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.     

The analysis of spin Hamiltonian and crystal field tensors for Fe3+ in crystals of LiCaAlF6 and LiSrAlF6

In single crystals of LiCaAlF₆ and LiSrAlF₆ doped with Fe³⁺ the trigonal EPR spectra with multiplicityK _M = 1 were observed due to Fe³⁺ substituted for Al³⁺. The spin Hamiltonian parameters describing the fine structure and the superhyperfine structure were determined. It is argued that the tensorsB ₂ andB ₄ of the spin Hamiltonian for Fe³⁺ ions are essentially determined by the quadratic contributions of the crystal field at the substitution site. The signs and the relative values of the elements in the spin Hamiltonian tensorB ₄ of rankL = 4 for Fe³⁺ are determined by the irreducible tensor product [V ₄ ?V ₄]₂ of the crystal field tensorV ₄ of rankL = 4 at the substitution sites. The ratio between the invariant sum of the spin Hamiltonian tensorB ₄ for Fe³⁺ in oxygen octahedra [FeO₆] and that in fluorine octahedra [FeF₆] is directly proportional to the fourth power of the ratio between the effective charges of surrounding ions. The sign of the spin Hamiltonian parameterB ₂₀ corresponds to the sign of the element [V ₄ ?V ₄]₂₀ in the irreducible tensor product [V ₄ ?V ₄]₂ of rankL = 2.     

A Low Temperature (10 K) High-Frequency (208 GHz) EPR Study of the Non-Kramers Ion Mn3+ in a MnMo6Se8 Single Crystal

A high-frequency (208 GHz) electron paramagnetic resonance (EPR) study on Mn³⁺ (3d⁴, S = 2) ions embedded in a MnMo₆Se₈ single crystal has been performed at 10 K. The experimental spectra reveal the presence of only one set of EPR lines from Mn³⁺ ions, whose magnetic axes are oriented along the crystal axes. The spin-Hamiltonian parameters are evaluated by the method of least-squares, fitting all the observed line positions simultaneously, for the three orthogonal orientations of the external magnetic field. The symmetry of the spin Hamiltonian at the site of the Mn³⁺ ions has been deduced from the EPR spectra.     

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.     

Quadratic crystal field tensors and spin Hamiltonian tensors of Fe3+ in Li2Ge7O15 above and below the phase transition temperature

Dopant Fe³⁺ ions in tetrahedral and octahedral positions of Ge⁴⁺ in the crystal Li₂Ge₇O₁₅ were studied using EPR. Fe³⁺ substitutes for Ge⁴⁺ with a local charge compensation. The octahedral site and the tetrahedral sites significantly differ by the value of the invariant sumS(B ₄) of theB ₄ tensor of the spin Hamiltonian of Fe³⁺. The irreducible tensor products {V ₄ ? V ₄}₄ and {V ₄?V ₄}₂ theV ₄ tensor of the crystal field calculated using the point-charge model for octahedral and tetrahedral complexes provide the predominant contribution of the crystal field to theB ₄ andB ₂ tensors of the spin Hamiltonian of Fe³⁺, respectively. A comparison of the fourth-rank tensorsB ₄ of the spin Hamiltonian and {V ₄?V ₄}₄ of the crystal field determined at 300 K with those determined at 77 K supports the conclusion that the phase transition is accompanied by combined rotation of the [GeO₄] tetrahedra with the [Ge(1)O₆] octahedron almost unaltered. The spectrum lines are narrow and the variety of point defects in the vicinity of the paramagnetic impurity ions Fe³⁺, Cr³⁺ and Cu²⁺ is not detected. These facts are inconsistent with the statistically distributed model for the Li(2) atom. In specific cases at 300 K, when the wings of the two spectrum lines of theM→M+1 and theM+2 →M+3 transitions of Fe³⁺ ions belonging to one system of translationally equivalent positions overlap an extra line appears in the center between these lines. It is suggested that this effect is due to the soft phonon mode above the phase transition temperature.     

Wide-band EPR spectroscopy of YAlO3: Tm3+ single crystals

The YAlO₃: Tm³⁺ single crystal has been studied on a wide-band EPR spectrometer. The EPR spectra of Tm³⁺ ions in the frequency range of 90–160 GHz have been detected for the first time. It has been confirmed that thulium ions substitute the position of Y³⁺ in the crystal lattice. The detected spectra have been described with the use of a spin Hamiltonian with the effective spin S = 1/2. A comparative analysis of the orientation of the magnetic axes of the Tm³⁺ paramagnetic center with earlier data on other rare-earth ions has been performed.     

