Crystal Field Research Articles

Quantum Effects at a Spin-Flop Transition in the Antiferromagnetic Topological Insulator MnBi2Te4

It is shown that the experimentally detected features in the low-temperature behavior of the magnetization in an external magnetic field perpendicular to the layers of manganese ions of the topological antiferromagnet MnBi2Te4 are due to quantum effects induced by the off-diagonal nature of the trigonal component of the crystal field. In this case, the anomalous increase in the magnetization of the material before the spin-flop transition, as well as after it in the phase of “collapsed” sublattices, is explained by the suppression of contributions from quantum effects. The comparison of the results of the theoretical analysis with experimental data has made it possible to refine the parameters of the effective spin model of MnBi2Te4 and to establish the important role of the noted trigonal component.

Magnetic and thermodynamic properties of mixed spin-3/2 and spin-3 Ising ferrimagnets on a 2D triangular lattice: Monte Carlo study

Monte Carlo methods in the presence and absence of an external magnetic field were used to investigate the thermodynamic and magnetic properties of the mixed spins (3/2, 3) Ising ferrimagnets in a 2D triangular lattice consisting of sublattices A, B, and C. Two types of mixing were considered: S→=(SA,SB,SC)=(3/2,3,3) (Model I) and S→=(SA,SB,SC)=(3/2,3,3/2) (Model II). In contrast to bipartite lattices, the antiferromagnetic coupling between spin-3/2 and spin-3 in the triangular lattice leads to geometric frustrations that impact magnetic properties. Determination of ground state phase diagrams, combined with verification of whether or not there was magnetic hysteresis when crossing each transition line, revealed first- and second-order phase transition lines, as well as multicritical points. The temperature investigation in the absence of an external magnetic field revealed rich magnetic properties such as 1st- and 2nd-order phase transitions, N- and L-type compensation points, tricritical points, and critical endpoints, as well as M-, P-, Q-, R- and S-type total magnetization behaviours. The effect of the crystal field on finite-temperature phase diagrams and compensation behaviour was also carried out. As a result, the critical temperatures of Model II become constant when the crystal field is sufficiently strong. In contrast, several critical values of the crystal field for which the critical temperature is zero were identified for Model I. In the presence of a magnetic field, hysteresis behaviour and associated magnetic properties such as coercivity and magnetic remanence were investigated. The effect of crystal field and temperature was explored. Hysteresis of one to four loops were found at low temperatures when the crystal field varied. Finally, as the temperature rises, the hysteresis loops' area decreases to zero at high temperatures.

Thermal expansion, quasi-metamagnetism and spin-reorientation in Fe7Se8 substituted with chromium

The properties of the selenide compound Fe7Se8 with a layered crystal structure of the NiAs type are strongly influenced by substitutions and the distribution of vacancies. The Cr-substituted compound Fe6.5Cr0.5Se8 was obtained in single-crystalline form and studied by x-ray diffraction, energy-dispersive x-ray spectroscopy, thermal expansion and magnetization measurements. It was observed that the partial replacement of iron with chromium led to a twofold decrease in spontaneous volume magnetostriction due to changes in competing magnetoelastic contributions to thermal expansion along and perpendicular to the c axis of the crystal. The replacement of iron with chromium slightly decreases the Néel temperature (from 440 to 435 K) and significantly enhances the critical temperature of spin reorientation transition Tsr (from 115 to 160 K), apparently due to a change in the crystal electric field. Below 160 K, the Fe6.5Cr0.5Se crystal is found to exhibit metamagnetic-like behavior of the magnetization when the magnetic field is applied along the c axis. A jump-like change of the magnetization at a critical field up to ∼10 kOe is attributed to the presence of pinning centers of domain walls presumably due the ordering of chromium atoms substituting iron in cationic layers.

