EPR of Yb<Superscript>3+</Superscript> ions in a monoclinic KY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> single crystal

The electron paramagnetic resonance (EPR) of Yb³⁺ ions in a KY(WO₄)₂ single crystal was investigated at T=4.2 K and fixed frequency of 9.38 GHz. The resonance absorption observed on the lowest Kramers doublet represents the complex superposition of three spectra, corresponding to the ytterbium isotopes with different nuclear moments. The EPR spectrum is characterized by a strong anisotropy of the g-factors. The temperature dependence of the g-factors is shown to be caused by the strong spin-orbital and orbital-lattice coupling. The resonance lines broaden with increasing temperature due to the short spin-lattice relaxation times.     

Electron paramagnetic resonance of Ce<Superscript>3+</Superscript> and Nd<Superscript>3+</Superscript> impurity ions in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.13</Subscript>

The electron paramagnetic resonance (EPR) spectra of Ce³⁺ and Nd³⁺ impurity ions in unoriented powders of the YBa₂Cu₃O_6.13 compound are observed and interpreted for the first time. It is demonstrated that, upon long-term storage of the samples at room temperature, the EPR signals of these ions are masked by the spectral line (with the g factor of approximately 2) associated with the intrinsic magnetic centers due to the significant increase in its intensity.     

Superhyperfine Structure of the EPR Spectra of Nd<Superscript>3+</Superscript> Impurity Ions in Fluorite CaF<Subscript>2</Subscript>

EPR spectra of a CaF₂ single crystal that was grown from melt containing a small addition of NdF₃ were studied. Signals corresponding to tetragonal centers of Nd³⁺ ions and cubic centers of Er3+ and Yb3+ ions were found. Superhyperfine structure (SHFS) in the spectra of the Nd³⁺ ions was observed for the first time in this crystal; parameters of the superhyperfine interaction of the Nd³⁺ ions with the nearest nine fluorine ions were determined. The dependence of the resolution of the Nd³⁺ EPR spectrum SHFS on the incident microwave power at the temperature of T ≈ 6 K was studied. Obtained results are discussed and compared with the literature data.     

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     

Luminescence thermometry using Gd<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript>:Eu<Superscript>3+</Superscript>

In this paper we study the possibility of using the synthesized nanopowder samples of Gd₂Zr₂O₇:Eu³⁺ for temperature measurements by analyzing the temperature effects on its photoluminescence. The nanopowder was prepared by solution combustion synthesis method. The photoluminescence spectra used for analysis of Gd₂Zr₂O₇:Eu³⁺ nano phosphor optical emission temperature dependence were acquired using continuous laser diode excitation at 405 nm. The temperature dependencies of line emission intensities of transitions from ⁵D₀ and ⁵D₁ energy levels to the ground state were analyzed. Based on this analysis we use the two lines intensity ratio method for temperature sensing. Our results show that the synthesized material can be efficiently used as thermographic phosphor up to 650 K.     

EPR and optical absorption studies of Cu<Superscript>2+</Superscript> ions in [ZnPd(CN)<Subscript>4</Subscript>(C<Subscript>4</Subscript>H<Subscript>12</Subscript>N<Subscript>2</Subscript>O<Subscript>2</Subscript>)] single crystals

A Cu²⁺-doped single crystal of catena-trans-bis(N-(2-hydroxyethyl)-ethylenediamine) zinc(II)-tetra-m-cyanopaladate(II) [ZnPd(CN)₄(C₄H₁₂N₂O₂)] complex has been investigated by electron paramagnetic resonance (EPR) technique at room temperature. EPR spectra indicate that Cu²⁺ ions substitute for magnetically equivalent Zn²⁺ ions and form octahedral complexes in [ZnPd(CN)₄(C₄H₁₂N₂O₂)] hosts. The crystal field affecting the Cu²⁺ ion is nearly axial. The optical absorption studies show two bands at 322 nm (30864 cm⁻¹) and 634 nm (15337 cm⁻¹) which confirm the axial symmetry. The spin Hamiltonian parameters and the relevant wave function are determined.     

High-frequency EPR of Tb<Superscript>3+</Superscript>-doped KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> crystal

High-frequency electron paramagnetic resonance (EPR) spectra of the KPb₂Cl₅:Tb³⁺ crystal have been investigated. Three types of spectra were observed in the frequency range of 74–200 GHz. The most intensive spectrum with the resolved hyperfine structure corresponded to transitions between sublevels of the¹⁵⁹Tb³⁺ ground quasi-doublet with the zero-field splitting (ZFS) close to 48 GHz. Experimental results were analyzed by the exchange charge model of the crystal field affecting terbium ions in low-symmetry Pb²⁺ positions with the chlorine sevenfold coordination and the charge compensating vacancy in the nearest potassium site. The calculated values ofg-factors and ZFS were in agreement with the experimental data. The nature of a broad EPR line with ZFS of about 180 GHz and of additional weak EPR lines observed as satellites of the main Tb³⁺ lines was discussed.     

