Temperature features of the EPR spectrum of GdMnO3: Single crystal and thin film GdMnO3/LaAlO3

Electron paramagnetic resonance (EPR) spectra of GdMnO₃ single crystal and GdMnO₃/LaAlO₃ thin film are measured at Q-band frequency in the temperature range of 4.2 to 300 K. The temperature dependence of parameters of the spin Hamiltonian corresponding to the monoclinic symmetry of such paramagnetic centers as Gd³⁺ ions in the GdMnO₃/LaAlO₃ film is determined.     

EPR spectra of a GdMnO3 thin film on a SrTiO3 substrate

Electron paramagnetic resonance (EPR) spectra of a GdMnO₃/SrTiO₃ thin film in the X band have been measured in the temperature interval from 200 to 450 K. Signals from two types of paramagnetic centers have been observed in the spectra. The first paramagnetic center is a subsystem of Gd³⁺ ions, in the EPR spectrum of which the fine structure lines are resolved below 350 K. The second paramagnetic center is a system of manganese and gadolinium ions, in the EPR spectrum of which an exchange-narrowed line is observed with the width ΔH several times less than the width ΔH of an exchange-narrowed line observed in the GdMnO₃ single crystal. Unusual magnetic properties are due to the mismatch of the lattice parameters of the GdMnO₃ thin film and the SrTiO₃ substrate.     

EPR of Gd3+ in single crystal colquiriite and analysis of the spin Hamiltonian tensorsB 4 andB 6

In single crystal colquiriite LiCaAlF₆ doped with Gd³⁺ ions two EPR spectra of the Gd³⁺ ions with the Laue site-symmetry groups C_i and C_3i were observed. The spectrum angular dependence for trigonal Gd³⁺ centre was investigated in detail and corresponding spin Hamiltonian parameters were fitted. From analysis of the spin Hamiltonian tensorsB ₄ andB ₆ it was established that Gd³⁺ with the Laue group C_3i substitutes at Ca²⁺ site with the excess charge compensation by an ion located along the threefold axis from this site. The transformation formulas for a sixth-rank irreducible Hermitian tensor under coordinate rotation are tabulated in an explicit form. By using the EPR data for Gd³⁺ substituted in a variety of host crystals, the fourth-rank and sixth-rank tensors of Gd³⁺ spin Hamiltonians were tabulated and correlated with structures of the coordination polyhedra at substitution sites. The results suppose a predominance of quadratic crystal field contributions into the spin Hamiltonian tensorB ₄ of Gd³⁺.     

The domain structure of ferroelastic BiVO4 crystal studied by Gd EPR

The domain structure of ferroelastic BiVO₄ single crystal has been investigated using the electron paramagnetic resonance (EPR) of the Gd³⁺ ions existing as an impurity in the crystal. Two sets of Gd³⁺ EPR signals were obtained in the crystallographic ca-plane. These two sets of signals originated not from the two kinds of substitutional sites but from the twin-domain structure in the host crystal. It is found that the BiVO₄ crystal investigated with Gd³⁺ EPR has the prominent (W-plane) domain wall. The domain structure is stable in contrast with a previous report by Baran et al. From the observed W-plane of the domain wall, it is suggested that a ferroelastic transition in BiVO₄ is 4/mmm F 2/m instead of 4/m F 2/m. The model of twinning mechanism improved in a previous report by Mn²⁺ EPR is confirmed by Gd³⁺ EPR.     

Electron paramagnetic resonance of Gd<Superscript>3+</Superscript> aqua complexes in vitrified aqueous solutions and Gd<Superscript>3+</Superscript> aqua ions adsorbed on the capillary surface

Electron paramagnetic resonance (EPR) spectra of Gd³⁺ aqua complexes are measured in dilute aqueous solutions of Gd(NO₃)₃ (C<0.2 M) at room temperature. A partial resolution of the fine structure observed in the spectra is characteristic of solid disordered systems and results in an increase in the effective width of the EPR line with a decrease in the Gd³⁺ concentration. This phenomenon is explained in terms of adsorption of Gd³⁺ aqua ions on the surface of the measuring capillaries. The fine structure is revealed in the EPR spectra of Gd(NO₃)₃ aqueous solutions, namely, the Gd(NO₃)₃ solutions vitrified at a temperature of 77 K (with an addition of 10–15 vol % glycerol) and Gd(NO₃)₃ solutions quasi-vitrified at 298 K (with an addition of 70–90 vol % glycerol). Analysis of the EPR spectra demonstrates that these solutions contain two types of aqua complexes with fine structure parameters D₁=180 G and D₂=580 G. Reasoning from a comparison with x-ray diffraction data, the fine structure parameters D₁ and D₂ are assigned to higher symmetric eight-coordinate and lower symmetric nine-coordinate Gd³⁺ aqua complexes, respectively.     

