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Growth and characterization of Cu (II) doped negatively soluble lithium sulfate monohydrate crystals
Journal of Crystal Growth 386 (2014) 32–37
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth and characterization of Cu (II) doped negatively soluble lithium sulfate monohydrate crystals K. Boopathi a, P. Ramasamy a,n, G. Bhagavannarayana b a b
Centre for Crystal Growth, Siva Subramaniya Nadar College of Engineering, Kalavakkam 603110, India Materials Characterization Division, National Physical Laboratory, Council of Scientific and Industrial Research, New Delhi 110012, India
art ic l e i nf o
a b s t r a c t
Article history: Received 17 May 2013 Received in revised form 7 September 2013 Accepted 18 September 2013 Communicated by S.R. Qiu Available online 25 September 2013
Single crystals of pure and Cu (II) doped negatively soluble lithium sulfate monohydrate have been grown by slow evaporation solution technique. In the present work, to improve the crystalline quality of lithium sulfate monohydrate crystal, metal dopant was incorporated into the pure crystals. The as grown crystals are clear, transparent and the sizes of the crystals were up to 18 12 3 mm3 and 50 15 5 mm3. The presence of metal dopant has been confirmed by energy dispersive spectroscopy, atomic absorption spectroscopy analysis. Single crystal and powder X-ray diffraction studies were carried out to ascertain lattice parameters and identify different phase nature. Optical transmission spectrum of the grown crystals was recorded. FT-IR and thermal analysis were carried out to investigate the functional group and thermal behavior of the grown crystals respectively. The grown crystal was subjected to Vickers micro hardness, HRXRD, piezoelectric, laser damage threshold measurements and second harmonic generation efficiency studies. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Single crystal Growth A2. Growth from solutions B1. Lithium compounds B2. Nonlinear optical material B2. Piezoelectric crystals
1. Introduction Nonlinear optical materials find a variety of applications to perform functions like frequency conversion, light modulation, optical memory storage, second harmonic generation, and optical switching. In recent years, there has been extensive investigation in the growth of nonlinear optical (NLO) materials because of their wide applications in optoelectronics and photonic applications [1,2]. Lithium sulfate monohydrate, Li2SO4 H2O, in its monoclinic point group P21 was first described by Groth (1908) hundred years ago [3]. The first measurements of the pyroelectric coefficient were published by Ackermann in 1912 [4]. In the phase matched direction Bohaty et al., [5] have observed high efficiency conversion from pump power to Stokes and anti-Stokes lines by the cascaded self-stimulated Raman scattering effect. In addition, lithium sulfate monohydrate has remarkable piezoelectric and electro-optic properties [6]. Recently, Li2SO4 H2O was classified as a promising material for Raman laser frequency converters. Inorganic materials like KDP, ZnSO4, doped ZnSO4, niobate crystals such as potassium lithium niobate, borate crystals like lithium triborate have already been reported [6,7] to be NLO active materials. It is seen from the literature that lithium on its combination with materials like glycine, selenate [8,9] proves to be highly NLO active. Also, many new crystals in the lithium sulfate family [10] are excellent NLO materials. In earlier work, we have reported solubility and
unidirectional growth of lithium sulfate monohydrate [11]. In the present work, attempts have been made to improve the physicochemical properties of lithium sulfate monohydrate by incorporating metal dopant. A systematic study has been carried out on the growth of pure and metal (Cu (II)) doped LSMH crystals. In solution grown method it has been observed that certain dopants are playing vital role to enhance the physical properties of single crystals. The presence of very low concentration of certain dopant enhances crystalline perfection. Suitable additives are known to reduce or eliminate the defects in a crystal thus enhancing crystalline perfection [12]. Single crystals were grown from aqueous lithium sulfate solution containing 0.1 and 1 mol% of copper sulfate pentahydrate (CuSO4 5H2O) using deionized water as a solvent by slow evaporation method. Both the crystals were harvested after 20–25 days. The 1 mol% doped crystal had many inclusions and was of poor quality. Hence, we have studied the 0.1 mol% Cu (II) doped lithium sulfate monohydrate crystal alone for the present communication. Single crystal X-ray diffraction studies were carried out and the lattice parameters of the grown pure and doped crystals are evaluated. The content of Cu(II) has been determined by energy dispersive X-ray analysis (EDS), atomic absorption spectroscopy studies (AAS).
