Internal strain and the ferroelectric transition in ferroelastic NH4HSO4 crystals as studied by EPR of VO2+ impurity probes

The ferroelectric phase transition and its relation to the spontaneous strain in ferroelastic NH₄HSO₄ crystals were investigated using VO²⁺ ions as an EPR probe. The impurity ions were found to be interstitially trapped at sites surrounded by crystallographically inequivalent NH₄⁺ and SO₄^2? ions. The polar VO²⁺ axes exhibited temperature-dependent displacements in two distinct directions with different energies. The differential properties of VO²⁺ ions in NH₄HSO₄ crystals were used to verify the presence of internal stress in the ferroelastic phases, and the corresponding strain was studied in the range between ?120 and + 100°C. The results indicate that the ferroelectric phase transition occurs as a consequence of lattice instability caused by the internal strain. At the second-order structural transition a dipolar lattice emerges in the crystal and the spontaneous polarization appears as a result of internal entropy transfer to the strained lattice.     

EPR and UV studies of VO ions in potassium d-gluconate monohydrate single crystals

Electron paramagnetic resonance (EPR) of VO²⁺ doped potassium hydrogen d-gluconate single crystals and powder have been examined at room temperature. Single crystal rotations in each of the three mutually orthogonal crystalline planes namely ac, ba and ca indicate two different VO²⁺ complexes. Each complex is located in different chemical environments, each environment containing two magnetically inequivalent VO²⁺ sites in distinct orientations occupying substitutional positions in the lattice and showing a very large angular dependence. The powder spectrum also clearly indicates four different VO²⁺ complexes, confirming the single crystal analysis. Crystalline field around the VO²⁺ ion is nearly axial. The optical absorption spectrum of VO²⁺ ions in the crystal lattice is also studied at room temperature. The characteristic spectrum of the VO²⁺ ions has two absorption bonds. The bond positions are at 17 857 and 11 235 cm^-1. Spin Hamiltonian parameters and molecular orbital coefficients are calculated from the EPR and the optical data, and results are discussed.     

Static and dynamic disordered states in VO2+-doped BaCl2·2H2O crystals

From the EPR investigation, two different crystalline states were found in the VO²⁺ ion-doped BaCl₂·H₂O crystal. One is a dynamic disordered state, where the trapped VO⁺² ion exhibits hindered rotational motion in a wide temperature region. This state is transformed to a static disordered state even at ambient temperature. The orientation of the doped ion reveals random distribution in the lattice sites. Such polymorphism does not come from phase transition of the host crystal, but from the existence of the metastable state of the VO²⁺ ion trapped in the lattice. The absence of the phase transition of pure BaCl₂·2H₂O was confirmed by the measurement of ¹³⁷Ba NQR and by differential thermal analysis.     

EPR and Optical Absorption Study of VO2+ Ions Doped Potassium Hexaaquazinc (II) Sulfate Single Crystals

X-band single-crystal electron paramagnetic resonance (EPR) studies are done on VO²⁺ ions doped in potassium hexaaquazinc (II) sulfate, K₂[Zn (H₂O)₆] (SO₄)₂ (PHZS) at room temperature. The spin Hamiltonian parameters, i.e., g and A tensors and their direction cosines, are evaluated by the standard diagonalization procedure using angular variation of the EPR spectra in three planes (ab, bc* and c*a), with the help of a computer program. The EPR spectrum is simulated using the EasySpin program to verify the calculations. The detailed EPR analysis indicates the presence of two magnetically inequivalent VO²⁺ sites. Both the vanadyl complexes are found to take up the substitutional position in the host lattice. The optical absorption spectrum of VO²⁺ ions doped in PHZS single crystal at room temperature is also recorded and four main d–d transfer bands in the visible region are assigned. The theoretical band positions are obtained using energy expressions and a good agreement is found with the experimental values. With the help of assigned bands the crystal-field parameters (Dq, Ds and Dt) are evaluated. Finally, with the optical and EPR data, the nature of bonding in the complex is discussed.     

Ferrielectric phase transition in ammonium sulphate: A study of two-sublattice model by EPR of polar VO2+ probes

EPR spectra from VO²⁺ -doped (NH₄)₂SO₄ crystals were studied in the range between ?40 and ?150°C. VO²⁺ impurities substituted fog NK₄⁺ ions are accompanied with lattice defects, forming complexes oriented in crystals. Below the transition temperature (?50°C) the spectra exhibited splittings into two branches with unequal intensities, as previously observed in ferroelectric TGS. In ferrielectrie (NH₄)₂SO₄, however, the temperature dependence of the spectra was markedly different from the TGS case. While the internal field determined from the dipolar energies of VO²⁺-probes was consistent with anomalous polarization observed by Unruh[1], the displacement in VO²⁺-directions showed a monotonie increase when the temperature was lowered. We consider that the local order in the molecular arrangement is reflected in the VO²⁺-displacement. The two-sublattice model for the ferrielectrie (NH₄)₂SO₄ is reviewed on the basis of the present EPR results, and the validity of the concept of sublattice polarization is discussed.     

