Crystal Field Energy Research Articles

Laser site-selective spectroscopy of Nd3+-doped Y2SiO5

We report on laser site-selective spectroscopy, as well as high resolution, infrared to optical absorption, of both the six and seven -fold coordinated, C1 symmetry sites for Nd3+ doped Y2SiO5. A total of fifty three crystal-field energy levels have been determined for both sites. We also present the temperature dependencies of the spectral linewidth and line position as measured in absorption. The coupling coefficients due to Raman two-phonon and direct one-phonon processes are interpreted in terms of the phonon energies contributing and their roles in the phonon energy distribution of Y2SiO5. Finally, we analyse direct non-radiative relaxation processes as inferred from the temperature-dependent fluorescence lifetimes.

First principle insight on physical characteristics of Mn doped BeS compound

The electronic, optical and magnetic properties of Be1-xMnxS (x = 3.125%, 6.25%, 12.5% and 18.75%) semiconductors have been performed by PBEsol + GGA approximations. The investigation was done by using full potential-linearized augmented plane wave (FP-LAPW) method with in DFT as implemented in the WIEN2K code. Stability of Be1-xMnxS compounds is confirmed by the heat of enthalpy formation. In electronic properties, the spin-polarized electronic band structure and density of states elucidate the semiconductor nature of material in both spin channels at different concentration of dopant atoms. The nature and origin of ferromagnetism have been observed in term of crystal-field energies, exchange constant and the exchange energies. Optical parameters like extinction coefficient, refractive index, dielectric constant, reflectivity, optical conductivity and absorption coefficient are analyzed which active in visible to ultraviolet region. The refractive index less than unity illustrate that the group velocity of the incident photons is greater than the speed of light. Furthermore, the total magnetic moment of Be1-xMnxS alloys is mainly due to the Mn site with very small contribution of Be and S and increases with increasingconcentration of Mn. Calculated results indicate that the electronic, magnetic and optical properties provide a new route to the experimentalist for the potential applications in optoelectronic and spintronics devices.

In situ and tunable structuring of semiconductor-in-glass transparent composite.

SummarySemiconductor-in-glass composites are an exciting class of photonic materials for various fundamental applications. The significant challenge is the scalable elaboration of composite with the desirable combination of tunable structure, high semiconductor loading ratio, and excellent transparency. Here we report that the topological engineering strategy via hybridization of the glass network former enables to surmount the aforementioned challenge. It not only facilitates the in situ precipitation of (Ga2-xAlx)O3 domains with continuously tunable composition but also allows to simultaneously refine the grain size and enhance the crystallinity. In addition, the composites exhibit excellent transparency and can host various active dopants. We demonstrate the attractive broadband optical response of the composite and achieve the pulse laser operation in mid-infrared waveband. The findings are expected to provide a fundamental principle of in situ modification in hybrid system for generation of high-performance semiconductor-in-glass composites.

Theoretical analysis of ferromagnetism and electronic transport aspects of CaM2S4 (M = Ti, Cr) spinels for the application of spintronic and energy storage system

By using the density functional theory (DFT), we explored ferromagnetic and electronic transport aspects of CaM2S4 (M = Ti, Cr) spinels. For both spinels, we used PBEsol generalized gradient approximation (GGA) to investigate structural parameters and noted that studied spinels are good compared with the existing parameters. A novel exchange correlation potential, modified Becke and Johnson (mBJ), was utilized for the investigation of electronic, magnetic and transport aspects because we assure our predicting results of electronic bandgap to be consistent to the experiments. The energy difference between ferromagnetic and nonmagnetic states is calculated to check the structural stability of ferromagnetic state. Further, the calculations of band structures and density of state have been explored to check ferromagnetic nature of semiconductor spinels that was further confirmed on the basis of exchange splitting energy and magnetic parameters. In addition, exchange constants ([Formula: see text] and [Formula: see text]) and crystal field energy [Formula: see text] along with magnetic moments were also calculated. Our calculated results of magnetic parameters indicate that these spinels are considered as suitable candidate for spintronic applications. Furthermore, to check the reliability of these spinels in energy storage systems, the electronic transport aspects were calculated in detail.