2. Experimental 2.1. Crystal growth
n
Corresponding author. Tel.: þ 91 9283105760; fax: þ 91 44 27475166. E-mail addresses: [email protected], [email protected] (P. Ramasamy). 0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.09.028
Single crystals of pure and Cu2 þ added lithium sulfate monohydrate were grown by slow evaporation method using water as
K. Boopathi et al. / Journal of Crystal Growth 386 (2014) 32–37
solvent. According to the solubility data (39.9 g at 30 1C) the saturated solution of lithium sulfate monohydrate was prepared [11]. Repeated recrystallization was carried out in order to eliminate the impurities in the LSMH crystal. An ion in the form of CuSO4 5H2O was used as dopant. The concentration of the dopant is 0.1 mol% of CuSO4 5H2O. The saturated solution was prepared for pure and doped LSMH separately and each was stirred well for 4 h to get homogenous solution. The transparent, good quality crystals were collected after 20–25 days. The grown pure and doped crystals are shown in Fig. 1, which possess the crystallography-dependent growth shape as confirmed by the chemical bonding theory of single crystal growth [13]. The morphology of the crystal has been already reported [5]. The morphology of the grown crystal is given in Fig. 2. Lithium sulfate monohydrate and Copper sulfate penta hydrate used in the present study were bought from M/S. SRL (AR grade), Merck (GR grade) India and the deionized water got from Millipore water purification unit. The resistivity of used deionized water is 18.2 MΩ cm.
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(pC/N) was measured using piezometer (PM300, PiezoTest, UK). The powder technique of Kurtz and Perry [15] was used for the comprehensive analysis of second-order nonlinearity, which is regarded as the simplest method to assess the nonlinearities. An Nd: YAG laser with fundamental radiation of 1064 nm was used as the optical source and directed onto these powder samples.
3. Results and discussion 3.1. Atomic absorption spectroscopy (AAS) analysis 10 mg of fine powder of the doped LSMH crystals was dissolved in 100 ml of triple distilled water. The prepared solution was analyzed using atomic absorption spectroscopy. From the results, the amount of Cu (II) was found to be 1.9 mg/L. Experiments were repeated and reproducibility was ascertained. The calculation shows that only 1.9 mg of Cu (II) is present in the samples for 100 mg (0.1 mol%) of the crystal.
2.2. Characterization The presence of metals in the crystal lattice of grown crystals was determined by using AA6300 Shimadzu Atomic Absorption Spectrophotometer. Quanta 200 FEG scanning electron microscope was used to evaluate qualitative and quantitative determinations of the elements present in the sampled volume. This method can detect elements from the presence of elements down to boron (B) in the periodic table. The single crystal X-ray diffraction studies of pure and doped LSMH single crystals were carried out. Bruker Axs (Kappa Apex II) diffractometer with Mo Kα (0.71073 Å) radiation was used to obtain the cell parameters of the grown crystal at room temperature. Powder form of the above mentioned crystal was taken for the powder X-ray diffraction analysis using a REICH SEIFERT X-ray diffractometer employing Cu Kα (1.54058 Ǻ) with a scan speed of 1 1/min. The transmittance spectra were taken on flat polished crystal samples of about 3 mm thickness employing a Perkin-Elmer Lambda 35 UV–vis–NIR spectrometer in a wide wavelength range 200–1100 nm at room temperature. The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) experiments were carried out using Perkin-Elmer Diamond TG/DTA instrument with a heating rate of 10 1C min 1 from 35 1C to 500 1C. The FT-IR spectra recorded for pure and doped crystals were obtained from KBr pellets on a JASCO FT-IR 410 spectrometer by the KBr pellet method to study the functional groups in sample in the region 4000–400 cm 1. Identically Cut and polished (100) crystal samples were subjected to indentation along their most developed plane surface using MATSUZAWA model MMT-X series micro hardness tester fitted with diamond indenter and load was varied from 1 to 50 g with dwell time of 5 s. A multicrystal XRD developed at NPL [14] has been used to record high-resolution diffraction curves (DCs). The piezoelectric charge coefficient d33
3.2. EDS analysis The incorporation of Cu (II) into the crystalline matrix was confirmed by EDS performed on LSMH crystal (Fig. 3). It appears that the accommodating capability of host crystal is limited and only a small quantity is incorporated into the LSMH crystalline matrix. Further, analysis of surface at different sites indicates that the incorporation is non-uniform over the surface, connected with adsorption mechanisms at ledges of steps.
