Defect structure of Cu2+ center in K2SO4‒Na2SO4‒ZnSO4 glasses

The spin Hamiltonian parameters (g factors g _//, g _⊥ and hyperfine structure constants A _//, A _⊥) of Cu²⁺ in K₂SO₄?Na₂SO₄?ZnSO₄ glasses are calculated from the high-order perturbation formulas of 3d⁹ ion in tetragonal octahedral sites. The calculated results are in agreement with the observed values. Since the EPR parameters are sensitive to the local structure of a paramagnetic impurity center, the defect structure of Cu²⁺ center in K₂SO₄?Na₂SO₄?ZnSO₄ glasses is estimated. The validity of results is discussed.     

Laser Raman spectra of mixed crystals of [(NH4)1−x Kx]2 SO4

The Raman spectra of mixed crystals of [(NH₄)_1?x K _x ]₂ SO₄ in the region 50–3400 cm^?1 at 293 K and below 223 K have been reported. At room temperature 293 K, as the concentration of K⁺ ion increases in the crystal up to 50%, the frequencies of the totally symmetric vibrations of SO ₄ ^2? and NH ₄ ⁺ ions increase and thereafter the frequency of SO ₄ ^2? vibration decreases and attains the value in K₂SO₄. This change in frequency up to 50% of potassium concentration is due to the breaking of hydrogen bonds of the type N-H...O. The behaviour of Raman intensities of A _g (v ₁) mode of SO ₄ ^2? for various concentrations (x=0, 0·03, 0·11, 0·5, 0·85) suggest that the phase transition changes from first order type to one of second order. The phase transition in mixed crystals of [(NH₄)_1?x K _x ]₂ SO₄ can be a cooperative phenomenon arising from a coupling between (NH₄)⁺ ions through hydrogen bonds with the distorted SO ₄ ^2? ions in the low temperature phase.     

γ-irradiation effect on the spectroscopic and electrical properties of K2SO4—Na2SO4 mixed system

Abstract

Measurements of electron paramagnetic resonance, infrared and electrical properties were carried out for the K₂SO₄—Na₂SO₄ mixed system before and after γ-irradiation. EPR measurements revealed the presence of a quartet of lines characterized by an isotropic g-value of 2.0034. These lines are mainly attributed to the formation of a SO^? ₃ center which results from the interaction of γ-rays with the sulfate ion. A decrease in the absorption intensity of the Infrared radiation was observed after γ-irradiation due to radiation damage in the sulfate group. The electrical conductivity, σ, was measured for the K₂SO₄—Na₂SO₄ system before and after γ-irradiation in the temperature range from 30 up to 430°C. A considerable decrease in the conductivity value accompanied by an increase in the activation energy was observed after γ-irradiation. The energy of formation of Frenkel defects was estimated to be 2.94eV. The current-voltage characteristics were measured at different temperatures in order to estimate the type of conduction in the samples. Isothermal annealing kinetics was investigated at different temperatures before and after γ-irradiation. The electrical conductivity decreases with increasing time of annealing and the annealing process is dominated by a unique rate process.     

Superionic phase transition in (NH4)2SeO4·2NH4HSeO4 single crystal at 378 K

Abstract

DTA, structural and electric conductivity investigations were made for (NH₄)₄H₂(SeO₄)₃ single crystals. A high-temperature phase transition at 378 K to a superionic phase was found. The phase is characterized by a high electrical conductivity (～4.10^?3 Ω^?1 cm^?1) and a low activation energy (0.11 eV).     

Low-temperature phase transitions and dynamics of ammonium in (NH4)3H(SO4)2 and [(NH4)1−x Rbx]3H(SO4)2 crystals

The (NH₄)₃H(SO₄)₂ and [(NH₄)_0.82Rb_0.18]₃H(SO₄)₂ crystals are investigated by dielectric spectroscopy, inelastic incoherent neutron scattering (IINS), and neutron powder diffraction. A comparative analysis of the data obtained is given. It is shown that the phase transitions II ? III, III ? IV, IV ? V, and V ? VII in the (NH₄)₃H(SO₄)₂ crystal are accompanied by changes in the orientation ordering of the NH ₄ ⁺ ions. In the [(NH₄)_0.82Rb_0.18]₃H(SO₄)₂ crystal, these phase transitions are completely suppressed and the long-range order inherent in the II phase is retained over the entire temperature range covered (6–300 K). It is revealed that this crystal at the temperature T _g≈70 K undergoes a transition to the dipole glass phase, which is attended by “freezing” the orientation disordering of the ammonium ions.     

