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.     

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.     

Family of paramagnetic centers of Ce3+ ions in yttrium aluminum garnet

The electron paramagnetic resonance (EPR) spectra of Ce³⁺ ions in single crystals of yttrium aluminum garnet have been investigated. It has been found that, in addition to the usually observed EPR signals of Ce³⁺ ions located in the regular environment at dodecahedral sites of the crystal lattice, the spectra contain a group of less intense anisotropic lines with g-factors close to the corresponding parameters of Ce³⁺ ions in the regular environment. It has been concluded that the observed satellite lines belong to the family of Ce³⁺ ions in the immediate vicinity of which there are permutation defects that lead to a change in the strength and symmetry of the crystal field in the vicinity of the paramagnetic center.     

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.     

Fe2+ spin transition in [Fe(trz)(Htrz)2](BF4) as evidenced by a variable temperature EPR study

The EPR of Fe³⁺ ions has been used for the first time to evidence a low-spin (S=0) to high-spin (S=2) transition of Fe²⁺ ions in an octahedral ferrous complex [Fe(trz)(Htrz)₂](BF₄). The temperature dependence of the intensity of the Fe³⁺ EPR line atg=4.3 reveals a spin transition which occurs for the Fe²⁺ ions, with hysteresis. The transition temperatures areT _c↑=374 K in the warming mode andT _c↓=345 K in the cooling mode. The analysis of the EPR spectral data indicates the presence of a structural phase transition accompanying the spin transition.     

Electron paramagnetic resonance and photoluminescence of NaBi(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals doped with gadolinium ions

The results of electron paramagnetic resonance (EPR) and photoluminescence studies of large NaBi(MoO₄)₂ crystals grown by the low-gradient Czochralski method and doped with gadolinium ions (0.1 wt %) have been presented. It has been found from the analysis of the angular dependence of EPR spectra that the gadolinium ions enter into the crystal structure in the state Gd³⁺ and occupy the bismuth position. The parameters of the EPR spectra of the gadolinium ions have been calculated and the analogy has been drawn based on these data between the specific features of the incorporation of gadolinium ions into the structures of double tungstates and molybdates. The observed shift of the maximum of the photoluminescence band of the NaBi(MoO₄)₂ crystals doped with Gd³⁺ ions with respect to the spectrum of the undoped crystal suggests the influence of gadolinium ions on the formation of the bottom of the conduction band caused by the states of the (MoO₄)^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.     

Electron paramagnetic resonance of Cr3+ ions in ferroelastic CsLiSO4

The X-band EPR studies of Cr³⁺-doped LCS have been performed. The values of the spin-Hamiltonian parameters for Cr³⁺ ions have been estimated at room temperature. The effect of crystal twinning on the EPR spectra has also been described. The temperature behaviour of EPR lines originated from two structurally different chromium complexes has been studied close to the ferroelastic phase transition. From the temperature dependence of positions of the split resonance lines a value of the critical exponent β of the order parameter was found to be β = 0.25.     

Variable temperature EPR spectra of Copper (II) ions in kainite crystals

X-band electron paramagnetic resonance (EPR) studies on divalent copper ions embedded in KMgClSO₄·3H₂O single crystals have been performed at low temperature (123 K). The angular variation of the EPR spectra reveals the presence of two Cu²⁺ sites, which have different orientations. The spin-Hamiltonian parameters of this six-coordinated cupric ion have been evaluated from the EPR spectra at 123 K. The forbidden lines due to Δm_I=±1 transitions are observed in between allowed transitions. The temperature variation EPR studies have also been performed both for a single crystal and a polycrystalline sample. The ground state wavefunction of Cu²⁺ ions has been estimated and is found to be an admixture of d_3z²−r² and d_x²−y². The temperature variation of the EPR spectra reveals that Cu²⁺ ions exhibit dynamic Jahn-Teller effect. From the polycrystalline EPR data, the temperature dependent magnetic susceptibilities are evaluated and discussed.     

