (3A2⇄3T2) ZPL, strength of the octahedral crystal field, 10Dq (3A2→3T2)ZPL, are calculated for the KMgF3:Ni2+ phosphor. Any changes of the Em(3A2⇄1E)ZPL transition energy of the KMgF3:Ni2+ phosphor with pressure increasing from 0 to 20 GPa are not detected, while the crystal‐field strength increases linearly with increasing pressure. Present results bring a foresight tool for predicting physicochemical properties of undoped and doped wide‐gap fluorides; KMgF3:Ni2+, without any toxic/harmful or expensive rare‐earth can be effectively used as an optical manometer in 0–20 GPa, which covers the almost whole pressure range available at present in Diamond anvil cell experiments.

Emerging Anomalous Hall Effect in (111)‐Oriented Artificial Iridate Honeycomb Lattices

AbstractComplex Ir oxides provide a promising platform for investigating cooperativity and competition between Mott physics and spin‐orbit coupling in condensed matter physics. These materials have the potential to reveal intriguing phases, such as topological phases and the Kitaev spin liquid state, an exactly solvable S = 1/2 spin model describing Ir4+ ions on a 2D honeycomb lattice. However, experimental confirmation of such phases in iridate films has been hindered by the difficulty of preparing (111)‐oriented samples of the perovskite phase. Herein, by constructing SrIrO3/SrTiO3 superlattices on SrTiO3 (111) substrates, (111)‐oriented perovskite phase SrIrO3 films are achieved with the unprecedentedly large thickness of 6 unit cells. Electrical and magnetic characterizations reveal that these films exhibit anomalous Hall insulating behavior, significant charge fluctuations, and long‐range antiferromagnetic order. Moreover, these properties can be tuned by varying the thickness of the SrIrO₃ layer. These emergent phenomena underscore the complex interactions among spin‐orbit coupling, electron–electron correlations, and crystal structure in (111)‐oriented artificial iridate honeycomb lattices. Further, spin fractionalization and topological quantum spin liquid may be achieved by tuning the spin–orbit coupling and crystal field intensity from interface engineering.

High‐Pressure Modulation of NIR Luminescence in Cr3+‐Doped Cs2AgInCl6 Double Perovskites: The Role of Ultrafast Energy Transfer

AbstractLead‐free halide double perovskites (DPs) have attracted much attention due to their potential applications in the field of near‐infrared (NIR) optoelectronics. However, regulating the NIR luminescence properties of such materials remains a challenge. In this study, it is found that Cs2AgInCl6:Cr3+ DPs exhibit a broad NIR emission under ultraviolet excitation, with a peak value of 980 nm. In the range of 9 GPa, the emission intensity increases significantly. As the pressure increases, a transition from broadband NIR emission to narrow‐line emission occurs, accompanied by a decrease in the full width at half maximum (FWHM) from 260 to 40 nm, attributed to increased local structural asymmetry and enhanced crystal field around Cr3+ ions. Ultrafast transient absorption experiments reveal the changes of energy transfer under pressure and the emergence of new radiation channels, demonstrating the dynamic evolution of emission characteristics under high pressure. The research results deepen the understanding of high pressure luminescence in Cr3⁺‐doped halide DPs, providing strategies for designing highly efficient NIR emitters in metal halide DPs.

Evolution of critical current density in CaKFe4As4 with La-doping

Abstract Single crystals of (Ca1-xLax)KFe4As4 (0 ≤ x ≤0.16) have been grown by using the self-flux method, and the evolution of physical properties including the critical current density (Jc) with La-doping has been investigated. Tc decreases monotonically with increasing x, while Jc at the same temperature and magnetic field increases initially and reach its maximum at x = 0.082. The increase in Jc is more obvious at low temperatures and high fields. At T = 5 K and H = 40 kOe, Jc reaches 0.34 MA/cm2, which is ~4 times larger than that for pure crystals. It is also found that anomalous temperature dependence of Jc in CaKFe4As4 is wiped away as the La content is increased. However, Jc shows non-monotonic field dependence (peak effect) at high fields in crystals with large x. In addition, we found that despite weak anisotropy of Hc2, there is extremely large anisotropy of Jc up to ~15, which is most likely caused by novel planar defects in the crystal, similar to CaKFe4As4. Jc characteristics in (Ca1-xLax)KFe4As4 with disorder outside FeAs planes is compared with that in CaK(Fe1-xCox)4As4 with disorder within FeAs planes.