The crystal structure of tetrammine copper sulphate Cu(NH3)4SO4·H2O

The crystal structure of tetrammine copper sulphate was determined. The orthorhombic unit cell with dimensions a=12.12, b=10.66, c==7.07 Åcontains four molecules of Cu(NH ₃)₄ SO ₄.H ₂ O, the space group is Pbnm (D _2h ¹⁶ ).The atomic coordinates were determined from the projections of the Patterson function and the electron density onto the (001) and (010) planes. The complex cation Cu(NH ₃)₄ ⁺⁺ is approximately planar, the groups of NH ₃ are linked by co-valent bonds to the central Cu atom. The average distance of the groups of NH ₃,which are approximately at the corners of a square surrounding the central Cu atom, is 2.90 Å, the distance Cu-NH ₃,corresponding to the co-valent bond, is 2.05 Å.     

Electron paramagnetic resonance,infrared, and electrical conductivity studies of γ-ray irradiated Li2SO4-K2SO4 mixed system

Abstract

Electron paramagnetic resonance spectra of 7-irradiated Li:₂SO₄-K₂SO₄ mixed system are measured at X-band and 300 K. Two types of centers, A and B, identified as LiOH⁺ ₂ and SO^? ₃ are studied. The EPR spectrum of LiOH⁺ ₂ has a well-resolved hyperfine structure. The effective g-value and the hyperfine structure constant of it are calculated. On the other hand, the SO^? ₃ center is characterized by an intense isotropic signal at a g-value ≈ 2. 0035. Mechanisms of center formation are proposed. Infrared transmittance spectra were recorded for unirradiated and γ-ray irradiated samples. A decrease in the transmittance spectra was observed after irradiation. The ionic conductivity, σ, has been measured for the Li₂SO₄-K₂SO₄ system before and after irradiation in the temperature range 30–400°C. The activation energy was found to be 1. 1 eV. A considerable decrease in the conductivity value accompanied by an increase in the activation energy was observed after γ-irradiation.     

Studying the dielectric, electromechanical, and elastic properties of K1 − x (NH4) x H2PO4 mixed crystals

Temperature dependences of the longitudinal permittivity, piezoelectric coefficient d ₃₆, and elastic constant c ₆₆ ^E of K_{1 ? x} (NH₄) _x H₂PO₄ mixed crystals are studied experimentally. A microscopic model is proposed for crystals of the K_{1 ? x} (NH₄) _x H₂PO₄ type that includes the piezoelectric contribution to the effective pseudospin cluster Hamiltonian. Dielectric elastic and electromechanical properties of these crystals with ammonium concentrations x below 0.40 are calculated in a wide temperature range using the cluster approach. The calculation results are in qualitative agreement with experimental data.     

Ionic transport in the Na2SO4Na2WO4 and Na2SO4M2(SO4)3 (M=La,Dy, Sm,In) systems

Premelted, predried Na₂SO₄, premelted Na₂WO₄, Na₂SO₄Na₂WO₄ composites and Na₂SO₄M₂(SO₄)₃ (M = La, Dy, Sm, In) have been studied by means of X-ray diffraction, DTA and electrical conductivity measurements. The high temperature, highly conducting Na₂SO₄ phase I has been successfully stabilised at room temperature; the Na₂SO₄ containing 4 mole% La₂(SO₄)₃ exhibits the highest conductivity (σ) and lowest activation energy (E_a) (σ=1.08 × 10⁻³ω⁻¹ cm⁻¹ at 290°C and E_a=0.50 eV) and therefore this system appears promising for further development.     

