A Variable Temperature X- and W-Band EPR Study of Fe-Doped SiCN Ceramics Annealed at 1000, 1100, and 1285 °C: Dangling Bonds,Ferromagnetism and Superparamagnetism

Polymer-derived SiCN ceramics, annealed (also referred to as pyrolyzed) at 1000, 1100, and 1285 °C, and doped with Fe(III) acetylacetonate, are investigated by electron paramagnetic resonance (EPR) from 4 to 120 K at X-band (9.425 GHz). In addition, the SiCN ceramic, annealed at 1100 °C, was studied by EPR at 300 K at W-band (93.96 GHz). There was observed a significant increase in EPR linewidth due to dangling bonds (g = 2.001) below 20 K at X-band. The low-field X-band FMR line (g ≈ 12) indicated the presence of ferromagnetic Fe₅Si₃ crystallites. There were found two EPR lines due to carbon-related dangling bonds: (1) those present as defects on the surface of the free-carbon phase (as sp² carbon-related dangling bonds with g = 2.0011) and (2) those present within the bulk of carbon phase (as sp³ carbon-related dangling bonds with g = 2.0033). On the other hand, the intense low-field EPR signal observed at X-band was not observed at W-band. As well, there was observed splitting of the single broad EPR signal observed at g = 2.05 at X-band into two signals at W-band at g = 1.99 and g = 2.06, due to two different Fe-containing superparamagnetic nanocrystallites. Two new EPR signals, not observed at X-band, were observed at W-band, namely at g = 2.28 and g = 3.00, which are also due to g_∥ of these superparamagnetic nanocrystallites.     

EPR and Optical Absorption Study of Cu2+-Doped Diammonium Hexaaqua Magnesium Sulphate Single Crystals

Electron paramagnetic resonance (EPR) study of Cu²⁺ ions doped in diammonium hexaaqua magnesium sulphate single crystal over the temperature range of 4.2–320 K is reported. Copper enters the lattice substitutionally and is trapped at two magnetically equivalent sites. The spin Hamiltonian parameters are evaluated at 320, 300, 77, and 4.2 K. The angular variations of the resonance lines in three mutually perpendicular planes ab, bc* and c*a are used to determine principal g and A values. The observed spectra are fitted to a spin Hamiltonian of rhombic symmetry with parameters of Cu²⁺ at 77 and 4.2 K: g _xx  = 2.089, g _yy  = 2.112, g _zz  = 2.437 (±0.002) and A _xx  = 38, A _yy  = 14, A _zz  = 110 (±2) × 10^?4 cm^?1. The ground state wave function of Cu²⁺ ion in this lattice is determined. The g-factor anisotropy is calculated and compared with the experimental value. The optical absorption spectra of the crystal are also recorded at room temperature. With the help of assigned bands the crystal-field parameters (Dq, Ds and Dt) are evaluated. By correlating the optical and EPR data, the nature of bonding in the complex is discussed. The temperature dependence of the g values is explained to conclude the occurrence of both static and dynamic Jahn–Teller effects over the temperature range of investigation.     

EPR and Optical Absorption Study of Cr3+-Doped Dipotassium Tetrachloropalladate

X-band electron paramagnetic resonance (EPR) study of Cr³⁺-doped dipotassium tetrachloropalladate single crystal is done at liquid nitrogen temperature. EPR spectrum shows two sites. The spin-Hamiltonian parameters have been evaluated by employing hyperfine resonance lines observed in EPR spectra for different orientations of crystal in externally applied magnetic field. The values of spin-Hamiltonian and zero-field splitting (ZFS) parameters of Cr³⁺ ion-doped DTP for site I are: g _x  = 2.096 ± 0.002, g _y  = 2.167 ± 0.002, g _z  = 2.220 ± 0.002, D = (89 ± 2) × 10^?4 cm^?1, E = (16 ± 2) × 10^?4 cm^?1. EPR study indicates that Cr³⁺ ion enters the host lattice substitutionally replacing K⁺ ion and local site symmetry reduces to orthorhombic. Optical absorption spectra are recorded at room temperature. From the optical absorption study, the Racah parameters (B = 521 cm^?1, C = 2,861 cm^?1), cubic crystal field splitting parameter (Dq = 1,851 cm^?1) and nephelauxetic parameters (h = 2.06, k = 0.21) are determined. These parameters together with EPR data are used to discuss the nature of bonding in the crystal.     

