Study of optical and transport properties of K2CuCl4 · 2H2O single crystal

Optical absorption, transport properties and EPR of K₂CuCl₄ · 2H₂O single crystals have been studied. The optical absorption in UV, and visible region are characterized by a charge transfer band, and in the near infrared region at 3998, 4336, and 4480 cm⁻¹ are attributed to transitions between the stark levels of copper(II) ion in an extended octahedral crystal field. An anisotrophic ‘g’ value was observed with g_∥ = 2.12 and g_∥ = 2.24 by EPR method. The spin orbit coupling constant is found to be 500 cm₋₁. D.C. electrical conductivity measurements with temperature reveal an anisotropy characteristic of a two-dimensional layered structure and exhibit a first order irreversible structural phase-transition at 377 K, i.e. from tetragonal to monoclinic crystal system. X-ray diffraction studies and density calculations from the crystal structure data in both the phases suggest that the first order irreversible transition occurs following the loss of the two water molecules of hydration.     

Electrical and optical properties of CuGaTe2

The electrical and optical properties of CuGaTe₂ single crystals were investigated by resistivity and Hall effect measurements in the temperatur range T = 77… 300 K and optical transmission measurements in the temperature range T = 20… 300 K at photon energies hν = 1.15…1.50 eV. All samples were p-type conducting due to shallow acceptors with ionization energies E_A1 ≈︁ 10⁻³ eV and concentrations N_A1 ≈︁ p = (2…4). 10¹⁸ cm⁻³. The absorption spectra could be described by simultaneous consideration of acceptor - to - conduction band transitions with E_A2 = 360 meV and N_A2 ≈︁ 10²² cm⁻³ and valence band - to - conduction band transitions with E_G = 1.24 eV at room temperaure. The temperature coefficient of the fundamental edge is dE_G/dT = −4.0. 10⁻⁴ eV/K. The results are discussed with regard to some general trends found in the Cu-III–VI₂ compounds.     

EPR and pulsed NMR study of (NH4)2ZnBr4:Mn2+

The EPR and pulsed NMR measurements of (NH₄)₂ZnBr₄:Mn²⁺ have been carried out in the temperature range 110–473 K and 83–284 K, respectively. EPR spectra from three different paramagnetic entities have been observed. Intensity changes of certain lines in the EPR spectrum observed at ≈ 430 K indicate the occurrence of a phase transition. Another phase transition at ≈ 235 K is indicated as a change of slope in the T₁ vs 1/T curve.     

EPR and Optical Absorption Spectra of Cr3+ Ions in Lithium Sulphate Single Crystals

Optical absorption and EPR spectra of Li₂SO₄ · H₂O crystals doped with Cr³⁺ are studied at liquid nitrogen temperature. The bands are found in absorption spectra with maxima about 17000, 23 800 and 37 200 cm⁻¹, assigned to the ⁴A₂ → ⁴T₂, ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₁ (⁴P) transitions, respectively. The crystal field theory parameters were determined and appeared to be as follows: Dq = 1700 cm⁻¹, B = 667 cm⁻¹, C = 3002 cm⁻¹. The lines resulting from Cr³⁺ ions are found in EPR spectra. All lines are doublets, which is indicative of presence of two magnetically unequivalent centre positions, and have the hyperfine structure resulting from interaction of the unpaired electron spin with Cr⁵³ isotope nucleus. Centres are oriented in such a way, that z-axes, corresponding to two centre positions, are situated at both sides of a-axis at an angle of about 3°. Spin Hamiltonian parameters were found as follows: g_x = 1.985, g_y = 1.984, g_z = 1.988, D = 0.130 cm⁻¹, E = 0.016 cm⁻¹, |A| = 17.8 · 10⁻⁴ cm⁻¹.     

Optical Absorption Spectrum of Cu2+ Ions in Lithium Sodium Potassium Sulphate Single Crystal

Single crystals of Copper-doped Lithium sodium potassium sulphate were grown at room temperature. The optical absorption spectra were recorded at room and liquid-nitrogen temperatures. The observed absorption bands have been attributed to an ion of Cu²⁺ in tetragonal symmetry with ²B₁ as the ground state. A successful interpretation of all the observed bands have been made by taking into consideration spin-orbit coupling associated with a tetragonal field. The crystal field parameters derived are D_q = 1045 cm⁻¹, Ds = 1352 cm⁻¹ and Dt = 457 cm⁻¹.     

Vibrational properties of CdGa2S4

Polarization-dependent infrared reflectivity spectra of CdGa₂S₄ are measured at 300 K in the wavenumber range from 180 to 500 cm⁻¹. The analysis of the spectra yields three E and four B modes in this frequency range. The results are compared with previously published data and a final identification of the infrared active modes in CdGa₂S₄ is proposed. It is shown that the two-phonon absorption spectra of CdGa₂S₄ can be interpreted in terms of zone-centre two-phonon combination modes. The relation between the lattice vibrational properties of chalcopyrite and defect-chalcopyrite compounds is discussed.     