Fig. 2. Morphology of the crystal.
Fig. 1. Photograph of grown crystals (a) pure lithium sulfate monohydrate and (b) Cu (II) doped crystal.
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K. Boopathi et al. / Journal of Crystal Growth 386 (2014) 32–37
Cu-LSMH
10
20
(305)
(20-1) (031)
0
(020)
(012) (1-1 2)
(010) (101) (110)
200
(100)
400
(102)
600
(10-2)
(10 -1)
800
(001)
Intensity (Cps)
1000
30
40
50
60
10
20
(305)
LSMH
30
(031)
(020)
(1-1 2) (10 2) (20-1)
200
(100)
400
(012)
600
0
Fig. 3. EDS spectrum of Cu (II) doped Lithium sulfate monohydrate.
(10-1)
800
(001)
Intensity (Cps)
1000
(010) (101) (10-2) (110)
2 degree
40
50
60
2 degree Fig. 4. Powder XRD pattern of pure and Cu doped lithium sulfate monohydrate crystal. Table 1 Crystal lattice parameters of the grown crystal. Parameters
Single crystal XRD (present work)
Single crystal XRD [2]
a (Ǻ) b (Ǻ) c (Ǻ) α (1) ß (1) γ (1) Volume (Ǻ3) Crystal system Space group
5.49 4.88 8.18 90.00 107.56 90.00 845.00 Monoclinic P21
5.45 (2) 4.85 (2) 8.16 (3) 90.00 107.37 90.00 831.3 (2) monoclinic P21
3.3. Single crystal and powder X-ray diffraction The calculated parameter values are tabulated in Table 1. It is observed that both the pure and the doped LSMH crystallize into monoclinic system and belong to the P21 space group. However, there are slight variations in the lattice parameters as well as in the cell volume values. It is believed that the incorporation of metal ions in LSMH lattice account for these variations. The powder XRD pattern is recorded by REICH SEIFERT X-ray diffractometer employing Cu Kα (1.54058 Å) with a scan speed of 1 deg/ min and analyzed by using ‘TWO THETA’ refinement software and Fig. 4(a) and (b) shows powder X-ray diffraction pattern of pure and doped crystal. It shows single phase with a slight reduction in peak intensities accompanied by a small shift in their positions and it could be due to lattice strain as a result of metal doping. 3.4. UV–vis–NIR analysis It is evident from the spectrum (Fig. 5) that the percentage of transmission is high for doped LSMH crystals, which is a desirable property for the crystals used for NLO applications. In addition, the light absorbing properties of doped ones are less that the parent LSMH. In the transmission spectrum, the absorption at 968 nm is due to the presence of Cu ions. Normally Cu containing compounds have light absorption in the range between 800 nm and 1000 nm [16]. According to the newly proposed electronegativity scale for crystallized ions [17], we can conveniently select proper metal ions as dopants in crystal lattice to meet our demands for various property modifications. However, by doping with a variety
Fig. 5. UV–vis–NIR transmission spectra of pure and Cu (II) doped crystals.
