Electric field effects in EPR of the SrTiO3 :Cr5+ Jahn-Teller system

It is shown that a static electric field affects the intensity ratio of the EPR lines of Cr⁵⁺ (d₁) in SrTiO₃ at 77K. The experimental results can be accounted for quantitatively by considering for the ${}^2\text{T}{}_{\mathrm{2g}}$ electronic ground state of Cr⁵⁺ a strong Jahn-Teller interaction with localized ρ_g mode. The degeneracy of the resulting vibronic ground state is lifted appreciably by a macroscopic strain which, as a result of the large electrostrictive coupling in SrTiO₃, is induced at moderate electric field strengths.     

EPR of V2+ in SrCl2

The X-band EPR spectrum of SrCl₂:V has been measured at liquid nitrogen temperature. A signal associated with V²⁺ in a site of trigonal symmetry is observed. The EPR data have been explained using the spin hamiltonian $\textcolor{red}{}\text{= μ}{}_{\mathrm{\beta }}\text{H}\text{g}\text{?}\text{S}\text{+ D[S}{}^2{}_{\mathrm{z}}\text{?}\text{1}\text{3}\text{S(SH)] +}\text{S}\text{A}\text{?}\text{I}$, with D ? hv, g_∥ = 1.957 ± 0.004, g₆ = 1.954 ± 0.004, A_∥ = 230 ± 5 MHz, A₆ = 235 ± 5 MHz. This V²⁺ defect is similar to those previously reported in fluoride crystals with the fluorite structure.     

Tetravalent vanadium in CaF2 and SrF2 crystals: An EPR study

An EPR study of tetravalent vanadium centers created by room temperature X-irradiation in CaF₂ and SrF₂ is presented. The production efficiency of these centers is enhanced by previous annealing of the samples at 1000 K in air. The symmetry of V⁴⁺ ions is tetragonal and its EPR spectrum can be described by an axial spin Hamiltonian including a Zeeman and hyperfine term with $\text{S =}\text{1}\text{2}$ and $\text{I =}\text{7}\text{2}$ (corresponding to ⁵¹V nuclei). The following values for the spin Hamiltonian parameters are obtained g_∥ = 1.947 ± 0.002, g_⊥ = 1.935 ± 0.005, A_∥ = 500 ± 5 MHz, A_⊥ = 150 ± 10 MHz in the case of SrF₂ and g_∥ = 1.945 ± 0.002, A_∥ = 505 ± 5 MHz and A_⊥ < 200 MHz, in the case of CaF₂. A model for the center including an interstitial O^2? ion is tentatively proposed.     

Cr5+ in SrTiO3: An example of a static Jahn-Teller effect in A d1 system

The effect of uniaxial stress on the EPR spectrum of Cr⁵⁺ in SrTiO₃ has been studied. It is concluded that SrTiO₃:Cr⁵⁺ is a static Jahn-Teller system. The strain-coupling coefficient V₂ is found to be 2 × 10⁴ cm^?1. Our results show that in the absence of external stress the intensity ratio of the EPR lines, at temperatures below the cubic-to-tetragonal phase transition, is related to the macroscopic strain, present in SrTiO₃ at these temperatures.     

Interpretation of charge transfer bands in Fe doped SrTiO3 crystals

The electronic structures of SrTiO₃ crystals doped with Fe³⁺, Fe⁴⁺ and Fe⁵⁺ ions have been investigated using the Xα cluster approach. The ground-state eigenvalues show the lower Fe acceptor level, of t_2g↓ symmetry, localized inside the SrTiO₃ band gap, respectively at 2.8 eV $\text{(}\text{Fe}{}^{\mathrm{3+}}\text{, S =}\text{5}\text{2}\text{), 1.6}\text{eV}\text{(}\text{Fe}{}^{\mathrm{4+}}\text{, S = 1)}\text{or}\text{1.1}\text{eV}\text{(}\text{Fe}{}^{\mathrm{4+}}\text{, S = 0)}\text{and}\text{1.1}\text{eV}\text{(}\text{Fe}{}^{\mathrm{5+}}\text{, S =}\text{3}\text{2}\text{)}$ above the valence band edge. Other acceptor levels, with eg↓ and eg↑ symmetries, appear inside the gap when the Fe nominal ionicity increases.The theoretical Xα excitation energies of O 2p-Fe 3d transitions confirms the experimental interpretations of acceptor charge transfer bands for the optical absorption spectra of SrTiO₃:Fe⁴⁺ and SrTiO₃:Fe⁵⁺ crystals.The large optical excitation energies compared with the thermal transitions are partly due to the O 2p band width.     