Investigation of the EPR parameters of the rhombic Cu2+ center in (NH4)2Mg(SO4)2·6H2O single crystal

The electron paramagnetic resonance (EPR) parameters (g factors g_x, g_y, g_z and hyperfine structure constants A_x, A_y, A_z) for Cu²⁺ in (NH₄)₂Mg(SO₄)₂·6H₂O (DHMS) crystal are theoretically investigated using the high-order perturbation formulas of these parameters. In the calculations, the ligand orbital and spin–orbit coupling for the impurity Cu²⁺ are taken into account; the required crystal-field parameters are estimated from the superposition model which enables correlation of the crystal-field parameters and hence the EPR parameters with the local structure of the impurity center. The ligand orbital and the spin–orbit coupling contributions are included on the basis of the cluster approach. Based on the calculation, the theoretical EPR parameters show good agreement with the observed values. The results are discussed.     

Thermoluminescence of γ-irradiated BaCa(SO4)2:Eu,Dy

In this paper, thermoluminescence (TL) studies of BaCa(SO₄)₂:Eu,Dy phosphor are reported. A microcrystalline sample of BaCa(SO₄)₂:Eu,Dy was prepared by a solid state diffusion method and the formation of the compound was confirmed by the X-ray diffraction study. Morphology of the phosphor was analyzed by scanning electron microscopy (SEM). The sample is found to have an average particle size of 5?µm. TL glow curves of the γ-irradiated samples with different concentrations of Eu and Dy were studied and compared with BaCa(SO₄)₂:Eu and BaCa(SO₄)₂:Dy. It has been found that a single peak was located at around 230°C with the highest TL intensity in BaCa(SO₄)₂:Eu,Dy which is eight times and two times more than singly Dy- and Eu-doped BaCa(SO₄)₂ phosphor, respectively. For TL analysis, BaCa(SO₄)₂:Eu,Dy (0.2?mol%, 1?mol%) is annealed at different temperatures ranging from 900°C to 1100°C. Analysis of the TL glow curve was carried out by a glow curve deconvoluted method. Trapping parameters (activation energy and frequency factor) of all TL glow curves were evaluated by Chen's peak shape method. A comparison of trapping parameters between BaCa(SO₄)₂:Eu,Dy; BaCa(SO₄)₂:Eu and BaCa(SO₄)₂:Dy phosphors at 900°C, 1000°C and 1100°C is also reported in this paper.     

Determination of Chemical Effect on the Kβ1/Kα, Kβ2/Kα, Kβ2/Kβ1 and Kβ/Kα X-Ray Intensity Ratios of 4d Transition Metals

It is clear that detailed studies performed under identical experimental conditions are desirable to determine the effect of the chemical environment on the value of intensity ratios. To better understand this effect, we conducted measurements on Kβ₁/Kα, Kβ₂/Kα, Kβ₂/Kβ₁ and Kβ/Kα x-ray intensity ratios. The Kα and Kβ_1,2 emission spectra for compounds of 4d transition metals Y, Zr, Nb, and Mo were measured using a Si(Li) solid-state detector. The samples were excited by 22.69 keV x-rays emitted from a ¹⁰⁹Cd radioisotopes source. The experimental results for pure elements are compared with the other experimental and theoretical values.     

E.P.R. study of γ-irradiated single crystals of K2C2O4.H2O and (NH4)2C2O4.H2O

The electron paramagnetic resonance of γ-irradiated single crystals of K₂C₂O₄.H₂O and (NH₄)₂C₂O₄.H₂O has been studied. The spectra show interesting microwave power saturation effects. The singlet spectrum is attributed to the C₂O₄ ^- radical derived from the C₂O₄ ^-- ion. The principal g-values are determined to be 1·998, 2·0028 and 2·0004. Certain weak lines are observed with different power saturation and interpreted as due to OH radicals derived from water molecules in the crystal lattice.     

