Optical and electrical characterization of cadmium selenide doped with cobalt

The optical and electrical properties of Co²⁺ ions in CdSe have been investigated. Absorption, photoluminescence, electron paramagnetic resonance, and Hall measurements were used to characterize a cobalt-doped (1×10¹⁹ cm⁻³) single crystal. Infrared absorption and emission spectra associated with transitions between the ⁴A₂(F) ground state and the ⁴T₁(F) and ⁴T₂(F) excited states are described. At 10 K, spin-orbit splittings result in three structured absorption bands associated with the ⁴A₂(F) to ⁴T₁(F) transition having zero-phonon lines at 4926, 5101, and 5724 cm⁻¹. The ⁴A₂(F) to ⁴T₂(F) transition shows two zero-phonon lines at 2874 and 3286 cm⁻¹, also accompanied by vibronic structure. Intrinsic lattice modes explain most of the sharp-line structure observed at low temperature, except for a subset of peaks where local modes (25-30 cm⁻¹) are invoked. Using below-band-gap light, selective excitation allows detection of the ⁴T₁(F) to ⁴A₂(F) recombination at liquid-helium temperatures. The activation of free carriers in our n-type material containing shallow donors is affected by the presence of cobalt, and we find the Co^+/++ level to be about 34 meV below the conduction band of CdSe.     

Energy levels of vanadium ions in CdTe

In CdTe doped with vanadium the photoluminescence due to the ³ T ₂(F) ³ A ₂(F) transition of V³⁺(d ²) is detected. Its decay time is determined as (630±20) s, a result comparable to the analogous emissions in various host lattices. Further emissions around 5000 cm^–1 and 9000 cm^–1 are caused by charge-transfer transitions or bound-exciton decay. Excitation and sensitization spectra yield information on the positions of the energy levels within the gap, which are discussed using two different models. At T=4.2 K, the distance of the V²⁺/V³⁺ donor level is 7300 cm^–1 and 5700 cm^–1 referred to the valence and the conduction band edges, respectively. The absence of V²⁺(d ³) centres is tentatively ascribed to the existence of deeply bound excitons.     

Dielectric spectroscopy characteristics of ferroelectric Pb0.77K0.26Li0.2Ti0.25Nb1.8O6 ceramics

Temperature and frequency dependence of dielectric constant and conductivity properties of Pb_0.77K_0.26Li_0.2Ti_0.25Nb_1.8O₆ (PKLTN) ceramics are modelled through the universal dielectric response (UDR). Partial substitution of Ti⁴⁺ for Nb⁵⁺ was compensated by charge and the creation of oxygen vacancies according to the Kroger-Vink notation. The electrons released by this reaction are captured by Nb⁵⁺ and Ti⁴⁺ to generate Nb³⁺ and/or Ti³⁺. The hopping of electrons between Nb⁵⁺–Nb³⁺ and Ti⁴⁺–Ti³⁺ are believed to participate in conductivity. Characterization of the dielectric constant has been performed from room temperature to 590°C in the frequency range from 45 Hz to 5 MHz. The measured dielectric constant obeyed Jonscher's dielectric dispersion relations: ε ^l = ε _∞ + sin(n(T)π/2)(a(T)/ε ₀)(ω^{n ( T )?1}) and ε ^ll = σ/iε ₀ ω + cos(n(T)π/2)(a(T)/ε ₀)(ω^{n ( T )?1}). Cole-Cole plots inclined at an angle (1 ? n(T))π/2 and followed the trend of universal material behavior ε _∞ + A(T)(ω^{n ( T )?1}). The exponent n(T) and coefficient A(T) = (a(T)S/L) exhibited a minimum and maximum at T _c = 425°C, respectively. The conductivity studies show the contribution of the hopping of bound charge carriers to conduction in PKLTN.     