A study of valency changes of vanadium ions in Al2O3 by γ irradiation

The effect of γ irradiation at 300 K on the concentrations of vanadium ions V³⁺, V⁴⁺ and V²⁺ in Al₂O₃ has been studied quantitatively, using three techniques: optical absorption (V³⁺), low temperature thermal conductivity measurements (V⁴⁺) and EPR (V²⁺). Several single crystals of Al₂O₃ doped with vanadium in a large range of concentration $\text{(2.8 × 10}{}^{18}\text{? 1.3 × 10}{}^{20}\text{at.}\text{cm}{}^3\text{)}$ have been measured. The evolution of the respective concentrations by γ irradiation as a function of the total vanadium content C is quite different in the two regions $\text{C< 1.2 × 10}{}^{19}\text{at.}\text{cm}{}^3$ and C larger than this value. A consistent analysis of the results has nevertheless been achieved, leading to the determination of the absolute concentrations of the three ions in the as-received and γ irradiated states for all samples with $\text{C<4.2 × 10}{}^{19}\text{at.}\text{cm}{}^3$ (room temperature annealing is observed above this value). The concentrations of V⁴⁺ and V²⁺ ions are always small, but V⁴⁺ ions are more stable: they are present in the as-received state at a level of 1% of the total concentration and a maximum value of $\text{/}\text{?}\text{2.3 × 10}{}^{18}\text{at.}\text{cm}{}^3$ is observed in the γ irradiated state; on the other hand there are less than 4.7 × 10¹⁵V²⁺ ions per cm³ in the as-received state and the maximum value is only $\text{4.2 × 10}{}^{17}\text{at.}\text{cm}{}^3$. Charge transfer between V ions only is not sufficient to explain the experimental results and other defects must be involved in the γ irradiation effect.     

Radiation-induced paramagnetic centres in glass-ceramics derived from the MgO-Al2O3-SiO2-TiO2 system

MgO-Al₂O₃-SiO₂-TiO₂ glass-ceramics have been γ-irradiated and examined by electron spin resonance. The spectra observed arise from two principle sites; Ti⁴⁺ ions radiochemically reduced to Ti³⁺ and holes trapped at the π-type orbitals of oxygen ions bridging between SiO₄ and AlO₄ units. The Ti³⁺ line, although very similar to its form in the parent glasses, is in general a composite of two distinct lineshapes, each of which is associated with one of the two major crystalline phases Cordierite or Enstatite. The hole centre has a characteristic hyperfine interaction similar to that of the Boron Oxygen Hole Centre of Borate glasses. The Hamiltonian parameters of this centre are $\text{S =}\text{1}\text{2}$, $\text{I =}\text{5}\text{2}$, g₁ = 2.0023, g₂ = 2.0148, g₃ = 2.035 and |A₁| = |A₂| = |A₃| = 8.41 × 10^-4cm^-1.     

Crystal growth,structure, electron paramagnetic resonance and magnetic properties of (NH4)2V3O8

