The influence of Mn-doping on the nonlinear optical properties and crystalline perfection of tris(thiourea)zinc(II) sulphate crystals: Concentration effects

doi:10.1016/j.jcrysgro.2008.09.116

Journal of Crystal Growth

Volume 311, Issue 3, 15 January 2009, Pages 960-965

https://doi.org/10.1016/j.jcrysgro.2008.09.116 Get rights and content

Abstract

The influence of Mn-doping on the nonlinear optical (NLO) properties and crystalline perfection of tris(thiourea)zinc(II) sulphate (ZTS) single crystals grown at 30 °C by slow evaporation solution growth technique (SEST) has been investigated. Incorporation of Mn-dopant into the crystal lattice even at the low concentrations was well confirmed by energy dispersive X-ray spectroscopy (EDS). Powder XRD and FT-IR analyses confirm the slight distortion in the structure of the crystal at high concentrations of dopants. Close observation of powder XRD of pure and doped specimens reveals some interesting features in the XRD profiles with changes in intensity and position of some peaks due to stress developed in the crystal because of doping. A nominal increase in cell parameter values is also observed. High-resolution X-ray diffraction (HRXRD) studies reveal many interesting aspects of the effect of dopants on crystalline perfection which in turn shows a good correlation with second harmonic generation (SHG) efficiency. The present investigation shows that the full width at half maximum (FWHM) of the diffraction curves (DC) increased with the dopant concentration. However, at the higher concentrations of the dopants (>10 mol%), the DCs exhibit additional peaks due to the formation of structural grain boundaries. The SHG efficiency is enhanced greatly when the concentration of dopants is low and no significant enhancement is observed at high concentrations due to deterioration of the crystalline perfection.

Introduction

ZTS is a promising semiorganic NLO material with high SHG efficiency and high laser damage threshold suitable for Nd:YAG laser. The relative SHG and laser damage threshold values for ZTS are, respectively, 1.2 [1] times and nearly equal to that of KDP [2]. ZTS belongs to the orthorhombic system with the space group Pca2₁ (point group mm2). Its growth from the aqueous solution and characterization have been reported in a number of recent publications [3], [4], [5], [6]. In our previous study, it has been demonstrated that a small quantity of doping of suitable organic additives enhances the crystalline perfection of ZTS, which lead to enhance the SHG efficiency [7]. Transition metal impurity in the crystalline matrix generally influences the physical properties of the crystal [8], [9], [10], [11], [12], [13]. Mn was found as a very efficient dopant in enhancing the nonlinear properties of ZnO varistor [14]. The Mn²⁺ d-electron states act as efficient luminescent centers. Mn²⁺ emission was red shifted from 584 to 600 nm in bulk ZnS:Mn [15]. Further, because of its transparency for λ>480 nm and low thermal expansion, it may be of use in laser production in the green region based on (3d)⁵ ⁴T₁→⁶A₁ transition of Mn(II) [16].

In the present investigation, the effect of Mn-doping on ZTS crystals for a wide range of dopant concentrations has been studied using FT-IR, XRD, HRXRD, EDS, and Kurtz powder SHG measurements. Doping affects the NLO properties of the crystals and an interesting correlation has been observed between the crystalline perfection and SHG efficiency.

Section snippets

Synthesis and crystal growth

The material of the title compound was synthesized in the aqueous medium from zinc sulphate heptahydrate (EM) and thiourea (SQ) with 1:3 stoichiometric ratio according to the following chemical reaction. To avoid decomposition, low temperature (<70 °C) was maintained during preparation of the solution in the deionized water. ${ZnSO}_{4} \cdot 7 H_{2} O + 3 (CS ({NH}_{2})_{2}) \to Zn (CS ({NH}_{2})_{2})_{3} {SO}_{4}$ The product was purified by repeated recrystallization. Different concentrations of dopants between 1 and 20 mol% of MnSO₄ were used in

