α-LiIO3 Crystals Research Articles

Determination of energy band gap of α-LiIO3 doped with L-Arginine and L-Nitroarginine amino acids using diffuse reflectance spectroscopy

An accurate determination of the band gap energy is crucial for predicting the photophysical and photochemical properties of investigated materials. In the present work, the dependence of the energy band gap (Eg ) of α-LiIO3 crystals doped with L-arginine and L-nitroarginine amino acids was obtained. The method, based on the simultaneous fitting of many absorption mechanisms to the spectral dependence of the Kubelka-Munk function estimated from diffuse reflection data, as well as Tauc's method, has been applied to determine the energy band gap. In the case of a small dopant concentration, additional electron states appear within the band gap of the crystals. As a result, a broad absorption band appears in the material spectrum. The optical transmittance spectra were recorded using an Agilent Cary 60 spectrophotometer in the spectral range 190–1100 nm.

Growth and improved properties of single crystal α-LiIO3 doped with amino acids

In this study, the effect of doping the amino acids L-arginine and L-nitroarginine on α-LiIO3 single crystal were presented. Bulk crystals were grown both from solutions doped with L-arginine and L-nitroarginine and from pure solution of α-LiIO3.When doped with L-arginine and L-nitroarginine the optical quality of the crystals was improved and growth rate was increased. These grown crystals were studied by single-crystal X-ray diffraction, IR and UV–Vis spectroscopy, Vickers microhardness, diffuse reflection, and second harmonic generation methods. Studies of microhardness and nonlinear optical activity showed that the properties of α-LiIO3 crystals were improved in the presence of L-arginine and L-nitroarginine. UV–Vis transmittance was improved in the presence of L-arginine (range 370–800 nm) and L-nitroarginine (range 290–370 nm). The second harmonic generation activity of the crystals α-LiIO3 grown with L-arginine (1.16 times) and L-nitroarginine (1.18 times) dopants is higher than that of pure α-LiIO3. The energy of the band gap increases by 1.2 times in the case of arginine dopants, and it decreases by 1.37 times in the case of the nitroarginine dopants, compared to that for the pure α-LiIO3.

Determination of the polarization plane specific rotation in gyrotropic crystals of the middle category by the spectrophotometric method

A large number of modern functional single crystals of the middle category belong to gyrotropic media. In these crystals, when light propagates along the optical axis, rotation of the plane of its polarization is observed. In this work a spectrophotometric method was used to obtain the dispersion dependences of the rotation angle of the polarization plane. This method is based on measuring the intensity of light passing through the polarizer–crystal–analyzer system, the crystal is a polished plane-parallel plate of a uniaxial gyrotropic crystal cut perpendicular to the optical axis. Measurements were carried out on a UV-Vis-NIR spectrophotometer Cary-5000 in the wavelength range of 200—1200 nm using polarizers — Glan–Taylor prisms. Polished plane-parallel plates of known SiO2 and α-LiIO3 crystals were used as samples. The obtained dispersion dependences of the spectral transmission coefficients are oscillating. Discrete values of the specific angles of rotation of the plane of polarization of light are calculated from the extremes on these dependencies. These discrete values can be approximated by the formulas Drude, Chandrasekhar and Vyshina, depending on what determines the nature of the rotational ability of the plane of polarization of light in each particular material. For the studied crystals, dependences of the modified Drude formula of the form 1/ρ = f(λ2) are plotted, these dependences should have a linear character in the case of an ideal crystal. The obtained experimental results correlate well with the available literature data. The advantages of this method are efficiency, the possibility of obtaining dispersion dependences of the specific rotation angle of the polarization plane, the need for a single sample, the possibility of assessing the nature of the rotational ability of specific crystals, the possibility of evaluating the structural perfection of the studied crystals.

Thermally induced charge relaxation in α-LiIO3 superionic conductor

Electrical properties of single crystals of α-LiIO3 superionic conductor (hexagonal symmetry, sp. gr. P6322, z cut) have been studied by the methods of impedance spectroscopy and thermally induced depolarization. The mechanism of thermally induced charge relaxation in α-LiIO3 crystal is related to “hopping” migration of mobile Li+ ions via crystallographic sites in the channels of the [IO3]– crystalline framework oriented along the c crystallographic axis.

Impact of impurities on the α-LiIO3 crystal growth: Technique for measuring the “dead zone”

Lithium iodate (α-LiIO3) single crystal is a promising nonlinear optical material, which used for efficient laser radiation conversion in the visible and near-IR regions. A technique for measuring the “dead zone” (ΔТ dz) of LiIO3 solutions has been developed; data on the impact of Fe(IO3)2, AgIO3, CsOH, H3PO4, and methyl methacrylate on the growth of the (100) face of α-LiIO3 crystals have been presented; and the dependences of the nucleation and motion of single steps on the degree of supersaturation have been measured. It is shown that the ΔТ dz value makes it possible to estimate the validity of solutions for growing α-LiIO3 crystals. The results of measuring the face growth rates and step velocities for KDP and α-LiIO3 crystals are compared.

