First principles study of structural stability, electronic and related properties of (NH4)2SO4

The electronic structure and related physical properties of crystalline ammonium sulfate, (NH₄)₂SO₄, have been studied using the first principles code CRYSTAL06 at the B3LYP level of theory. The title compound has been found to possess one stable and three metastable configurations, all within the polar space group Pna2₁ (no. 33). Two of the metastable polymorphs are newly predicted and have not yet been observed experimentally. The different configurations show considerably varying magnitudes of the spontaneous polarization P_s. All coefficients of the elastic stiffness tensor, c_kl, and elasto-electrical tensor, e_ki have been calculated for the first time and have been found to agree satisfactorily with experimental data, as far as available.     

Laser Raman Spectrum of Triglycine Sulfate

Abstract

The laser Raman spectrum of ferroelectric triglycine sulfate has been determined by the use of an argon ion gas laser. The observed Raman lines are compared with reported assignments in the literature by Taurel¹ and Krishnan², Determination of depolarization ratio of the sulfate line at 980 cm^?1 shows that the Raman band is highly polarizable and the molecular vibration is totally symmetrical v₁.

The role played by the glycine groups in the spontaneous polarization and its reversal can be indirectly confirmed by Raman spectrum of TGS. Interpretation of the Raman spectrum indicates that the SO₄ groups do not have tetrahedral symmetry at roan temperature. Ferroelectric behavior of TGS is attributed to the glycine groups.     

Simulation of elasto optical properties of K2SO4 crystals

The electronic optical spectra of the mechanically free and stressed crystals of potassium sulfate, K₂SO₄, in the orthorhombic phase Pnma have been calculated by the Cambridge Serial Total Energy Package (CASTEP) code. On the basis of these calculations, the components of stress elasto optical tensors based on the changes of refractive index n (π_im) and birefringence Δn () (i, k, m=1, 2, …, 6) have been obtained for the indices i, k ,m=1, 2, 3. Absolute magnitudes of the calculated tensor π_im are probably underestimated because the magnitudes of the calculated elastic stiffness tensor c_rm are found to be overestimated about two times. Features of the spectral dependences n(E) and k(E) of refractive and absorption indices of the mechanically free and stressed potassium sulfate crystals have been analyzed.     

Band structure and optical spectra of RbNH4SO4 crystals

First-principal density functional theory (DFT) calculations of the band structure, density of states and dielectric functions ε(E) of the rubidium ammonium sulfate (RAS) crystal, RbNH₄SO₄, in the orthorhombic phase Pnma have been carried out using the CASTEP code. Valence electron bands of the crystal are flat in k-space, that responds to the relatively great effective mass, m*?5m_e. The top valence band of the crystal has been found to be the most flat, what might be an evidence of a weak chemical bonding of the sulfate complexes (SO₄) in the crystal and therefore for the predisposition to structural instability and phase transitions. The characteristic feature is that two top valence bands are originated almost entirely from p-electrons of oxygen. The bottom part of the conduction band is formed mainly by the hydrogen atoms, the higher parts of this band—by a mixed set of chemical elements and orbital moments. The calculated refractive indices in the range of crystal's transparency agree satisfactorily with the experiment considering that the infrared absorption is not taken into account in calculations.     

Effect of a temperature gradient on the polarization distribution in the surface layer of TGS crystals

Effect of variable temperature gradients on the polarization of the surface layers of triglycine-sulphate (TGS) crystals has been analyzed. A fundamental difference in the impact of these gradients on the sample sides corresponding to the positive (“+” P _s ) and negative (“?” P _s ) yield of the polarization vector has been discovered. In particular, it has been found that repolarization of the surface layer under the impact of thermal radiation occurs only on the “?” P _s side.     

Temperature dependence of the elastic constant c16 in Ca2Sr(C2H5CO2)6

The temperature dependence of the elastic constant c₁₆ in Ca₂Sr(C₂H₅CO₂)₆ is experimentally obtained and found to obey a simple relation c₁₆ = e₁₆P_s (e₁₆ = ?2.5 x 10¹⁰dyne/μC), where P_s is the spontaneous polarization.     