Low-temperature x-ray diffraction studies of the crystallographic parameters and thermal expansion of (CH3)2NH2Al(SO4)2 · 6H2O crystals

The a, b, c, and β crystallographic parameters of the (CH₃)₂NH₂Al(SO₄)₂ · 6H₂O crystal (DMAAS) have been measured by x-ray diffraction in the 90–300-K temperature range. The thermal expansion coefficients along the principal crystallographic axes α_a, α_b, and α_c have been determined. It was shown that, as the temperature is increased, the parameter α decreases and b increases, whereas c decreases for T<T _c (where T _c is the transition temperature) and increases for T>T _c, so that one observes a minimum in the c=f(T) curve in the region of the phase transition (PT) temperature T _c ～ 152 K. The thermal expansion coefficients α_a, α_b, and α_c vary in a complicated manner with increasing temperature, more specifically, α_a and α_c assume negative values at low temperatures, and the α_a=f(T), α_b=f(T), and α_c=f(T) curves exhibit anomalies at the PT point. The crystal has been found to be substantially anisotropic in thermal expansion.     

Effect of γ-radiation on TL,ESR and evaluation of trapping parameters of K2Ca(SO4)2:X (X = Dy or Eu) phosphors

ABSTRACT

The present phosphor K₂Ca(SO₄)_2, doped by dysprosium and europium, is synthesized by the solid-state diffusion method. The doping concentration varied from 0.1 to 0.5?mol% by weight. A phosphor is studied for X-ray powder diffraction, surface morphology analytical scanning electron microscopy and analyzed by energy-dispersive X-ray spectroscopy. The prepared phosphor K₂Ca(SO₄)_2, doped by Dy and Eu, has been characterized for thermoluminescence (TL) glow curve, showing maximum peak temperatures at 176°C and at 200°C, respectively. TL peak intensity of K₂Ca(SO₄)₂: Dy and Eu was compared with the standard TLD CaSO₄:Dy phosphor. Both phosphors show the dose linearity ranging from 20 to 240?Gy doses of γ-rays of ⁶⁰Co source at room temperature. Negligible fading has been observed when irradiated with γ-rays and stored for 60 days without taking any care from sunlight. The TL materials were used in powder forms. The linearity of ESR response with dose for powder forms of K₂Ca(SO₄)₂: Dy was also studied using the signals at g?=?2:0039 (SO₃^?) and at g?=?2:02282 (SO₄^?). It was observed that the range of linearity of dose response extended between 20 and 240?Gy. Kinetic parameters have been calculated using three different methods: Chen's peak shape method, various heating rate method and initial rise method. To study the heating rate method, the glow curve was recorded for the heating rate as 1°C, 3°C, 5°C, 7°C, 9°C each time. Electron spin resonance (ESR) shows the ionic radical formation during γ-irradiation, which is responsible for TL. The effect of temperature and microwave power on the ESR signal was also studied.     

Ultrasonic Studies of (CH3)2NH2Al(SO4)2 · 6H2O crystals irradiated by γ quanta and electrons

The temperature dependence of the longitudinal-ultrasound velocities in (CH₃)₂NH₂Al(SO₄)₂ · 6H₂O crystals was studied using the echo-pulse technique in the 90–300 K range. The measurements were carried out along mutually perpendicular crystallographic directions X, Y, Z on samples both unirradiated and irradiated to various doses by γ quanta and an electron beam. The ultrasound velocity V in this crystal was shown to be anisotropic, with V _YY >V _XX >V _ZZ . The V _XX =f(T), V _YY =f(T), and V _ZZ =f(T) curves exhibit anomalies in the form of breaks at the ferroelectric phase transition (PT) at T _c1=152 K, as well as in the region of T _c2=218 K. It was established that as the irradiation dose increases, the PT temperature T _c1 decreases and the anomalies in the temperature dependences of the ultrasound velocities are smeared.     

