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.     

Ferroelasticity in BiVO4

A transition in BiVO₄ crystals has been observed at 255°C. Optical and X-ray studies indicate that this is a ferroelastic transition of the type 4/m F2/m.     

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.     

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 vanadyl impurity in zinc potassium phosphate hexahydrate single crystal

Electron paramagnetic resonance (EPR) study of VO²⁺ doped zinc potassium phosphate hexahydrate single crystal is carried out. The angular variation of the spectra is studied in the three crystallographic planes. The principal value of spin Hamiltonian parameters g and A and the direction cosines which principal axes make with the crystallographic axes are determined. The observed values are site I: g_∥=1.9664±0.0002, g_⊥=1.9973±0.0002, A_∥=150±2×10⁻⁴, A_⊥=60±2×10⁻⁴ cm⁻¹; site II: g_∥=1.9276±0.0002, g_⊥=1.9921±0.0002, A_∥=155±2×10⁻⁴ and A_⊥=62±2×10⁻⁴ cm⁻¹. By comparison of direction cosines of g from EPR with the direction cosines of different bonds obtained from crystal structure data it is ascertained that the VO²⁺ ion occupies Zn²⁺ substitutional sites. The optical absorption study of the crystal at room temperature is also carried out. The bands observed in the optical absorption spectrum are attributed to d-d transitions. The EPR results together with the optical data are employed to estimate the molecular orbital (MO) coefficients. These MO coefficients (also called bonding coefficients) are further used to discuss the nature of bonding of VO²⁺ ion with different ligands in the crystal.     

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     

Twinned EPR spectra of Fe centers in ternary layered TlGaS2 crystal

TlGaS₂ single crystal doped by paramagnetic Fe³⁺ ions has been studied by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe³⁺ ions was observed. The spectra reveal a nearly orthorhombic symmetry of the crystal field (CF) on the Fe³⁺ ions. Two groups each consisting of four equivalent Fe³⁺ centers were observed in the EPR spectra. The local symmetry of the crystal field on the Fe³⁺ centers and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of the GaS₄ tetrahedrons. The rhombic distortion of the sulfur ligand CF is attributed to the effect of Tl ions located in the trigonal cavities between the tetrahedral complexes. The observed twinning of the resonance lines indicates a presence of two non-equivalent positions of Tl ions that confirms their zigzag alignment in the TlGaS₂ crystal structure.     

Temperature dependence of the EPR lines in weakly doped LiNbO3:Yb—possible evidence of Yb ion pairs formation

The electron paramagnetic resonance (EPR) studies of LiNbO₃ single crystal doped with 1 wt% of Yb³⁺ are reported. To put the EPR results in perspective, a brief discussion of optical absorption spectroscopy investigations of LiNbO₃:Yb³⁺ is provided. The temperature behavior of the EPR lines intensity and linewidth for LiNbO₃:Yb³⁺ reveals antiferromagnetic coupling between Yb³⁺ ions. The deconvolution of the EPR lines indicates that EPR signals arise from both the isolated Yb³⁺ ions as well as the Yb³⁺-Yb³⁺ ion pairs; the latter signals dominate. Based on this indication, EPR spectra are interpreted using a spin Hamiltonian for the Yb³⁺ dissimilar ion pairs. The negative sign of the isotropic parameter J confirms the existence of the antiferromagnetic interactions within Yb³⁺-Yb³⁺ pairs. The value of J obtained based on the proposed pair model, assuming the dipole-dipole interactions, is used to identify the positions of the Yb³⁺-Yb³⁺ pairs in the unit cell. Our results suggest the ^evenYb³⁺-^evenYb³⁺ pairs are located at the neighboring Li⁺ and Nb⁵⁺ positions, whereas the pair axis is not parallel to the optical c-axis. Some alternative explanations of the observed EPR spectra are also considered.     

EPR theoretical study of local lattice structure in ZnS:Cr system

By analyzing the EPR parameters a, D and F of Cr²⁺ ion located at tetrahedral site in ZnS, the local structure around Cr²⁺ in the crystal has been investigated on the basis of the complete energy matrix for a d⁴ configuration in a tetragonal ligand-field within a strong-field-representation. It is shown that there exists an expansion distortion in the local lattice structure. From EPR calculation, the distortion parameters ΔR=0.13 Å and Δθ=1.417° are determined.     

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⁺.     