Refractometry of uniaxially compressed triglycine sulphate crystals doped with L-valine

The temperature and spectral dependences of the refractive indices n _i of triglycine sulphate (TGS) crystals doped with L-valine have been investigated. Doping is found to weaken the temperature dependence of n _i of TGS crystals. The electronic polarizabilities ?? _i , refractions R _i , and parameters of UV oscillators (??_0i , B _1i ) of mechanically distorted doped TGS crystals have been calculated. The temperature coefficients of the shift of the phase-transition point, ?T _c /??? _m , are found to be somewhat smaller than those for pure TGS crystals, which is confirmed by the increase in the hardness of TGS crystals after doping.     

Electro-optical features of antiferroelectric liquid crystal material (S)-(+)-4-(1-methylheptyloxycarbonyl) phenyl 4′-(6-octanoyloxyhex-1-oxy) biphenyl-4-carboxylate in its ferro and antiferroelectric phases

A detailed investigation of the electro-optical switching parameters of an antiferroelectric liquid crystal (S)-(+)-4-(1-methylheptyloxycarbonyl) phenyl 4′-(6-octanoyloxyhex-1-oxy) biphenyl-4-carboxylate (abbreviated as S-7H6Bi) has been carried out. S-7H6Bi has paraelectric (SmA^?) and ferroelectric (SmC^?) phases in addition to antiferroelectric (SmC^?_A) phase. Switching parameters viz. spontaneous polarization and switching time were determined by polarization reversal method. The spontaneous polarization (P_s) is found to be highly temperature dependent and decreases with temperature. The maximum value of P_s is found to be ∼90 nC/cm² whereas the switching time (t_s) is found to be of the order 1-2 ms. The temperature dependent torsional viscosity (γ_t) is of the order 10 Pa sec. It increases with decrease in temperature.     

Aspects of symmetry in the problem of phase transitions in improper ferroelectrics

The term ‘improper’ ferroelectrics has recently come to be used for ferroelectrics in which the spontaneous polarizationP _s is not a transition parameter. From the standpoint of symmetry therefore, the new (polar) symmetry group is no longer the highest common sub-group of the crystal symmetry and the spontaneous polarization symmetry (polar vector symmetry group ∞mm). The concept of an improper ferroelectric has therefore been revised to include transitions from polar to polar groups. It is shown that in the general cases where other parameters are permissible, the problem has to be solved using the symmetry of directions. The present analysis on the bases of irreducible representation of polar crystals establishes all the possible cases where spontaneous polarization is accompanied by spontaneous magnetisation, optical activity and more complex phenomena. The symmetry changes and the polarization configurations in Gd(MoO₄)₃, boracites, (NH₄)₂BeF₄, alkali trihydrogen selenite family have been discussed in the light of the extended concept of an ‘improper’ ferroelectric.     

Electro-optic characterization of tetragonal (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 single crystals by a modified Sénarmont setup

The refractive indices of tetragonal (1−x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-xPT) single crystals were measured with a prism coupler and their linear electro-optic (EO) properties were investigated from 20 to 80 °C by the automated scanning Sénarmont system with an ac field. The composition and temperature effect on the EO coefficients were also discussed. It has been found that their EO coefficients are much larger than that of widely used LiNbO₃ single crystal and the calculated half-wave voltages are also much lower, which enable the operation at lower voltages and the smaller device dimensions. Since the excellent EO properties are very stable and such high quality single crystals with large-size have been obtained, the PMN-xPT single crystals are a very promising candidate for EO modulation applications. By linking to the polarization-related quadratic EO coefficients, we find that the linear EO properties are related with the spontaneous polarization and dielectric constants.     