of metal ions it is easy to generate suitable defects in a crystal structure in order to increase a range for NLO applications [18]. 3.5. FT-IR spectral analysis Infrared spectrum is an important record, which provides more information about the functional group of the compound. In this technique, almost all functional groups in a molecule absorb characteristically a definite range of frequency [19]. The peak appearing at 3494 cm 1 is attributed to the presence of water molecule in the title compound. The characteristic stretching vibrations of SO4 group appear at 1118, 887 and 648 cm 1. The FT-IR spectrum of Cu (II) (0.1 mol%) doped LSMH crystal is shown in Fig. 6. Although it provides similar features as that of pure LSMH spectrum, there is a peak suggesting wide range of interactions for the groupings. 3.6. Thermal studies The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out using Perkin-Elmer Diamond TG/ DTA instrument in the presence of nitrogen atmosphere. A trace of TG/DTA curve is shown in Fig. 7(a) and (b). For pure LSMH, the
K. Boopathi et al. / Journal of Crystal Growth 386 (2014) 32–37
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decomposition temperature. As a result of Cu (II) doping, the peak maximum in DTA corresponding to first stage of decomposition is shifted to 79.16 1C that indicates loss of water molecules which is present in the dopant and second stage decomposition values is shifted to higher values by 11 1C. Recently, interests in the influence of dopants on the thermal behavior of optical materials have attracted many researchers to study the mechanisms between them. 3.7. Microhardness studies The structure and nature of bonding of the crystalline solids have influence on their mechanical hardness [20]. Microhardness testing is one of the methods of understanding the mechanical properties of materials such as fracture behavior, yield strength, brittleness index, and temperature of cracking [21]. The (100) crystal slices are well polished with a thickness 3 mm to avoid the surface defects which may influence the hardness value strongly. Fig. 8 shows the variation of P versus Vickers hardness number (Hv). Vickers microhardness was calculated from the relation Hv ¼ 1:8544P=d Fig. 6. FT-IR spectrum of (a) pure and (b) Cu (II) doped crystals.
2
where P is the applied load in gram and d is the average diagonal length of the indented impressions in mm. It is evident from the plot that the Vickers hardness number increases with the applied load. The hardness experiment on pure and doped crystals suggests that the sample has a VHN of 107.4 and 136.5 kg mm2 for an applied load of 50 g on the [100] plane. In solution grown crystal, it has been observed in all the cases [22,23] that a more prefect crystal has higher hardness than a crystal with less perfection. In the case of solution grown crystal the microparticles of solution that get into the crystal are responsible for crystal defects and the inclusion of solution micro-particles into the crystal results in decrease of hardness. 3.8. HRXRD studies Fig. 9(a) shows the diffraction curve (DC) recorded for a typical SEST grown lithium sulfate monohydrate (LSMH) single crystal specimen using (100) diffracting planes in symmetrical Bragg geometry. As seen in the figure, the DC contains a single sharp peak and indicates that the specimen is free from structural grain boundaries. The FWHM (full width at half maximum) of the curve is 18″ which is somewhat more than that expected from the plane wave theory of dynamical X-ray diffraction [24] for an ideally
Fig. 7. (a) TG and DTA curve of pure crystal. (b) TG and DTA curve of Cu(II) doped crystal.
endothermic peak appears at 142 1C. The residue in terms of percentage after release of water molecule is calculated for pure sample as 85.6%. The Cu doped LSMH crystals show same features as that of pure LSMH, but there is a distinct shift in the
Fig. 8. Variation of microhardness with load of pure and doped crystal.