Electron paramagnetic resonance studies of GdMnO3 single crystal and thin film deposited onto a LaAlO3 substrate

Electronic paramagnetic resonance (EPR) spectra of a GdMnO₃ single crystal and GdMnO₃/LaAlO₃ thin film have been measured at X- and Q-band frequencies in the temperature range from 4.2 to 300 K. It is found that the EPR spectrum of a GdMnO₃ single crystal consists of only one broad exchange-narrowed line. Unusual magnetism is observed at the interface between the GdMnO₃ thin film and LaAlO₃ substrate, where it is possible to see the fine structure of the EPR spectrum for a Gd³⁺ ion. The parameters characterizing the fine structure related to the Gd³⁺ ion in the GdMnO₃ film deposited onto the LaAlO₃ substrate are determined.     

EPR and optical absorption study of Cr-doped ammonium oxalate monohydrate single crystals

The electron paramagnetic resonance (EPR) study of the Cr³⁺-doped ammonium oxalate monohydrate (AOM) single crystal is done at room temperature. Two magnetically inequivalent sites for chromium are observed. The hyperfine structure for Cr⁵³ isotope is also obtained. The spin Hamiltonian parameters are evaluated as: D=(309±2)×10⁻⁴ cm⁻¹, E=(103±2)×10⁻⁴ cm⁻¹, g=1.9820±0.0002, A=(161±2)×10⁻⁴ cm⁻¹ for site I and D=(309±2)×10⁻⁴ cm⁻¹, E=(103±2)×10⁻⁴ cm⁻¹, g=1.9791±0.0002, A=(160±2)×10⁻⁴ cm⁻¹ for site II, respectively. On the basis of EPR data the site symmetry of Cr³⁺ doped single crystal is discussed. The optical absorption spectra are recorded in 195-925 nm wavelength range at room temperature. The energy values of different orbital levels are determined. On the basis of EPR and optical data, the nature of bonding in the crystal is discussed. The values of different parameters are B=803, C=3531, D_q=2208 cm⁻¹, h=0.59 and k=0.21, where B and C are Racah parameters, D_q is crystal field parameter and h and k are nephelauxetic parameters, respectively.     

Growth, EPR and optical absorption spectra of l-threonine single crystals doped with Cu ions

We investigated the crystal growth, electron paramagnetic resonance (EPR) and optical absorption spectra of l-threonine doped with Cu²⁺. The quality, size and habit of the single crystals grown from aqueous solution by the slow solvent evaporation and by the cooling methods vary when the impurities are introduced during the growth process. The variations with the magnetic field orientation of the EPR spectra of single-crystal samples at room temperature and 9.77 GHz in three crystal planes (ab, bc and ac) show the presence of copper impurities in four symmetry-related sites of the unit cell. These spectra display well resolved hyperfine couplings of the of Cu²⁺ with the I_Cu= of the copper nuclei. Additional hyperfine splittings, well-resolved only for specific orientations of the magnetic field, indicate that the copper impurity ions in the interstitial sites have two N ligands with similar hyperfine couplings. The principal values of the g and A_Cu tensors calculated from the EPR data are g₁=2.051(1), g₂=2.062(2), g₃=2.260(2), A_Cu,1=16.9(5)×10⁻⁴ cm⁻¹, A_Cu,2=21.8(6)×10⁻⁴ cm⁻¹, A_Cu,3=180.0(5)×10⁻⁴ cm⁻¹. The principal directions corresponding to g₃ and to A_Cu,3 are coincident within the experimental errors, reflecting the orientation of the bonding planes of the copper ions in the crystal. The values of the crystal field energies are evaluated from the optical absorption spectrum, and the crystal field and bonding parameters of the Cu impurities in the crystal are calculated and analyzed. The EPR and optical absorption results are discussed in terms of the crystal structure of l-threonine and the electronic structure of the Cu²⁺ ions, and compared with data reported for other systems. The effects of the impurities in the growth and habit of the crystals are also discussed.     

Synthesis and characterization of novel red emitting nanocrystal Gd6WO12:Eu phosphors

Novel nanosized Gd₆WO₁₂:Eu³⁺ phosphors were synthesized via a co-precipitation reaction. The crystal structure and morphology of the phosphors were characterized by using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). It was found that the resultant powders show a regular and sphere-like shape with average particle size of 60 nm. Intrinsic red emission originating from Eu³⁺ was observed while excited at the W⁶⁺→O²⁻ and Eu³⁺→O²⁻ charge transfer bands or f-f absorption bands. The color coordinates of the phosphors were calculated to be x=0.625, y=0.375. The concentration dependence of the luminescence was studied, and optimum doping concentration for obtaining maximum emitting intensity was confirmed to be around 12 mol%. It was also found that the electric dipole-dipole interaction plays an important role for quenching luminescence of Eu³⁺.     