Ferroelastoelectric phenomena in a uniaxial ferroelectric TGS crystal

Possibility of the ferroelastoelectric behavior manifestations in a uniaxial ferroelectric TGS crystal was investigated. Analysis of the spontaneous piezoelectric moduli tensors for each of the 180°-domains formed as a result of the phase transition in TGS revealed possible directions for simultaneous application of the electric field and mechanical stresses to induce the switching effect in TGS. Influence of the uniaxial mechanical stresses σ₁₁, σ₂₂, σ₃₃ on the parameters of saturated and unsaturated dielectric hysteresis loops in TGS is considered. It was found that a nontraditional E ₃σ₁₂ combination of fields may lead to the domain switching in TGS crystals.     

Investigation of piezoelectric properties of crystals using surface excitation of ultrasonic waves and some results for ferroelectric triglycine sulphate

The surface excitation of ultrasonic waves as a new method of investigation of piezoelectric properties of materials is suggested and experimental results specified to theZ-direction of a single crystal of triglycine sulphate (TGS) are reported. It has been found that the piezoelectric stress constant e₂₃ of TGS depends on temperatureθ as:e ₂₃=K ₂₃(θ_c?θ)^?1 whereK ₂₃=(31±0.5) As m^?2 deg andθ _c=49.6°C. The influence of an externally applied d.c. electric field has been studied in order to account for a residual excitation of ultrasonic waves in TGS at temperatures above the Curie point.     

Memory effect in the triglycine sulphate crystal polarized not parallel to the ferroelectric axis

The experimental results concerning the electric susceptibility χ₂₂ of the triglycine sulphate (TGS) crystal polarized not parallel to the ferroelectric axis at different temperatures have been shown. The process of partial rejuvenation in the ferroelectric phase has been presented. It has been found that the temperature changes of the slope correspond to step-wise changes of a pyroelectric coefficient p=dP/dT (P=P₂, P₃), where P₂ and P₃ are the longitudinal and transverse polarizations, respectively. An interpretation of temperature dependences of P₂ and P₃ as well as χ₂₂ of the triglycine sulphate ferroelectric previously exposed to a prolonged transverse electric field has been proposed. It has been concluded that the number of elementary cells containing molecules in metastable states depends on temperature in such a way that some kind of memory effect can be observed.     

Thermally stimulated electron emission from chromium-doped triglycine sulfate in the paraelectric phase

Thermally stimulated electron emission from a ferroelectric chromium-doped triglycine sulfate (TGS) crystal was experimentally observed to occur in a temperature range 6 K above the Curie point from samples heated at a relatively high rate. Increasing the heating rate q was shown to cause the emission current density to increase throughout the temperature range studied. The emission onset temperature in chromium-doped TGS depends only weakly on the rate q and is close to that for pure TGS, and the emission cutoff temperature grows monotonically with q at comparatively low heating rates and stabilizes at high q. At the same time, the interval of emission extension into the paraelectric phase here is about one half that for pure TGS heated at the same rate. The specific features of emission observed for this crystal can be assigned to relaxation of the charges screening the spontaneous polarization. The lower emission cutoff temperature for the chromium-doped TGS compared to that for pure TGS is accounted for by the shorter Maxwellian relaxation time in the doped crystal.     

Electrochemical determinations of the chemical diffusion coefficient and non-stoichiometry in AgCl

Measurements were made of the electronic conductivities, σ_e and σ_h, of AgCl as a function of P_Cl2 and of the chemical diffusion coefficient of n-type and p-type AgCl, respectively, in the temperature range of 360–440°C by means of the improved dc polarization technique. The deviation from stoichiometry, δ, in the formula Ag_1+δCl was calculated from the measured electronic conductivities and chemical diffusion coefficients. It was found that δ ranges from -10^?4 to 10^?6 according to P_Cl2. Based on the determined δ values, the imperfection equilibrium in AgCl are discussed.     