Influence of Na2O addition on the alkali borochromate glasses: structure and ligand field

Trivalent chromium is a very common ion used for coloring in the industry of glass. As glasses are melted under high oxidizing circumstances, hexavalent chromium Cr(VI) can be fixed and found in certain advertising applications of UV-protection receptacles. Melt quenching technique is employed to prepare oxide glasses with a general formula xNa2O–(30 − x) Li2O–10K2O–10ZnO–49.7B2O3–0.3Cr2O3 (where x = 0, 5, 10 and 15 mol%). The amorphous nature of the samples is confirmed using X-ray diffraction. The optical absorption spectra have been recorded, and the resulting data were used for the ligand field theory analysis, from which the crystal-field energy (10Dq), Racah parameters (B and C) and nephelauxetic function (h) were evaluated. The variations in the optical bandgap and the band tail with Na2O content have been discussed in relation to the glass structure. FTIR of these glasses revealed that the borate network is affected by the replacement of Na2O by Li2O content. It showed also reduction in N4 with increasing Na2O. This result indicates the increase of non-bridging oxygens (NBOs) which in turn reduces the rigidity of the glass matrix and opens up the structure. The density and the molar volume (VM) measurements were also employed to investigate the structure. These measurements show a linear increase as Na2O content increases. This can be attributed to the differences in the ionic radii of sodium and lithium ions, being larger for sodium. UV–Visible optical absorption spectra show three characteristics bands around ~ 588, 435 and 360 nm which are assigned to the transitions 4A2g → 4T2g, 4A2g → 4T1g and 4A2g → 2A1g, respectively. Racah parameters (B and C) show an opposite behavior. The obtained ratio (10Dq/B) reveals a strong ligand field around chromium ions. Also, the nephelauxetic parameter of the ligand shows the same 10Dq behavior. The increase of chromium ions in the trivalent state shows that it acts as modifiers, yielding the higher concentration of NBOs in the glass matrix and confirming the FTIR and optical results. Electron spin resonance spectroscopy was measured and studied to confirm the existence of Cr3+ and Cr6+.

Electronic, optical and magnetic properties of PrXO3(X = V, Cr): first-principle calculations

ABSTRACT Electronic, optical and magnetic properties of perovskite oxides PrXO3(X = V, Cr) are calculated by first-principle calculations. The calculations of electronic band structure and density of states (DOS) show that the PrVO3 exhibits metallic behaviour, while PrCrO3 exhibits half-metallic nature with direct band gap. The calculated band gap of PrCrO3 is 2.7 in the minority spin state. The origin and nature of ferromagnetism has been observed in terms of crystal field energies, exchange constant and exchange energies. The magnetic moment of PrVO3 is 3.42491 μB, while that of PrCrO3 is 5.00003 μB. Moreover, the evolution of PrCrO3 compound based on its electronic and magnetic properties confirms that the compound is suitable for spintronics applications. Results revealed that PrCrO3 has low reflectivity, high dielectric constant, high optical conductivity and high absorption coefficient which indicate that the material is a potential candidate in the wide range of optoelectronic applications.