Thin <Emphasis Type="Italic">W</Emphasis>′ and <Emphasis Type="Italic">W</Emphasis> domain walls in ferroelastic Pb<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

A model of ferroelastic domain walls consisting of matching interlayers of crystal lattices is proposed. The dependences of the parameters of the interlayers and of the parameters of the equations for W′ and W domain walls on the crystal lattice parameters of the ferroelastic phase in Pb₃(PO₄)₂ are determined. The problem concerning the number of possible orientational states and their interaction in a polydomain crystal is considered.     

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.     

Optical spectra and luminescence dynamics of the Dy-doped Gd2SiO5 single crystal

Spectroscopic properties of the Dy³⁺:Gd₂SiO₅ (GSO) single crystal were investigated. The polarized absorption and unpolarized emission spectra were measured at temperature ranging from 10 K to 300 K. Experimental oscillator strengths were determined from room temperature polarized absorption spectra and phenomenological intensity parameters Ω _t were calculated by using the standard Judd–Ofelt theory. Low-temperature measurements were used to determine the energy level structure of two nonequivalent Dy³⁺ sites in the GSO crystalline host. Analysis of spectra and decay curves of the ⁴F_9/2 emission revealed that Dy³⁺ ions entering nine-coordinated sites with C _3v symmetry and Dy³⁺ ions entering the seven-coordinated sites with C _s symmetry form two distinct, well-isolated subsystems weakly coupled by the spectral energy migration process. In addition to dissimilar crystal field splitting of multiplets, the two subsystems differ significantly in the efficiency of excitation energy transfer between dysprosium ions, thereby showing dissimilar self-quenching of the ⁴F_9/2 emission. Besides, only one of the two Dy³⁺ subsystems is coupled to Gd³⁺ ions by nonradiative Gd³⁺–Dy³⁺ energy transfer process. Laser potential related to the ⁴F_9/2→⁶H_13/2 yellow luminescence of dysprosium ions was assessed based on evaluation of the emission cross-section values. It was concluded that the Dy:Gd₂SiO₅ (Dy:GSO) is a promising material for the visible laser operation.     

EPR and optical absorption study of Cr-doped tetramethyl ammonium cadmium chloride single crystals

EPR study of Cr³⁺-doped tetramethyl cadmium chloride (TMCC) single crystals is carried out at room temperature. The crystal field and spin-Hamiltonian parameters are evaluated from the resonance line positions of different lines observed in the EPR spectra. The g and D parameter values are found to be g=1.9741±0.0002 and D=553±2×10⁻⁴ cm⁻¹, respectively. EPR data indicate that the site symmetry of Cr³⁺ ion in the crystal is distorted octahedron. Cr³⁺ ions enter the lattice substitutionally replacing Cd²⁺ sites and bind to the neighboring extra Cd vacancies necessary for charge compensation. The optical absorption spectra are measured in 195–925 nm wavelength range at room temperature. From optical study the energy values of different orbital levels are estimated. Further, the bonding parameters are obtained by correlating optical and EPR data and the nature of bonding in the crystal is discussed. The values of Racah parameters (B and C), crystal field parameter (Dq) and nephelauxetic parameters (h and k) are obtained to be B=722, C=2845, Dq=2043 cm⁻¹, h=1.015 and k=0.21.     

Short-range order changes induced by heat treatment in yttrium-aluminosilicate glasses

Short-range order and local atomic configuration in yttrium-aluminosilicate glasses doped with gadolinium were studied by infrared (IR) spectroscopy, ²⁷Al magic-angle-spinning nuclear magnetic resonance (MAS-NMR) and Gd³⁺ electron spin resonance (EPR) on as-prepared and heat-treated samples.A small amount of yttrium was replaced by gadolinium in the host glass because Y³⁺ and Gd³⁺ cations are quite similar and gadolinium ions can be used as structural sensor in electron paramagnetic resonance measurements. The results evidence weak changes in the structure of as-prepared glasses with respect to the coordination of aluminium atoms produced by gadolinium doping (0.2 and 0.5 mol%). New IR bands recorded from heat-treated samples are associated with stretching modes of hexacoordinated aluminium in AlO₆ octahedra. The effect of the heat treatment on aluminium environment is estimated by analysing the relative intensity of the component lines of simulated ²⁷Al MAS-NMR spectra. High-coordinated AlO_n species were identified in all samples. EPR results evidence the increase of the number of gadolinium sites with weak crystal field as effect of the structural relaxation.     

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.