Giant nonlinear absorption in the NiO antiferromagnet

A study of the spectrum of nonlinear two-photon and two-step absorption in NiO single crystals, carried out in the energy region ?ω₁ + ?ω₂ = 2.45–4.575 eV, showed it to have a complex shape and consist of very strong peaks (from 0.05 to 2.7 cm/MW). Within the energy interval 2.45–3.3 eV, the spectrum is due to d-d transitions in the Ni²⁺ ion. The band gap width was determined to be E _g =3.466 eV. The spectral features seen above this energy originate from interband transitions from three valence subbands to the conduction band bottom.     

Effects of nickel doping on structural and optical properties of spinel lithium manganate thin films

Spinel LiNi_xMn_2−xO₄ (x≤0.9) thin films were synthesized by a sol-gel method employing spin-coating. The Ni-doped films were found to maintain cubic structure at low x but to exhibit a phase transition to tetragonal structure for x≥0.6. Such cubic-tetragonal phase transition can be explained in terms of Ni³⁺(d⁷) ions with low-spin (t_2g⁶,e_g¹) configuration occupying the octahedral sites of the compound, thus being subject to the Jahn-Teller effect. By X-ray photoelectron spectroscopy both Ni³⁺ and Ni²⁺ ions were detected where Ni²⁺ is more populated than Ni³⁺. Optical properties of the LiNi_xMn_2−xO₄ films were investigated by spectroscopic ellipsometry in the visible-ultraviolet range. The measured dielectric function spectra mainly consist of broad absorption structures attributed to charge-transfer transitions, O²⁻(2p)→Mn⁴⁺(3d) for 1.9 (t_2g) and 2.8-3.0 eV (e_g) structures and O²⁻(2p)→Mn³⁺(3d) for 2.3 (t_2g) and 3.4-3.6 eV (e_g) structures. Also, sharp absorption structures were observed at about 1.6, 1.7, and 1.9 eV, interpreted as being due to d-d crystal-field transitions within the octahedral Mn³⁺ ion. In terms of these transitions, the evolution of the optical absorption spectrum of LiMn₂O₄ by Ni doping could be explained and the related electronic structure parameters were obtained.     

Magnetic properties of Ni-substituted BiFeO3

BiFe_1−xNi_xO₃ ceramic powders with x up to 0.10 have been prepared by the sol-gel technique. The band gap of BiFeO₃ is 2.23 eV, and decreases to 2.09 eV for BiFe_0.95Ni_0.05O₃ and BiFe_0.90Ni_0.10O₃. The Mössbauer spectra show sextet at room temperature, indicating the magnetic ordering and the presence of only Fe³⁺ ions. Superparamagnetism with blocking temperature of 31 K for BiFe_0.95Ni_0.05O₃ and 100 K for BiFe_0.90Ni_0.10O₃ was observed. Enhanced magnetization at room temperature have been observed (1.0 emu/g for BiFe_0.95Ni_0.05O₃ and 2.9 emu/g for BiFe_0.90Ni_0.10O₃ under magnetic field of 10,000 Oe), which is one order larger than that of BiFeO₃ (0.1 emu/g under magnetic field of 10,000 Oe). The enhanced magnetization was attributed to the suppression of the cycloidal spin structure by Ni³⁺ substitution and the ferrimagnetic interaction between Fe³⁺ and Ni³⁺ ions.     