Optical and EPR Properties of Mn- and Eu-Activated LaMgAl11O19 Phosphor

LaMgAl₁₁O₁₉ phosphor doped with Eu and Mn ions has been prepared by the urea combustion route. The as-prepared phosphor was studied using X-ray diffraction, electron paramagnetic resonance (EPR), diffuse reflectance and photoluminescence studies. The EPR spectra of LaMgAl₁₁O₁₉:Eu, Mn phosphor exhibit signals with the effective g values at g = 1.98, 4.29 and 7.23. The resonance signals at g = 1.98 and 4.29 were attributed to Mn²⁺ ions in tetrahedral and rhombic environment, respectively. The resonance signal at g = 7.23 was attributed to Eu²⁺ ions. The optical spectrum of this phosphor exhibits an intense band in the visible region and this band has been attributed to spin-allowed ⁵E_g → ⁵T_2g transition of Mn³⁺ ions. Upon excitation at 324 nm, the material displays emission in the blue, green and red spectral region.     

Variable Temperature Single Crystal EPR Studies of Cu(II) in Dipotassium Diaquabis(Malonato-<Emphasis Type="Italic">κ</Emphasis><Superscript>2</Superscript><Emphasis Type="Italic">O,O</Emphasis>′) Nickelate Dihydrate: An Example of Contaminated Ground State

Single crystal X-band electron paramagnetic resonance (EPR) studies on divalent copper ions embedded in dipotassium diaquabis(malonato-κ ² O,O′) nickelate dihydrate have been performed at 300, 123 and 77 K to understand the nature of Jahn–Teller distortion in the paramagnetic host lattice. The angular variation of the EPR spectra reveals the presence of two sites, with one site not showing hyperfine resolution even at 77 K. The spin-Hamiltonian parameters of this six-coordinated Cu(II) ion, evaluated from EPR spectra at various temperatures, are:

• 300 K: g ₁₁ = 2.125, g ₂₂ = 2.118, g ₃₃ = 2.290, no copper hyperfine resolution
• 123 K: g ₁₁ = 2.229, g ₂₂ = 2.113, g ₃₃ = 2.319 and A ₁₁ = 5.02, A ₂₂ = 3.82, A ₃₃ = 6.87 mT
• 77 K: g ₁₁ = 2.224, g ₂₂ = 2.114, g ₃₃ = 2.324 and A ₁₁ = 5.32, A ₂₂ = 3.90, A ₃₃ = 7.06 mT

respectively. The low value observed for A ₃₃ at 123 and 77 K has been explained by assuming a ground state \({\text{d}}_{{x^{2} - y^{2} }}\) wave function for Cu(II) ions, contaminated with the excited state \({\text{d}}_{{z^{2} }}\). From the temperature dependence of the EPR spectra, the Cu(II) ions can be considered as a static Jahn–Teller system, with contaminated ground state. The admixture coefficients and bonding parameters have also been calculated by combining EPR and optical data. The EPR spectrum of powder sample confirms single crystal data.

     

EPR Study of the Vanadium Ions in Mg2SiO4 Crystal

Vanadium-doped forsterite crystal has been studied with X-band electron paramagnetic resonance (EPR) spectroscopy. The sample was grown by the Czochralski technique in an argon atmosphere with 2 vol% of hydrogen. The EPR spectrum of the sample at T = 15 K is predominantly represented by the V⁴⁺ ion signals that possess a characteristic eight-line hyperfine structure and are observed close to g = 2. The observation of the two magnetically nonequivalent centers in the angular dependence in the (ab) crystal plane and one center in the (ac) and (bc) planes, combined with the published optical spectroscopy data, unambiguously show that the V⁴⁺ ions are located at the silicon lattice site. Principal values of the hyperfine A and g-tensor and magnetic axes orientations of the V⁴⁺ centers have been determined. The orientation disorder of the V⁴⁺ centers has been found around the crystalline c axis but not in the (ab) crystal plane. The angular variation of the hyperfine component linewidth is described best with a disorder range of ±3.0°.     