A transient two-photon–one-photon double resonance on interband transitions in crystals: I. General relations

A theory of the transient absorption of femtosecond light pulses at a two-photon–one-photon double resonance on adjacent interband transitions in the electronic system has been developed. Approximate expressions for the reactive component of the nonlinear polarization of the three-level system, P _s, which determines the absorbed power in the medium, have been obtained, and the dependences of P _s on the light intensity and detunings of resonances have been calculated.     

Thermally stimulated current study of electron-irradiation induced defects in semi-insulating InP obtained by multiple-step wafer annealing

Electron-irradiation induced defects in semi-insulating (SI) InP wafers with Fe concentration ranging from 1.5×10¹⁵ to 2.5×10¹⁵ cm⁻³, which have been obtained by multiple-step wafer annealing (MWA) under phosphorus vapor pressure, were studied using a thermally stimulated current (TSC) method. New traps, e₁, e₂, e₃, e₄ and e₅, with activation energies of 0.22, 0.28, 0.37, 0.44 and 0.46 eV, respectively, were observed. Based upon the annealing behavior of traps and the calculated defect levels, traps e₁ and e₅ produced by the irradiation with electron doses above 1×10¹⁵ cm⁻² were linked to In_P and P_In antisite defects, respectively, that probably form complexes. Traps e₃ and e₄ produced by the irradiation with doses above 1×10¹⁴ cm⁻² were associated with In and P vacancy related defects, respectively.     

Specific Heat and Thermal Expansion of Triglycine Sulfate–Porous Glass Nanocomposites

The effect of restricted geometry on specific heat capacity and thermal expansion of the triglycine sulfate (TGS)–borosilicate glass composites have been studied first. A decrease in the entropy and temperature of the P2₁ ? P2₁/m phase transition in the TGS component with decreasing the glass matrix pore diameter at the invariable specific heat and thermal expansion coefficient has been observed. The estimates are indicative of the minor effect of internal pressure on the TGS pressure coefficient dT_C/dp in the composites.     

Origin of the multiferroicity in BiMn2O5 from first-principles calculations

Electronic structure, Born effective charges, and spontaneous polarization of multiferroic single crystal BiMn₂O₅ have been investigated in the framework of density functional theory. The relative stability of the ground state and the origin of multiferroicity for magnetism and ferroelectricity are addressed. The results reveal that the stability of antiferromagnetic (AFM) state is better than the ferromagnetic (FM) and ferrimagnetic configurations. The Born effective charge tensors (Z^?) have been calculated for this compound using a Berry-phase approach, compared to their nominal ionic values, the Z^* of Mn atoms show anomalous difference. By investigating the electric structure of BiMn₂O₅, there exists obviously hybridization between Bi 6s and O 2p states, our calculations indicate that the 6s² lone pair on the formally trivalent Bi ion plays an important role in inducing the ferroelectric distortion.     

Experimental and theoretical studies on bis(glycine) lithium nitrate (BGLiN): A physico-chemical approach

Good quality and bulk size single crystal (size: 20×13×8 mm³) of bis(glycine) lithium nitrate (BGLiN) was grown by a slow evaporation solution technique from the aqueous solutions at constant temperature i.e. 27 °C using synthesized materials. Crystal system and lattice parameters were determined by single crystals as well as powder X-ray diffraction analysis. The lattice parameters of the titled compound are a=10.0223 Å, b=5.0343 Å, c=17.0510 Å, and V=860.312 Å³ and it crystallized in an orthorhombic system with space group Pca2₁ obtained by single crystal XRD. Elemental composition was confirmed by energy dispersive X-ray spectroscopic analysis. Optical absorption spectrum was recorded and various optical parameters such as optical transmission (~60%), and optical band gap (4.998 eV) were calculated. Photoluminescence study shows that the grown crystal is free from major defects. Crystalline perfection of the grown crystal was assessed and found good. Ground state optimized geometry has been obtained by using DFT with 6-31G(d,p) basis set. HOMO and LUMO energy gap was found to be 6.01 eV and dipole moment was 1.65 D.