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K. Boopathi et al. / Journal of Crystal Growth 386 (2014) 32–37
Fig. 9(b) shows the high-resolution diffraction curve recorded for a typical Cu (II) doped LSMH single crystal using (100) diffracting planes in symmetrical Bragg geometry by employing the multicrystal X-ray diffractometer with MoKα1 radiation. As seen in the figure, the DC contains a single peak and indicates that the specimen is free from structural grain boundaries. The FWHM of the curve is 13″ which is a bit more than that expected for an ideally perfect crystal from the plane wave theory of dynamical X-ray diffraction [24], but close to that expected for a nearly perfect real life crystal. For a particular angular deviation (Δθ) of glancing angle with respect to the peak position, the scattered intensity is much more in the positive direction in comparison to that of the negative direction. This feature indicates occupancy of bigger size Cu (II) in place of Li þ . According to the electronegativity scale for crystallized ions [17], we cannot rule out the predominant occupancy of interstitial type of point defects than that of vacancy defects which might have come right from the raw material or self-interstitials. This is because of the fact that if the former one alone is the reason, (replacement of Li þ by Cu2 þ ), one K þ vacancy defect is also expected for each substitutional incorporation of Cu2 þ , where the ionic electronegativity values for Li þ , Cu 2 þ , and K þ are 1.009, 1.372, and 0.998, respectively. Li þ and K þ have very similar ability to occupy the same lattice [17]. Due to predominant occupation of defects at interstitial sites or replacement of a lattice atom by a bigger size dopant, the lattice around these defects undergoes compressive stress [26].” However, the single diffraction curve with reasonably low FWHM indicates that the crystalline perfection is fairly good. 3.9. Piezoelectric measurements
Fig. 9. High-resolution X-ray diffraction curves recorded for (100) diffracting planes. In symmetrical Bragg geometry using MoKα1 radiation for lithium sulfate monohydrate single crystals: Pure and (b) Cu (II) doped specimens.
perfect crystal. The broadening of diffraction curve without the presence of any splitting can be attributed to variety of defects like randomly oriented mosaic blocks, dislocations, Frankel defects, implantation induced defects (due to simultaneous existence of vacancies as well as interstitial defects) etc. But depending upon the nature of asymmetry, as investigated in the Refs. [14,25,26], one can expect predominant occupation of vacancy or interstitial defects, which can be realized in the following way. For a particular angular deviation (Δθ) of glancing angle with respect to the peak position, the scattered intensity is much more in the negative direction in comparison to that of the positive direction. This feature clearly indicates that the crystal contains predominantly vacancy type of defects than that of interstitial defects. This can be well understood by the fact that due to vacancy defects, as shown schematically in the inset, the lattice around these defects undergo tensile stress and the lattice parameter d (interplanar spacing) increases and leads to give more scattered (also known as diffuse X-ray scattering) intensity at slightly lower Bragg angles (θB) as d and sin θB are inversely proportional to each other in the Bragg equation (2dsin θB ¼nλ; n and λ being the order of reflection and wavelength respectively which are fixed). However, these point defects with much lesser density as in the present case hardly give any effect on the performance of the devices based on such crystals. If the concentration is high, the FWHM would be much higher and often lead to structural grain boundaries [26]. Point defects up to some extent are unavoidable due to thermodynamical considerations and growth conditions [25].
The crystals were subjected to piezoelectric characterizations. Finely polished basal surface of the crystal was coated with high grade silver paste and dried at 60 1C for various measurements. Piezoelectric charge coefficients (d33 pC/N) is an important parameter associated with piezoelectric material which is defined as the amount of charge developed at the opposite surfaces of the material when one unit force is applied across it. The piezoelectric charge coefficient d33 (pC/N) was measured using piezometer (PM300, PiezoTest, UK). A constant tapping force of 0.25 N at a frequency of 110 Hz was applied on the electroded surfaces of as grown and poled LSMH crystal. After poling under a field of 10 kV/cm for 30 min at room temperature, d33 values of pure LSMH crystal is 48 pC/N and for Cu (II) doped LSMH crystal is 68 pC/N. The piezoelectric properties are largely affected by the dislocations [27]. Any defect irrespective of their origin may cause slowing down of the domain wall mobility and tends to reduce the d33 coefficient [28]. Due to the addition of Cu2 þ the crystalline perfection of the crystal has been enhanced. The greater crystalline perfection may be one reason for the increasing piezoelectric coefficient. 3.10. Laser damage threshold measurements The laser induced breakdown in the crystals caused by various physical processes such as electron avalanche, multiphoton absorption, and photo ionization for the transparent materials whereas in case of high absorbing materials, the damage threshold is mainly due to the temperature rise, which leads to straininduced fracture [29,30]. It also depends upon the specific properties of material, pulse width, and wavelength of laser used. For the long-pulse regime τ 4100 ps, the damage process occurs mainly by the rate of thermal conduction through the atomic lattice and for the short-pulse regime τ r10 ps, the optical breakdown is a nonthermal process and various nonlinear ionization mechanisms multiphoton, avalanche multiplication, and tunneling become important [26].