Crystal growth, structure and crystal field splitting and fitting of Yb:GdScO3

Crystal growth, structure and crystal field splitting and fitting of Yb:GdScO3

Crystal field and microscopic spin Hamiltonians approach for V3+ ions in α-ZnAl2S4 single crystals

Crystal field and microscopic spin Hamiltonians approach for V3+ ions in α-ZnAl2S4 single crystals

Manipulating photoluminescence properties of neodymium-doped fluorites through local structure regulation

The long upper level lifetimes are highly advantageous for laser diode pumped high energy laser systems. However, lifetimes of the rare earth neodymium ions, one of the most important active ions in 1 μm, are generally around 300 μs or less, which limits the development of laser diode pumped high energy lasers on neodymium gains. In this work, Nd:CaF2 and Nd:SrF2 were chosen and introduced with appropriate codopants to adjust the local structures, thereby modulating the coupling of active ions with the crystal fields. The transition rate was slowed down and a long lifetime nearly 600 μs was obtained. Neodymium-doped fluorites with broad emission bandwidths and high thermal conductivities are potential laser gains for generation of high repetition rate high power lasers.

Dynamic magnetic properties of core-shell nanoparticles in an oscillating magnetic field

The Monte Carlo simulation is employed to study the dynamic magnetic properties of Ising core-shell nanoparticles in an oscillating magnetic field. The effects of time-dependent magnetic field, exchange coupling, crystal field, concentration, and temperature on the instantaneous magnetization of the system are presented. In addition, the dynamic hysteresis behaviors are investigated. According to the results, the system displays relaxation behavior at low temperatures, which is affected by various parameters, and reduced significantly with temperature increasing.

Optimization of batch cooling crystallization systems considering crystal growth, nucleation and dissolution. Part I: Simulation

Optimal control of batch crystallization systems is still a focus and hot topic in the field of industrial crystallization, which seriously affects the consistency of batch product quality. In this paper, a new method with a new objective function and improved optimization algorithm was proposed for optimization of crystal size distribution (CSD) in case of fine crystals occurrence. The new objective function was developed with better margin metric and weighting technique to minimize fine crystal mass, meanwhile, a newly constructed sinusoidal weight function was introduced to improve the particle swarm optimization (PSO) algorithm. A precise control of CSD with suppressed numerical discrepancy caused by fine crystals removal was developed by combining seed recipe and temperature-swing. In addition, the effects of temperature curve segments on CSD during process optimization were systematically investigated to achieve optimal results. Two typical batch cooling crystallization systems were used to verify the effectiveness of the proposed method in controlling product CSD while minimizing fine crystal mass. Results demonstrated that the desired product CSD can be achieved with minor errors while the fine crystals could be shrunk to be negligible, i.e., the fine crystal mass and number can be reduced by over 90%. This work has an important guiding significance for the removal of fine crystals in industrial crystallization processes, especially when only operational optimization rather than equipment updating is considered.

Unusual Manganese Luminescence Channels in Low Doped MgAl2O4

We have done the rigorous characterizing of the Mn-doped MgAl2O4 (Mn = 0, 0.02, 0.04, 0.1 wt.%) single crystals and the comprehensive analysis of their optical properties theoretically supported by the Modified Crystal Field Theory to obtain the most complete picture, to date, of the diverse absorption and luminescence behavior of MgAl2O4:Mn spinels. The study has focused on exploring the formation and coexistence of multivalent states of Mn and identifying the conditions needed for activating luminescence channels. We found that Mn takes the charge state +2, +3 and +4 and occupies both tetrahedral and octahedral positions. Six luminescence channels related to Mn ions were identified for the first time. Two channels activate the luminescence at 520 nm: intra-atomic transitions in tetrahedral coordinated Mn2+ ions under VIS-excitation and a recombination Mn3+→Mn2+→Mn3+ process involving Mn3+ in tetrahedral positions under X-ray. Mn3+ in tetrahedrons and octahedrons activate luminescence channels at 733 and 926 nm under UV-C and VIS excitations. Two distinct processes involving Mn3+ and Mn4+ ions in octahedrons result in the luminescence at 651 nm. The first is an intra-atomic transition in the Mn4+ occurring under VIS-excitation. The second appears through a charge transfer Mn3+→Mn4+→Mn3+ in the octahedrons under UV-B-excitation.