Rapid growth of α-LiIO3 crystal

A fundamental possibility of significantly increasing the growth rate of α-LiIO3 crystals without the formation of visible defects in them is demonstrated. The corresponding equipment is designed and a technique for obtaining intermediate-size samples is developed. The crystals grown with an average rate of up to 4 mm/days along the Z axis are of high enough quality to be used as materials of optical elements for laser frequency doubling. α-LiIO3 crystals have been grown both in an open volume via natural faceting and in a rectangular habit, where growth occurs basically through the pyramid (101) face. It is shown that that amount of limiting impurities in the raw material used can be reduced and that the applicability of this material for rapid growth can be verified.

Synthesis, Growth and Optical Studies of Mn: α-LiIO<sub>3</sub> Single Crystal

Manganese doped alpha-lithium iodate have been synthesized and single crystals have been grown for nonlinear optical applications. The grown crystal has been subjected to single crystal X-ray diffraction to confirm the structure. The crystalline perfection was assessed by high-resolution X- ray diffractometer (HRXRD). Second harmonic generation (SHG) efficiency was found to be 71 times than that of KDP. The UV–Vis-NIR spectroscopic study revealed that the grown crystal has good optical transparency in the visible region clearly indicates that Mn doped α-LiIO3 crystals can be used as window material in optical instruments. Presence of dopant was confirmed by energy-dispersive spectrometry.

Dielectric studies of a α-LiIO3 crystals grown from neutral and alkaline solutions

The dielectric properties of lithium iodate crystals of the hexagonal modification grown from solutions with pH = 6.2, 7.1, and 11.0 in the frequency range from 25 Hz to 1 MHz at temperatures of 290–420 K were investigated. The temperature dependence of the imaginary part of the permittivity was found to exhibit clearly pronounced relaxation peaks originating from ionic mobility. The shift in the position of the relaxation maxima was used to calculate the activation energy and attempt frequency for mobile ions. The Cole-Cole diagrams revealed the existence of a spectrum of relaxation times. The presence of the specific feature in the ionic mobility of lithium iodate crystals grown from solutions with pH ∼ 7 was confirmed.

The mechanism of dielectric relaxation and proton conductivity in an α-LiIO3 nanostructure

Thermostimulated depolarization currents are studied and analyzed. As a result, a mechanism of dielectric relaxation is developed for α-LiIO3 crystals with the interatomic distance about 0.55 nm. This mechanism is due to formation of the Н3О+ and OН− defects resulting from the displacement of a proton along the valence bond in water molecules and its following migration through tunneling both inside the IO 3 − ions and between these ions in the neighboring layers. In addition, the НIO3 and НI3О8 impurities can serve as proton donors. The negative maximum of thermostimulated depolarization currents caused by false hysteresis loops due to a conductivity lag is explained.

Studies of the defect structure from the calculations of optical and electron paramagnetic resonance spectra for Ni2+ centre in α-LiIO3 crystal

By calculating the optical spectrum band positions and EPR parameters (g factors, g‖, g⊥ and zero-field splitting D) by diagonalizing the complete energy matrix of 3d8 ions in trigonal symmetry, the defect structure of Ni2+ centre in α-LiIO3 crystal is studied. It is found that to reach the good fits of optical and EPR data between calculation and experiment, the Ni2+ ion should shift by Δz ≈ 0.298 A along C3-axis and the O2− ions between the Ni2+ ion and Li+ vacancy (V Li) should be displaced away from the V Li by Δx ≈ 0.097 A because of the electrostatic interaction. The results are discussed.

Electrostatic model of the α-LiIO3 pyroelectric

The range of possible combinations of the components of the electronic-polarizability tensors of oxygen and iodine ions and the effective charges of these ions is established on the basis of the experimental study of the parameters of the electric-field gradient tensor at 7Li nuclei in an α-LiIO3 crystal and the calculation of the electric-field gradient from the classical electrostatic model. The I-O bonds in α-LiIO3 are shown to be mixed ionic-covalent bonds with low ionicity. Spontaneous polarization of the crystal is estimated.