Spectral studies on Cu ions in sodium–lead borophosphate glasses

Electron Paramagnetic Resonance (EPR) and optical absorption spectra of Cu²⁺ ions in sodium–lead borophosphate glasses doped with different concentrations of Cu²⁺ ions have been studied. EPR spectra of all the glass samples exhibit resonance signals characteristic of Cu²⁺ ions. The values of spin-Hamiltonian parameters indicate that the Cu²⁺ ions in sodium–lead borophosphate glasses are present in octahedral sites with tetragonal distortion. The optical absorption spectra of all the glass samples show a single broad band, which has been assigned to the ²B_1g→²B_2g transition of Cu²⁺ ions. The optical band gap energy (E_opt) and Urbach energy (ΔE) are calculated from their ultraviolet absorption edges. The emission bands observed in the ultraviolet and blue region are attributed to 3d⁹4s→3d¹⁰ triplet transition in Cu⁺ ion. The FT-IR spectra show that the glass system contains BO₃, BO₄ and PO₄ structural units.     

Oxygen - 17 superhyperfine structure of transition metal ions in MgO

EPR measurements of the oxygen-17 superhyperfine structure of Fe³⁺ Cr³⁺, Ni³⁺ and Rh²⁺ impurities in isotopically labeled single crystals of MgO have been made. In connection with the interpretation of the spectra of the Jahn-Teller ions Ni³⁺ and Rh²⁺, expressions are derived for the superhyperfine Hamiltonian for the ²E state in octahedral symmetry.     

Electron paramegnetic detection of ionic motional effects in the super ionic conductor β-sodium gallate

The EPR spectra of Cu²⁺, Mn²⁺ and Eu²⁺, incorporated into the conducting mirror planes of β-sodium gallate, have been studied. For all three impurities, the EPR spectra at or near room temperature show pronounced relaxation effects caused by the motion of the ions. From the relaxation effects the correlation time of the motion is found to be of the order of 10^-11 sec. This time corresponds to a residence time for the paramagnetic ion at a lattice site in the conducting plane. From this time and the thermal activation energy an attempt frequency of 10¹² Hz is determined.     

An EPR study of local fluctuations in the vicinity of the ferroelectric phase transition in Li2Ge7O15

EPR spectra of Mn²⁺ ions have been studied in the temperature interval containing the ferroelectric transition in crystalline lithium heptagermanate Li₂Ge₇O₁₅. The EPR linewidth and shape were measured as functions of temperature and orientation. It is shown that the critical resonance-line broadening observed to occur in the vicinity of the phase transition is due to fluctuations in the local order parameter with frequencies below 10⁷ Hz.     

The effects of added foreign ions in Gd2O2S : Tb; crystal field calculations, lifetimes, photo-luminescence and absorption spectra

A spectroscopic study is carried out in which the effects of added Ca²⁺ and Ru⁴⁺ transition metal ions on some characteristics of the emission of Gd₂O₂S : Tb³⁺ phosphors (energy levels, intensities, lifetimes) are examined and compared. In order to distinguish the Tb³⁺ emissions from impurity ones, the electronic energy levels of trivalent terbium are determined and the energy level scheme is completed by a crystal field analysis. The optical spectra reveal no terbium doped impurity phase; however, other rare earth ions present as impurities in the starting materials are detected. They are identified, and the influence of the added Ca²⁺ and Ru⁴⁺ on their emission lines is also examined.     

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 and optical spectroscopy of Yb<Superscript>3+</Superscript> ions in CaF<Subscript>2</Subscript>: an analysis of the structure of tetragonal centers

CaF₂ crystals doped with Yb³⁺ ions have been studied by electron paramagnetic resonance (EPR) and optical spectroscopy. EPR spectra of paramagnetic centers (PCs) for cubic (T_c) and tetragonal (T_tet) symmetries were identified. Empirical energy level diagrams were established and crystal field parameters were determined. Information on the CaF₂∶Yb³⁺ phonon spectra was obtained from the electron-vibrational structure of the optical spectra. The crystal field parameters were used to analyze the crystal lattice distortions in the vicinity of the Yb³⁺ ion. Within the framework of a superposition model, it is established that four F⁻ ions located symmetrically with respect to the fourfold axis from the side of the ion-compensator approach the impurity ion and deviate from the PC axis. The second set of four fluorine ions also approaches the Yb³⁺ ion and the PC axis. The ion-compensator F⁻ also approaches considerably the impurity ion.     