A novel Cr3+-doped MgGaBO4 phosphor: Expanding the horizons in plant illumination

Near-infrared (NIR) spectroscopy technology has demonstrated unparalleled efficacy in night vision, iris authentication, plant illumination, food inspection, and various other domains. Nonetheless, the quest for novel phosphors exhibiting superior luminescent efficiency and enduring thermal stability persists as a formidable obstacle. In this study, phosphors MgGaBO4: Cr3+, Al3+ were successfully synthesized by high-temperature solid-phase reaction method. Firstly, starting from MgGaBO4: Cr3+, the crystal field environment around Cr3+ ions was modulated by replacing Ga3+ with Al3+ to further improve the luminescence performance of the phosphor. The emission peak wavelength of MgGaBO4: 0.0075Cr3+,0.005Al3+ phosphor was about 756 nm, the full width at half-maximum (FWHM) was about 174 nm under the blue light excitation of 440 nm. In addition, the luminescence intensity at 100 °C remains at 77.41 % of that at room temperature. Finally, the excellent moisture resistance of this phosphor was demonstrated. This work not only introduces a novel broadband NIR phosphor with promising application potential in advanced agricultural lighting but also proposes a synergistic enhancement strategy for effectively improving the performance of broadband NIR phosphor-converted LED light sources.

Study of Curie Temperature, Ferromagnetism, and thermoelectric properties BaCo2Z4 (Z = S, Se, Te) for spintronic and energy applications

Spintronics is an emerging technology that harnesses electron spin to speed up data transfer, manipulation, and storage. In our thorough examination of BaCo2Z4 (Z = S, Se, Te), we have probed into electronic behaviour, Curie temperature, ferromagnetism, and thermoelectric behaviour. Our optimization analysis underscored the superior energy release of ferromagnetic states over antiferromagnetic counterparts, a finding substantiated by formation energy. We calculated spin polarization and Curie temperature using the Heisenberg model, confirming that ferromagnetism is prevalent at temperatures higher than room temperature. Including hybridization, crystal field energy, states density, band structures, exchange constants and energies, and the double exchange model, the investigation examined the complex nature of ferromagnetism. Importantly, shifting the magnetic moment away from Co and toward Ba and S/Se/Te sites revealed that electron spin, not clustering of Co’s magnetic ions, is the fundamental property of ferromagnetism. We also investigated thermoelectric metrics such as the Seebeck coefficient, conductivities, and power factor for spin (↑) and spin (↓), which helped us to understand the complex role of electron spin at high temperatures and their role in energy harvesting applications.

Elucidating correlation-driven insulating state in an effective spin—½ antiferromagnet NdVO4

TheJeff= ½ state: a result of interplay of strong electronic correlations (U) with spin-orbit coupling (SOC) and crystal field splitting, offers a platform in the research of quantum materials. In this context, 4frare-earth based materials offer a fertile playground. Here, strong experimental and theoretical evidences for aJeff= ½ state is established in a three-dimensional spin system NdVO4. Magnetic measurements show the signatures of a SOC drivenJeff= ½ state along with the presence of antiferromagnetic (AFM) interaction between Nd3+moments, whereas, heat capacity reveals the presence of an AFM ordering around 0.8 K, within this state. An entropy of Rln2 (equivalent toJ= ½) is released around 4 K which implies the presence ofJeff= ½ state at low temperatures. Total energy calculations within the density functional theory (DFT) framework reflect the central role of SOC in driving the Nd3+ions to host such a state with AFM correlations between them, which is in agreement with experimental results. Further, DFT + SOC calculations with and without the inclusion ofU, points that electron-electron correlations give rise to the insulating state making NdVO4a potential candidate forU-driven correlated Mott insulator.

Synthesis, Structure and Luminescence Properties of Mn-doped MgAl2O4 Red-Emitting Phosphors with Varying Sintering Temperature.