An Electron Paramagnetic Resonance Study of the Orbital Ordering Compound MgTi<Subscript>2</Subscript>O<Subscript>4</Subscript>

We report the electron paramagnetic resonance (EPR) studies of MgTi₂O₄ in the 300–140 K range. Above the transition temperature T _t (~258 K), the EPR results indicate that MgTi₂O₄ is paramagnetic. The parameters of the EPR spectra show an anomalous change at T _t. The clear EPR lines can be observed in temperature between T _t and 220 K. Besides that the EPR intensity, g value, and EPR linewidth increase with decreasing temperature; in temperature range below 220 K, no clear EPR line can be detected. The EPR spectra results demonstrate that magnetic spin-singlet state and the orbital density wave of MgTi₂O₄ system are formed gradually with decreasing temperature at low temperature range.     

Li<Superscript>+</Superscript> ion conductivity and transport properties of LiYP<Subscript>2</Subscript>O<Subscript>7</Subscript> compound

A lithium yttrium diphosphate LiYP₂O₇ was prepared by a solid-state reaction method. Rietveld refinement of the X-ray diffraction pattern suggests the formation of the single phase desired compound with monoclinic structure at room temperature. The infrared and Raman spectrum of this compound was interpreted on the basis of P₂O₇ ^4? vibrations. The AC conductivity was measured in the frequency range from 100 to 10⁶ Hz and temperatures between 473 and 673 K using impedance spectroscopy technique. The obtained results were analyzed by fitting the experimental data to the equivalent circuit model. The Cole–Cole diagram determined complex impedance for different temperatures. The angular frequency dependence of the AC conductivity is found to obey Jonscher’s relation. The temperature dependence of σ _AC could be described in terms of Arrhenius relation with two activation energies, 0.87 eV in region I and 1.36 eV in region II. The study of temperature variation of the exponent(s) reveals two conduction models: the AC conduction dependence upon temperature is governed by the correlated barrier hopping (CBH) model in region I (T < 540 K) and non-overlapping small polaron tunneling (NSPT) model in region II (T > 540 K). The near value of activation energies obtained from the equivalent circuit and DC conductivity confirms that the transport is through ion hopping mechanism dominated by the motion of the Li⁺ ion in the structure of the investigated material.     

EPR study of copper ions in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals

The EPR spectra of Cu²⁺ ions (² D _5/2) located at two structurally nonequivalent positions Cu1 and Cu2 in crystals of lithium heptagermanate Li₂Ge₇O₁₅ are recorded. The angular dependences of the EPR spectrum are measured in the paraelectric phase of the Li₂Ge₇O₁₅ compound (T = 300 K). The components of the g factor and the hyperfine interaction tensor A are determined, and the orientation of the magnetic axes with respect to the crystallographic basis is established. The EPR spectra are recorded in the temperature range in the vicinity of the temperature T _C = 283 K of the transition from the paraelectric phase to the ferroelectric phase. The position symmetry of the Cu1 and Cu2 centers is determined at temperatures above and below the phase transition temperature T _C . The localization of paramagnetic centers in the structure is discussed, An analysis of the results obtained demonstrates that the Cu1 and Cu2 centers in the Li₂Ge₇O₁₅ crystal lattice replace lithium ions located at two structurally nonequivalent positions with the symmetries described at temperatures above T _C by the triclinic C _i and monoclinic C ₂ point groups, respectively.     

Frequency measurements in the b <Superscript>3</Superscript>Π(0<Subscript>u</Subscript><Superscript>+</Superscript>) - X <Superscript>1</Superscript>Σ<Subscript>g</Subscript><Superscript>+</Superscript> system of K<Subscript>2</Subscript>

Absolute transition frequencies of the b ³Π(0_u ⁺) - X ¹Σ_g ⁺ system of K₂ were measured in a molecular beam with Lamb dip absorption spectroscopy applying a frequency comb from a femtosecond pulsed laser. Both, K atoms and K₂ molecules are present in the beam and are expected to interact by collisions. The atoms can be deflected optically out of the beam, and thus the collision rate between K atoms and K₂ molecules is changed by about an order of magnitude. The molecular transition frequencies for low collisional rate are compared with those for high one. Limits for the collisional frequency shift within the beam are determined.     