Magnetic interaction in Mg, Ti, Nb doped manganites

An effect of Mn substitution with Me=Mg²⁺, Ti⁴⁺, Nb⁵⁺ in manganites has been investigated by preparing La_0.7Sr_0.3(Mn_1-xMe_x)O₃ and La_1-xSr_x(Mn_{1 - x/2}Nb_x/2)O₃ series. It was established that substitution of manganese with magnesium up to x = 0.16 leads to a collapse of a long-range ferromagnetic order whereas La_0.7Sr_0.3(Mn ^{3 +} _0.85Nb ^{5 +} _0.15)O₃ is ferromagnet with T _C = 123 K and exhibits a large magnetoresistance below Curie point despite an absence of four-valent manganese. Hypothetical magnetic phase diagrams are constructed for La_0.7Sr_0.3(Mn_1-xMe_x)O₃ and La_1-xSr_x(Mn_{1 - x/2}Nb_x/2)O₃. Our results show that Mn³⁺-O-Mn³⁺ exchange interaction is ferromagnetic in the orbitally disordered manganites as well as an increase of Mn⁴⁺ content above 50% from a total amount of manganese ions leads to formation of a spin glass state due to a competition between antiferromagnetic Mn⁴⁺-O-Mn⁴⁺ and ferromagnetic Mn³⁺-O-Mn⁴⁺(Mn³⁺) superexchange interactions. Received 24 January 2002 Published online 9 July 2002     

EPR parameters and defect structures of the off-center Ti ion on the Sr site in neutron-irradiated SrTiO3 crystal

The EPR parameters (g factors and hyperfine structure constants A) for the tetragonal Ti³⁺ center in cubic phase and the rhombic Ti³⁺ center in tetragonal phase in the neutron-irradiated SrTiO₃ crystals are calculated from the third-order perturbation formulas of EPR parameters for d¹ ions. These low-symmetry Ti³⁺ centers in both phases of SrTiO₃ are due to the Ti³⁺ ion at “off center” on the Sr²⁺ site. From the calculation, the defect models (including the direction and magnitude of the Ti³⁺ off-center displacement) of the two Ti³⁺ centers in SrTiO₃ are estimated and the EPR parameters of both Ti³⁺ centers are reasonably explained on the basis of the defect models. The results are discussed.     

(T1u+T1g)⊗(hg+τ1u)vibronic interaction and superconductivity in C 60 n− fullerides

The closeness of low-lying T_1u and T_1g levels of C ₆₀ ⁻ could enable their mixing under an odd parity vibration of (T_{1 u} + T_{1 g} ⊗ (h_g + τ_{1 u})type. In addition, the two levels are susceptible to Jahn-Teller interaction due to five-fold degenerate h_g vibrations. This complex problem of (T_1u+T_1g)⊗(h_g+τ_1u) vibronic interaction is transformed to a form similar to T_2g ⊗ (ε_g + τ_2g) vibronic problem of octahedral symmetry. The problem is analysed in an infinite coupling model and compared with the experimental spectroscopic results for the C ₆₀ ⁻ radical. The resulting parameters are used to calculate the pair-binding energy and superconducting transition temperature in C ₆₀ ⁿ⁻ fullerides. Vibronic mixing with the T_1g level is found to be responsible for maximising the pair-binding energy at the doping level n=3. It is also found to be an important source of T_c enhancement.     

Structural Phase Transition and the Dielectric Permittivity of the Model Lipid Bilayer [(CH2)12(NH3)2]CuCl4

Structural phase transitions in the lipid-like bilayer material [(CH₂)₁₂(NH₃)₂]CuCl₄ have been observed using differential thermal scanning. The compound shows an irreversible thermochromic transition at ? 465 K and three reversible transitions at T ₁ = 433 ± 4 K and T ₂ = 411 ± 2 K and T ₃ = 358 K. The transition at 350 K is ascribed to chain melting. The other two correspond to crystalline phase transformation.

Phase (IV) T₃ = 358 ± 2K Phase (III) T₂ = 411 ± 2K Phase (II) T₁ = 433 ± 4K Phase (I)

Dielectric permittivity is studied as a function of temperature in the range 300-440 K and frequency, range (60 Hz-100 kHz). It confirms the observed transitions. The dielectric permittivity reflects rotational and conformational transitions for the compound. The variation of the real part of the conductivity with temperature is thermally activated in the temperature range above 350 K, with frequency-dependent activation energy, the values of activation energy lie in the range of ionic hopping. The dependence of the conductivity on frequency follows the universal power law σ = σ₀ + A(T) ω ^{s ( T )} with 0<s<1. Comparison of this material with other members of the series is discussed     

A study of the temperature dependence of the rate constant for the reaction of O<Subscript>2</Subscript> with surface alkoxy radicals