Transport Properties of (NH4)2 CuCl4.2H2O Low Dimensional Single Crystals

The X‐ray diffraction and Infrared (IR) spectral studies of (NH₄)₂ CuCl₄.2H₂O single crystals reveals that these crystals contains tetragonal crystal structure with the unit cell dimensions of a = 7.58Å, c = 7.95Å, z= 2, β =90° and two water molecules in the unit cell. The temperature dependence of thermally stimulated depolarization current (TSDC) and dc electrical conductivity (σ) studies of this two‐dimensional (NH₄)₂ CuCl₄.2H₂O single crystal have been carried out in 77K–300K temperature region. The TSDC thermograph shows only one sharp peak at 248K with a peak current of 130nA, which is attributed to the Maxwell‐Wagner peak. The activation energy (U), relaxation time (τ) are calculated as 0.78eV and 3.44×10^‐15 s respectively. Dc electrical conductivity studies of these crystals show a first order phase transition at about 248K.     

Absorption and electrical conductivity of CaMoO4 crystals

The electrical conductivity σ of CaMoO₄ crystals was investigated between room temperature and 850°C. The mobility v_v and the diffusion coefficient D_v of the O⁻ ion vacancies have been derived from σ: v_vT = 3300 exp (−1.52 eV/kT) cm² K/Vs, D_v = (0.1…1) × exp (−1.52 eV/kT) cm²/s. An absorption occuring in crystals which are reduced or X-irradiated at low temperatures is dichroitic and caused by Mo⁵⁺ ions. For measurement in c direction the oscillator strength of the 680 nm absorption band is found to be about 0.1.     

Electric conductivity and lattice disorder of PbMoO4 crystals

From differential thermal analysis (DTA), thermal etching, perfectly reversable redox treatments and electric conductivity it is concluded that the Pb/Mo ratio of the PbMoO₄ crystals is always 1 and that phase transitions do not occur. Pb³⁺ ions detectable by an absorption band at 435 nm cause a p-conductivity due to the reaction Pb³⁺ ⇌ Pb⁺⁺ + e⁺. At elevated temperatures the p-conductivity increases with increasing oxygen partial pressure of the surrounding atmosphere. The influence of foreign ions on the concentration of ionic and electronic defects in PbMoO₄, CaMoO₄, PbO, and PbTiO₃ can be explained by an anti-Frenkel disorder of the oxygen ion sublattice. For PbMoO₄ crystals the mobility O⁻ ion vacancies and the free formation enthalpy of anti-Frenkel defects are found to be v_v = \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop - \limits_T^{9160} $\end{document} exp (−1.15 eV/kT) cm² K/Vs and g_AF = 3.6kT −2.2 eV, respectively.     

Electron irradiation‐induced effects on optical spectra of (NH4)2ZnCl4: x Sr2+ single crystals

The effect of electron‐beam irradiation with different doses on optical constants of (NH₄)₂ZnCl₄: x Sr²⁺ crystals with x=0.000, 0.020, 0.039, 0.087 or 0.144 wt% has been studied. The optical transmission in the energy range 3.4‐6.4 eV was measured hence the absorption coefficient was computed as a frequency function. The absorption coefficient was also calculated as a function of electron‐beam dose. Irradiation with e‐beam did not affect the allowed indirect type of transition responsible for interband transitions of (NH₄)₂ZnCl₄: x Sr²⁺ crystals. Values of the optical energy gap E_g and optical moment E_p for electronic interband transition of unexposed and (NH₄)₂ZnCl₄: x Sr²⁺ crystals after e‐beam exposure were deduced. The area under the absorption band at 5.30 eV was used to evaluate the effect of e‐irradiation on optical parameters of samples with x=0.00, 0.020 or 0.039. A shift in the position and a nonmonotonic change in the intensity of this band with increasing e‐beam dose was observed. Changes in the E_g value were used to evaluate the effect of e‐beam exposure dose on (NH₄)₂ZnCl₄: x Sr²⁺ samples with x=0.087 or 0.144. The obtained results were compared with those obtained for the same crystals after irradiation with different γ‐doses.     

