Octahedral Crystal Field Research Articles

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.

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.

Ionic substitutions in the Cu3TeO6 structure type and magnetic properties of “medium entropy” Cu3/2Mn1/2Co1/2Fe1/2SbO6

Cubic antiferromagnet Cu3TeO6 demonstrates interesting magnetic properties. Aimed at modification of them, we tried multiple ionic substitutions in its structure. However, single-phase materials could only be prepared with large fraction of the Jahn-Teller ions (Cu2+ and Mn3+), although formally isostructural bixbyites R2O3 (R4O6) with R = Sc, In, Tl, Sm…Lu exist with no Jahn-Teller ions. Moreover, ions having highest octahedral crystal field stabilization energy (Ni2+ and Cr3+) were found least tolerable. This points to Cu3TeO6 as a separate structure type, distinct from classical bixbyites. We report crystal structure, magnetic and thermodynamic properties of a rare single-phase multicomponent preparation, Cu3/2Mn1/2Co1/2Fe1/2SbO6. The dc magnetic studies show that the formation of the ground spin-cluster state at T = 18 K is preceded by a broad anomaly at ∼122 K. Both specific heat and ac susceptibility data rule out the long-range magnetic ordering, in contrast to closely related Cu2MSbO6 (M = Mn or Fe).

LaMg6Ga6S16: a chemical stable divalent lanthanide chalcogenide

Divalent lanthanide inorganic compounds can exhibit unique electronic configurations and physicochemical properties, yet their synthesis remains a great challenge because of the weak chemical stability. To the best of our knowledge, although several lanthanide monoxides epitaxial thin films have been reported, there is no chemically stable crystalline divalent lanthanide chalcogenide synthesized up to now. Herein, by using octahedra coupling tetrahedra single/double chains to construct an octahedral crystal field, we synthesized the stable crystalline La(II)-chalcogenide, LaMg6Ga6S16. The nature of the divalent La2+ cations can be identified by X-ray photoelectron spectroscopy, X-ray absorption near-edge structure and electron paramagnetic resonance, while the stability is confirmed by the differential thermal scanning, in-situ variable-temperature powder X-ray diffraction and a series of solid-state reactions. Owing to the particular electronic characteristics of La2+(5d1), LaMg6Ga6S16 displays an ultrabroad-band green emission at 500 nm, which is the inaugural instance of La(II)-based compounds demonstrating luminescent properties. Furthermore, as LaMg6Ga6S16 crystallizes in the non-centrosymmetric space group, P−6, it is the second-harmonic generation (SHG) active, possessing a comparable SHG response with classical AgGaS2. In consideration of its wider band gap (Eg = 3.0 eV) and higher laser-induced damage threshold (5×AgGaS2), LaMg6Ga6S16 is also a promising nonlinear optical material.

Structure Responsible for the Superconducting State in La3Ni2O7 at High-Pressure and Low-Temperature Conditions.

Very recently, a new superconductor with Tc = 80 K has been reported in nickelate (La3Ni2O7) at around 15-40 GPa conditions (Nature, 621, 493, 2023), which is the second type of unconventional superconductor, besides cuprates, with Tc above liquid nitrogen temperature. However, the phase diagram plotted in this report was mostly based on the transport measurement under low-temperature and high-pressure conditions, and the assumed corresponding X-ray diffraction (XRD) results were carried out at room temperature. This encouraged us to carry out in situ high-pressure and low-temperature synchrotron XRD experiments to determine which phase is responsible for the high Tc state. In addition to the phase transition from the orthorhombic Amam structure to the orthorhombic Fmmm structure, a tetragonal phase with the space group of I4/mmm was discovered when the sample was compressed to around 19 GPa at 40 K where the superconductivity takes place in La3Ni2O7. The calculations based on this tetragonal structure reveal that the electronic states that approached the Fermi energy were mainly dominated by the eg orbitals (3dz2 and 3dx2-y2) of Ni atoms, which are located in the oxygen octahedral crystal field. The correlation between Tc and this structural evolution, especially Ni-O octahedra regularity and the in-plane Ni-O-Ni bonding angles, is analyzed. This work sheds new light to identify what is the most likely phase responsible for superconductivity in double-layered nickelate.