A 236 GHz Fe3+ EPR Study of Nanoparticles of the Ferromagnetic Room-Temperature Semiconductor Sn1−x Fe x O2 (x = 0.005)

High-frequency (236 GHz) electron paramagnetic resonance (EPR) studies of Fe³⁺ ions at 255 K are reported in a Sn_1?x Fe _x O₂ powder with x = 0.005, which is a ferromagnetic semiconductor at room temperature. The observed EPR spectrum can be simulated reasonably well as the overlap of spectra due to four magnetically inequivalent high-spin (HS) Fe³⁺ ions (S = 5/2). The spectrum intensity is calculated, using the overlap I(BL) + (I(HS1) + I(HS2) + I(HS3) + I(HS4)) × exp(?0.00001B), where B is the magnetic field intensity in Gauss, I represents the intensity of an EPR line (HS1, HS2, HS3, HS4), and BL stands for the baseline (the exponential factor, as found by fitting to the experimental spectrum, is related to the Boltzmann population distribution of energy levels at 255 K, which is the temperature of the sample in the spectrometer). These high-frequency EPR results are significantly different from those at X-band. The large values of the zero-field splitting parameter (D) observed here for the four centers at the high frequency of 236 GHz are beyond the capability of X-band, which can only record spectra of ions with much smaller D values than those reported here.     

Shifts of g Values in the Excited Triplet States of Metal Complexes Studied by Time-Resolved W-band EPR

A highly time-resolved high-frequency/high-field W-band electron paramagnetic resonance (EPR) (ν ~ 94 GHz) is a powerful technique to determine small g anisotropies of transient paramagnetic species. We applied this method to studies of the lowest excited triplet (T₁)³ ππ* states in metal complexes such as a platinum (Pt) diimine complex and metal (Zn and Mg) porphines in rigid glasses. From the analyses of time-resolved EPR spectra, g anisotropies were obtained as g _z  = 2.0048, g _x  = g _y  = 2.0035 for Pt(b-iq)(CN)₂ (b-iq = 3,3′bi-isoquinoline) and g _z  = 1.9968, g _x  = g _y  = 2.0022 for zinc tetraphenylporphine (ZnTPP). No measurable anisotropies were found for magnesium (Mg) TPP. The g values of the Pt complex are larger than g _e (=2.0023, g value of free electron) and that g _z of ZnTPP is smaller than g _e. These results were interpreted in terms of the nature of the perturbed states: the higher triplet ππ′* state mixes with T₁(ππ*) via spin–orbit coupling in ZnTPP. In contrast, the higher triplet dπ* state is involved in this coupling for the Pt complex. Thus, the nature of the perturbed state can be distinguished from the anisotropic g values of the T₁(ππ*) state.     

Electron Paramagnetic Resonance and Electron Spin Echo Studies of Co2+ Coordination by Nicotinamide Adenine Dinucleotide (NAD+) in Water Solution

Co²⁺ binding to the nicotinamide adenine dinucleotide (NAD⁺) molecule in water solution was studied by electron paramagnetic resonance (EPR) and electron spin echo at low temperatures. Cobalt is coordinated by NAD⁺ when the metal is in excess only, but even in such conditions, the Co/NAD⁺ complexes coexist with Co(H₂O)₆ complexes. EPR spin-Hamiltonian parameters of the Co/NAD⁺ complex at 6 K are g _z  = 2.01, g _x  = 2.38, g _y  = 3.06, A _z  = 94 × 10^?4 cm^?1, A _x  = 33 × 10^?4 cm^?1 and A _y  = 71 × 10^?4 cm^?1. They indicate the low-spin Co²⁺ configuration with S = 1/2. Electron spin echo envelope modulation spectroscopy with Fourier transform of the modulated spin echo decay shows a strong coordination by nitrogen atoms and excludes the coordination by phosphate and/or amide groups. Thus, Co²⁺ ion is coordinated in pseudo-tetrahedral geometry by four nitrogen atoms of adenine rings of two NAD⁺ molecules.     