A method for studying the reactions of surface alkoxy radicals with O₂ at temperatures of 230 to 300 K is described. Alkoxy radicals were generated directly in the cavity of an EPR spectrometer. Surface organic radicals, prepared from paraffin wax ((CH₃)₂(CH₂) _n , n = 16–20), were applied to Aerosil particles from a solution in heptane. The Aerosil sample was placed in the cavity of the EPR spectrometer in a cylindrical cup with a central hole for pumping out gases and exposed to H atoms. In this way, it is possible observe a steady increase in the EPR signal from the surface radicals. To measure the rate constant at tropospheric temperatures, the reaction tube was placed in a Teflon jacket, through which cool nitrogen vapor was pumped. The temperature in the reactor was varied from 230 to 300 K. The recorded EPR spectra belong to the (RO) _s radical. After obtaining a stable EPR signal from the surface radicals, treatment with H atoms was stopped, additional flow of O₂ was introduced ([O₂] = 10¹⁴–10¹⁶ cm⁻³), and the reaction of O₂ with the surface organic radicals was studied by monitoring the EPR signal decay. The temperature dependence of the rate constant for the (RO) _s + O₂ → HO₂ + ketone was obtained within T = 230–300 K. The extrapolation of the data to real tropospheric conditions ([O₂] = 10¹⁸ cm⁻³) was performed.     

Electronic pair excitations of Mn2+ and Ni2+ in perovskite fluorides: involvement of the 3 T 1g a state of Ni2+

Crystals of KZnF₃ and KMgF₃ doped with Mn²⁺ and Ni²⁺ were used to study the spectroscopic properties of Mn²⁺-F^--Ni²⁺ pairs. Pair transitions to the doubly excited states ⁴ E_g ^a , ⁴ A _1g (Mn)³ T _1g ^a (Ni) and ⁴ E_g ^b (Mn)³ T _1g ^a (Ni) were observed. The participation of the spin-allowed ³ A _2g →³ T _1g ^a excitation on Ni²⁺ in the pair transition is explained by spin-orbit mixing between ³ T _1g ^a and ¹ E_g . The prominent electronic origins are assigned to the double spin-flip transitions ⁶ A _1g (Mn)³ A _2g (Ni) →⁴ E_gu(Mn)³ T _1g ^a (Γ₃)v(Ni) and ⁴ A _1g (Mn)³ T _1g ^a (Γ₃)v(Ni). The former lie at lower energy and are more intense than the corresponding ⁶ A _1g (Mn)³ A _2g (Ni) →⁴ E_gv(Mn)³ T _1g ^a (Γ₃)u(Ni) transitions involving two orbital jumps. The well-resolved vibronic structure is composed of three basic vibrations of ～ 150 cm^-1, ～ 294 cm^-1 and ～ 508 cm^-1 in the KZnF₃ host.     

Vibronic coupling and intersystem crossing in aromatic amines

The fluorescence and phosphorescence spectra of the aromatic amines acridan, iminobibenzyl, and carbazole have been measured in Shpolskii matrices at 10 K. Under these conditions the emission exhibits a detailed vibrational structure which has been analyzed. The change of the polarization degree observed within the fluorescence spectra at 77 K, particularly pronounced in acridan and iminobibenzyl, is attributed to vibronic interaction between the closely lying S₁(¹A₁) and S₂(¹B₁) excited states. This process activates a b₁ vibration with a frequency of 1200 cm⁻¹ in the ground state. The appearance of a long-axis (b₁) polarized vibration (700 cm⁻¹) following the out-of-plane polarized 0-0 band of the phosphorescence of these amines at 77 K is suggested to arise from vibronic interactions in the triplet manifold. This second-order spin-orbit coupling (soc) process is superimposed upon the dominant first-order electronic soc mechanism, which couples the lowest π, π^* triplet with high-energy (σ, π)^* singlet states.     

A novel high-k ‘Y5V’ barium titanate ceramics co-doped with lanthanum and cerium

Structural, dielectric, and ferroelectric properties of a novel high-k ‘Y5V’ (Ba_1−xLa_x)(Ti_1−x/4−yCe_y)O₃ ceramics (where x=0.03 and y=0.05, denoted by BL3TC5) with the highest ‘Y5V’ dielectric response (ε′>10 000) among rare-earth-doped BaTiO₃ ceramics to date are investigated in detail using SEM, TEM, XRD, DSC, EPR, Raman spectroscopy (RS), temperature and frequency, electric field dependences of dielectric permittivity (ε′), and temperature and electric field dependences of ferroelectric hysteresis loops. The BL3TC5 diffusion of ferroelectric phase transition occurs around dielectric peak temperatures (T_m) near a room temperature characteristic of dielectric thermal relaxation. Powder XRD data and defect complex model were given. “Relaxor” behavior associated with an order/disorder model and formation of a solid solution were discussed. The EPR results provided the evidence of Ti vacancies as compensating for lattice defects. High-k relaxor nature of BL3TC5 is characterized by an average cubic structure with long-range lattice disordering and local polar ordering; a slow change of the ε′ (T) and P_r(T) curves around T_m; no phase transition observed by DSC; and a broad, red-shifted A₁ (TO₂) Raman phonon mode at 251 cm⁻¹ accompanying the disappearance of the “silent” mode at 305 cm⁻¹ and a clear anti-resonance effect at 126 cm⁻¹ at room temperature.     