Superhyperfine Structure of EPR Spectra in LiLuF<Subscript>4</Subscript>:U<Superscript>3+</Superscript> and LiYF<Subscript>4</Subscript>:Yb<Superscript>3+</Superscript> Single Crystals

Electron paramagnetic resonance (EPR) spectra of doped paramagnetic crystals LiLuF₄:U³⁺ and LiYF₄:Yb³⁺ have been investigated at a frequency of about 9.42 GHz in the temperature range of 10–20 K. The U³⁺ ion spectrum is characterized by g-factors g _∥ = 1.228 and g _⊥ = 2.516, and contains the hyperfine structure due to the ²³⁵U isotope with nuclear spin I = 7/2 and natural abundance of 0.71%. The observed hyperfine interaction constants are A _∥ = 81 G and A _⊥ = 83.8 G. Moreover, the spectrum reveals the well-resolved superhyperfine structure (SHFS) due to two groups of four fluorine ions forming the nearest surrounding of the U³⁺ ion. This SHFS contains up to nine components with the spacing between components being about 12.7 G. The SHFS is observed also in the EPR spectrum of the LiYF₄:Yb³⁺ crystal; up to 17 components with spacing of about 3.7 G may be traced. Some parameters of the effective Hamiltonian of the SHF interaction are estimated, the contribution of covalent bonding of f-electrons with ligands into these parameters is discussed. Authors' address: Igor N. Kurkin, Kazan State University, Kremlevskaya ulitsa 18, Kazan 420008, Russian Federation     

luminescence of BaGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> crystals activated by Eu<Superscript>2+</Superscript> and Ce<Superscript>3+</Superscript> ions

We have studied the effect of doping with Eu²⁺ and Ce³⁺ ions on the photoluminescence (PL) of BaGa₂Se₄ crystals in the temperature range 77–300 K. We have established that the broad bands with maxima at wavelengths 456 nm and 506 nm observed in the photoluminescence spectra of BaGa₂Se₄:Ce³⁺ crystals are due to intracenter transitions 5d → ²F_7/2 and 5d →²F_5/2 of the Ce³⁺ ions, while the broad photoluminescence band with maximum at 521 nm in the spectrum of BaGa₂Se₄:Eu²⁺ is associated with 4f⁶ 5d → 4f⁷ (⁸S_7/2) transitions of the Eu²⁺ ion. We show that in BaGa₂Se₄:Eu²⁺,Ce³⁺ crystals, excitation energy is transferred from the Ce³⁺ ions to the Eu²⁺ ions.     

EPR of Gd<Superscript>3+</Superscript> ion in mixed CeO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocrystals

This paper reports on the results of EPR studies of mixed CeO₂-Y₂O₃ crystals (including nanosized crystals) doped by gadolinium ions. It is revealed that the width of the line corresponding to the allowed transition 1/2 ↔ −1/2 between the Kramers-conjugated states |±1/2〉 of the Gd³⁺ ion decreases with a decrease in the powder size from macrosizes to nanosizes. The observed dependence can be due to the increase in the unit cell size during grinding of the samples.     

A novel white light-emitting diode (w-LED) fabricated with Sr<Subscript>6</Subscript>BP<Subscript>5</Subscript>O<Subscript>20</Subscript>:Eu<Superscript>2+</Superscript> phosphor

Sr₆BP₅O₂₀:Eu²⁺ phosphor was prepared by the solid-state reaction method under a weak reductive atmosphere and the photoluminescence properties were studied systematically. The bluish-green emission band of Sr₆BP₅O₂₀:Eu²⁺ phosphor is peaking at 475 nm, and the excitation bands are broad with peaks at about 290 and 365 nm with a shoulder around 390 nm, respectively. By combining with Ga(In)N-based near-ultraviolet LEDs, a bluish-green LED was fabricated based on the Sr₆BP₅O₂₀:Eu²⁺ phosphor, and a novel intense white LED was fabricated based on the bluish-green phosphor Sr₆BP₅O₂₀:Eu²⁺ and the red phosphor (Sr,Ca)₅(PO₄)₃Cl:Eu²⁺,Mn²⁺. When this two-phosphor white LED is operated under 20-mA forward-bias current at room temperature, the Commission Internationale de l’Eclairage(CIE) chromaticity coordinates (x,y), the correlated color temperature Tc, and the color rendering index Ra are calculated to be (0.3281,0.3071), 5687 K, and 87.3, respectively. The dependence of the bluish-green and two-phosphor white LEDs on different forward-bias currents from 5 mA to 50 mA shows a similar behavior. As the current increases, the relative intensity simultaneously increases. The CIE chromaticity coordinates (x,y) of the two-phosphor white LED tend to decrease. Consequently, the correlated color temperature Tc increases from 3800 K to 9400 K and the color rendering index Ra of the two-phosphor white LED increases from 83.4 to 91.8 simultaneously. PACS 07.60.-j; 42.70.-a; 71.55.Eq     

Intense White Luminescence from Combustion Synthesized Ca<Subscript>12</Subscript>Al<Subscript>14</Subscript>O<Subscript>33</Subscript>: Yb<Superscript>3+</Superscript>/Yb<Superscript>2+</Superscript> Single Phase Phosphor