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.     

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.     

ESR Investigation of Gd3+ Ion Introduction into the Structure of Simple and Double Tungstates

In this paper the spectra of Gd³⁺ ions studied by electron spin resonance (ESR) in four types of tungstate crystals KY(WO₄)₂, CdWO₄, CsLa(WO₄)₂ and La₂(WO₄)₃ are discussed. The tungstate crystals were grown by the low-temperature gradient Czochralski technique with Gd₂O₃ addition in melt. The estimation of spin Hamiltonian (SH) parameters was realized using an original simulation program developed. The parameters of crystalline field in oxide crystals are comparable with Zeeman interaction so the solution of the Schr?dinger equation was realized numerically. We have estimated 36 SH parameters from ESR angular diagrams of Gd³⁺, including g _x , g _y , g _z , g _i (where i = 1, …, 6), B _m ⁿ , C _m ⁿ (where m = 2, …, 6, n = 0 (or 1 for C _m ⁿ ), …, m), and the Euler angles. The positions of Gd³⁺ ions in the crystal structures, the charge compensation and the influence of lattice parameters on the SH parameters B ₂ ⁰ and B ₂ ² are discussed in this paper as well. Authors' address: Nikolay V. Cherney, Institute of Inorganic Chemistry, Russian Academy of Sciences, Lavrentyev av. 3, Novosibirsk 630090, Russian Federation     

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.     

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.     

Effects of Gd<Superscript>3+</Superscript> doping on structural and dielectric properties of PZT (Zr:Ti = 52:48) piezoceramics

The purpose of this research is to study the effect of doping Gd into Pb(Zr_0.52Ti_0.48)O₃ ceramics prepared by solid state reaction. X-ray diffraction patterns show that all PGZT samples are of tetragonal structure and the highest doping should be no more than 2 mole % Gd at which the unreacted oxides start to appear. The electron spin resonance (ESR) spectra of PGZT's indicate that Gd³⁺ can enter both A site of the perovskite structure instead of only A site as widely believed. The ESR peaks resonance shift towards low fields as the concentration is higher, which is due to the change in crystal field experienced by Gd³⁺ ions. At x =0.001, 0.005 and 0.01 dopings, two sets of powder ESR signals arising from Gd³⁺ (4f⁷, spin 7/2) ions at A site. The first set shows some fine structure having strong absorption peaks centered at 76.26 mT (g = 8.550). The second is a seven-peak spectrum centered at 206.01 mT (g = 3.165), which belongs to the Gd³⁺ ions at B sites. Furthermore, the overlapped ESR strong absorption peaks from 309.17 mT to 314.49 mT (g = 2.2818-2.1087) belong to Gd³⁺ of unreacted Gd₂O₃. The local environments of Gd³⁺ ions were verified from the calculated ESR spectra using appropriate spin Hamiltonian parameter, i.e. gyromagnetic tensor g, zero-field splitting D and hyperfine tensor A.     

Magnetic anisotropy of antiferromagnet (CH3)4NMnCl3

The parameters of the electron paramagnetic resonance (EPR) spectra of S ion pairs in diamagnetic crystals are analyzed. A relation between the spin Hamiltonian constants is established for solitary ions and pairs for (CH₃)₄NCdCl₃: Mn²⁺ crystals. In contrast to solitary ions, an additional contribution (which is a linear function of the exchange field) to the “single-ion” spin Hamiltonian constants appears in the case of pairs. It is shown that anisotropic exchange mechanisms do not play a significant part in the formation of the axial constant of the spin Hamiltonian for this crystal. Some aspects of the method of studying “single-ion” anisotropy predicted by the two-ion model are developed with the help of an isostructural diamagnetic analog with impurity concentration of the paramagnetic ions of a magnetically concentrated substance sufficiently high for observing the EPR spectrum of the pairs. It is found that the microscopic quantities determined partially from the EPR spectra for pairs and solitary Mn²⁺ ions in (CH₃)₄NCdCl₃ are in accord with the experimental value of the effective field for the (CH₃)₄NMnCl₃ crystal anisotropy which can be described primarily by the dipole and “single-ion” mechanisms of the exchange origin.     