Luminescent Ni+ centres and changes of the charge state of nickel ions in ZnS and ZnSe

In addition to Ni²⁺(3d ⁸) with its known internal transitions, Ni⁺(3d ⁹) is identified here by detecting its²E(D)?² T ₂(D) transition in ZnS and ZnSe both in absorption and emission. An analysis of vibronic satellites in these spectra indicates a moderate Jahn-Teller coupling in the² T ₂ ground state. Thermal annealing procedures which raise the Fermi level allow to increase the intensity ratio between the characteristic low-temperature optical bands of Ni⁺ and Ni²⁺. Ionisation and capture processes at these centres are studied by using the intensity of internal transitions as a probe for the concentration of the respective charge state of nickel. They are induced by additional optical irradiation of the samples in various spectral ranges. These photoionisation and radiative recombination processes manifest themselves in absorption, excitation and luminescence spectra as well as in the spectral response of photosensitive EPR signals. In addition to the threshold for the process [Ni²⁺]^x+hv→[Ni⁺]′+e _vb ^/ which had previously been determined, the experiments yield another threshold for the reaction [Ni⁺] +hv→[Ni²⁺]^x+e _cb ^/′ . A well-known but hitherto unsettled emission band of ZnS:Ni is attributed to the radiative recombination which is the reverse of this photoionisation. The corresponding transition in ZnSe:Ni is observed as well.     

Relaxor-like dielectric behavior in La2NiMnO6 double perovskite ceramics

The dielectric and conductive characteristics of La₂NiMnO₆ double perovskite ceramics were investigated together with the crystal structure. La₂NiMnO₆ ceramics crystallized in the monoclinic P2₁/n structure in which the Ni²⁺ and Mn⁴⁺ ions ordered periodically. Relaxor-like dielectric behavior combined with a giant dielectric constant step was observed in the present ceramics, and these unique dielectric characteristics should be attributed to the charge ordering of Ni²⁺ and Mn⁴⁺. The dielectric relaxation was well fitted by the modified Debye equation and Arrhenius law with the activation energy of 0.17 eV. The dc conductivity of La₂NiMnO₆ could be well fitted using a variable-range hopping mechanism instead of a band conduction mechanism.     

Ferromagnetic half-metallic characteristic in bulk Ni0.5M0.5O (M=Cu,Zn and Cd): A GGA+U study

Ferromagnetic half metallicity with a high spin polarization of 100% was predicted in the bulk Ni_0.5Cu_0.5O using density-functional theory method. The band gap of majority spin is 3.45 eV for Ni_0.5Cu_0.5O. The density of states of minority spin at the Fermi level are mainly from Cu 3d and O 2p in the Ni_0.5Cu_0.5O. The magnetic moments are from Ni 3d states. Ni_0.5Zn_0.5O and Ni_0.5Cd_0.5O systems are ferromagnetic insulators, but the magnetic moment of Ni²⁺ ions is enhanced by the Zn and Cd incorporation. Therefore, Ni_0.5Cu_0.5O is the potential candidate for spintronics devices because of the predicted high spin polarization.     

An ESCA investigation of some thiocyanato complexes

A series of simple thiocyanato complex ions have been investigated as their tetraphenylphosphonium and potassium salts. The binding energies of N, C and S were determined as well as those of the metals. From the first-mentioned data effective charge-values were estimated for the atoms of the ligands. For this purpose linear relations E_b = kq + E_b0 were used that had been previously established within a scheme having C_1s (phenyl) as the internal standard.From the data thus obtained the effective charge on the metal atoms was estimated. For elements where we have sufficient data the same type of linear relation seems to he followed. Tentatively valid examples are E_b(Ni) = 6.74q_Ni + 848.3 eV and E_b(Pd) = 4.45q_Pd + 333.9 eV.In this interpretation the atoms of the metals are considered to be positively charged and surrounded by the negative charge of the electrons occupying the s band.It is further suggested for complexes with pronounced π backbonding (Pt(SCN)₄^2?, Pd(SCN)₄^2? and Hg(SCN)₄^2?) that the C 1s binding energy measured for the carbon atom of the SCN^? ligand is composed of the ionisation energy from the 1s level and an additional term corresponding to the intraligand π → π* transition.     