Some stereochemical criteria concerning the structural stability of A2BX4 compounds of type β-K2SO4

In A₂BX₄ structures which are isostructural to β-K₂SO₄ (with A being a monoatomic cation) there are two crystallographically independent cations surrounded by 11 and 9 X-atoms. The 11-coordinated cation is less firmly bound and the arrangement of its five closest neighbours is irregular. One of these contacts is approximately parallel to the pseudohexagonal axis of the structure and is often shorter than the sum of the corresponding ionic radii. A survey of available structural data indicates that the low-temperature structural instability of a good number of these compounds is related to the coordination of this 11-coordinated cation and especially to the bonding strength of this short bond, which is often the shortest cation-anion contact in the structure. Typically, the relative contribution of this contact to the bond-valence sum of the 11-coordinated cation is larger in the compounds which undergo phase transitions at lower temperatures. The presence of this short contact is correlated with the ratio of the lattice parameters a/b (Pnma-setting). In general, the Pnma phase is unstable at low temperatures in those compounds where this ratio is smaller. On the other hand, the value of a/b can be related to the ratio of the effective sizes of cations and BX₄ tetrahedra, so that typical low-temperature instabilities of the β-K₂SO₄ structure occur for smaller values of the ratio between cation radius and the sum of the ionic radii of atoms A and X. In most cases, the resulting phase transitions stabilize modulated structures (frequently incommensurate), with slight distortions with respect to the β-K₂SO₄ structure. However, when the bond valence sum of the eleven-coordinated cation is extremely low, more drastic (first-order) structural changes are observed (e.g. phase transitions into the Sr₂GeS₄ structure type). In addition, the survey indicates, especially in complex oxides, that low-temperature phase transitions are more probable in those structures with looser packing. Considering the criteria proposed, a set of compounds is indicated where low-temperature phase transitions are plausible.     

Electrical conductivity and phase diagram of the Na2SO4CaSO4 system

Very high vacancy concentrations may be obtained in solid solutions of the high temperature phase of Na₂SO₄. In this paper the Na₂SO₄CaSO₄ system has been studied using differential scanning calorimetry (DSC), impedance spectroscopy and X-ray powder diffraction. The phase diagram, especially the stability range of the solid solution of the high temperature Na₂SO₄ phase, has been redetermined. The electrical conductivity of this solid solution increases rapidly with increasing CaSO₄ content and reaches a maximum for about 5 mol% CaSO₄; the conductivity at e.g. 300°C, 3.5×10⁻³ (μ cm)⁻¹, is almost three orders of magnitude higher than that of pure Na₂SO₄.     

Rotational Spectra of CH3CCH–NH3, NCCCH–NH3, and NCCCH–OH2

Microwave spectra of NCCCH–NH₃, CH₃CCH–NH₃, and NCCCH–OH₂have been recorded using a pulsed-nozzle Fourier-transform microwave spectrometer. The complexes NCCCH–NH₃and CH₃CCH–NH₃are found to have symmetric-top structures with the acetylenic proton hydrogen bonded to the nitrogen of the NH₃. The data for CH₃CCH–NH₃are further consistent with free or nearly free internal rotation of the methyl top against the ammonia top. For NCCCH–OH₂, the acetylenic proton is hydrogen bonded to the oxygen of the water. The complex has a dynamicalC_2vstructure, as evidenced by the presence of two nuclear-spin modifications of the complex. The hydrogen bond lengths and hydrogen-bond stretching force constants are 2.212 Å and 10.8 N/m, 2.322 Å and 6.0 N/m, and 2.125 Å and 9.6 N/m for NCCCH–NH₃, CH₃CCH–NH₃, and NCCCH–OH₂, respectively. For the cyanoacetylene complexes, these bond lengths and force constants lie between the values for the related hydrogen cyanide and acetylene complexes of NH₃and H₂O. The NH₃bending and weak-bond stretching force constants for CH₃CCH–NH₃are less than those found in NCCCH–NH₃, NCH–NH₃, and HCCH–NH₃, suggesting that the hydrogen bonding interaction is particularly weak in CH₃CCH–NH₃. The weakness of this hydrogen bond is partially a consequence of the orientation of the monomer electric dipole moments in the complex. In CH₃CCH–NH₃the antialigned monomer dipole moments lead to a repulsive dipole–dipole interaction energy, while in NCH–NH₃and NCCCH–NH₃the aligned dipoles give an attraction interaction.