Vanadium-51 solid-state NMR electric field gradient tensors: A DFT-embedded ion and isolated cluster study of crystalline vanadium oxides

Density functional theory (DFT) calculations (6–311+G(2d,p)/B3LYP level of theory) of ⁵¹V electric field gradient (EFG) tensor elements are performed for embedded and isolated cluster models of orthovanadates. The structural models used to calculate the EFGs of ⁵¹V are (I) an isolated H₄VO₄⁺ cluster, (II) an isolated H_nVO₄ⁿ⁻³ cluster (n=number of next-neighbor cations) (III) an isolated orthovanadate anion, VO₄^−x, and (IV) a VO₄^−x ion embedded in a finite point-charge array whose electrostatic potential, at the embedded ion, is equivalent to that of the infinite lattice. For models III and IV, a charge x is assigned estimating the covalence of the system. Models III and IV provide results in good agreement with the experiment. Calculations, employing the embedded and isolated VO₄^−x models, are used to discuss site assignments for AlVO₄. Correlations between quadrupole coupling parameters and deviations of the orthovanadate structure from ideal tetrahedral symmetry are shown.     

Spectroscopic investigation of structure transformation in wavellite

The Electron Spin Resonance of VO²⁺, and the Infrared and Nuclear Magnetic Resonance of protons were studied in wavellite. Interpretation of¹H NMR by shape function calculation allows us to determine the distribution of protons between molecular water, OH-groups, the three-spin magnetic configuration H₃O⁺ and to ascertain the characteristic distances between protons. The spin-Hamiltonian parameters, Fermi contact interaction parameters and dipolar hyperfine coupling parameterPK were calculated. Three types of VO₂₊ complexes were detected in wavellite. It is found that VO²⁺(1)- and VO²⁺(2)-centres disappear and a new VO²⁺(3)-centre appears during annealing. VO²⁺(1) is more stable than VO²⁺(2). The parameters of the VO²⁺(3) complex change in the process of annealing. The transformation of VO²⁺(1) is connected with the destruction of acidic H₃O⁺-groups, and the transformation of VO²⁺(2) correlates with the loss of weakly-bonded water molecules in the wavellite structure. ThePK parameter and Δg?Δg _⊥ ratio can be used as markers of disorder in the wavellite structure.     

Theoretical investigation of the optical and EPR parameters for VOion in some complexes

The molecular orbital coefficients and the EPR parameters of trisodium citrate dihydrate, sodium hydrogen oxalate monohydrate, potassium d-gluconate monohydrate and L-Alanine vanadyl complexes are calculated theoretically. Two d-d transition spectra and EPR parameters for the VO²⁺ complex are calculated theoretically by using crystal-field theory. The calculated g and A paramaters have indicated that paramagnetic center is axially symmetric. Having the relations of g_∥〈g_⊥〈g_e and A_∥〉A_⊥ for VO²⁺ ions, it can be concluded that VO²⁺ ions are located in distorted octahedral sites (C_4v) elongated along the z-axis and the ground state of the paramagnetic electron is d_xy.     

Atomistic computer simulation studies of La1−xSrxVO3

Static computer simulation techniques have been employed for structural investigation of the La_1−xSr_xVO₃ series. Potential parameters for V³⁺-O²⁻ and V⁴⁺-O²⁻ have been derived which reproduces the crystal structures of end members with sufficient accuracy. Variations of lattice parameters and bond distances with Sr concentration have been studied. The calculated lattice parameters decrease with increase in the Sr concentration. A structural phase transition from orthorhombic to cubic is observed at 50% Sr doping level.     