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.     

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.     

EPR and optical absorption studies of Cr ions in potassium sodium dl-tartrate tetrahydrate

EPR spectra of Cr³⁺ ions doped in potassium sodium dl-tartrate tetrahydrate single crystals are recorded at 77 K. The spin Hamiltonian and zero field parameters g, |D| and |E| are measured from the resonance lines obtained at various rotations of the magnetic field. The values obtained are: g_x=1.9257±0.0002, g_y=1.9720±0.0002, g_z=2.0102±0.0002, |D|=313±2 (×10⁻⁴) cm⁻¹ and |E|=101±2 (×10⁻⁴) cm⁻¹. From the results of EPR study, the site symmetry of Cr³⁺ ion in the crystal is discussed. The optical absorption at room temperature is also studied. From the observed band positions, the crystal field splitting parameter (D_q) and the Racah inter-electronic repulsion parameters (B and C) are evaluated. The bonding parameters are obtained by correlating optical and EPR data and the nature of bonding in the crystal is discussed.     

Electron paramagnetic resonance of Ce3+ ions in lead thiogallate single crystals

The results of electron paramagnetic resonance (EPR) studies of Ce³⁺ impurity ions in single crystals of lead thiogallate PbGa₂S₄ have been reported. The Ce³⁺ ions substitute for Pb²⁺ ions in the crystal lattice of PbGa₂S₄. A number of paramagnetic cerium centers in lead thiogallate have been observed. The spectra are described by the spin Hamiltonian of rhombic symmetry with the effective spin S = 1/2. The g factors of the main cerium centers have been determined. A large number of paramagnetic centers are due to both nonequivalent positions of lead and local charge compensation under the substitution Ce³⁺ ?? Pb²⁺.     

A Variable-Temperature X-Band EPR Study of the Gd<Superscript>3+</Superscript> Ion in a La<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> Crystal Characterized by Monoclinic Site Symmetry

Electron paramagnetic resonance (EPR) studies on a single crystal of diamagnetic compound La₂Si₂O₇, potentially a phosphorescent/luminescent/laser material, with the Gd³⁺ ion substituting for the La³⁺ ion, were carried out at X-band (9.61 GHz) over the 4–295 K temperature range. The asymmetry exhibited by the Gd³⁺ EPR line positions for the orientations of the external magnetic field about the magnetic Z- and Y-axes in the ZY-plane was ascribed to the existence of monoclinic site symmetry at the site of the Gd³⁺ ion, as confirmed by the significant values of the spin Hamiltonian parameters g _YZ , b ₂ ⁻¹, b ₄ ^−m (m = 1, 3), b ₆ ^−m (m = 1, 3, 5), estimated by fitting all EPR line positions observed at room temperature for the orientation of the magnetic field in the magnetic ZX- and ZY-planes using a rigorous least-squares fitting procedure. At 8 K measurements were only carried out for orientation of B in the magnetic ZX-plane, due to difficulty in orientation of the crystal inside the cryostat, enabling estimation of all spin Hamiltonian parameters b _n ^m except those characterized by negative m values and g _YZ . The absolute sign of the zero-field splitting parameter b ₂ ⁰ was determined to be negative from the relative intensities of the lines at 8 K. Authors' address: Sushil K. Misra, Physics Department, Concordia University, 1455 de Maisonneuve Boulevard West, Montreal, Quebec H3G 1M8, Canada     

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.     

Theoretical studies of the optical and EPR spectra for V in Y2O3 crystal

In this paper, we give an alternative suggestion that both the observed optical and electron paramagnetic resonance (EPR) spectra of Yttrium oxide (Y₂O₃):V³⁺ are attributed to V³⁺ ions at the S₆ site of Y₂O₃. This suggestion is different from the opinion in the previous paper that the optical and EPR spectra are attributed to V³⁺ ions at the C₂ and S₆ sites, respectively. From the suggestion, the optical band positions and spin-Hamiltonian parameters are calculated by diagonalizing the complete energy matrix for 3d² ions in trigonal symmetry. The results are in good agreement with the experimental values, suggesting that both the observed optical and EPR spectra in Y₂O₃:V³⁺ may be due to V³⁺ at S₆ site of Y₂O₃ crystal.