FT-IR analysis

The characteristic vibrational frequencies of pure and low [Mn] doped ZTS are very similar. The symmetric and asymmetric C=S stretching vibrations at 740 and 1417 cm⁻¹ of thiourea are shifted to lower frequencies [20]. The band at ∼1500 cm⁻¹ is assigned to N−C−N stretching vibration. Heavy doping caused some variations in FT-IR spectra. Extra bands observed below 1000 cm⁻¹ are marked by asterisks (Fig. 2). The minor structural changes indicate the incorporation of Mn into the ZTS crystalline

Conclusions

Using XRD, FT-IR, EDS, HRXRD and Kurtz powder techniques, the influence of Mn-doping on ZTS crystals has been investigated. Concentration of the dopant plays a decisive role in influencing the properties of ZTS crystals. EDS and chemical analysis confirm the presence of Mn in the doped specimen. A close observation of XRD profiles of doped and undoped samples reveals some minor structural variations. Changes in intensity patterns and slight shift in peak positions are observed because of

Acknowledgments

The author GB acknowledges the encouragement given by Dr. Vikram Kumar, Director, NPL for carrying out the present investigations. We thank the authorities of Annamalai University for providing the necessary facilities.

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The crystal structure, growth, kinetics, morphology, Z-scan study, high pressure electrical resistivity, neutron diffraction study, doping studies and second harmonic generation (SHG) efficiency on ZTS have been extensively studied [5–42]. Recently, we have investigated the effect of doping on the growth and properties of ZTS [23–33]. The molecular structure, spectroscopic (FT-IR, FT Raman, UV–vis), NBO, thermochemical analysis of bis(thiourea)zinc(II) chloride crystals [43] and crystal growth, characterization and theoretical studies of alkaline earth metal-doped tetrakis(thiourea)nickel(II) chloride [44] have been reported from our laboratories.
Transparent single crystals of tris(thiourea)zinc(II) sulfate (ZTS) were grown by slow evaporation technique at room temperature from an aqueous solution containing zinc sulfate and thiourea in the molar ratio 1:3. The experimental and theoretical studies on the molecular structure and vibrational spectra of ZTS were investigated by single crystal X-ray diffraction, FT-IR and density functional theory (DFT). The recorded X-ray diffraction bond parameters are compared with theoretical values calculated at B3LYP/LANL2DZ level. The observed vibrational patterns were compared with the computed wave numbers. The energy and oscillator strength calculated by TD-DFT results complement with the experimental findings. The first-order molecular hyperpolarizability, polarizability, dipole moment and HOMO–LUMO band gap energies were derived. The molecular stability and bond strength were investigated by applying the natural bond orbital analysis (NBO). Information about the size, shape, charge density distribution and site of chemical reactivity of the molecule has been obtained by mapping electron density with molecular electrostatic potential (MEP) using the same level of basis set. Intermolecular hydrogen bonding was investigated by means of the Hirshfeld surfaces, and the role of the NH⋯O interactions as driving force for crystal structure formation has been demonstrated. The percentages of hydrogen bonding interactions are analyzed by Fingerprint plots of Hirshfeld surface.
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The crystal growth, kinetics, morphology, Z-scan study, and high pressure electrical resistivity study on ZTS crystals have been extensively studied [3–9]. The effect of doping on the properties of ZTS crystals has been investigated in the past few years [10–26]. Incorporation of Mg into pure α-LiIO3 [27], SnO2 nanoparticles [28], bis(thiourea)zinc(II) chloride [29] and bis(thiourea)cadmium(II) chloride [30,31] crystals significantly alters the electrical conductivity, optical, microhardness and the dielectric properties.
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The influence of Mn-doping on the nonlinear optical properties and crystalline perfection of tris(thiourea)zinc(II) sulphate crystals: Concentration effects

Abstract

Introduction

Section snippets

Synthesis and crystal growth

FT-IR analysis

Conclusions

Acknowledgments

J. Cryst. Growth

J. Cryst. Growth

J. Lumin.

J. Magn. Magn. Mater.

J. Crystal Growth

J. Crystal Growth

J. Crystal Growth

Appl. Opt.

J. Appl. Phys.

Cryst. Res. Technol.

Cryst. Res. Technol.

Pramana J. Phys.