Electric field properties at cationic sites in normal, incommensurate and commensurate K2ZnCl4 crystals

The local electric properties at K and Zn sites in the normal, incommensurate and commensurate phases of K₂ZnCl₄, as derived from a numerical computation of the lattice contributions to the electric potential V(r), electric field intensityE(r) and electric field gradient tensorV _αβ(r) are reported. The numerical data obtained at each cationic position were correlated with the experimental³⁹K NMR, Cu²⁺ and Mn²⁺ EPR and⁵⁷Fe Mössbauer results in pure and doped K₂ZnCl₄. A proportionality between crystal field and zero-field splitting was taken into account for Mn²⁺, whereas for K⁺, Cu²⁺ and Fe³⁺ ions the electric field gradient is directly related to the crystal field parameter. By this comparison, on computations done in the ionic fractional charge and relaxed lattice approximations, the insertion of probe-species of iron, copper and manganese ions on off-center Zn sites is proposed. The³⁹K electric field gradient tensor calculations in the incommensurate phase fit well with the NMR data reported recently.     

EPR and optical absorption studies on VO ions in zinc lactate trihydrate

EPR and optical absorption studies of VO²⁺-doped zinc lactate trihydrate single crystals are done at room temperature. The EPR spectra of VO²⁺ are characteristic of tetragonally compressed octahedral site. The angular variation of the EPR spectra shows single site occupying interstitial position in the lattice. The spin Hamiltonian parameters are evaluated as g_x=1.9771, g_y=2.0229, g_z=1.9236 and A_x=76, A_y=104, A_z=197 (×10⁻⁴) cm⁻¹. Using these parameters and optical absorption data various bonding parameters are determined and the nature of bonding in the complex is discussed.     

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.     

Optical spectra of Eu3+ ion pairs in Y2O3

All the optical transitions of Eu³⁺ ions substituted on Y³⁺ lattice sites of point symmetryC ₂ orC _3i in Y₂O₃ show weak extra lines (satellites) extending over a range of few wavenumbers on both sides of the main line. It is proved by a study of the concentration dependence of the intensities of these satellites that they arise from pairs of two identical Eu³⁺ ions, interacting with each other when their separation is less then ～9 Å. By means of absorption and fluorescence excitation spectra of the transitions of Eu³⁺ at lattice sites of point symmetryC _3i , first, second and third nearestC _3i -C _3i neighbour pairs could be identified. The mean pair interaction is of the order of 5 cm^?1 and is assumed to be due to superexchange via the oxygen ions.     

Preparation and characterization of nanosized (Y,Bi)VO4:Eu and Y(V,P)O4:Eu red phosphors

Nanosized luminescent (Y,Bi)VO₄:Eu³⁺ and Y(V,P)O₄:Eu³⁺ were synthesized at low temperatures either by a coprecipitation method or by a hydrothermal method from aqueous solutions. The effect of Bi³⁺ ion or P⁵⁺ ion content in the lattice, annealing temperature effects on the crystal structure and the particle size, and the luminescence property of (Y,Bi)VO₄:Eu³⁺ and Y(V,P)O₄:Eu³⁺ nanoparticles were examined with a field-enhanced scanning electron microscopy, XRD, and a spectrofluorometer. The pristine YVO₄:Eu³⁺, (Y,Bi)VO₄:Eu³⁺, or Y(V,P)O₄:Eu³⁺ nanoparticles are 35-50 nm in size. The luminescence spectrum of the Eu³⁺ ion was used to probe its position in the crystal lattice. The dopant ions enter the same lattice sites in the nanocrystalline as in the corresponding bulk material, resulting similar spectral features between them. Photoluminescence intensity is weak for the pristine nanoparticles. Annealing the nanoparticles at temperatures up to 1000 °C results in the increased luminescence intensity (>80% of micrometer-sized phosphors) with the minimal particle growth and the improved particle crystallinity.     

CsLiSO4/NH4LiSO4 system investigated by EPR

The single crystals of CsLiSO₄ and the solid solutions Cs _x (NH₄)_{(1? x )}LiSO₄ have been investigated using the EPR technique. Two kinds of vanadyl doped samples were considered – as-grown from the aqueous solutions of appropriate compounds and after a thermal treatment. It has been established that in as-grown crystals the EPR spectra of vanadyl ions were very complex due to the presence of more than one paramagnetic site in the lattice. At least two such sites were identified and spin Hamiltonian parameters were calculated for all the sites. After annealing the crystals at about 400?K, a significant decrease in the intensities of the EPR lines of VO²⁺ complexes was observed. In well-annealed samples the complexity of the EPR spectra has been found to disappear. Additionally, the third kind of vanadyl complexes were created during this treatment. It has also been shown that the EPR spectra of Cr³⁺ ions in CsLiSO₄ could be observed without any additional sample treatment reported in literature like an irradiation or thermal decomposition. It was established that the VO²⁺ and Cr³⁺, being sensitive to structural phase transition in the CsLiSO₄, can be used as a probe for the structural changes in the CsLiSO₄/NH₄LiSO₄ system.     