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.     

Production and Laser Characteristics of Fe<Superscript>2+</Superscript>:ZnS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> Polycrystals

A method is developed for producing active laser elements (spectrum range 4 to 5 μm) based on polycrystalline solid solutions ZnS_xSe_x-1 doped with iron ions. Bilateral diffusion doping of the elements by Fe²⁺ ions is performed during hot isostatic pressing. Spectral and energy characteristics of the laser are investigated with the Fe²⁺:ZnS_0.1Se_0.9 active element kept at room temperature. It is found that the absorption band of the Fe²⁺:ZnS_0.1Se_0.9 crystal is blueshifted with respect to the Fe²⁺:ZnSe absorption band, while the lasing spectra of the Fe²⁺:ZnSe and Fe²⁺:ZnS_0.1Se_0.9 lasers and their energy parameters are almost identical. The lasing energy of 580 mJ is obtained at the slope efficiency with respect to the absorbed energy of 46%. Further increase in the lasing energy is limited by development of transversal parasitic oscillation at a large size of the pump beam spot.     

Absorption and emission spectra of yttria-stabilized zirconia and magnesium oxide

We have investigated the luminescence and absorption spectra of doped and undoped ZrO₂-Y₂O₃ and MgO crystals at room- and low temperatures. The crystals used are partly doped with the transition metals Ni, Co, Cr and the rare earth Pr. The emission spectra were obtained under laser excitation at different wavelengths. The observed optical emission and absorption bands of the MgO crystals doped with Ni, Co and Cr correspond to transitions between spin-orbit split crystal field levels of the transition metals. Luminescence and absorption bands of undoped yttria-stabilized zirconia (YSZ) crystals are due to color centers, absorption bands of the doped YSZ correspond to the well known transitions of the Ni²⁺, Co²⁺ and Pr³⁺ ions, respectively. The emission spectra of the doped YSZ obtained under various laser excitations can be explained by an energy transfer process between the color center and the doping materials. The influence of annealing on the absorption and emission of Pr³⁺/Pr⁴⁺ is investigated.     

EPR, luminescence and IR studies of Mn activated ZnGa2O4 phosphor

Electron paramagnetic resonance (EPR), luminescence and infrared spectra of Mn²⁺ ions doped in zinc gallate (ZnGa₂O₄) powder phosphor have been studied. The EPR spectra have been recorded for zinc gallate phosphor doped with different concentrations of Mn²⁺ ions. The EPR spectra exhibit characteristic spectrum of Mn²⁺ ions (S=I=5/2) with a sextet hyperfine pattern, centered at g_eff=2.00. At higher concentrations of Mn²⁺ ions, the intensity of the resonance signals decreases. The number of spins participating in the resonance has been measured as a function of temperature and the activation energy (E_a) is calculated. The EPR spectra of ZnGa₂O₄: Mn²⁺ have been recorded at various temperatures. From the EPR data, the paramagnetic susceptibility (χ) at various temperatures, the Curie constant (C) and the Curie temperature (θ) have been evaluated. The emission spectrum of ZnGa₂O₄: Mn²⁺ (0.08 mol%) exhibits two bands centered at 468 and 502 nm. The band observed at 502 nm is attributed to ⁴T₁→⁶A₁ transition of Mn²⁺ ions. The band observed at 468 nm is attributed to the trap-state transitions. The excitation spectrum exhibits two bands centered at 228 and 280 nm. The strong band at 228 nm is attributed to host-lattice absorption and the weak band at 280 nm is attributed to the charge-transfer absorption or d⁵→d⁴s transition band. The observed bands in the FT-IR spectrum are assigned to the stretching vibrations of M-O groups at octahedral and tetrahedral sites.