A method to grow single crystals of ammonium vanadate (IV, V) (NH₄)₂V₃O₈ has been devised. The crystal structure is tetragonal P4bm; residual factor is R = 0.030. Cell parameters are $\text{a = 8.891 ± 0.004}\text{A}\text{?}$ and $\text{c = 5.582 + 0.002}\text{A}\text{?}$. The V⁵⁺ atom lies at the center of a triangular pyramide (VO₄ tetrahedron) while the V⁴⁺ atom is on A 4-fold rotation axis at the center of a square-based pyramide VO₅ whose symmetry point group is almost C_4v with the short V = O bond lying along the 4-fold axis parallel to the c edge of the tetragonal cell. Crystals are thin platlets with (001) cleavage planes. The platlets have very often a square or rectangular shape limited by {100} or {110} planes. Each single crystal was not large enough to record a good e.p.r. spectrum, but by sticking on the same quartz plate a score of them it was possible to gather enough crystals so to record correct spectra and by orienting the plate to obtain resonance lines separately for g_∥ = 1.9263 et g_τ = 1.9755. Measurements at 283 K on powder samples gave times for spin-spin relaxation T₂ = 0.4 × 10^?7s and for spin-lattice relaxation T₁ = 1.6 × 10^?7s. The magnetic structure is characterized by an exchange narrowing ω_e = 3 × 10¹⁰rad/s which corresponds to a transition temperature of about 0.5 K. Static susceptibility measurements at high magnetic field show a paramagnetic behaviour with an antiferromagnetic interaction which is interpreted in the magnetic space group P_2c4bm as the interaction between V⁴⁺ ions from consecutive planes parallel to (001).     

ESR and photoacoustic spectra of V2O5−MO2 (M = Se, Te, Ge) glasses

ESR spectra of V₂O₅?MO₂ (M = Ge, Se, Te) glasses are investigated in the range 298–498 K. The spectra at 298 K are characteristic of V⁴⁺ with the 3d¹ electron localized on a single ${}^{51}\text{V}\text{(I =}\text{7}\text{2}$) in the glass network. At higher temperature, the hyperfine structure progressively broadens, leading eventually to a broad, single ESR peak. These results are consistent with thermally-induced electron hopping from V⁴⁺ to V⁵⁺. Photoacoustic spectra of the glass at 298 K are characteristic of V⁴⁺ in a distorted octa environment. A correlation of ESR and PAS data suggests that covalency increased as M is charged from Ge through Te to Se.     

Non-cubic Mn2+-centers in BaF2

The EPR spectra of thermally treated BaF₂: Mn samples is reported. After thermal annealing at 900 K a trigonal Mn²⁺ center with g=2.000±0.005, |D|=2725±40MHz, |A|=265±10MHz, $\text{D}\text{A}\text{>0}$, is observed. Annealing at 1200 K produces an orthorhombic Mn²⁺ center with g=2.00±0.01, |D|=2430±40MHz, |E|=570±20MHz, |A|=265±10MHz, $\text{D}\text{A}\text{<0}$. The superhyperfine (SHF) structures due to interactions with the neighbouring fluorines indicates that the trigonal manganese interacts with four fluorines, three of them equivalent. The orthorhombic Mn²⁺ shows interaction with four equivalent fluorine nuclei.     

EPR and magnetic susceptibility studies of xCuO·(1−x)[2B2O3·Li2O] glasses

The results of EPR and magnetic susceptibility studies on xCuO·(1?x)? [2B₂O₃·Li₂O] glasses with $\text{0?x?30 mol \%}$, are reported. The modification of EPR spectra with the increasing of CuO content are explained supposing that these are the result of the superposition of two EPR signals, one showing the hyperfine structure typical for isolated Cu²⁺ ions and other consisting from a broad line centered at g ～ 2 typical for the clustered Cu²⁺ ions. The values of the EPR parameters prove that the coordination of isolated Cu²⁺ -complexes remains approximately the same and show that Cu²⁺ ions are situated in axially distorted octahedral vicinities. EPR measurements have shown that the Cu²⁺ ions are present mostly as the isolated species when $\text{x?5 mol \%}$. Beside the dipole-dipole coupling between Cu²⁺ ions, the magnetic measurements suggest that for x>10 mol % superexchange interactions appear, too. From Curie constant is established that in this glass system the copper ions are in Cu²⁺ and Cu⁺ valence states. Also, the amounts of the copper ions in bivalent state are determined.     

Observation of electronic Raman transitions from the transition metal ions Ti3+ and V4+ highly diluted in α-Al2O3

α-Al₂O₃ : Ti³⁺ (300 mass ppm concentration) at 10 K shows two electronic Raman transitions at 38 cm^-1 and 109 cm^-1. The E symmetry of the scattering matrix agrees with the selection rules for transitions between the ground state B_{$\text{3}\text{2}$} and the E_{$\text{1}\text{2}$} of the ground state manifold ²t_2g.α-Al₂o₃ : V⁴⁺ shows two very weak electronic Raman transitions at about 30 cm^-1 and 56 cm^-1 respectively.     