EPR and Optical Absorption Studies of VO2+-Doped Diammonium Hexaaqua Magnesium(II) Sulfate

Electron paramagnetic resonance (EPR) studies of VO²⁺ impurity ion in single crystals of diammonium hexaaqua magnesium(II) sulfate are carried out at 9.5 GHz (X-band) at room temperature. Different spin-Hamiltonian parameters are determined. VO²⁺ is expected to enter the lattice substitutionally. Superhyperfine splitting is also observed. An EPR study of a powder sample is done that supports the data obtained from single crystal studies. Optical absorption studies are also performed at room temperature. The crystal field parameter (D_q), tetragonal parameters (D_s and D_t), and various bonding parameters are evaluated to estimate the covalency and nature of bonding of VO²⁺ with its different ligands.     

Simulation of Al/Fe disorder in Ca2FexAl2−xO5

A method is described which is capable of simulating disorder across a range of composition. It is used to study the disorder of iron and aluminium on the octahedral and tetrahedral cation sites in the Brownmillerite phase Ca₂Fe_xAl_2−xO₅. The arrangements with the lowest lattice energies are those that maximize the number of Fe³⁺ on octahedral sites throughout the composition range. An exchange of one Fe³⁺ with one Al³⁺, which results in a decrease in the number of iron ions on octahedral sites, increases the lattice energy by an amount that is dependent on x but is independent of the number of exchanges. The exchange of trivalent cations also results in an expansion in the b lattice direction, accompanied by a decrease in the a and c directions across the full range of composition.     

Electron paramagnetic resonance study of exchange coupled Ce3+ ions in Lu2SiO5 single crystal scintillator

The Ce³⁺ ions incorporation inside lutetium oxyorthosilicate (Lu₂SiO₅) single crystals was studied by electron paramagnetic resonance. Already known Ce1 and Ce2 centers originating from the lattice peculiarity allowing two lutetium sites coordinated by different number of the oxygen ions were detected. Remarkably, for the Ce2 center, the determined g² tensor is asymmetric and could not be diagonalized as compared to the Ce1 center, for which the three principal values and corresponding axes orientation have been determined and reported previously. Besides, the much weaker resonance lines found in spectra close to those coming from the Ce1 and Ce2, and following them under crystal rotation with respect to the direction of an external magnetic field, have been revealed as well. They were classified as doublets produced by the exchange coupled Ce³⁺ ions, creating the Ce1–Ce1, Ce2–Ce2 and Ce1–Ce2-like dimers. The corresponding spin–spin coupling constants were estimated. They are in the range 0.04–0.4 cm⁻¹. The Ce1, Ce2 and total dimer centers populations were calculated as 89%, 4.5% and 6.5%, comparing integral intensities of corresponding resonance lines.     

Time resolved investigations of energy transfer in Eu3+∶Y2O3

The concentration dependence of the energy transfer from the Eu³⁺ ions at lattice sites of point symmetryC ₃ i to the Eu³⁺ ions at lattice sites of point symmetryC ₂ in Y₂O₃ has been investigated by analyzing the nonexponential fluorescence decay curves of the donors measured after excitation by a short optical light pulse. The simple model of a fixed interaction rangeR ₀ proved to be adequate to explain the experimental observations. An interaction range ofR ₀=8.7 Å was found in agreement with previous steady state experiments.     

Correlation of photoluminescence of (Y, Ln)VO4:Eu (Ln=Gd and La) phosphors with their crystal structures

We have studied the photoluminescence (PL) of (Y, Ln)VO₄:Eu³⁺ (Ln=La and Gd) phosphors and the correlation of the PL of those phosphor with their crystal structure. It is found that (Y, Gd)VO₄:Eu³⁺ phosphors have the same crystal structure as YVO₄:Eu³⁺, which is tetragonal with a little different lattice parameters. In the case of (Y, La)VO₄:Eu³⁺ phosphors, however, the gradual change from tetragonal to monoclinic structure of host lattice was observed as the amount of La ion increased. To investigate the PL property of (Y, Ln)VO₄:Eu³⁺ (Ln=La and Gd) phosphors, vacuum ultraviolet (VUV) and ultraviolet (UV) excitation were used. The favorable crystal structure for the PL intensity of orthovanadate phosphor under 147 and 254 nm excitation was tetragonal containing Gd ion and under 365 nm excitation was monoclinic containing La ion which might have the lowest site symmetry for Eu³⁺ ion.