High-order EPR transitions of Gd3+ in Ce2Zn3(NO3)12·24H2O

The angular variation of the EPR spectrum of Gd³⁺ in Ce₂Zn₃(NO₃)₁₂·24H₂O has been studied. In addition to the allowed fine structure lines some weak low-field lines, identified as forbidden transitions (ΔM = ±2 , ± 3), have been observed.     

Temperature and pressure dependences of EPR spectra of Gd ion doped in the EuAl3(BO3)4 monocrystal

The ground state of Gd³⁺ ions substituting for trivalent europium in the EuAl₃(BO₃)₄ single crystal was studied by electron paramagnetic resonance (EPR) over the temperature range of 300-4.2 K and at pressures up to 9 kbar. The EPR spectra were analysed using the spin Hamiltonian of axial symmetry. The following parameters are reported: g=1.981±0.002, b₂⁰=280.18±0.12, b₄⁰=−12.95±0.08 and b₆⁰=0.61±0.12 (at Т=298 K). The distortions of the nearest environment of Gd³⁺ ion were analysed within the framework of the superposition model of crystal field.     

Electron paramagnetic resonance and luminescence of Gd3+ in the mixed fluorides Ca1-xLaxF2+x and Ca1

Abstract

In this paper we present an study on the EPR and photoluminiscence spectra of Gd³⁺ in the mixed crystals Ca_0.998-xR_xGd_0.002F_0.002+x(x≤0.35, R=La or Y). For x <0.1 the EPR spectrum shows a main signal due to cubic Gd³⁺ that is broadened as x increases. Different broadenings are found for La and Y samples For high x values the EPR spectrum is similar to that in glass matrices. The results are explained taking to account a random second rank crystal field due to the extra charges associated with R³⁺ and fluorine intersitials. The ⁶P→⁶S emission of Gd³⁺ also has been measured. The influence of R concentration on this luminiscence has been studied.     

EPR study of Gd3+ in a ferroelastic BiVO4 single crystal

Electron paramagnetic resonance of the Gd³⁺ ion in a ferroelastic BiVO₄ single crystal with a single domain, grown by the Czochralski method, has been investigated at room temperature using a Q-band spectrometer. The rotation patterns of the resonance fields measured in the crystallographic planes are analyzed using a monoclinic spin Hamiltonian. The principal Z-axis of the second-order zero-field splitting tensorD is found to be along the crystallographicb-axis. Spin Hamiltonian parameters together with the principal axes ofg andD tensors in the monoclinic plane show that the local site symmetry of Gd³⁺ ion in BiVO₄ crystal is monoclinic and that the Gd³⁺ ion substitutes for Bi³⁺ ion.     

EPR of Gd3+ - doped CeF3 single crystal: Linewidth variation with temperature

Detailed X-band EPR study of a Gd³⁺-doped CeF₃ single crystal has been made from 4.2 to 473 K, with particular attention to EPR linewidths. In general, it is found that there are four regions over which the log-log plot of the linewidth versus temperature is linear, implying separate power-law dependences of the linewidth. Gd³⁺ spin Hamiltonian parameters in CeF₃ have been estimated at various temperatures from the line positions. From the linewidth variation with temperature the Debye temperature has determined to be about 140 K.     

An EPR study of different Gd3+ centers in LiNbO3

Electron Paramagnetic Resonance (EPR) and Zero-(magnetic)-Field (paramagnetic) Resonance (ZFR) spectra have been observed for LiNbO₃:Gd. EPR spectra measured at 9 and 35 GHz consisted of at least seven independent patterns; all of predominantly axial symmetry and all ascribed to Gd³⁺. The two most intense patterns have been analyzed in detail, and while one was found to be axially symmetric the other, though predominantly axial, had a small rhombic crystal field term. The ZFR spectrum, measured between 1 and 18 GHz, consisted of at least two patterns of zero field transitions. Both EPR and ZFR spectra were found to be insensitive to variations in LiNbO₃ stoichiometry, c- of a-axis crystal growth direction, and various sample treatments.     