Quasi-two-dimensional magnetic correlations inNi2P2S6probed byP31NMR

Detailed $^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) measurements are presented on well-characterized single-crystals of antiferromagnetic van der Waals ${\mathrm{Ni}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$. An anomalous breakdown is observed in the proportionality of the NMR shift $K$ with the bulk susceptibility $\ensuremath{\chi}$. This so-called $K$--$\ensuremath{\chi}$ anomaly occurs in close proximity to the broad peak in $\ensuremath{\chi}(T)$, thereby implying a connection to quasi-two-dimensional (2D) magnetic correlations known to be responsible for this maximum. Quantum chemistry calculations show that crystal field energy level depopulation effects cannot be responsible for the $K$--$\ensuremath{\chi}$ anomaly. Appreciable transferred hyperfine coupling is observed, which is consistent with the proposed Ni--S--Ni super- and Ni--S--S--Ni super-super-exchange coupling mechanisms. Magnetization and spin-lattice relaxation rate (${T}_{1}^{\ensuremath{-}1}$) measurements indicate little to no magnetic field dependence of the N\'eel temperature. Finally, ${T}_{1}^{\ensuremath{-}1}(T)$ evidences relaxation driven by three-magnon scattering in the antiferromagnetic state.

Investigation of optical polarization characteristics for an AlGaN-based quantum well structure

The anisotropic optical properties of an AlGaN-based quantum well (QW) structure are investigated by using the effective-mass theory. Firstly, the crystal-field bowing parameter (bcr = −0.087) and the slope (bs = 8) of the crystal-field splitting energy (∆cr) — in-plane strain (εxx) curve are obtained by comparing the calculated degree of polarization (DOP) of the AlGaN alloy with the experimental values. Then, the DOP of Al0.35Ga0.65N/AlxGa1-xN single QW structures with various Al-content (x = 0.5, 0.45, 0.55) in the barrier layer is calculated by employing the obtained parameters. The DOP is enhanced from −43.15% to −4.47% with increasing Al-content at J = 100 A cm−2. By increasing the injection current density or the residual strain in the active region, the DOP can be increased/decreased by modulating the Al component of the barrier layer due to the different polarization characteristics of the transition levels. In addition, the AlGaN-based QW structure with lower Al-content in the barrier layer shows the larger spontaneous emission rate.

A new microwave approach for the synthesis of green emitting Mn2+-doped ZnAl2O4: A detailed study on its structural and optical properties

A simple recipe for synthesizing green emitting Mn2+-doped ZnAl2O4 phosphor has been developed. Metal-organic complexes, with their unique properties, were employed as precursors to obtain phase-pure, nanocrystalline material in the as-prepared form within just 5 min of microwave irradiation. The Mn2+ doping concentration that showed the highest photoluminescence (PL) intensity was optimized and a comprehensive investigation of the structural and optical properties were made for various annealing temperatures. Rietveld refinement of the samples annealed at 1200 °C and 1400 °C, showed that the cationic inversion in the spinel decreased from 3.4 to 2.1% and this change was validated by the X-ray photoelectron spectroscopy results. XPS confirmed that the inversion for Zn2+, Al3+, and Mn2+ cations decreased with annealing temperature, despite of which, inversion remained at 20%, 10%, and 15%, respectively for the sample annealed at 1400 °C, emphasizing the fact that synthesis plays an important role in controlling the amount of inversion in an otherwise normal spinel. Electron paramagnetic resonance spectra of the as-prepared and the samples annealed at high temperatures confirmed that the Mn2+ hyperfine spectrum was not just a function of the crystal field environment but also strongly depends on the doping concentration. The PL spectrum taken at different annealing temperatures, comprised of the characteristic 4T1 (G) → 6A1 (S) spin-forbidden Mn2+ transitions, showed that the emission intensity depends on the material crystallinity. The sample annealed at 1400 °C displayed a significantly higher PL intensity compared to those annealed at lower temperatures. The variation of PL spectrum of this sample was investigated between 9 K and 300 K to determine the origins of the asymmetry at room temperature and the vibrational sidebands at lower temperatures. The energy levels of the Mn2+ dopant, calculated theoretically and verified experimentally, were used to determine the spectroscopic parameters such as the Racah B and C values and the crystal field energy, Dq. These values showed that the Mn2+ was in a weak tetrahedral field. This work demonstrates a technologically important, green, and swift technique in synthesizing phosphors for various applications in displays, bioimaging, solid state lighting, etc.