Azomethine Fe3+ coordination compounds containing carbazole units: Synthetic approach, spectral characterization, and magnetic studies

The main aim of our study is to investigate various types of hybrid systems with a paramagnetic iron (III) ion as a core and photoactive derivatives of carbazole at the periphery. We have synthesized a variety of biligand coordination compounds with the composition [FeL2]A (where L is 2‐[2‐[(E)‐[4‐[4‐(3,6‐di‐tert‐butylcarbazol‐9‐yl)benzoyl]oxy‐2‐phenolate]methyleneamino]ethylamino]ethyl 4‐(3,6‐di‐tert‐butylcarbazol‐9‐yl)benzoate, A− is NO3− (I), Cl− (II), PF6− (III)). These systems were studied by superconducting quantum interference device (SQUID) magnetometry and X‐band electron paramagnetic resonance (EPR) spectroscopy. Magnetic measurements revealed mixed spin states (high spin [HS], low spin [LS]) of Fe (III) ions at room temperatures. Estimates of the corresponding spin contributions were made. It was found that all samples exhibit AFM exchange interactions between iron (III) ions. The ground spin state at 2.0 K was established and analyzed. EPR measurements confirmed the HS states of iron (III) and revealed two types of them: with weak distorted and strong low‐symmetry octahedral crystal fields.

Metal exsolution from perovskite-based anodes in solid oxide fuel cells.

Solid oxide fuel cells (SOFCs) are highly efficient and environmentally friendly devices for converting fuel into electrical energy. In this regard, metal nanoparticles (NPs) loaded onto the anode oxide play a crucial role due to their exceptional catalytic activity. NPs synthesized through exsolution exhibit excellent dispersion and stability, garnering significant attention for comprehending the exsolution process mechanism and consequently improving synthesis effectiveness. This review presents recent advancements in the exsolution process, focusing on the influence of oxygen vacancies, A-site defects, lattice strain, and phase transformation on the variation of the octahedral crystal field in perovskites. Moreover, we offer insights into future research directions to further enhance our understanding of the mechanism and achieve significant exsolution of NPs on perovskites.

Luminescence Enhancement of CsMnBr3 Nanocrystals through Heterometallic Doping

The absorption and photoluminescence (PL) of CsMnBr3 with Mn(II) in octahedral crystal fields are extremely weak due to a d-d forbidden transition. Herein, we introduce a facile and general synthetic procedure that can prepare undoped and heterometallic doped CsMnBr3 NCs at room temperature. Importantly, both PL and absorption of CsMnBr3 NCs were significantly improved after doping a small amount of Pb2+ (4.9%). The absolute photoluminescence quantum yield (PL QY) of Pb-doped CsMnBr3 NCs is up to 41.5%, 11-fold higher than undoped CsMnBr3 NCs (3.7%). The PL enhancement is attributed to the synergistic effects between [MnBr6]4- units and [PbBr6]4- units. Furthermore, we verified the similar synergistic effects between [MnBr6]4- units and [SbBr6]4- units in Sb-doped CsMnBr3 NCs. Our results highlight the potential of tailoring luminescence properties of manganese halides through heterometallic doping.

Spectrally tunable near-infrared photoluminescence in MP3O9:Cr3+ (M = Al, Ga, In) phosphate phosphors.

Modulation of the octahedral crystal field environment of Cr3+ is an effective approach to achieve tunable emission. Here, we prepared a series of broadband MP3O9:Cr3+ (M = Al, Ga, In) near-infrared (NIR) phosphors, and cubic AlP3O9:Cr3+ (APO-c:Cr3+) and monoclinic AlP3O9:Cr3+ (APO-m:Cr3+) phosphors were prepared by controlling the synthesis temperature. The emission wavelength was tuned from 787 nm for APO-c:Cr3+ to 894 nm for monoclinic InP3O9:Cr3+ (IPO:Cr3+) by regulating the M ion and reducing the crystal field intensity. Excitingly, the MP3O9:Cr3+ (M = Al, Ga, In) family shows excellent thermal stability; the emission intensity of APO-c:Cr3+, APO-m:Cr3+ and monoclinic GaP3O9:Cr3+ (GPO:Cr3+) can still maintain 95.6%, 86% and 86% of that at room temperature when heating to 423 K, respectively. An NIR LED device was prepared by incorporating GPO:Cr3+ and a blue light LED, demonstrating the potential application in night vision and non-destructive testing.