Pulsed High-Frequency EPR Investigation of Gadolinium-Doped PbZrO3 and PbTiO3

Using a resonator-free setup, pulsed high-frequency (240 GHz) electron paramagnetic resonance (EPR) experiments on gadolinium-doped PbTiO₃ and PbZrO₃ samples have been performed. It could be demonstrated that echo-detected EPR spectra can be recorded routinely from these materials. These compounds are highly absorptive at microwave frequencies, thus preventing the use of microwave resonators at very high frequencies. As echo-detected EPR allows us to record the EPR absorption directly, the effect of relative suppression of broad unstructured spectral components in conventional field-derivative EPR is avoided. The analysis of the high-frequency EPR spectra indicated that Gd³⁺ ions are additionally also positioned at highly distorted sites. This might indicate that charge compensation leads to the formation of closely correlated Gd³⁺-V _Pb′′-Gd³⁺ defects under high doping conditions in addition to Gd³⁺ inserted substitutionally at Pb²⁺ sites with undistorted oxygen and lead neighboring shells. For the orthorhombic low-temperature phase of PbZrO₃ two crystallographically inequivalent Pb²⁺ sites of equal abundance are present. The contribution of Gd³⁺ inserted substitutionally at these sites could be confirmed.     

Local structure and the electron paramagnetic resonance parameters for the Cr3+ ions at K+-vacancy site in Cr3+:KZnF3 laser crystal

The quantitative relationship between the electron paramagnetic resonance (EPR) parameters D,g_∥,g_⊥ and the local structure parameters of Cr³⁺ ion in KZnF₃ crystals is established. The local structure for Cr³⁺ paramagnetic center in KZnF₃:Cr³⁺ crystal has been determined from EPR parameters of Cr³⁺ ion. This work shows that the trigonal crystal field of Cr³⁺ ion in KZnF₃ crystals comes from following two origins: (1) the nearest-neighbor K⁺ vacancy caused by the charge compensation in the [1 1 1]-axis direction; and (2) the lattice distortions of the nearest-neighbor fluorine coordination caused by the K⁺ vacancy and the differences in mass, charge, and radius between Cr³⁺ ion and Zn²⁺ ion. The unified calculation of the EPR zero-field splitting and g factors, taking into account the K⁺ vacancy and the lattice distortions, has been carried out on the basis of the complete diagonalization procedure and the superposition crystal-field model, all calculation results are in excellent agreement with the experimental data. Although the main source of the trigonal crystal field comes from the K⁺ vacancy caused by the charge compensation, the contribution of the lattice distortion cannot be neglected.     

Pulsed EPR spectroscopy of Gd3+ centers in Ca1−x R x F2+x (R=La, Y) mixed crystals

A study of Gd³⁺ centers in Ca_1?x R _x F_2+x (R=La, Y) crystals using pulsed EPR spectroscopy is presented. The echo-induced EPR (ESE-EPR) spectrum shows, besides the signal of slightly perturbed cubic Gd³⁺ centers, a broad signal at g≈2 due to Gd³⁺ centers at low symmetry sites. To describe the effects of R³⁺ ions on the EPR Gd³⁺, a model, including cubic and linear R³⁺?2F _i ^? centers, is developed. Its predictions are compared with the experimental results. The composition dependence of the EPR signal due to slightly perturbed cubic Gd³⁺ centers in mixed Ca_1?x R _x F_2+x crystals is explained taking into account the different clustering tendency in La and Y crystals. Moreover, the formation of mixed clusters involving R³⁺ and Gd³⁺ ions is proposed for both series of samples. A greater clustering trend is found in the Y crystals than in the La ones. Gd³⁺ ions are found to be a “non innocent” paramagnetic probe for structural studies in these mixed crystals.     

Influence of photoexcitation on the EPR spectra of Mo5+ in Li2Zn2(MoO4)3: Ce3+,Cu2+ crystals annealed in a CO2 atmosphere

The influence of recovery annealing in a CO₂ atmosphere at 700°C on the properties of Li₂Zn₂(MoO₄)₃ crystals doped with cerium and copper ions has been studied. The EPR investigation of Li₂Zn₂(MoO₄)₃ crystals annealed in a CO₂ atmosphere has revealed that the annealing leads to the formation of oxygen vacancies in positions adjacent to the oxygen octahedron of lithium, M3, and the oxygen tetrahedron of molybdenum, Mo1. In this case, the charge state of molybdenum becomes Mo⁵⁺ and appears in the EPR spectra in the form of one magnetically nonequivalent position. The analysis of the angular dependence of the EPR spectrum of Mo⁵⁺ made it possible to calculate the spectral parameters g _∥ = 1.862, g _⊥ = 1.933, A _∥ = 71.8 G, and A _⊥ = 34.1 G. The cross relaxation on the hyperfine structure from the molybdenum isotope ⁹⁷Mo is found in the EPR spectra. The photoexcitation of Li₂Zn₂(MoO₄)₃ crystal doped with cerium ions leads to the saturation of the EPR spectrum of Mo⁵⁺ and to the formation of the hyperfine structure from one lithium ion with a hyperfine structure constant of 14 G. For Li₂Zn₂(MoO₄)₃ crystals doped with copper ions, a very weak EPR spectrum of Mo⁵⁺ is observed in the initial crystals. As a result of the photoexcitation, an increase in the intensity of this spectrum by an order of magnitude and manifestation of the EPR spectrum of Cu²⁺ ions take place. It is assumed that such a behavior of the EPR spectra of molybdenum ions in Li₂Zn₂(MoO₄)₃ crystals doped with cerium and copper ions under photoexcitation is caused by different positions of the energy levels of cerium and copper ions with respect to the energy level of the molybdenum ion.     