Optical absorption, EPR, infrared and Raman spectral studies of clinochlore mineral

Optical absorption, EPR, Infrared and Raman spectral studies have been carried out on natural clinochlore mineral. The optical absorption spectrum exhibits bands characteristic of Fe²⁺ and Fe³⁺ ions. A band observed in the NIR region is attributed to an intervalence charge transfer (Fe²⁺-Fe³⁺) band. The room temperature EPR spectrum of single crystal of clinochlore mineral reveals the dominance of Fe³⁺ ion exhibiting resonance signals at g=2.66; 3.68 and 4.31 besides one isotropic resonance signal at g=2.0. The EPR studies have been carried out for a polycrystalline sample in the temperature range from 103 to 443 K and for a single crystal of clinochlore mineral in the temperature range 123-297 K. The number of spins (N) participating in resonance at g=4.3 signal of the single crystal of clinochlore mineral has been calculated at different temperatures. The paramagnetic susceptibility (χ) is calculated from the EPR data at different temperatures for single crystal of clinochlore mineral. The Curie constant and Curie temperature values are evaluated from 1/χ versus T graph. The infrared spectral studies reveal the formation of Fe³⁺-OH complexes due to the presence of higher amount of iron in this mineral. The Raman spectrum exhibits bands characteristic of Si-O-Si stretching and Si-O bending modes.     

Table-like magnetocaloric effect involving the enhancement of refrigerant capacity in (AMn0.9Ti0.1O3)1−x/(AMn0.85Ti0.15O3)x composite

In this paper, a table-like magnetocaloric effect (MCE) is obtained in the (AMn_0.9Ti_0.1O₃)_1?x/(AMn_0.85Ti_0.15O₃)_x composite system. A flattening behaviour of the entropy change is investigated from the isothermal magnetic entropy change versus temperature curves ?S(T) of AMn_0.9Ti_0.1O₃ and AMn_0.85Ti_0.15O₃ (A = La_0.57Nd_0.1Pb_0.33) manganite materials. Both compounds exhibit nearly some large MCE, 5.2–4.85 J kg^?1 K^?1, under an applied field of 0–5 T, around their respective Curie temperature (T_C) ranged from 279 to 299 K. The temperature dependence of the isothermal magnetic entropy change ΔS(T) is calculated for (AMn_0.9Ti_0.1O₃)_1?x/(AMn_0.85Ti_0.15O₃)_x composite with 0 ≤ x ≤ 1. The optimum ΔS(T) of the composite with x = 0.495 approaches a nearly constant value of ～3.63 J kg^?1 K^?1 in a field change of 0–5 T in a wide temperature span over 15 K, resulting in large refrigerant capacity value of ～172.3 J kg^?1. The (AMn_0.9Ti_0.1O₃)_0.505/(AMn_0.85Ti_0.15O₃)_0.495 composite system indicates that this approach can be used to design magnetic refrigerant materials with enhanced magnetocaloric response in a magnetic refrigerator performing an Ericsson cycle near room temperature.     

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.     

Raman light scattering,infrared absorption and DSC studies of the phase transition and vibrational and reorientational dynamics of H2O ligands and ClO4– anions in [Ba(H2O)3](ClO4)2