EPR study of the Ag6S3O4 compound prepared by two methods

The temperature dependence of the EPR spectra of the recently discovered Ag₆S₃O₄ phase in the Ag-O-S system prepared by two methods, the known method of co-precipitation from aqueous solution and a new method depending on the interaction of Ag₂S and Ag₂SO₄ solid reagents, has been investigated. No EPR spectra were observed at room temperature, while at liquid helium temperature a number of EPR spectra have been recorded, which disappeared upon increasing temperature up to liquid nitrogen temperature. The sample obtained by the co-precipitation method revealed an intense, rich EPR spectrum that has been tentatively interpreted assuming the presence of at least two different Ag²⁺ ion complexes, one monomer resulting in an intense anisotropic, rhombic EPR powder pattern with g₁ = 1.93(1), g₂ = 2.025(3), g₃ = 2.094(5), hyperfine constants A₁ = = A₂ = 60(5) · 10^?4 cm^?1,A ₃ = 90(5) · 10^?4 cm^?1 and one dimer EPR pattern presumably involving a pair of Ag²⁺ ions with internuclear separation of 4.3 Å. However, the presence of larger clusters could not be excluded. On the other hand, the sample obtained by a solid state reaction method has given rise to two rather weak EPR lines centered at g_eff = 2.105(3) and g_eff = 4.213(3), respectively.     

EPR of Cu2+ Impurities in Cytosine Hydrochloride: A Case of Superhyperfine Structure

Electron paramagnetic resonance spectra of Cu²⁺ impurities in cytosine hydrochloride single crystals are observed at liquid nitrogen temperature. Two magnetically equivalent sites for Cu²⁺ have been observed. The parameters of ⁶³Cu obtained with the fitting of spectra to rhombic symmetry spin Hamiltonian are: g _x  = 2.047 ± 0.002, g _y  = 2.187 ± 0.002, g _z  = 2.390 ± 0.002, A _x  = (86 ± 3) × 10^?4 cm^?1, A _y  = (87 ± 3) × 10^?4 cm^?1, and A _z  = (138 ± 3) × 10^?4 cm^?1. The observed bands in optical spectra of the single crystal recorded at room temperature are assigned to various d–d and charge-transfer transitions. Using both EPR and optical data, the nature of bonding of metal ion with different ligands is discussed.     

EPR, Optical Absorption and Superposition Model Study of Fe3+-Doped Ammonium Dihydrogen Phosphate Single Crystals

X-band electron paramagnetic resonance (EPR) studies are carried out on Fe³⁺ ions doped in ammonium dihydrogen phosphate (ADP) single crystals at room temperature. The crystal field and spin Hamiltonian parameters are evaluated from the resonance lines obtained at different angular rotations. The obtained values of spin Hamiltonian and zero-field parameters of the Fe³⁺ ion in ADP are: g = 1.994 ± 0.002, |D| = (220 ± 5) × 10^?4 cm^?1 and a = (640 ± 5) × 10^?4 cm^?1. On the basis of EPR data, the site symmetry of the Fe³⁺ ion in the crystal is discussed. The Fe³⁺ ion enters the lattice substitutionally replacing the NH₄ ⁺ sites. The optical absorption of the crystal is also studied at room temperature in the wavelength range of 195–925 nm. The energy values of different orbital levels are calculated. The observed bands are assigned as transitions from the ⁶ A _1g (S) ground state to various excited quartet levels of the Fe³⁺ ion in a cubic crystalline field. From the observed band positions, Racah interelectronic repulsion parameters (B and C), cubic crystal field splitting parameter (D _q ) and Trees correction are calculated. There values are: B = 970, C = 1,923, D _q  = 1,380 cm^?1 and α = 90 cm^?1, respectively. On the basis of EPR and optical data, the nature of bonding in the crystal is discussed. The zero-field splitting (ZFS) parameters are also determined theoretically using B _kq parameters estimated from the superposition model. The values of ZFS parameters thus obtained are |D| = (213 ± 5) × 10^?4 cm^?1 and |E| = (21 ± 5) × 10^?4 cm^?1.     