EPR, Optical Absorption and Superposition Model Studies of Fe3+-Doped Diammonium Hexaaqua Magnesium Sulfate: A Case of Hyperfine Structure

An electron paramagnetic resonance (EPR) study of Fe³⁺-doped diammonium hexaaqua magnesium sulphate single crystal is carried out at liquid nitrogen temperature. EPR spectrum shows two sites. The spin-Hamiltonian parameters are evaluated from angular variation of observed hyperfine lines. Fe³⁺ ion enters the host lattice substitutionally at site I, replacing Mg²⁺, whereas it enters interstitially at site II. The local site symmetry of Fe³⁺ ion within the host lattice is orthorhombic. An optical absorption study is performed at room temperature. Using the optical absorption spectrum the bands are assigned and the Racah parameters (B and C) and cubic crystal field splitting parameter Dq are determined. The nature of metal–ligand bonding in the crystal is determined using EPR and optical data. Crystal field parameters and zero-field splitting parameters (ZFSPs) are evaluated theoretically for both the sites using superposition model and microscopic spin Hamiltonian together with perturbation equations, respectively. The theoretically evaluated ZFSPs are in good agreement with the experimental values.     

Crystal field splitting of gadolinium in single crystals of metallic LaBi

Resolved fine structure has been observed in the ESR of Gd³⁺ in metallic LaBi single crystals. The parameter b₄=+63.7 Oe characterizes a very large cubic crystal field splitting. The g factor of g=2.002 is interpreted as evidence of s-f exchange interaction.     

EPR spectra of Fe centers in layered TlGaSe2 single crystal

A single-crystal TlGaSe₂ doped by paramagnetic Fe ions has been studied at room temperature by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe³⁺ ions was observed. The spectra were interpreted to correspond to the transitions among spin multiplet (S=5/2, L=0) of Fe³⁺ ion, which are splitted by the local ligand crystal field (CF) of orthorhombic symmetry. Four equivalent Fe³⁺ centers have been observed in the EPR spectra and the local symmetry of crystal field at the Fe³⁺ site and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of GaSe₄ tetrahedrons, and the rhombic distortion of the CF is caused by the Tl ions located in the trigonal cavities between the tetrahedral complexes.     

EPR and optical absorption studies on VO ions in KZnClSO4·3H2O single crystals—an observation of superhyperfine structure

EPR spectra of VO²⁺ ions doped in KZnClSO₄·3H₂O single crystals have been studied at different temperatures. The EPR spectrum shows a well-resolved hyperfine and superhyperfine structure patterns. The angular variation of EPR spectra reveals the presence of more than three magnetic complexes, which correspond to distinct sites of VO²⁺ ion. From the angular variation EPR data, the spin-Hamiltonian parameters are evaluated and discussed. The optical absorption spectrum studied at room temperature shows bands corresponding to C_4v symmetry. From the EPR and optical data, the molecular-orbital bonding coefficient ε² and β² are evaluated and discussed. The observed five-line superhyperfine structure has been attributed to four protons (with I=1/2) from the surrounding water molecules of one of the vanadyl sites.     

Estimation of T1 for Co2+ ions from temperature variation of the EPR linewidths for Gd3+ in Ce2Co3(NO3)12.24H2O single crystals

EPR of Gd³⁺ doped in Ce₂M″₃(NO₃)₁₂.24H₂O (M″ = Mg, Zn, Co) single crystals has been studied at various temperatures from room temperature to 77 K using ∼ 9.45 GHz EPR spectrometer. The observation of resolved Gd³⁺ spectra at room temperature in Ce₃Co₂(NO₃)₁₂.24H₂O has been interpreted in terms of a random modulation of the interaction between the Gd³⁺ and the Co²⁺ ions by the rapid spin-lattice relaxation of Co²⁺ ions. It is found that the effective spin-lattice relaxation time T₁ ∝ T⁻ⁿ where n = 1.85 (B∥z axis) and n = 1.75 (B⊥z axis) if 103 < T < 283 K.