Absorption spectra of Ni2+ in (NH4)2Mg(SO4)2·6H2O and Co2+ in Na2Zn(SO4)2·4H2O

Optical absorption spectra of Ni²⁺ in (NH₄)₂Mg(SO₄)₂·6H₂O and Co²⁺ in Na₂Zn(SO₄)₂·4H₂O single crystals have been studied at room and liquid nitrogen temperatures. From the nature and position of the observed bands, a successful interpretation could be made assuming octahedral symmetry for both the ions in the crystals. The splitting observed for ${}^3\text{T}{}_{\mathrm{1g}}\text{(F)}$ band in Ni²⁺ and ${}^4\text{T}{}_{\mathrm{2g}}\text{(F)}$ band in Co²⁺ at liquid nitrogen temperature have been explained as due to spin-orbit interaction. The extra band observed at 16,325 cm^-1 in the case of Ni²⁺ at low temperature has been interpreted to be the superposition of vibrational mode of SO^2-₄ radical on ${}^3\text{T}{}_{\mathrm{1g}}\text{(F)}$ band. The observed band positions in both the crystals have been fitted with four parameters B, C, Dq and ζ.     

Decoupled magnetic subsystems in the random mixture FepCo1-p(C5H5NO)6(ClO4)2

Specific heat data on the random mixtures Fe_pCo_1-pL₆(ClO₄)₂, where L = C₅H₅NO, are presented. The Fe and Co magnetic atoms have competing anisotropies since the pure Fe and Co compounds are known to be good examples of the simple cubic, $\text{S =}\text{1}\text{2}$, Ising and XY magnet, respectively. The experimental data show the two magnetic subsystems in the mixtures to be almost completely decoupled, which is a consequence of the fact that the crystal field anisotropies of the Fe²⁺ and Co²⁺ ions, yielding g_∥ ? g_⊥ and g_⊥ ? g_∥, respectively, are very strong compared to the magnetic exchange interactions. Consequently the two magnetic subsystems experience one another as nonmagnetic impurities. A model is presented which explains these results, as well as those previously found for related random mixtures, in terms of two interpenetrating percolation clusters.     

The average structure of 5.10-Dihydro-5.10-Diethylphenazinium-Iodide (E2P·I1.6): A new linear chain Polyiodide compound

Upon oxidation of 5.10-dihydro-5.10-diethylphenazine (E₂P) with iodine golden-green lustrous crystals of a compound with stoichiometry E₂P.I_1.6 were isolated. The compound crystallizes in the tetragonal space group D₄² with $\text{a = 12.321(2)}\text{A}\text{?}$ and $\text{c = 5.330(2)}\text{A}\text{?}$. The E₂P and I form interpenetrating incommensurate sublattices along c, with an iodine repeat distance of 9.7 Å. Static susceptibility measurements at room temperature give χ_g = + 0.994 × 10^?6g^?1 × cm³. This corresponds to one unpaired electron spin per two formular units. Single-crystal EPR indicates that the paramagnetism is associated with weakly interacting E₂P⁺ cation radicals. The 300K-d.c. conductivity of 3×10^?2Ω^?1cm^?1 and activation energy of 0.17±0.02eV for single crystals is consequently associated with the polyiodide chains, and not with the E₂P⁺ cation radicals.     

EPR parameters and local lattice structure study of V ions in CdCl2 and CsMgCl3 crystals

The local lattice structure and EPR parameters (D, g_‖, g_⊥) have been studied systematically on the basis of the complete energy matrix for a d³ configuration ion in a trigonal ligand field. By simulating the calculated optical and EPR spectra data to the experimental results, the local distortion parameters (ΔR, Δθ) are determined for V²⁺ ions in CdCl₂ and CsMgCl₃ crystals, respectively. The results show that the local lattice structure of CdCl₂:V²⁺ system exhibits a compression distortion (ΔR=−0.0868 Å) while that of CsMgCl₃:V²⁺ system exists an elongation distortion (ΔR=0.0165 Å). The different distortion may be ascribed to the fact that the radius of V²⁺ ion is smaller than that of Cd²⁺ ion or larger than that of Mg²⁺ ion. Moreover, the relationships between EPR parameter D and local structure parameters (R, θ) as well as the orbital reduction factor k and g‐factors (g_‖, g_⊥) are discussed.     