Probing of mechanical behaviour, quantum mechanism of spin exchange and magnetism of SnV2O4 and SnCr2O4 spinel oxides by DFT

ABSTRACT The spinel oxides SnV2O4 and SnCr2O4 are analysed to reveal the mechanical, electronic and magnetic properties using Wien2k software. The negative formation energy and positive elastic constants shows the studied materials are thermodynamically and mechanically favorable. In ferromagnetic (FM) state more energy release than antiferromagnetic (AFM) state. The above room temperature magnetism is elaborated by spin polarisation, exchange mechanism, direct Δx(d) and indirect Δx(pd) exchange energy and crystal field energy Δx(CF). The shifting of magnetic moment form V/Cr sites to Sn/V sites illustrate the ferromagnetism is caused by pd hybridisation that affects the net orientation of electrons spin and hence, it increases the importance of the materials for spin-dependent electronic applications.

Investigations of crystal-field energy levels and microscopic spin-Hamiltonian parameters of Mn4+ ions in pyrochlore hosts Y2Ti2O7 and Y2Sn2O7

We investigate the crystal-field energy levels and the microscopic origin of spin-Hamiltonian parameters (SHPs), i.e. g-factors and zero-field splitting parameter D, for the ground state 4A2g(4F) of Mn4+ impurity ions at trigonal sites in host pyrochlores, Y2Ti2O7 and Y2Sn2O7. The total Hamiltonian includes the electrostatic and crystal-field (CF) Hamiltonians, and the magnetic interactions, i.e. spin-orbit, spin-spin, spin-other-orbit, and orbit-orbit couplings. The calculations are performed under the complete diagonalization method and microscopic spin-Hamiltonian (CDM/MSH) program. The calculated CF levels agree reasonably well with the observed energy levels for Mn4+ ions in these compounds. The SHPs are evaluated with the CDM/MSH wave functions as well as by a perturbation loop theory method (PLTM). The g-factors of Mn4+ in Y2Ti2O7 and Y2Sn2O7 using PLTM are close to those evaluated by CDM, while the PLTM value of D shows appreciable difference with the CDM value. This evidences that the g-factors depend predominately on the lower states, while the higher energy states significantly affect the D value. The contributions of spin-spin, spin-other-orbit, and orbit-orbit couplings to the CF energy levels and the SHPs are discussed.

Exploring the band structure of Wurtzite InAs nanowires using photocurrent spectroscopy

We use polarized photocurrent spectroscopy in a nanowire device to investigate the band structure of hexagonal Wurtzite InAs. Signatures of optical transitions between four valence bands and two conduction bands are observed which are consistent with the symmetries expected from group theory. The ground state transition energy identified from photocurrent spectra is seen to be consistent with photoluminescence emitted from a cluster of nanowires from the same growth substrate. From the energies of the observed bands we determine the spin orbit and crystal field energies in Wurtzite InAs. This information is essential to the development of crystal phase engineering of this important III-V semiconductor.

Ab initio calculations of the crystal field and phonon dispersions in CePd2Al2 and LaPd2Al2

CePd2Al2 crystallizes in the CaBe2Ge2-type tetragonal structure (P4/nmm, 129) and undergoes a phase transition to the orthorhombic Cmme structure at around 13 K. Its inelastic neutron spectra reveal an additional magnetic excitation that was ascribed to electron-phonon interaction leading to a formation of a new quantum quasi-bound vibron state. We present the first-principles calculations of the crystal field excitations and lattice dynamics calculations of the phonon dispersions to compare with the experimental data. The calculated crystal field energy splitting in CePd2Al2 agrees well with the model used to describe the experimental neutron scattering spectra. The first excited crystal field level moves to higher energies when undergoing the transformation from tetragonal to orthorhombic structure, in agreement with the experiment. The analysis based on calculated elastic constants and lattice dynamics calculations show that in both tetragonal and orthorhombic structures there are no imaginary modes for any q-wave vector within the Brillouin zone, and therefore the lattice structures are stable. The phonon dispersions and density of states are calculated for both crystal structures of CePd2Al2 and its nonmagnetic counterpart LaPd2Al2. The results generally agree well with the experimental data including the high phonon density of states around 12 meV. The phonon density of states is also used to calculate the mean squared displacement, Debye temperature, lattice heat capacity and compared with similar properties of the available experiment.