K. Boopathi et al. / Journal of Crystal Growth 386 (2014) 32–37
In the present investigation, Q switched Nd: YAG laser operating at 1064 nm radiation was used. The laser was operated at the repetition rate 10 Hz with pulse width of the laser beam 10 ns. For the LDT measurement diameter of the laser beam is 1 mm, laser beam was focused on the crystal with a 20 cm focal length convex lens and the crystal was placed just at the focal point. LDT values for (100) planes of pure and doped crystals were recorded when the clear visible spot occurred on surface with audible sound. As given in the experimental details, to get the bulk LDT values, the beam was focused inside the crystal, but close to the surface with a predetermined spot size of diameter 0.20 mm in air. The pulse energy of each shot was measured using the combination of phototube and oscilloscope. The surface damage threshold of the crystal was calculated using the expression: Power density ðP d Þ ¼ E=τπ r 2 where E is the input energy (mJ), τ is the pulse width (ns) and r is the radius of the spot (mm). The measured multiple shot laser damage threshold value for pure and Cu (II) added LSMH is 2.8 and 3.1 GW/cm2. The higher crystalline perfection of grown crystal may be responsible for the larger laser damage threshold. Azarov et al. [31] reported that the damage threshold was influenced by the dislocation in the crystal and the crystal with many dislocations presented low damage threshold. 3.11. NLO property studies The NLO property of the crystal was confirmed by the Kurtz Perry powder technique [15]. The crystals are ground to powder and packed between two transparent glass slides. The first harmonic output of 1064 nm from Nd: YAG laser was made to fall normally, passed through the pure and doped LSMH powder sample. The SHG behavior in these crystals was confirmed from the emission of intense green radiation (λ ¼532 nm) by the sample. The Cu2 þ doped crystal has lower intensity of the green radiation than pure LSMH. The depressed SHG efficiency may be due to the disturbance of electronic charge distribution in doped crystal [32]. 4. Conclusions Good quality single crystals of pure, Cu (II) doped LSMH were grown successfully by slow evaporation solution technique. AAS, EDS result reveals that the amount of dopant incorporated into the crystal lattice is less than the concentration of the dopant in the corresponding solution. Single crystal X-ray diffraction study shows that the small variation in lattice parameter values is due to the contribution of metal dopant in the interstitial sites. The powder X-ray diffraction analysis shows small shift in intensity and peak position. The pure and doped LSMH crystals have good transmission in the range 200–1100 nm and the doped crystals possess increased transmission. Using the FT-IR spectroscopy functional group of the compounds has been confirmed. It is seen from thermal analysis that the decomposition temperature of the doped LSMH crystals increases, when compared to pure LSMH.
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The HRXRD result shows that the crystalline perfection of the doped crystal is considerably good compared to pure crystal. The piezoelectric studies show that doped crystals have higher piezoelectric coefficient than pure LSMH crystal. The laser damage threshold value for pure and Cu (II) doped LSMH was found to be 2.8 and 3.1 GW/cm2 at 1064 nm wavelength of Nd: YAG laser radiation. Form the SHG efficiency analysis doped LSMH crystal has less efficiency than that of pure LSMH.