Mössbauer and magnetic studies of BaVS3 and V5S8 doped with 57Fe

Iron can be easily introduced in BaVS₃ and V₅S₈. It is located at the vanadium sites and has been used as a probe to analyse by Mössbauer effect the magnetic properties of its surrounding matrix. The electronic state of iron in this matrix has also been studied. It was found that in BaVS₃, the iron is in a low spin Fe³⁺ configuration $\text{(S =}\text{1}\text{2}\text{)}$. In V₅S₈ at 4.2 K, the iron is in low spin Fe²⁺ configuration (S = 0). The rapid decrease of quadrupole splitting observed between 50 and 200 K is attributed to a thermally activated change in electronic structure. The high temperature configuration (above 200 K) seems to be neither pure low spin Fe³⁺ nor high spin Fe²⁺, but a mixture of configurations fluctuating at a rate which is faster than the characteristic time of Mössbauer measurements.     

Nuclear g-factor of the 172+ isomeric state in 63Cu

The nuclear g-factor of the 4498 keV $\text{17}\text{2}$⁺ isomeric state in ⁶³Cu was measured with the in-bearn perturbed angular distribution method, through the ⁶²Ni(α, p2nγ)⁶³Cu reaction, to be g_exp = 0.184 ± 0.012. This value is in good agreement with a semiempirical g-factor for the three-quasiparticle configuration [π2p_{$\text{3}\text{2}$}(v1f_{$\text{5}\text{2}$}, 1g_{$\text{9}\text{2}$})₇]_{$\text{17}\text{2}$⁺} calculated using the experimental single-particle g-factors of neighbouring nuclei. At the same time the internal magnetic field at the Cu nuclei in Ni metal was obtained to be B_int = ?46.6 ± 1.3 kG.     

Defect model and spin-Hamiltonian parameters for the tetragonal Nd center in the tetragonal phase of SrTiO3 crystal

The spin-Hamiltonian parameters (g factors g_∥, g_⊥ and hyperfine structure constants ¹⁴³A_∥, ¹⁴³A_⊥, ¹⁴⁵A_∥ and ¹⁴⁵A_⊥) of the tetragonal Nd³⁺ center in the low-temperature (T≈4.2 K) tetragonal phase of SrTiO₃ are calculated from a diagonalization (of energy matrix) method. In the method, the Zeeman and hyperfine interaction terms are attached to the conventional Hamiltonian and a 52×52 energy matrix concerning the ground term ⁴H_J (J=9/2, 11/2, 13/2, 15/2) is constructed. The Nd³⁺ center is attributed to Nd³⁺ occupying the 12-fold coordinated Sr²⁺ site in SrTiO₃. Differing from the defect model assumed in the previous paper that the tetragonal distortion of this Nd³⁺ center is due to the association of one interstitial oxygen ion at a nearest neighborhood of Nd³⁺ and the Nd³⁺ displacement Δz along C₄ axis, we suggest that it is due to the distortion of SrTiO₃ lattice in the tetragonal phase. The calculated g factors g_∥ and g_⊥ show good agreement with the experimental values, suggesting that our defect model of Nd³⁺ center in SrTiO₃ is reasonable. The experimental hyperfine structure constants were not reported and so our calculated results remain to be checked by EPR experiment.     

An O23− radiation defect in AlPO4 and GaPO4

Hole centers with C₂ site symmetry, very similar g-matrices and small hyperfine splittings due to two nuclei with $\text{I =}\text{1}\text{2}$ were detected and analyzed by EPR in synthetic single crystals of AlPO₄ and GaPO₄ after X-ray irradiation. A very similar center, but without any detectable hyperfine splitting, was reported in natural quartz. These centers are ascribed to O₂^3? ions with two equivalent ³¹P nuclei as the source of the hyperfine splitting in the phosphates. This hyperfine splitting is considerably smaller than for O? adjacent to phosphorous (i.e. PO₄^2? centers). The electronic structures of these defects are discussed and compared with those reported for the same centers in other compounds.