Electron spin relaxation studies of La@C82 in the solid state by EPR

The temperature dependence of EPR spectrum of La@C₈₂ in the powder of empty C₆₀ and C₇₀ mixed crystals was studied by EPR spectroscopy employing X- and Q-band microwave frequencies. The rigid limit spectra (at 4.2 K for the X-band and at 132 K for the Q-band) could be analyzed by static spectral simulation which yielded the EPR parameters,g _‖=2.0021,g _⊥=1.9970,^La A _⊥=7.8 MHz,^La A _‖～0 MHz and an isotropic¹³C coupling value of about 3 MHz. For higher temperatures an appreciable motional averaging effect was observed and the spectra were analyzed by using dynamic spectral simulation based on the stochastic Liouville equation, where we assumed an isotropic rotational motion with the Brownian diffusion. The calculated spectra reproduced the dominant feature of the temperature dependence of the spectra almost satisfactorily for both the X-and Q-band frequencies with the appropriate rotational correlation times. The Arrhenius plots of the correlation time gave two activation energies of 0.9 kcal/mol and 2.9–3.8 kcal/mol for the temperatures below and above 200 K, respectively.     

EPR study of Fe3+ and Mn2+ in CeO2 and ThO2

Attempts were made to grow CeO₂ and ThO₂ single crystals doped with transition metal ions. Only Fe³⁺ and Mn²⁺ could be detected by the EPR technique. The EPR spectrum of Fe³⁺ in CeO₂ exhibits the well-known fine structure in cubic fields. The parameters areg=2.0044(1) anda=15.6(1)·10^?4 cm^?1. The hyperfine constantA for⁵⁷Fe in hexahedral coordination was found to be 8.9(1)·10^?4 cm^?1. The EPR spectrum of Mn²⁺ in CeO₂ reveals two cubic Mn²⁺ centers. The parameters for center 1 areg=1.9999(1) andA=86.9(1)·10^?4 cm^?1 and for center 2g=1.9984(1) andA=87.0(1)·10^?4 cm^?1. Heating the Mn doped CeO₂ samples in hydrogen, the Mn²⁺ centers transform from cubic into trigonal centers with approximate values ofg=1.9988(2),A=84.5(6)·10^?4 cm^?1 andD=203(1)·10^?4 cm^?1. The two observed Mn²⁺ centers in ThO₂ exhibita priori axial symmetry with approximate values ofg=2.0006(2),A=88.9(4)·10^?4 cm^?1 andD=33(3)·10^?4 cm^?1.     

The electronic properties of [CH(FeCl4)0.061]x

The temperature dependence of d.c. conductivity, thermopower and electron paramagnetic resonance (EPR) results on [CH(FeCl₄)_0.061]_x are reported. The d.c. conductivity and thermopower measurements indicate metallic charge transport along the polyacetylene chain interrupted by the interfibril contact resistances and also ‘dragged’ primarily by the dopant ions. The initial measurements of EPR show Dysonian lineshape with very broad linewidth (ΔH ≈ 600 G at room temperature). The temperature dependence of EPR absorption intensity implies that there exist localized magnetic moments. The observed g value (g ≈ 2.03) suggests the dopant anion is in a form of (FeCl₄)^-.     

EPR and Optical Absorption Study of VO2+-Doped Lithium Hydroxylammonium Sulphate

ABSTRACT

Single-crystal and powder EPR studies of VO²⁺-doped lithium hydroxylammonium sulphate (LiNH₃OHSO₄) were carried out at room temperature. The results indicate the presence of two magnetically inequivalent VO²⁺ sites. The VO²⁺ ion takes up a substitutional position in the host lattice. The angular variation of EPR spectra in three mutually perpendicular planes were used to determine the spin Hamiltonian parameters, and the values obtained were the following: For Site 1, g_x = 2.0249 ± 0.0002, g_y = 1.9698 ± 0.0002, g_z = 1.9552 ± 0.0002, A_x = (51 ± 2) × 10^?4 cm^?1, A_y = (93 ± 2) × 10^?4 cm^?1, and A_z = (165 ± 2) × 10^?4 cm^?1; and for Site 2, g_x = 2.0267 ± 0.0002, g_y = 1.9743 ± 0.0002, g_z = 1.9213 ± 0.0002, A_x = (40 ± 2) × 10^?4 cm^?1, A_y = (80 ± 2) × 10^?4 cm^?1, and A_z = (155 ± 2) × 10^?4 cm^?1. The optical absorption spectrum recorded at room temperature shows four bands. From the optical and EPR data, various molecular coefficients are evaluated, and the nature of bonding in the crystal is discussed.     