Crystal Structure Determination of Nickel (2,2-Dipyridyl) Sulphate Hexahydrate [Ni(C10H8N2) · 4 H2O]SO4 · 2H2O

[Ni(C₁₀H₈N₂) · 4 H₂O]SO₄ · 2 H₂O, Mr = 418.7, monoclinic, P1 , a = 7.781(3), b = 9.421(3), c = 11.457(4) Å, α = 87.54(2)º, β = 96.40(2)º, γ = 102.58(2)º, ν = 814.5 Å, D_x = 1.26 g/cm³, Z = 2, (MoKx) = 0.71073 Å, μ = 7.0 cm⁻¹: T = 23 °C, R = 0.047 for 2139 reflections having intensities greater than 36. The structure was solved using the Patterson heavy-atom method and the remaining atoms were located in succeeding difference Fourier syntheses. The structure was then refined by full matrix least-squares using anisotropic temperature factors. Hydrogen atoms were located and their positions and isotropic parameters were refined.     

Electrical and infrared optical properties of CdIn2S4 single crystals grown by chemical transport

Single crystals of CdIn₂S₄ were grown by chemical transport with iodine as transporting agent using different transporter concentrations and temperature gradients. It is found that the electron concentration increases with increasing transporter concentration and decreases after annealing in a sulphur atmosphere. Infrared reflectivity and absorption spectra are measured at room temperature in the wavenumber range from 180 to 4000 cm⁻¹ in order to determine the optical mode parameters of the compound and to evaluate the scattering mechanism of the electrons from the free carrier absorption. Lattice scattering is found to be predominant at room temperature for electron concentrations below about 4 · 10¹⁷ cm⁻³.     

Identification of crystal symmetry in Kramers and non‐Kramers ions by optical absorption: Divalent copper and nickel ions in diamagnetic lattices

The optical absorption spectra of Ni(II) doped hexaimidazole zinc(II) dichloride tetrahydrate (HZDT) and Cu(II) doped magnesium potassium phosphate hexahydrate (MPPH) have been studied at room temperature. Ni(II)/HZDT spectrum consists of three strong and one weak band. The calculated value of Dq is 1051 cm^‐1 and the interelectron repulsion parameters B and C are 854 and 3626 cm^‐1 respectively. The correlation of optical work with EPR has yielded the spin‐orbit interaction parameters λ and ξ as –225 and –450 cm^‐1 respectively. The symmetry around the Ni(II) ion is distorted octahedral, as suggested by EPR results. The estimated percentage covalency of the nickel‐nitrogen bond is around 30%. The optical absorption studies of Cu(II)/MPPH show three bands, which are identified as ²B_1g → ²A_1g, ²B_1g → ²B_2g and ²B_1g → ²E_g transitions. The octahedral field parameter Dq and the tetragonal field parameters have been evaluated from the observed adsorption bands. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An EPR Study of Scapolite

In single crystals of scapolite from two different localities, three paramagnetic centres are detected by electron paramagnetic resonance (EPR): 1. One isotropic singlet with g_iso = 2.005; 2. One triclinic singlet with g‖ = 2.005 ± 0.001 and g⟂= 2.009; 3. One triclinic sextet with g‖= 2.005 ± 0.001, g⟂ = 2.011; A‖ = 85.4 × 10⁻⁴ cm⁻⁴, A⟂ = 85.3 × 10⁻⁴ cm⁻¹. Centres 1 and 2 can be attributed to colour centres as they are bleached after annealing. Centre 3 can be due to Mn²⁺ (only the central M_s = ± 1/2 transition is observable) most likely substituting for Ca²⁺ The site symmetry must be triclinic but due to Al, Si disorder and mixed Na, Ca composition the different components from magnetically non-equivalent sites are averaged out for many orientations.     

Optical absorption of Fe3+ Ion in LiF crystals

The absorption spectrum of LiF—Fe³⁺ single crystals was investigated at liquid nitrogen temperature. Six new absorption bands, unobserved earlier in other works, were found, which were located at 11 200, 16 000, 22 700, 27300, 30 500, and 41 900 cm⁻¹. The spectrum was interpreted in the cubic cristalline field approximation with D_q = 1397 cm⁻¹, B = 657 cm⁻¹, C = 3226 cm⁻¹. Above mentioned bands were assigned to the transitions from a ground state ⁶A_1g to the levels ⁴T_1g(G), ⁴T_2g(G), ⁴A_1g(G) (⁴E_g), ⁴E_g(D), ²T_1g(F), and ⁴T_1g(F), respectively.     