Oxide matrix red-emitting phosphors are deemed as excellent color converters for white light emitting diodes (WLEDs) and laser diodes (LDs). Manganese-doped MgAl2O4 powder was synthesized by a solid-state reaction method at different sintering temperatures. Microstructure shows that grain size is mainly in the range of 0.2-5μm, and grain agglomeration occurs with increased sintering temperature. XPS analysis indicates that the doped Mn ion exhibits a valence state of + 4 within the MgAl2O4 matrix. The diffraction peak of the phosphors is shifted by the sintering temperature, which affects lattice constant. Upon excitation by 300nm ultraviolet light, the samples emit asymmetric broadband red light within the range of 620-720nm, attributed to Mn4+ ion's transition from 2Eg to 4A2g states. With the increasing temperature, the main emission peak shifts from 677nm to 650nm, ascribed to the change in energy level (2Eg) resulting from the reduction of Al2O3 phase. Crystal field theory confirmed that Mn4+ ions are within a strong crystal field environment created by MgAl2O4 matrix. By affecting particle size and crystallinity, the sintering temperature influences the fluorescence lifetime of the Mn4+ ion. Notably, these red-emitting phosphors exhibits remarkable thermal stability as their emission intensity remains approximately at 58% of initial intensity even at elevated temperature (435K). Consequently, Mn4+: MgAl2O4 red-emitting phosphors with high thermal stability render them promising candidates for WLED applications.

Exploring magnetic behavior and compensation temperatures in spherical and hemispherical lattices: A Monte Carlo study

The magnetic behavior of spherical and hemispherical lattices under various conditions was insufficiently understood prior to this study, particularly about the influence of ferrimagnetic coupling parameters, external magnetic and crystal fields on the compensation and blocking temperatures. In current study Monte Carlo simulations provide new insight into how these factors affect the magnetic properties of spherical and hemispherical lattices, revealing critical size-related effects and dependencies. This study offers key insights into the magnetic properties of spherical and hemispherical lattices, critical for advancing magnetic materials in future nanotechnologies. The ferrimagnetic system studied, hold promise for applications such as MRI, cancer therapy, and next-generation memories, enhancing nanoscale sensor precision and driving progress in data storage and device miniaturization.

Monte Carlo simulations unveil magnetic differences in honeycomb, kagome, and triangular nanolattices

Monte Carlo simulations reveal distinct magnetic behaviors in honeycomb, kagome, and triangular nanolattices, crucial for magnetic material development. The honeycomb nanolattice shows the earliest magnetization decline, followed by the kagome and triangular nanolattices, due to differences in atomic arrangement and geometry. Increasing the linear coupling interaction (J), biquadratic coupling interaction (K), and external magnetic field raises the blocking temperature, while a higher crystal field (∣D∣) lowers it. These findings are pivotal for optimizing magnetic stability and behavior in applications like magnetic storage, sensors, and nanotechnologies.

A strategy for preparing broadband polymer based optical waveguide amplifiers: doping low crystal field symmetric nanocrystals in polymer matrix as gain media

A strategy for preparing broadband polymer based optical waveguide amplifiers: doping low crystal field symmetric nanocrystals in polymer matrix as gain media

Emergent Berezinskii-Kosterlitz-Thouless and Kugel-Khomskii Physics in the Triangular Lattice Bilayer Colbaltate.

Motivated by the experiments on the triangular lattice bilayer colbaltate K_{2}Co_{2}(SeO_{3})_{3}, we consider an extended XXZ model to explore the underlying physics. The model is composed of interacting Co^{2+} dimers on the triangular lattice, where the Co^{2+} ion provides an effective spin-1/2 local moment via the spin-orbit coupling and the crystal field effect. The intradimer interaction is dominant and would simply favor the local spin singlet, and the interdimer interactions compete with the interdimer interaction, leading to rich behaviors. With the easy-axis anisotropy, it is shown that, in the ground state manifold of the intradimer Ising interaction, the system realizes an effective quantum Ising model, where the ground state is either a three-sublattice order with a mixture of antiferromagnetic Ising order and a valence-bond spin singlet or Ising disordered. The finite temperature regime realizes the Berezinskii-Kosterlitz-Thouless physics. To explore the full excitations, we incorporate the excited state manifold of the intradimer Ising interaction and establish the emergent Kugel-Khomskii physics. Thus, the triangular lattice bilayer colbaltate is an excellent platform to explore the interplay between geometrical frustration and anisotropic interactions as well as the emergent effective models and the resulting physics.