Electron paramagnetic resonance and photoluminescence properties of α-Al2O3:Cr3+ phosphors

The red emitting Cr³⁺ activated α-Al₂O₃ powder phosphor has been prepared by easy combustion reactions from mixed metal nitrate reactants and urea with ignition temperatures of 500 °C. The as-synthesized powder was characterized by X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared techniques. The X-ray diffraction pattern reveals that the phosphor crystallized in the hexagonal α-Al₂O₃ phase directly from the combustion reaction. The EPR spectrum exhibits an intense resonance signal with effective g value at g=3.33 along with a few weak resonance signals with effective g values at g=13.7, 2.34, 1.95, 1.49, and 1.26. The spin concentration (N) and its paramagnetic susceptibility (χ) have been evaluated. The excitation spectrum consists of two broad intense bands at 415 nm and 555 nm and are assigned to ⁴A_2g (F)→⁴T_1g (F) and ⁴A_2g (F)→⁴T_2g (F) transitions, respectively. The intense fluorescence peak around 691 nm is attributed to ²E _g →⁴A_2g transition of Cr³⁺ ion. By correlating EPR and optical data, the crystal field splitting parameter (Dq), Racah inter-electronic repulsion parameter (B) have been evaluated and discussed. The EPR and optical studies reveal that Cr³⁺ ions are occupying in Al³⁺ sites in octahedral coordination.     

The temperature dependence of the angular variation of,and the critical point exponent for the EPR linewidth in the two-dimensional canted antiferromagnetic (C2H5NH3)2MnBr4: Evidence for a structural phase transition

The angular variation of the EPR linewidths in single crystals of (C₂H₅NH₃)₂MnBr₄ has been measured as a function of temperature. The angular dependence is well characterized by δH(θ) = a + b(3 cos²θ ? 1) + c(3 cos²θ ? 1)². The temperature dependence of the expansion coefficients is reported, and the effect of critical point fluctuations near the Néel temperature as well as a linear temperature dependence at high temperature are observed. A sharp decrease in linewidths at 160°K is attributed to a structural phase transition. The Néel temperature is determined to be 46°K (± 1°) from linewidth measurements of a powder sample. The linewidths diverge exponentially near the Néel temperature with a critical point exponent of 1.5.     

Supertransferred hyperfine structure in EPR of V4+ ion in Ca3In2Ge3O12 garnet

Interaction of an unpaired electron of the tetrahedral V⁴⁺ ion in Ca₃In₂Ge₃O₁₂ garnet with the four nearest In nuclei gives rise to clearly resolved structure of EPR spectra. We report the observation and analysis of these spectra at three frequencies and at temperature 77 K. The temperature dependence between liquid helium and the room temperatures was also studied. Parameters of the spin Hamiltonian are g_∥=1·8735; g_⊥=1·9825; ∣ ¦A∥|=150·4×10^?4cm^?1; ¦ A_⊥¦ = 36·7 × 10^?4 cm^?1. The supertransferred hyperfine interaction was found to be isotropic, absolute value of the corresponding parameter a is 22·1 × 10^?4 cm^?1.     

EPR and optical absorption studies of Fe ions in sodium borophosphate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectral investigations have been carried out on Fe³⁺ ions doped sodium borophosphate glasses (NaH₂PO₄-B₂O₃-Fe₂O₃). The EPR spectra exhibit resonance signals with effective g values at g=2.02, g=4.2 and g=6.4. The resonance signal at g=4.2 is due to isolated Fe³⁺ ions in site with rhombic symmetry whereas the g=2.02 resonance is due to Fe³⁺ ions coupled by exchange interaction in a distorted octahedral environment. The EPR spectra at different temperatures (123-295 K) have also been studied. The intensity of the resonance signals decreases with increase in temperature whereas linewidth is found to be independent of temperature. The paramagnetic susceptibility (χ) was calculated from the EPR data at various temperatures and the Curie constant (C) and paramagnetic Curie temperature (θ_p) have been evaluated from the 1/χ versus T graph. The optical absorption spectrum exhibits bands characteristic of Fe³⁺ ions in octahedral symmetry. The crystal field parameter (Dq) and the Racah interelectronic repulsion parameters (B and C) have also been evaluated and discussed.