Unifying analyses of the crystal field energy levels and EPR g factors for the trigonal Co2+ tetrahedral center in β˝-alumina

The crystal field energy levels (obtained from the optical spectra) and g factors g//, g⊥ (obtained from EPR spectra) for the Co2+ ion at the trigonal Al3+ tetrahedral site in β˝-alumina crystal are unifiedly analyzed with the full diagonalization (of energy matrix) method founded on the two-spin-orbit-parameter model (which contains the contributions from both the spin-orbit parameters of central dn ion and ligand ion via covalence effect). The results display that by applying only three adjustable parameters, the observed ten spectral data (eight crystal field energy levels and two g factors) can be rationally interpreted, confirming the availability of this full diagonalization method in the united analyses of optical and EPR data. The large impurity-generated angular distortion and hence the large trigonal distortion [which correspond to the large g-factor anisotropy and large ground state splitting ΔE(4A2)] of the Co2+ tetrahedral center in β˝-alumina are also gained. The outcomes are discussed.

First-principles calculation to investigate half metallic ferromagnetism and thermoelectric properties of Ca0.75Ti0.25X (X = S, Se) alloys

The electronic, magnetic and thermoelectric properties of Ca0.75Ti0.25S and Ca0.75Ti0.25Se alloys are explored comprehensively. The stabilityin ferromagnetic state (FM) has been ensured by energy released from optimized structures and enthalpy of formation (ΔH). The Heisenberg classical model has been used to calculate Curie temperature (Tc). The band structures (BS) as well as densities of states (DOS) are analyzed to explore half metallic ferromagnetism and 100% spin polarization. The Δx(d) (exchange energy) overcome ΔEcrystal (Crystal field energy) due to ferromagnetism induced by strong hybridization. The negative values of Δx(pd) and exchange constant N0β lesser exchange energy which satisfy the double exchange model. The thermoelectric properties of the studied compounds are elaborated in terms of electrical conductivity, thermal conductivity and Seebeck coefficient. The increase in power factor with the replacement of S by Se has been noted.

Investigation of the rare earth-based LaYO3 (Y = Cr and Mn) perovskites by ab-initio approach

Here, we report the ab-initio studies to explore physical aspects of Lanthanide-based perovskites LaYO3 (Y = Cr and Mn), using full-potential linearized augmented plane waves logic. The calculated values of lattice constants were 3.91 Å and 3.86 Å for LaCrO3 and LaMnO3 respectively with PBEsol-GGA, comparable with available experimental data. Band structure and density of states played pivotal role in determination of the electronic properties. Figuring the exchange splitting energies (Δx(d) and Δx (pd)), crystal field energy (ΔCF) and Jahn-Teller energy (ΔJT) traverse the ferromagnetic semiconducting nature of studied La-perovskites. In addition, BoltzTraP code implemented to interrogate the thermoelectric properties.

Study of magnetic, thermoelectric and optical behaviors of PbMO3 (M = V, Mn) perovskites using DFT approach

This work reports the magnetic, thermoelectric and optical behaviors of PbMO3 (M = V, Mn) perovskites analyzed using density functional theory based FP-LAPW method. The electronic structures are computed using mBJ potential. The antiferromagnetic state stability at the ground state is investigated by contrasting released energies in the ferromagnetic, antiferromagnetic and nonmagnetic states. The origin of magnetism is described by comparing the determined crystal field, Jahn-Teller and exchange energies. Thermoelectric response is investigated in terms of important parameters demonstrating that PbVO3 can exhibit higher figure of merit than PbMnO3. Various optical parameters are computed to reveal optical device applications and Penn’s model is also verified. The electronic properties elucidate reducing band gap in the semiconducting perovskites when moving from V to Mn cation, due to which, optical absorption edges are shifted to the lower energies suggesting energy harvesting device applications.