New phase-construction phosphors ceramics for warm-white solid-state lighting: Orange-yellow-emitting (Lu,Gd)3(Sc,Al)5O12: Ce3+

Phosphor-converted white light-emitting diodes (pc-wLEDs) and phosphor-converted white laser diodes (pc-wLDs) are extensively utilized in lighting applications, with their quality and reliability heavily dependent on color converters. Recent progress in all-inorganic color converters, particularly phosphor ceramics (PCs), have markedly enhanced performance. However, due to the difficulty of managing emissive components, achieving warm white light with substantial red emission still remains challenging. In this work, a series of pure-phase Ce3+-activated garnet-structured solid-solution PCs, (Lu,Gd)3(Sc,Al)5O12: Ce (LGSAG: Ce), were fabricated using vacuum sintering technology for the first time. Through systematic analysis, it can be revealed that the addition of Gd3+ leads to a red shift in emission through changes in crystal-field splitting, whereas the inclusion of Sc3+ primarily focuses on improving the microstructure of the developed PCs. Particularly, the pc-wLED constructed from typical LGSAG: Ce PC generated significantly warmer light with a correlated color temperature (CCT) of 2916 K, compared to the 9557 K from LuAG: Ce PC. Meanwhile, under 40.0 W (15.7 W/mm2) blue laser excitation, the maximum luminous flux and luminescence efficiency of one typical LGSAG: Ce PG are 1172 lm and 29.3 lm/W, respectively. Therefore, the developed LGSAG: Ce PCs can show notable potential as color converters for future high-power warm-white solid-state lighting applications.

Pseudosymmetry in Tetragonal Perovskite SrIrO3 Synthesized under High Pressure.

In this study, we report a tetragonal perovskite structure of SrIrO3 (P4/mmm, a = 3.9362(9) Å, c = 7.880(3) Å) synthesized at 6 GPa and 1400 °C, employing the ambient pressure monoclinic SrIrO3 with distorted 6H structure as a precursor. The crystal structure of tetragonal SrIrO3 was evaluated on the basis of single-crystal and powder X-ray diffraction. A cubic indexing was observed, which was attributed to overlooked superlattice reflections. Weak fractional peaks in the H and K dimensions suggest possible structure modulation by oxygen defects. Magnetization study reveals weak paramagnetic behavior down to 2 K, indicative of the interplay between spin-orbit coupling, electron correlations, and the crystal electric field. Additionally, measurements of electrical resistivity display metallic behavior with an upturn at about 54 K, which is ascribed to weak electron localization and possible structural defects.

Unveiling the tunable electronic, optoelectronic, and strain-sensitive gas sensing properties of Janus ZrBrCl: Insights from DFT study

Janus monolayers materials exhibit extraordinary physical and chemical properties because of crystal field changes caused by their asymmetric structures. Here, we investigate the electronic, switching, optoelectronic, and strain-sensitive gas-sensing properties of a Janus ZrBrCl monolayer via density-functional theory and the nonequilibrium Green’s function methods. The electronic properties can be tuned by applying biaxial strain and electric fields. In particular, the band gap can be changed from indirect to direct via applied a + 8 % biaxial tensile strain. It can be transformed from a semiconductor to a metal with an applied −14 % biaxial compressive strain, with a 1011 switching ratio. In addition, a biaxial strain and an electric field can modulate visible light absorption and photocurrents. Under a −12 % biaxial compressive strain, the absorption coefficient increased to 5.26 × 105 cm−1 from the intrinsic 4.08 × 105 cm−1. Across biaxial strains from −4% to + 4 %, the photocurrent density peak displayed red and blue shifts, respectively, with increasing tensile and compressive strains. NH3, NO2, and NO physically adsorb on the Br side of the ZrBrCl monolayer. Maximum gas sensitivity values of a ZrBrCl sensor are 52 % in the zigzag direction and 21 % in the armchair direction. After application of the −14 % biaxial strain, the maximum values respectively increased to 76 % and 179 %. The biaxial compressive strain-sensitive gas-sensing properties are driven by changes in the electrostatic potential difference between the Br and Cl surfaces. These findings indicate promising candidate materials for high-performance switching and optoelectronic devices, and also provide a strategy for improved gas sensors.

Degree of Crystal Structure Distortion-Induced Tunable LiGaO2 Long Persistent Luminescence for Optical Information Encryption.