Resolution of zigzag magnetic correlations in Na-deficient NaxIrO3 without long-range ordering

The materials search for Kitaev quantum spin liquids led to the discovery of many honeycomb lattice materials. Much attention has been paid to materials without magnetic order down to the lowest temperatures. The newly synthesized Na-deficient ${\mathrm{Na}}_{x}{\mathrm{IrO}}_{3}$ has been found to bear no sign of long-range magnetic order above 1 K from physical property measurements. In this paper, we report momentum- and energy-resolved excitation spectra in Na-deficient ${\mathrm{Na}}_{x}{\mathrm{IrO}}_{3}$ measured using a resonant inelastic x-ray scattering spectrometer. Orbital excitation spectra show that the octahedral and trigonal crystal field splittings are larger in ${\mathrm{Na}}_{x}{\mathrm{IrO}}_{3}$ than in ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$. On the other hand, the low-energy spectrum at low temperature shows a wave-vector dispersion and a spectral weight distribution that are similar to those of ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$, revealing that the two-dimensional zigzag magnetic correlations in ${\mathrm{Na}}_{x}{\mathrm{IrO}}_{3}$ are similar to those in ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ in terms of the ordered magnetic moment direction and three dynamically fluctuating zigzag orders. The azimuth angle dependence of the low-energy spectrum corroborates these results. The two-dimensional zigzag magnetic correlations rapidly weaken until 50 K. At high temperatures, the spectral weight distribution of the low-energy excitation resembles that of the pure Kitaev model, indicating that the Kitaev interaction dominates the dynamic magnetic response at high temperature. We suggest that the larger crystal field and distortion and the weakened longer-range Heisenberg exchange interactions due to the Na deficiency in ${\mathrm{Na}}_{x}{\mathrm{IrO}}_{3}$ contribute to bring ${\mathrm{Na}}_{x}{\mathrm{IrO}}_{3}$ away from the zigzag long-range magnetic order phase.

Energy Level Diagram of 3d2 Configuration in Tetrahedral Crystal Field and Its Applications to Cr4 + /Mn5 + ‐Doped Luminescent Materials

AbstractTanabe–Sugano diagram is a successful guideline for the energy levels of 3dn electrons for half a century. However, it is developed only under the regular octahedral crystal field. With the surging research interests in the near‐infra‐red luminescence of 3d block transition‐metal elements, it becomes an urgent demand to know the energy level diagram of Cr4 + and Mn5 + with 3d2 configuration, which are usually embedded in the tetrahedral site due to the smaller ionic size. Although the Tanabe–Sugano diagram of 3d8 configuration in octahedron is usually used as a substitution for that of 3d2 configuration in tetrahedron, the result mismatches with the crystal field strength and emission profile. In this work, irreducible tensor operator method is applied to derive the full analytical expressions of 3d2 energy levels in the tetrahedral crystal field, based on which the Tanabe–Sugano diagram of 3d2 configuration in tetrahedron is built for the first time. Also provided is a set of formulas for fast estimation of values, which shows consistency between the crystal field strength and the emission profile among Cr4+, Mn5+‐doped luminescent materials. As it is hoped, this work could provide more deep insights into the luminescence of 3d2 elements.

Magnetic properties of the mononuclear iron (III) complexes with biphenyl‐disubstituted Schiff base ligand: EPR and SQUID study

The magnetic features of the mononuclear iron (III) complexes [Fe(L)2]+X−with tridentate Schiff base ligands [where L is 4‐(4‐diphenylcarboxy)‐2′‐oxysalicylidene‐N′‐ethylene‐N‐ethylene‐2′‐amine‐(carboxy‐4‐diphenyl) and X− = Cl−(1), PF6−(2) and NO3−(3) counter‐ions] have been studied by electron paramagnetic resonance (EPR) spectroscopy, SQUID (superconducting quantum interference device) magnetometry (solid), and Evans1H nuclear magnetic resonance method (solution). EPR established that compounds (1) and (2) consist of two types high‐spin (HS, S = 5/2) centers with weak distorted and strong low‐symmetry octahedral crystal fields. The signals from low‐spin centers (LS, S = 1/2) of Fe (III) are observed in the spectra of compounds2and3. SQUID revealed permanent HS behavior in (1) and mixed LS + HS in (2), (3) in wide temperature range, while solution method—only HS. Magnetic measurements allowed to estimate part of HS and LS fractions and detected antiferromagnetic exchange interactions between the neighboring Fe (III) ions in complexes.“

SmI3:4f5 honeycomb magnet with spin-orbital entangled Γ7 Kramers doublet

We report magnetic properties of a 4f-honeycomb iodide SmI3 made up of edge-shared network of SmI6 octahedra. High temperature magnetic susceptibility indicates {\Gamma}7 Kramers doublet ground state of Sm3+ (4f5) ions stabilized by the spin-orbit coupling and octahedral crystal electric field, which interact with Sm-I-Sm bond angle nearly 90 degree. Magnetization measurements down to 0.1 K detected antiferromagnetic correlations and an anomaly in the magnetization curve before saturation without a sign of long-range order. Relevance between SmI3 and the antiferromagnetic Kitaev material proposed in the 4f-electron system is discussed.