Maximization of ferromagnetism in LaCoO3 films by competing symmetry

Spin state transition in perovskite cobaltite is delicately controlled by the competition between exchange interaction energy and crystal-field energy. The latter is mainly governed by the change of bond length and bonding angle. Previous work has revealed that the electronic configuration associated with spin state transition in $\mathrm{LaCo}{\mathrm{O}}_{3}$ (LCO) thin films can be effectively modulated by epitaxial strain. However, a systematic study on the spin state transition of $\mathrm{C}{\mathrm{o}}^{3+}$ ions in LCO films with different crystallographic symmetry is still missing. Here, keeping the in-plane strain unchanged, we report that the magnetization of LCO films can be manipulated with crystallographic symmetry. The ultrathin LCO layers, constrained by the cubic substrate, have pseudotetragonal structure and small magnetization. Upon increasing the layer thickness, the monoclinic structure dominates the LCO film and maximizes its ferromagnetism. For the LCO films with a thickness beyond 35 unit cells, the symmetry relaxes gradually towards its rhombohedral bulk form, and meanwhile the magnetization reduces. These results highlight the importance of spin-lattice entanglement in a ferroelastic material and provide a concise way to maximize its functionality using symmetry engineering.

A rare flattened tetrahedral Mn(II) salen type complex: Synthesis, crystal structure, biomimetic catalysis and DFT study

A new flattened tetrahedral high spin Mn(II) complex (1) has been synthesized using N2O4 donor Schiff base ligand. Complex 1 was characterized by X-ray diffraction and various spectroscopic techniques. For further understanding of electronic structure of the complex, DFT calculations and electrochemical studies have been performed. This is a rare example of a flattened tetrahedral Mn(II) salen type Schiff base complex. High-spin d5 configuration of the metal center provides no crystal-field stabilization energy to the system and that is the main reason behind the significant deviation of this salen-type ligand from planarity. Notably, the propylenic linker in the ligand provides adequate flexibility so that such an uncommon binding mode of the salen type Schiff base ligand becomes possible. Complex 1 exhibits excellent catalytic property towards oxidation of o-aminophenols in aerobic condition. Detailed kinetic investigations together with the mass spectrometry studies reveal several important information relating to biomimetic catalytic activity of the present complex.

Structural, electronic, magnetic, optical and thermoelectric response of half-metallic AMnTe2 (A = Li, Na, K): An ab-initio calculations

The structural, electronic, magnetic, optical and thermoelectric properties of manganese based half-metallic compounds AMnTe2 (A = Li, Na, K) are carried out by employing Full Potential Linearized Augmented Plane Wave (FP-LAPW) method. Perdew-Burke-Ernzerhof-Generalized-Gradient-Approximations (PBE-GGA) are used for the exchange-correlation potential. Various properties of the compounds (LiMnTe2, NaMnTe2 and KMnTe2) are investigated, such as the analysis of the density of states and spin-polarized band structures reveal that all the studied materials have an indirect bandgap in spin-down (↓) channel, while spin-up (↑) channel exhibit metallic behavior. The stability of ferromagnetic nature is predicted by exchange energies, crystal field energies and John-Teller distortion. The absorption and extinction coefficient, dielectric constant, reflectivity, refractive index, energy loss function and optical conductivity are calculated through optical properties. The thermoelectric response of these materials is computed by employing BoltzTraP code. In addition, the phonon dispersion curves are calculated to examine the thermal stability of the materials.