Growth and characterization of pure and Co2+‐doped Li2B4O7 single crystals

Pure and Co‐doped Li₂B₄O₇ (LBO) single crystals were grown by the Czochralski method. Starting concentrations of Co₂O₃ in the melt were: 0.5, 0.85 and 1 mol% relative to Li₂CO₃. Technological factors affecting the quality of both crystals were discussed. Optical absorption and EPR spectra were analyzed to define the oxidation states and lattice sites of cobalt ions. It was shown that Co²⁺ ions enter LBO crystal at octahedral Li⁺ site positions. Low‐temperature EPR measurements revealed that two types of Co²⁺ complexes can be distinguished in the Li₂B₄O₇:Co crystals. Additional absorption calculated for γ‐irradiated crystals showed V_k type defects suggesting the creation of cation vacancies during growth. The concentration of the defects decreases with an increase of intentional Co concentration. Introduction of cobalt ions to LBO crystal is limited probably by the formation of cobalt ion pairs or by the entrance of cobalt as Co⁺. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural Analysis of the Phase Transitions of (NH<Subscript>4</Subscript>)<Subscript>4</Subscript>HgBr<Subscript>6</Subscript>

Abstract Single crystal diffraction and differential scanning calorimetry DSC techniques have been used to investigate the different phases of (NH₄)₄HgBr₆, tetrammonium mercury hexabromide, from room temperature to 120 K. Two anomalies in thermal behaviour were detected for this compound at 190 and 268 K, by DSC experiment. X-ray diffraction measurements confirm these anomalies. At room temperature the structure is tetragonal P4/mnc (No. 128) with lattice parameters a = b = 9.25560(8) ?; c = 8.8657(11) ?; V = 759.49(9) ?³ and Z = 2. At T = 250 K the structure is orthorhombic Pnnm with lattice parameters a = 8.8436(8) ?; b = 9.2191(8) ?; c = 9.2232(7) ?; V = 751.97(11) ?³ and Z = 2. Below approximately 200 K the structure is monoclinic P2₁/n (No. 14) with: a = 8.8080(9) ?; b = 9.1608(8) ?; c = 9.1498(8) ?; β = 90.230(7)°; V = 738.28(12) ?³ and Z = 2 (T = 120 K). The structure of (NH₄)₄HgBr₆ consists of isolated HgBr₆-octahedra in the whole temperature range which are slightly compressed in c-direction. The ammonium groups are located between the octahedra ensuring the stability of the structure by hydrogen bonding contacts: N–H···Br. The structural phase transformations are described by a rotation of the [HgBr₆]²⁻ octahedra around the c-axis, and this behaviour is attributed to an orientational disorder of ammonium groups. Index abstract Structural analysis of the phase transitions of (NH₄)₄HgBr₆; M. Loukil, A. Kabadou, I. Svoboda, A. Ben Salah and H. Fuess; The phase transformations in (NH₄)₄HgBr₆ are explained by large rotation of [HgBr₆]²⁻ octahedra around the c-axis.      

Solubility of CaO in CaF2 crystals

By means of conductivity investigations the solubility of CaO in CaF₂ crystals was estimated to be C_L(CaO) ≈︁ 3.7 exp (-0.68 eV/kT) = 3.7 exp (-7900 K/T) and the diffusion constant of O⁻ ions in CaF₂ D(O⁻) ≈︁ 3 exp (-2.2 eV/kT) cm²/s = 3 exp (-25 500 K/T) cm²/s.     

Preparation and studies of new crystals in the K3H(SO4)2-(NH4)3H(SO4)2-H2O system

To elucidate the effect of isomorphic substitution on the kinetics of phase transitions, single crystals of (K _x (NH₄)_1?x ) _m H _n (SO₄)_{(m + n)/2} · yH₂O solid solutions are grown from the K₃H(SO₄)₂-(NH₄)₃H(SO₄)₂-H₂O system, whose end members are known to undergo superprotonic phase transitions of fundamentally different kinetics. The chemical composition of the single crystals grown is determined by energy dispersive X-ray microanalysis. The thermal and optical behavior of (K,NH₄)₉H₇(SO₄)₈ · H₂O single crystals is studied in the temperature range 295–420 K and the crystal structure at 295 K is determined. A comparison of the results of the studies with data for crystal K₉H₇(SO₄)₈ · H₂O published earlier shows that the substitution of ammonium for potassium atoms lowers the temperature of the structural phase transition by 8 K.     

Crystal structure of marmesin,C14H14O4, 2-(β-hydroxyisopropyl)-2,3-dihydro-6,7-furano-coumarin

The unit cell parameters of this compound were determined by least-squares calculations from the adjusted angular setting of 25 general reflections. The crystals are monoclinic with the following crystallographic data: a = 5.718(1), b = 13.794(4), c = 7.861(1) Å, β = 100.53(2)°, Z = 2 and D_x = 1.34 g · cm⁻³. The space group according to the systematic absences is P2₁. The structure of this compound was solved by direct methods and refined by full matrix least-squares with anisotropic temperature factors to R = 0.04. The refined atomic positions indicate a considerable strain within the molecule. The structure is built up from molecules connected by H-atoms forming infinite chains along b-axis.