Heat capacity of Heusler alloys: Ferromagnetic Ni2MnSb,Ni2MnSn,NiMnSb and antiferromagnetic CuMnSb

We report the results of the investigation of the specific heat of the ferromagnetic Heusler Ni₂MnSn, Ni₂MnSb, NiMnSb and antiferromagnetic CuMnSb alloys. The low-temperature behaviour of the specific heat may be described as C=γT+βT³ for ferromagnetic compounds and as C=γT+δ T²+βT³ for antiferromagnetic CuMnSb. The values of the density of states from the heat capacity measurements are higher than those from electronic band structure calculations. Debye temperatures are in a good agreement with those obtained from thermal expansion measurements. The Grüneisen parameter is calculated for Ni₂MnSn and CuMnSb from the magnetic contribution to the specific heat in the vicinity of T_C or T_N.     

Local coordination of Co2+ and Ni2+ cations in polyacrylate matrices

Parameters of the local coordination of Co²⁺ and Ni²⁺ cations in polymer matrices are determined by XAFS spectroscopy at the K edges of the corresponding elements. Ion-exchange complexes of Ni²⁺ and polymethacrylate, low-molecular acrylamide complexes [M(CH₂=CHC(O)NH₂)₆](NO₃)₂ (M = Co or Ni), and products of their thermal polymerization are investigated. The final results are in agreement with the octahedral coordination of M atoms (average distances, 2.10–2.11 (Co-O) and 2.05–2.06 Å (Ni-O).     

A superior microwave absorption material: Ni2+-Zr4+ Co-Doped barium ferrite ceramics with large reflection loss and broad bandwidth

Large reflection loss and wide bandwidth are significant targets, determining the microwave absorption ability. However, it is still a challenge to simultaneously satisfy the two conditions. As a multifunctional material, BaFe₁₂O₁₉ possess excellent electromagnetic properties in the microwave frequency band. Due to the natural resonance phenomenon of the material, BaFe₁₂O₁₉ can produce a large magnetic loss which correlates with Fe³⁺ content, and the microwave absorption characteristics of barium ferrite can be modulated by ion doping. As a typical magnetic metal, Ni coupled with high-valence state Zr⁴⁺ doping helps to produce double resonance peaks. In this work, Ni²⁺-Zr⁴⁺ co-doping M-type barium ferrites (BaFe_12-2xNi_xZr_xO₁₉, BNZFO-x, x = 0–0.8) were prepared conveniently by solid-state reaction method. Several necessary measurements to characterize its microwave absorption property have been operated such as morphology, magnetic performance and electromagnetic parameters. The results show that reflection loss and bandwidth can be simply tuned by tailoring Ni²⁺-Zr⁴⁺ content. The reflection loss peak drifts from 18 GHz to 9.76 GHz, which involves a half of the studied frequency range. The maximum reflection loss achieves −60.6 dB and the corresponding bandwidth over −10 dB is 7.68 GHz for BNZFO-0.6 ceramic with only 2.1 mm thickness. Thus, the doping of Ni²⁺-Zr⁴⁺ ion pairs is beneficial to improve the absorbing properties of the material, and the superior microwave absorption property may originate from its inner double natural resonance in micro-scale. The excellent microwave absorption properties suggest that BNZFO-x is a promising candidate applied for designing electromagnetic shielding devices.     

In situ evolution of Ni environment in magnesium aluminosilicate glasses and glass–ceramics–Influence of ZrO2 and TiO2 nucleating agents

The evolution of Ni²⁺ environment has been systematically investigated using optical and in situ X-ray absorption spectroscopy (XAS) to determine the influence of nucleating agents (TiO₂ and/or ZrO₂) during the formation of spinel in magnesium aluminosilicate glass–ceramics. The results were complemented by in situ X-ray diffraction data. According to XAS and optical spectroscopy, the nature of nucleating agents does not modify significantly the Ni environment in initial glasses. However, it has a relatively strong influence in the observed crystallization sequence. Ni²⁺ ions do not enter the Zr-containing crystalline phase of ZrO₂ or ZrTiO₄ but a Ni²⁺ coordination change from the fivefold coordinated sites, with a small amount of tetrahedral sites in parent glasses, to ^[6]Ni²⁺ and ^[4]Ni²⁺ sites in spinel (in glasses nucleated by ZrO₂ and/or TiO₂) or in β-quartz solid solutions (in glasses nucleated by ZrO₂) has been found.     