Tunable long persistent luminescence (LPL) phosphor materials have great potential for optoelectronic cryptographic applications. However, the mainstream techniques of modulating LPL generally have the characteristics of complex preparation processes, demanding crystal field environments, or expensive dopant ions, which restrict large-scale commercial application. Herein, we develop a simple, high-efficiency, and low-cost strategy to optimize the LPL of LiGaO2(LGO):Cu2+ by changing the sintering time to regulate the degree of crystal structure distortion. The Cu2+ as charge compensation will substantially enhance the emission intensity of LGO by a factor of 11.02 originating from the appropriate ionic size and coordination mode. Besides, the LPL time of LGO:Cu2+ can be extended effectively to 2 h by adjusting the sintering temperature and time (900 °C@24 h). The extension mechanism is that Li and Ga can be substituted for each other more easily and induce crystal structure distortion due to the special crystal structure of LGO, resulting in an optimal trap concentration in LGO:Cu2+. Thus, our findings provide a simple way to modulate long persistent luminescence and further consider their potential impact on optical information encryption.

Full-shell d-orbitals of interstitial Ni and anomalous electrical transport in Ni-based half-Heusler thermoelectric semiconductors

We systematically investigate the anomalous electronic properties and electrical transport induced by full-shell d10-orbitals of the extra interstitial Nii in Ni-based half-Heusler XNi1+xZ semiconductors. The orbitals from the interstitial Nii have the unique d10 configuration, split into high-energy eg4 orbitals and low-energy t2g6 orbitals under the octahedral crystal field. In XIVNi1+xZIV (XIV=Ti, Zr, Hf; ZIV=Sn, Pb), the localized Nii-eg4 states fall within the intrinsic bandgap leading to a reduced bandgap. In XIIINi1+xZV (XIII=Sc, Y; ZV=Sb, Bi), the Nii-eg4 states overlap with the intrinsic valence bands. Additionally, the interstitial Nii perturb the nearest neighbor X atomic coordination, leading to the splitting of degenerate conduction band minimum, which is stronger in XIIINi1+xZV than XIVNi1+xZIV. Trace amounts of interstitial Nii significantly impact the electrical transport properties. The introduction of the extra interstitial Nii reduces the density of states effective mass, the electron group velocity, and the relaxation time, leading to a decrease of the Seebeck coefficient and electrical conductivity at low and medium temperatures. Nevertheless, the introduction of localized Nii-eg4 states within the bandgap as new valence band maximum attenuate the high-temperature bipolar effect at low carrier concentration intervals, thus maintaining a high thermopower at elevated temperatures. Furthermore, the tuning of the Nii-d10 orbitals by solid solution of both XIVNi1+xZIV and XIIINi1+xZV is expected to further optimize the electrical transport.

A DFT theoretical prediction of new half-metallic ferromagnetism, mechanical stability, optoelectronic and thermoelectric properties of ZnCrO3 perovskites for spintronic applications

ZnCrO3 perovskites structural stability, optoelectronic, mechanical, along with thermoelectric properties, have all been to be described by calculations based on density functional theory (DFT). To determine the exchange correlation potential, the well-known generalized gradient approximation (GGA) and integration of the mBJ potential were used. The perovskite shows that this is in a cubic structure with Fm3m symmetry. Additionally, to check the stability, cohesive energy, structural optimization, and mechanical stability were requirements. This perovskite was found to be brittle, and their mechanical stability enhanced by the elastic constants. The perovskite has a half-metallic character, as evidenced by the spin-polarized electronic band profile, the behavior of the dielectric constant, and absorption coefficient in the spin-up and down channels. In this article, we also looked at magnetism and the origin of the half-metallic gap. The unpaired electrons in the split d-orbitals of the M-sited elements in the crystal field are responsible for the half-metallic as well as magnetic characteristics. Excellent spin polarization at the Fermi level encourages perovskite's possible use in spintronic technologies. Lastly, we computed the thermoelectric parameters within a chemical potential at various temperatures (300 K, 600, 900 K) to explore the potential application in spin electronic devices. Our finding shows that the ZnCrO3 alloy is exceptionally promising for the next generation of spintronics and thermoelectric devices at room and high temperatures.