[Ba(H₂O)₃](ClO₄)₂ between 90 and 300 K possesses two solid phases. One phase transition of the first‐order type at: = 211.3 K (on heating) and = 204.6 K (on cooling) was determined by differential scanning calorimetry. The entropy change value (ΔS ≈ 15 Jmol^–1 K^–1), associated with the observed phase transition, indicates a moderate degree of molecular dynamical disorder. Both, vibrational and reorientational motions of H₂O ligands and ClO₄^– anions, in the high‐temperature and low‐temperature phases, were investigated by Fourier transform far‐infrared and middle‐infrared and Raman light scattering spectroscopies. The temperature dependences of the full‐width at half‐maximum values of the bands associated with ρ_w(H₂O) mode, in both infrared (~570 cm^–1) and Raman light scattering (~535 cm^–1) spectra, suggest that the observed phase transition is not associated with a sudden change of a speed of the H₂O reorientational motions. Ligands reorient fast, with correlation time of the order of several picoseconds, with a mean activation energy value E_a = 5.1 kJ mol^–1 in both high and low temperature phases. On the other hand, measurements of temperature dependences of full‐width at half‐maximum values of the infrared band at ~460 cm^–1, associated with δ_d(OClO)E mode, and Raman band at ~1105 cm^–1, associated with ν_as(ClO)F₂ mode, revealed the existence of a fast ClO₄^– reorientation in phase I and in phase II, with the E_a(I) and E_a(II) values equal to 8.0 and 6.5 kJ mol^–1, respectively. These reorientational motions of ClO₄^– are slightly distorted at the T_C. Fourier transform far‐infrared and middle‐infrared spectra with decreasing of temperature indicated characteristic changes at the vicinity of PT at T_C, which suggested lowering of the crystal structure symmetry. All these experimental facts suggest that the discovered phase transition is associated with small change of H₂O ligands and somewhat major change of ClO₄^– anions reorientational dynamics, and with insignificant change of the crystal structure, too. Copyright © 2012 John Wiley & Sons, Ltd.     

Electronic absorption spectrum of Ni2+ in lithium potassium sulphate single crystal

Single crystals of nickel-doped lithium potassium sulphate were grown by slow evaporation method at room temperature. From the nature and position of the bands observed, a successful interpretation of all the bands could be made assumingO _h symmetry for the Ni²⁺ ion in the crystal. The bands have been assigned transitions from the ground³A_2g(F) state to the excited³T_2g(F),¹E_g(D),³T_1g(F),¹T_2g(D) and³T_1g(P) states. The crystal field parameters derived areDq=910cm^–1,B=890cm^–1 andC=3560cm^–1.The authors wish to express their thanks to Prof. K. Sreerama Murthy for his constant encouragement throughout this investigation. The authors are also thankful to Prof. Mihir Chowdhury, Indian Association for the cultivation of Science, Calcutta for giving permission to take the spectra.     

Estimation of T1 for Co2+ ions from temperature variation of the EPR linewidths for Gd3+ in Ce2Co3(NO3)12.24H2O single crystals

EPR of Gd³⁺ doped in Ce₂M″₃(NO₃)₁₂.24H₂O (M″ = Mg, Zn, Co) single crystals has been studied at various temperatures from room temperature to 77 K using ∼ 9.45 GHz EPR spectrometer. The observation of resolved Gd³⁺ spectra at room temperature in Ce₃Co₂(NO₃)₁₂.24H₂O has been interpreted in terms of a random modulation of the interaction between the Gd³⁺ and the Co²⁺ ions by the rapid spin-lattice relaxation of Co²⁺ ions. It is found that the effective spin-lattice relaxation time T₁ ∝ T⁻ⁿ where n = 1.85 (B∥z axis) and n = 1.75 (B⊥z axis) if 103 < T < 283 K.     

The potential barrier height for a Jahn-Teller center

The EPR spectrum of the Cu²⁺ ion in a ZnSiF₆·6H₂O crystal is studied in the temperature range T=5?300K. It is shown that the EPR spectrum can be represented in the form of a superposition of three contributions with essentially different properties. The first contribution is characterized by the maximal intensity at low temperatures and is described by a spin Hamiltonian with a large anisotropy of parameters. The second contribution has the maximal intensity at high temperatures and is described by a spin Hamiltonian with a low anisotropy of parameters. The third contribution cannot be described by a spin Hamiltonian and has the form of a partly orientationally averaged EPR spectrum. The reason for the emergence of these contributions is substantiated along with the form of the temperature dependence of their intensities on the basis of variation of the populations of vibronic states upon a change in temperature. The height (E₀=4±1cm_?1) of the potential barrier separating three equivalent Jahn-Teller potential wells of the Cu²⁺ ion is determined from analysis of the temperature dependence of the integrated intensity of the EPR spectrum. The obtained value of the barrier height substantially differs from the estimate (100 cm^?1) obtained earlier [2, 3] for the Cu²⁺ ion in ZnSiF₆·6H₂O on the basis of the tunneling model. It is shown that the forms of the temperature dependences of the linewidth of the low-and high-temperature EPR spectra are essentially different. This difference indicates that the contributions of the low-and high-temperature EPR spectra are associated with quantum-mechanical transitions between these states. It is proposed that the low-and high temperature contributions to the EPR spectrum are associated with the filling of under-the-barrier and above-the-barrier vibronic states, respectively.