Magnetic Ground State and Tunable Néel Temperature in the Spin ½ Linear Chain Antiferromagnet Co(OH)(2−x)Fx

The ground state of Co2+ in the octahedral CoO6 crystal field has often been assumed to be in high spin state with S = 3/2. As a result, the effective magnetic moments of Co2+ are overestimated in Co(OH)(2−x)F x . Herein, it is argued that the ground state of Co2+ in Co(OH)(2−x)F x has effective spin 1/2 but not S = 3/2, due to spin–orbit coupling in distorted octahedral crystal field. Analysis of the published magnetic susceptibility (χ) data on three samples of Co(OH)(2−x)F x is reported here with the new result that the Néel temperature T N decreases with an increase in χ as T N = 36.5, 30.5, and 26.5 K are determined for x = 1.09, 1.14, and 1.26, respectively. The employment of modified Curie–Weiss (CW) law along with S = 1/2 ground state eliminates the reported disagreement between the magnetic moment μ(Co2+) obtained from CW law above and neutron powder diffraction (NPD) below The fits of the data to the predictions of the linear chain model yield the exchange constant J/k B = −30.5, −28.1, and −24.7 K for x = 1.09, 1.14, and 1.26, respectively, supporting the chain‐like antiferromagnetic ordering observed using NPD.

Z2 nontrivial topology of rare-earth binary oxide superconductor LaO

Recently, superconductivity has been discovered in rock-salt structured binary lanthanum monoxide LaO through state-of-the-art oxide thin-film epitaxy. In this work, we reveal that the normal state of superconducting LaO is a $Z_2$ nontrivial topological metal, where the Dirac point protected by the crystal symmetry is located around the Fermi energy. By analysing the orbital characteristics, we show that the nature of the topological band structure of LaO originates from the intra-atomic transition from the outer shell La 5$d$ to the inner shell 4$f$ orbitals driven by the strong octahedral crystal-field. Furthermore, the appearance of novel surface states unambiguously demonstrates the topological signature of LaO superconductor. Our theoretical findings not only shed new light into the understanding of the exotic quantum behaviors in LaO superconductor with intimate correlation between 4$f$ and 5$d$ orbitals in La, but also provide an exciting platform to explore the interplay between nontrivial topology and superconductivity.

Resonant electronic Raman scattering from Ir4+ ions in β-Ga2O3

We report the observation of resonant electronic Raman scattering (ERS) originating from Ir4+ ions in bulk β-Ga2O3 crystals grown by the Czochralski method. The observed ERS peak at 5150 cm−1 at room temperature is attributed to an internal transition within the split 2T2g ground state of Ir4+ ions under strong octahedral crystal field conditions and combined action of spin–orbit coupling and low-symmetry field components. The ERS efficiency is found to strongly depend on the photon energy used for optical excitation and exhibits a maximum at about 2.9 eV. In accordance with the linear dependence of the ERS peak intensity on the optical excitation power, the enhancement around 2.9 eV is explained by Raman scattering in resonance with electronic transitions from the Ir4+ ground state 2T2g to the first excited state 4T1g. The optically induced Ir3+/4+ charge transfer is discussed as an alternative, but less likely the origin of the observed enhancement of the ERS efficiency.

Application of multi-edge HERFD-XAS to assess the uranium valence electronic structure in potassium uranate (KUO3).

The uranium valence electronic structure in the prototypical undistorted perovskite KUO3 is reported on the basis of a comprehensive experimental study using multi-edge HERFD-XAS and relativistic quantum chemistry calculations based on density functional theory. Very good agreement is obtained between theory and experiments, including the confirmation of previously reported Laporte forbidden f-f transitions and X-ray photoelectron spectroscopic measurements. Many spectral features are clearly identified in the probed U-f, U-p and U-d states and the contribution of the O-p states in those features could be assessed. The octahedral crystal field strength, 10Dq, was found to be6.6 (1.5) eV and 6.9 (4) eV from experiment and calculations, respectively. Calculated electron binding energies down to U-4f states are alsoreported.