Structure-optical properties of CdO glass matrix with low levels of cobalt impurities

The impacted role of cadmium oxide (CdO) on the structure-optical properties of current borate glasses with low cobalt oxide impurities was studied. The study was performed on a glass system of the formula [x CdO – (80-x) B2O3 – 0.4 CoO – 19.6 Na2O]; x = 0, 6, 12, 18 and 24 mol%, and produced by melt quenching method. Structure and optical absorption spectroscopy were done to explore the electronic transitions of Co cations inside the glass matrix. The obtained data indicated that structure and optical properties were significantly changed following CdO additives. Bands of optical absorption within both visible and near-infrared ranges showed signatures from Co2+ cations in the tetrahedral and octahedral sites, while Co3+ cations in the octahedral sites within the glass network. Optical absorption also changed significantly for the CdO-doped glasses which exhibited red shifts in visible absorption bands and blue shifts in the NIR absorption bands. The obtained absorption bands in both visible and NIR were employed to obtain the crystal field splitting parameter (Δt) showing augmented records from 3364 cm−1 to 3377 cm−1 with more CdO addition. The Tauc gap energy values were then attained showing decreased values from 3.50 to 3.27 eV when further CdO is added. Further, the impacted role of CdO addition on the nonlinearity optical properties of the present glasses was finally estimated. Potential increases in the linear and nonlinear optical are discussed based on polarizability and basicity increments inside the glass matrix.

Roles of Impurity Levels in 3d Transition Metal-Doped Two-Dimensional Ga2O3.

Doping engineering is crucial for both fundamental science and emerging applications. While transition metal (TM) dopants exhibit considerable advantages in the tuning of magnetism and conductivity in bulk Ga2O3, investigations on TM-doped two-dimensional (2D) Ga2O3 are scarce, both theoretically and experimentally. In this study, the detailed variations in impurity levels within 3d TM-doped 2D Ga2O3 systems have been explored via first-principles calculations using the generalized gradient approximation (GGA) +U method. Our results show that the Co impurity tends to incorporate on the tetrahedral GaII site, while the other dopants favor square pyramidal GaI sites in 2D Ga2O3. Moreover, Sc3+, Ti4+, V4+, Cr3+, Mn3+, Fe3+, Co3+, Ni3+, Cu2+, and Zn2+ are the energetically favorable charge states. Importantly, a transition from n-type to p-type conductivity occurs at the threshold Cu element as determined by the defect formation energies and partial density of states (PDOS), which can be ascribed to the shift from electron doping to hole doping with respect to the increase in the atomic number in the 3d TM group. Moreover, the spin configurations in the presence of the square pyramidal and tetrahedral coordinated crystal field effects are investigated in detail, and a transition from high-spin to low-spin arrangement is observed. As the atomic number of the 3d TM dopant increases, the percentage contribution of O ions to the total magnetic moment significantly increases due to the electronegativity effect. Additionally, the formed 3d bands for most TM dopants are located near the Fermi level, which can be of significant benefit to the transformation of the absorbing region from ultraviolet to visible/infrared light. Our results provide theoretical guidance for designing 2D Ga2O3 towards optoelectronic and spintronic applications.

A Four-Coordinate Pr4+ Imidophosphorane Complex.

The imidophosphorane ligand, [NPtBu3]- (tBu=tert-butyl), enables isolation of a pseudo-tetrahedral, tetravalent praseodymium complex, [Pr4+(NPtBu3)4] (1-Pr), which is characterized by a suite of physical characterization methods including single-crystal X-ray diffraction, electron paramagnetic resonance, and L3-edge X-ray near-edge spectroscopies. Variable-temperature direct-current magnetic susceptibility data, supported by multiconfigurational quantum chemical calculations, demonstrate that the electronic structure diverges from the isoelectronic Ce3+ analogue, driven by increased crystal field. The four-coordinate environment around Pr4+ in 1-Pr, which is unparalleled in reported extended solid systems, provides a unique opportunity to study the interplay between crystal field splitting and spin-orbit coupling in a molecular tetravalent lanthanide within a pseudo-tetrahedral coordination geometry.