EPR parameters and defect structure of paramagnetic Er3+ center in α-LiIO3 crystal

The EPR parameters (g factors g//, g⊥ and hyperfine structure constants A//, A⊥) of paramagnetic Er3+ centers in the α-LiIO3 crystal are studied from the perturbation formulas of EPR parameters for 4f11 ion in trigonal symmetry. In these formulas, besides the first-order perturbation contribution considered in previous papers, the second-order perturbation contribution is taken into account, and the crystal-field parameters are estimated from the structural data of Er3+ centers in α-LiIO3:Er3+ by using the superposition model. From the studies, it is found that in the Er3+ center the six O2− ions between the Er3+ ion and Li vacancies (VLi) are shifted away from the VLi by about 0.037 Å. The shift direction is in agreement with those of the trivalent iron-group ions Cr3+ and Fe3+ in α-LiIO3 as well as with the expectation based on the electrostatic interaction model. The EPR parameters g//, g⊥, A// and A⊥ for α-LiIO3: Er3+ are also reasonably well explained by considering the local lattice relaxation.

Defect Structure of Co2+ Center in α-LiIO3 Crystal

Abstract In this paper we establish the formulas of EPR g||, g⟂, for 3d7 ions in trigonal octahedral crystals from a cluster approach. In these formulas, the contributions from configuration interaction-and covalency-effects are considered. The parameters related to both effects can be determined from the optical spectra and the structural parameters of the studied crystal. With these formulas, the defect structure of a Co2+ center in α-LiIO3 crystal is studied. It is found that, to reach good fits between the calculated and observed g||, g⟂ , the O2_ ions between Co2+ and Li+ vacancy (VLi) should shift away from the VLi by about 0.49 Å. The displacement direction is consistent with those obtained for Cr3+ , Fe3+ , and Mn2+ centers in α-LiIO3 crystals as well as with the expectation based on the electrostatic interaction model.

Comparative study of electrical behavior and phase transitions in pure and chromium doped α-LiIO3 single crystals

The polymorphic phase transitions of undoped and chromium doped α-LiIO3 crystals have been investigated by means of DTA experiments at slow heating rates and electrical measurements vs temperature and frequency. Upon heating the well known α ⇔ γ ⇒ β sequence appears to be influenced by the sample's morphology, acidity of the growing solution and doping. Dielectric response at room temperature is close to the type “superposition of d.c. ionic conductivity and dipolar response”. We describe the thermal behavior of conductivity along the c polar axis at three frequencies. Electrical measurements also highlight a higher value of ionic conductivity and a less destructive transition for doped crystals.

Examination on the Optical Absorption and Electron Paramagnetic Resonance Spectra of Cr3+ ions in α-LiIO3 Crystal

The optical and electron paramagnetic resonance spectra of Cr3+ ions in the α-LiIO3 crystal are explained based on the complete diagonalization procedure and a crystal-field-like model. The results agree well with experimental data.

The phenomenon of electrochemical self-decomposition in polar dielectrics

The authors describe the behaviour of short circuit currents and the electromotive force in specimens of the Z-cut of LiIO3 crystals with identical deposited electrodes without preliminary polarisation. The results show that the formation of electromotive force and the short circuit current are the result of electrochemical processes taking place at the crystal electrode boundary and is determined by the anisotropy of α-LiIO3 crystals.

Investigation on second-order nonlinear optical polarizations in one-dimensional ionic conductive crystals

Under usual conditions a second-harmonic generation process generates a doubled frequency light which only consists of one polarized state for all nonlinear optical crystals. We found that a doubled frequency light containing two polarized states can be generated under the action of a dc or a low frequency ac field for α-LiIO3 and KTiOPO4 crystals. It is considered that the observed phenomenon is attributed to the one-dimensional ionic current.

Asymmetry in the scattered light by surface acoustic waves in lithium iodate crystals

The phenomenon of the asymmetry oflight diffraction by surface acoustic waves (SAW) occurring in α-LiIO3 crystals for parallel geometry of interaction is investigated on the basis of theoretical numerical calculations and experiments. The detailed analysis of this phenomenon is based on the consideration of the influence of the surface ripplings and periodical fluctuations of the refractive index arising on the close-to-surface layer, by means of elastooptical and electrooptical effects. General dependencies are formulated, on the basis of which it is possible to anticipate the occurrence of asymmetry of light diffraction by SAW propagating in a crystal with any symmetry and for any case of parallel geometry of interaction.

The Influence of Surface and Volume Effects on Light Diffraction by Surface Acoustic Waves in Lithium Iodate Crystals

The influence of surface ripplings and periodical fluctuations of the refractive index in the layer closest to the surface on the phenomenon of light diffraction by surface acoustic waves (SAW) is investigated. The analysis includes a study of parallel interactions in α-LiIO3 crystals. The cases of external reflection, internal reflection, and transmission of light from or by the surface on which the SAW is propagating are considered. A theoretical analysis, based on numerical calculations, makes it possible to determine which interaction mechanisms take part in this phenomenon and which components of the SAW amplitude contribute to the intensity of the diffracted light. For this reason it is of interest to study, at the same time, all the basic geometries of interactions.

Second-harmonic generation in α-LiIO3 crystals under the action of a DC field

Second-harmonic generation in α-LiIO3 crystals under the action of a DC field