Acknowledgment The authors are thankful to Dr. Binay Kumar, Department of Physics and Astrophysics, University of Delhi, India for the piezoelectric measurements, Dr. R. Gopalakrishnan, Anna University, Chennai for providing microhardness studies and SAIF, IIT Madras,
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth and characterization of Cu (II) doped negatively soluble lithium sulfate monohydrate crystals K. Boopathi a, P. Ramasamy a,n, G. Bhagavannarayana b a b
Centre for Crystal Growth, Siva Subramaniya Nadar College of Engineering, Kalavakkam 603110, India Materials Characterization Division, National Physical Laboratory, Council of Scientific and Industrial Research, New Delhi 110012, India
art ic l e i nf o
a b s t r a c t
Article history: Received 17 May 2013 Received in revised form 7 September 2013 Accepted 18 September 2013 Communicated by S.R. Qiu Available online 25 September 2013
Single crystals of pure and Cu (II) doped negatively soluble lithium sulfate monohydrate have been grown by slow evaporation solution technique. In the present work, to improve the crystalline quality of lithium sulfate monohydrate crystal, metal dopant was incorporated into the pure crystals. The as grown crystals are clear, transparent and the sizes of the crystals were up to 18 12 3 mm3 and 50 15 5 mm3. The presence of metal dopant has been confirmed by energy dispersive spectroscopy, atomic absorption spectroscopy analysis. Single crystal and powder X-ray diffraction studies were carried out to ascertain lattice parameters and identify different phase nature. Optical transmission spectrum of the grown crystals was recorded. FT-IR and thermal analysis were carried out to investigate the functional group and thermal behavior of the grown crystals respectively. The grown crystal was subjected to Vickers micro hardness, HRXRD, piezoelectric, laser damage threshold measurements and second harmonic generation efficiency studies. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Single crystal Growth A2. Growth from solutions B1. Lithium compounds B2. Nonlinear optical material B2. Piezoelectric crystals
1. Introduction Nonlinear optical materials find a variety of applications to perform functions like frequency conversion, light modulation, optical memory storage, second harmonic generation, and optical switching. In recent years, there has been extensive investigation in the growth of nonlinear optical (NLO) materials because of their wide applications in optoelectronics and photonic applications [1,2]. Lithium sulfate monohydrate, Li2SO4 H2O, in its monoclinic point group P21 was first described by Groth (1908) hundred years ago [3]. The first measurements of the pyroelectric coefficient were published by Ackermann in 1912 [4]. In the phase matched direction Bohaty et al., [5] have observed high efficiency conversion from pump power to Stokes and anti-Stokes lines by the cascaded self-stimulated Raman scattering effect. In addition, lithium sulfate monohydrate has remarkable piezoelectric and electro-optic properties [6]. Recently, Li2SO4 H2O was classified as a promising material for Raman laser frequency converters. Inorganic materials like KDP, ZnSO4, doped ZnSO4, niobate crystals such as potassium lithium niobate, borate crystals like lithium triborate have already been reported [6,7] to be NLO active materials. It is seen from the literature that lithium on its combination with materials like glycine, selenate [8,9] proves to be highly NLO active. Also, many new crystals in the lithium sulfate family [10] are excellent NLO materials. In earlier work, we have reported solubility and
unidirectional growth of lithium sulfate monohydrate [11]. In the present work, attempts have been made to improve the physicochemical properties of lithium sulfate monohydrate by incorporating metal dopant. A systematic study has been carried out on the growth of pure and metal (Cu (II)) doped LSMH crystals. In solution grown method it has been observed that certain dopants are playing vital role to enhance the physical properties of single crystals. The presence of very low concentration of certain dopant enhances crystalline perfection. Suitable additives are known to reduce or eliminate the defects in a crystal thus enhancing crystalline perfection [12]. Single crystals were grown from aqueous lithium sulfate solution containing 0.1 and 1 mol% of copper sulfate pentahydrate (CuSO4 5H2O) using deionized water as a solvent by slow evaporation method. Both the crystals were harvested after 20–25 days. The 1 mol% doped crystal had many inclusions and was of poor quality. Hence, we have studied the 0.1 mol% Cu (II) doped lithium sulfate monohydrate crystal alone for the present communication. Single crystal X-ray diffraction studies were carried out and the lattice parameters of the grown pure and doped crystals are evaluated. The content of Cu(II) has been determined by energy dispersive X-ray analysis (EDS), atomic absorption spectroscopy studies (AAS).