Theoretical study of EPR spectra and local structure for (NiO6) cluster in LiNbO3:Ni and Al2O3:Ni systems

For a d⁸ configuration ion, the 45×45 complete energy matrix, which contains the electron-electron repulsion interaction, the ligand-field interaction, the spin-orbit coupling interaction as well as the Zeeman interaction, has been established. By diagonalizing the complete energy matrix, the local lattice structure, EPR parameters (D, g_//, g_⊥) and optical absorption spectra for Ni²⁺ ions doped in LiNbO₃ and Al₂O₃ have been investigated. The local structure distortion parameters ΔR, Δθ₁ and Δθ₂ are determined for LiNbO₃:Ni²⁺ and Al₂O₃:Ni²⁺ systems, simultaneously. These results show that local structure of (NiO₆)¹⁰⁻ cluster exhibits an elongation distortion in both LiNbO₃:Ni²⁺ and Al₂O₃:Ni²⁺ systems, in spite of the different reasons of the elongation in both systems. In addition, we have found that the orbit reduction effect is very important to understand the anisotropic g-factors for Ni²⁺ ions doped in LiNbO₃ and Al₂O₃ crystals.     

Nuclear magnetic resonance of intermetallic compounds Gd2Ni17−xAlx

²⁷Al Knight shifts vs temperature and magnetic susceptibility for the intermetallic compounds Gd₂Ni_17?xAl_x (x = 17; 16.2; 16; 15) are presented. The results are discussed in terms of the uniform polarization model fo the conduction electrons by the 4f and 3d spins localized on the Gd and Ni ions. The phenomenological exchange constants J_sf and J_sd range between ?1.80×10^?3 and 1.19×10^?3 eV and ?0.63×10^?3 and ?0.52×10^?3 eV, respectively.     

Theoretical interpretation of optical and EPR spectra and the substitutional site of ferroelectric LiNbO3:Ni crystal

The optical absorption spectrum, zero-field splitting (ZFS) and EPR g factor of LiNbO₃:Ni²⁺ are explained uniformly on the basis of complete energy matrix diagonalization procedure (CDP) and Zhao's self-consistent field (SCF) d-orbit of free Ni²⁺ ions. The agreement between the calculated results and the experimental data shows quantitatively that impurities Ni²⁺ replace the Nb⁵⁺ rather than Li⁺ sites in LiNbO₃:Ni²⁺.     

Swift heavy ion induced modifications in cobalt doped ZnO thin films: Structural and optical studies

Modifications in the structural and optical properties of 100 MeV Ni⁷⁺ ions irradiated cobalt doped ZnO thin films (Zn_1−xCo_xO, x = 0.05) prepared by sol-gel route were studied. The films irradiated with a fluence of 1 × 10¹³ ions/cm² were single phase and show improved crystalline structure with preferred C-axis orientation as revealed from XRD analysis. Effects of irradiation on bond structure of thin films were studied by FTIR spectroscopy. The spectrum shows no change in bonding structure of Zn-O after irradiation. Improved quality of films is further supported by FTIR studies. Optical properties of the pristine and irradiated samples have been determined by using UV-vis spectroscopic technique. Optical absorption spectra show an appreciable red shift in the band gap of irradiated Zn_1−xCo_xO thin film due to sp-d interaction between Co²⁺ ions and ZnO band electrons. Transmission spectra show absorption band edges at 1.8 eV, 2.05 eV and 2.18 eV corresponding to d-d transition of Co²⁺ ions in tetrahedral field of ZnO. The AFM study shows a slight increase in grain size and surface roughness of the thin films after irradiation.     