Structural comparison of [Ce(OtBu)Cl(THF)5](BPh4) to smaller rare earth analogues.

The introduction of the cerium(III) analogue (1-Ce, Ln = Ce) of (tert-butoxido)chloridopentakis(tetrahydrofuran)lanthanide(III) tetraphenylborate tetrahydrofuran disolvate, [Ln{OC(CH3)3}Cl(C4H8O)5][B(C6H5)4]·2C4H8O or [Ln(OtBu)Cl(THF)5](BPh4)·2THF (1-Ln) has been achieved with a structural comparison between the existing solid-state structures of other rare earth analogues and the title compound at 100 and 180 K. The cation in 1-Ce is targeted as the cerium(III) ion possesses the criteria to exhibit slow magnetic relaxation in axial point-charge crystal fields, akin to the dysprosium(III) ion in 1-Dy. AC magnetic susceptibility experiments reveal no such behaviour for 1-Ce, putting the viability of cerium-based SMMs into question.

Effects of Lu-doping on the magnetic behaviour and ordering of (Ho[formula omitted]Lu[formula omitted])2Fe2Si2C

We have investigated the effects of Lu substitution on the magnetic behaviour and ordering of (Ho1−xLux)2Fe2Si2C (x = 0.32 and 0.46) by high-resolution neutron powder diffraction, magnetisation and specific heat over the temperature range 2 to 300 K. Our study has establised that the antiferromagnetic (AFM) state is weakened upon Lu substitution and that the Néel temperature TN shifts towards lower temperature with increasing Lu content. The replacement of the magnetic Ho3+ ion by the non-magnetic Lu3+ ion of smaller atomic radius, leads to a modification of the crystal field levels, as indicated by the specific heat measurements. Neutron diffraction data analysis reveals that the magnetic structure of undoped Ho2Fe2Si2C, which exhibits a commensurate, antiferromagnetic ordering of the Ho sublattice along the b-axis with a propagation vector k = [0, 0, 12], is maintained in the Lu-doped (Ho1−xLux)2Fe2Si2C compounds.

Study of Structural, Magnetic, and Thermoelectric Properties of Rare Earth-based CdCe2X4 (X = S, Se, Te) Spinels for Spintronic and Energy Harvesting Applications

Controlling the spin degree of freedom in electronics paves the way for novel approaches to employ, relocate, and store data at accelerated rates. In this regard, an in-depth examination of the structural, electronic, magnetic, and transport behaviour of CdCe2X4 (X = S, Se, Te) is undertaken. It is observed that ferromagnetic states exhibit higher energy release compared to antiferromagnetic states. Room temperature ferromagnetism is characterized by the Tc and spin-polarized density of states. The underlying mechanism in ferromagnetic behaviour is elicited in terms of crystal field energy, double exchange mechanism, exchange energies, and constants. The magnetic moment shift from Ce to other NM sites (Cd, X) is identified as a mechanism sustaining ferromagnetic character through electron exchange, thereby preventing clustering. Furthermore, the temperature-dependent thermoelectric properties are investigated, encompassing electrical and thermal conductivity, Seebeck coefficient, and power factor, recommending exploration of spinels as potential candidates for sustainable energy devices.

Influence of crystal field inhomogeneities on the spontaneous and laser tuning emission of a Nd3+-doped Li2O-BaO-Al2O3-P2O5 glass

The present work focuses on the spectroscopic and laser properties of Nd3+ in a Li2O-BaO-Al2O3-P2O5 aluminophosphate glass which includes Judd-Ofelt calculation, stimulated emission cross-section of the 4F3/2→4I11/2 emission laser transition, lifetime of the 4F3/2 state and quantum efficiency. Site-selective laser spectroscopy and stimulated emission under selective excitation has been performed to determine the crystal field inhomogeneity at the Nd3+ site together with its broadening effect on the spontaneous and laser emissions. However, a much narrowed laser emission has been achieved by using an intracavity tilting plate which acts as a tunable optical filter to adjust the laser wavelength.