The Ca₁₂Al₁₄O₃₃: Yb³⁺/Yb²⁺ single phase nano-phosphor has been synthesized through combustion route and its luminescence and lifetime studies have been carried out up to 20 K using 976 and 266 nm excitations. The samples heated in open atmosphere have shown the presence of Yb in Yb³⁺ and Yb²⁺ states. The 976 nm excitation results a cooperative upconversion emission at 486 nm due to the Yb³⁺ state and a broad band in the blue region and has been assigned to arise from the defect centers. The 266 nm excitation on the other hand results a broad emission band even from as-synthesized phosphor without doping of Yb, the width of which increases in presence of Yb due to the emission from Yb²⁺ ions formed in heated samples. The white emission covers almost whole visible region with bandwidth 190 nm. The ions in Yb²⁺ state has been found to increase with the increase in heating temperature up to 1,273 K. A back conversion of Yb²⁺ to Yb³⁺ has been observed for higher temperatures. Effect of boric and phosphoric acids as flux on the emission properties of Yb³⁺ and Yb²⁺ states have been examined and discussed. Quantum yield of emission has also been determined for different samples.     

Investigations of EPR parameters and impurity structure for Co<Superscript>2+</Superscript> in Ca(OH)<Subscript>2</Subscript> crystal

The electron paramagnetic resonance (EPR) parameters (g _‖ andg _⊥ factors and hyperfine structure constantsA _‖,A _⊥) for Co²⁺ in Ca(OH)₂ are studied from the second-order perturbation formulas on the basis of the cluster approach. In these formulas, the contributions to EPR parameters from the state interactions and covalency effects are considered and the parameters related to both effects are obtained from the optical spectra and impurity structure of the studied system. From the study, it is found that the β angle between the metal-ligand bond and the C₃ axis changes from 61° in a pure crystal to 53.68(26)° in the impurity center of a Co²⁺-doped Ca(OH)₂ crystal because of the impurity-induced local lattice relaxation. The reduction of the angle β in the impurity center is also supported by the result obtained by analyzing the EPR zero-field splitting for Mn²⁺ in the same Ca(OH)₂ crystal. The EPR parameters of Ca(OH)₂:Co²⁺ are also reasonably explained by considering the suitable local lattice relaxation.     

Migration of Mn<Superscript>2+</Superscript> excitations in Cd<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>Te crystal

The experimentally obtained intensity decay curves for the 2-eV intracenter luminescence band of Mn²⁺ ions in Cd_0.5Mn_0.5Te semiconductor solid solution at a temperature of 77 K have been simulated by the Monte Carlo method. The calculations show that the initial nonexponential behavior of the intensity decay curves at the band wings, as well as the time dependence of the band peak energy, are determined by the fast migration of excitations through the Mn²⁺ ion subsystem. There are more than 200 jumps per each emitted photon, and the migration rate increases by almost two orders of magnitude in comparison with the rate at 4 K. The analysis of the simulation results and the calculation based on the experimental data show the interaction between ions to be resonant. The estimate derived from the Anderson criterion suggests that the excited state is not delocalized. An increase in the migration rate with an increase in temperature significantly reduces the inhomogeneous broadening dispersion.     

Investigations of the electron paramagnetic resonance spectra of VO<Superscript>2+</Superscript> in CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> system

The EPR spectra for VO²⁺ in CaO-Al₂O₃-SiO₂ system are calculated using complete diagonalization method (CDM) and perturbation theory method (PTM). The calculated results are in good agreement with the observed values. By comparing the calculated results by CDM and PTM in a wide range of crystal field parameters, the validity of the PTM formulas has been studied. It is shown that, the PTM formulas are reasonable and reliable in a wide range of CF parameters.     

Magnetic properties of the tetranitrosyl-iron complex Fe<Subscript>2</Subscript>(SC<Subscript>3</Subscript>H<Subscript>5</Subscript>N<Subscript>2</Subscript>)<Subscript>2</Subscript>(NO)<Subscript>4</Subscript>

The magnetic properties of the binuclear nitrosyl-iron complexes Fe₂(SC₃H₅N₂)₂(NO)₄ are investigated. It is demonstrated that several types of particles, such as dimers with a pair of spins 1/2, dimers with a pair of spins 5/2, and paramagnetic particles with spin 3/2, make a contribution to the magnetic properties of the complexes. A decrease in the temperature below 25 K leads to a change in the shape of the EPR spectra corresponding to these dimers, so that Lorentzian lines (homogeneous broadening) transform into Gaussian lines (inhomogeneous broadening). This is accompanied by a stepwise change in the EPR line width and g factors. The change in the line shape indicates that complexes become asymmetric at low temperatures, possibly, due to the decrease in the spin exchange frequency below the frequency of the microwave field of the spectrometer.