Electrochemical behavior of Li intercalation processes into a Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 cathode

Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (X=0.17, 0.25, 0.33, 0.5) compounds are prepared by a simple combustion method. The Rietvelt analysis shows that these compounds could be classified as having the α-NaFeO₂ structure. The initial charge-discharge and irreversible capacity increases with the decrease of x in Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂. Indeed, Li[Ni_0.50Mn_0.50]O₂ compound shows relatively low initial discharge capacity of 200 mAh/g and large capacity loss during cycling, with Li[Ni_0.17Li_0.22Mn_0.61]O₂ and Li[Ni_0.25Li_0.17Mn₀.₅₈]O₂ compounds exhibit high initial discharge capacity over 245 mAh/g and stable cycle performance in the voltage range of 4.8 -2.0 V. On the other hand, XANES analysis shows that the oxidation state of Ni ion reversibly changes between Ni²⁺ and about Ni³⁺, while the oxidation state of Mn ion sustains Mn⁴⁺ during charge-discharge process. This result does not agree with the previously reported ‘electrochemistry model’ of Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂, in which Ni ion changes between Ni²⁺ and NI⁴⁺. Based on these results, we modified oxidation-state change of Mn and Ni ion during charge-discharge process.     

Optical absorption spectrum of Ni2+ ion doped in synthetic lecontite

The optical absorption spectrum of Ni²⁺ ion doped in lecontite (sodium ammonium sulphate dihydrate) single crystal has been studied at room and liquid air temperatures. All the bands could be assigned assumingO _h symmetry for the Ni²⁺ ion in the crystal. The splitting of³ T _1g (F) band at liquid air temperature has been attributed to spin-orbit interaction. The crystal field and spin-orbit parameters derived areD _q=1000 cm^?1;B=740 cm^?1;C/B=4.27 and ζ=600 cm^?1. All the bands observed show a blue shift when the crystal was cooled to liquid air temperature.     

The optical gap of NiO

The nature of the optical absorption gap in NiO at 4.0 eV is investigated. It is found that this gap is due to a band to band transition, where an electron is taken out of the valence band and placed into the conduction band. The optical gap of 6.0 eV found in NiMgO is of a nature, where an electron is taken out of the oxygen 2p band and placed into the first affinity level of the Ni²⁺ ion (3d ⁸L»3d ⁹L^–1). The impurity band created in Ni_1–x Li _x O by the Li ions is found 2.3 eV below the bottom of the conduction band in agreement with model predictions.     

Evidence for exciton binding at Ni impurity sites in ZnSe

A broad charge transfer band is observed in the photoluminescence excitation (PLE) spectrum of the 2.5 μ Ni²⁺ luminescence in ZnSe : Ni. This band lies above the highest energy d-d excitation bands and exhibits a ZPL at 1.8163 eV and LO(#38;0lambda;) phonon replicas at higher energy. In contrast, PLE spectra of Co²⁺ luminescence in ZnSe:Co contain only d-d excitation bands. The charge transfer band in ZnSe:Ni is interpreted as evidence for bound exciton formation at the Ni site. The recombination energy of this exciton is transferred efficiently to the excited d-band states of the Ni ion, leading to characteristic Ni²⁺d-d luminescence.     

Cation distribution in NixMg1−xFe2O4 studied by XPS and Mössbauer spectroscopy

Ni–Mg ferrites, synthesized by solid-state route, were analyzed by X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy to find the distribution of Mg²⁺ in tetrahedral (A) and octahedral (B) sites of the spinel. In Ni_xMg_1−xFe₂O₄, Ni²⁺ enters in B site resulting in a reduction in the availability of B sites for the distribution of Mg²⁺ and Fe³⁺ and thereby influences the distribution of Mg²⁺. It was observed through both techniques that higher percentage of Mg²⁺ occupied the tetrahedral sites of Ni rich spinel systems. Thermodynamic considerations showed that entropy played significant role in the distribution of Mg²⁺ in Ni_xMg_1−xFe₂O₄, in addition the difference in site preference energy between Fe³⁺ and Mg²⁺ was too small to cause any influence.