Ligand Field‐Induced Exotic Dopant for Infrared Transparent Electrode: W in Rutile SnO2

AbstractTransparent conductive oxides (TCOs) exhibiting high near‐infrared (NIR) transmittance are one of the key materials for highly efficient thin‐film solar cells with widened spectral sensitivity. To realize excellent NIR transparency in a TCO film, developing a dopant providing high mobility (µ) carriers is quite important. Herein, it is demonstrated that W is a high‐μ dopant in rutile SnO2, which is unexpected from the conventional strategy. A combination of electrical transport property measurements and hybrid density functional theory calculations reveals that W behaves as a singly charged donor (W5+) showing minimized ionized impurity scattering. This charge state is realized by the splitting of the W 5d t2g‐states originating not only from the octahedral crystal field but also hybridization with the O 2p orbitals, whose contribution has not been considered in transition metal‐doped TCOs. Hybridization between metal d orbital and O 2p orbitals would provide a new guide for designing a novel dopant of NIR transparent conductors.

Cr-doped ZnGa2O4: Simple synthesis of intense red-NIR emitting nanoparticles with enhanced quantum efficiency

Red-NIR emitting ZnGa2-xCrxO4 nanoparticles were synthesized using simple co-precipitation, and the effect of annealing was studied (600–1200 °C). Upon annealing, crystallite size increased (12 nm–73 nm), and color tone turned pink beyond 1000 °C, inferring dopant diffusion. Blue, green absorption bands in diffuse-reflectance spectra revealed Cr3+ in octahedral crystal field. Photoluminescence excitation spectra showed characteristic Cr3+ bands (∼400 nm and ∼550 nm), and the asymmetric 400 nm band suggested splitting the 4T1g(F) state due to trigonal distortion. Emission spectra consisted of a zero-phonon line (∼688 nm), along with associated multi-phonon sidebands. ZnGa1.98Cr0.02O4 showed intense red emission with λexc = 420 nm, while the tri-exponential fitting of the excited-state lifetime implied multiple decay processes due to Cr3+ occupying different environments. The internal quantum efficiency of the 1200 °C annealed sample was impressive (48%). Both UV and visible light up to 600 nm could induce persistent-luminescence (λem = 688 nm) for longer durations. The smaller size, intense emission, higher quantum yield, and visible light activation suggest that the nanoparticles are suitable for bioimaging applications.

Determination of the crystal field and nature of x-ray linear dichroism for Co-O with local octahedral, tetrahedral, and tetragonal symmetries

Cobalt oxides with multiple local Co-O coordination environments such as octahedral, tetrahedral, and tetragonal networks display versatile electronic and magnetic properties which have attracted great interest in many fields. Understanding the ground-state properties and determining the fundamental band gap remain challenges in cobalt-based compounds and thin films, which have been investigated here using synchrotron-based Co ${L}_{23}$-edge x-ray absorption measurements followed by configuration interaction cluster calculations. We focus on the detailed Co ${L}_{23}$-edge absorption spectral variations in different octahedral crystal fields as well as in the less investigated tetrahedral and tetragonal systems, taking into account Co ions with different valence states. From a quantitative comparison between the simulated spectrum and an accurately measured absorption spectrum of a specified compound, the crystal field value can be extracted from the Co ${L}_{23}$-edge absorption spectrum, which is complementary to the results obtained in optical measurements and other calculations. Furthermore, Co ${L}_{23}$-edge x-ray linear dichroism shows the same spectral evolutions as a result of either local ${\mathrm{CoO}}_{6}$ cluster with tetragonal symmetry or the magnetic exchange field, whereas both coexist in most antiferromagnetic cobalt oxide thin films. Detailed temperature and polarization-dependent Co ${L}_{23}$-edge absorption measurements have been proposed to distinguish both contributions, which show different spectral variations due to the specified modifications of the ground and final states at different temperatures. Our results offer theoretical guidance for understanding the multiplet structure of Co ${L}_{23}$-edge absorption spectrum, obtaining the precise crystal field value for cobalt oxides with versatile coordinations, and explaining the underlying mechanism of x-ray linear dichroism, as well as understanding the fundamental physical properties and their potential applicability of cobalt oxides and their thin films.