2. Experimental 2.1. Crystal growth
n
Corresponding author. Tel.: þ 91 9283105760; fax: þ 91 44 27475166. E-mail addresses: [email protected], [email protected] (P. Ramasamy). 0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.09.028
Single crystals of pure and Cu2 þ added lithium sulfate monohydrate were grown by slow evaporation method using water as
K. Boopathi et al. / Journal of Crystal Growth 386 (2014) 32–37
solvent. According to the solubility data (39.9 g at 30 1C) the saturated solution of lithium sulfate monohydrate was prepared [11]. Repeated recrystallization was carried out in order to eliminate the impurities in the LSMH crystal. An ion in the form of CuSO4 5H2O was used as dopant. The concentration of the dopant is 0.1 mol% of CuSO4 5H2O. The saturated solution was prepared for pure and doped LSMH separately and each was stirred well for 4 h to get homogenous solution. The transparent, good quality crystals were collected after 20–25 days. The grown pure and doped crystals are shown in Fig. 1, which possess the crystallography-dependent growth shape as confirmed by the chemical bonding theory of single crystal growth [13]. The morphology of the crystal has been already reported [5]. The morphology of the grown crystal is given in Fig. 2. Lithium sulfate monohydrate and Copper sulfate penta hydrate used in the present study were bought from M/S. SRL (AR grade), Merck (GR grade) India and the deionized water got from Millipore water purification unit. The resistivity of used deionized water is 18.2 MΩ cm.
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(pC/N) was measured using piezometer (PM300, PiezoTest, UK). The powder technique of Kurtz and Perry [15] was used for the comprehensive analysis of second-order nonlinearity, which is regarded as the simplest method to assess the nonlinearities. An Nd: YAG laser with fundamental radiation of 1064 nm was used as the optical source and directed onto these powder samples.
3. Results and discussion 3.1. Atomic absorption spectroscopy (AAS) analysis 10 mg of fine powder of the doped LSMH crystals was dissolved in 100 ml of triple distilled water. The prepared solution was analyzed using atomic absorption spectroscopy. From the results, the amount of Cu (II) was found to be 1.9 mg/L. Experiments were repeated and reproducibility was ascertained. The calculation shows that only 1.9 mg of Cu (II) is present in the samples for 100 mg (0.1 mol%) of the crystal.
2.2. Characterization The presence of metals in the crystal lattice of grown crystals was determined by using AA6300 Shimadzu Atomic Absorption Spectrophotometer. Quanta 200 FEG scanning electron microscope was used to evaluate qualitative and quantitative determinations of the elements present in the sampled volume. This method can detect elements from the presence of elements down to boron (B) in the periodic table. The single crystal X-ray diffraction studies of pure and doped LSMH single crystals were carried out. Bruker Axs (Kappa Apex II) diffractometer with Mo Kα (0.71073 Å) radiation was used to obtain the cell parameters of the grown crystal at room temperature. Powder form of the above mentioned crystal was taken for the powder X-ray diffraction analysis using a REICH SEIFERT X-ray diffractometer employing Cu Kα (1.54058 Ǻ) with a scan speed of 1 1/min. The transmittance spectra were taken on flat polished crystal samples of about 3 mm thickness employing a Perkin-Elmer Lambda 35 UV–vis–NIR spectrometer in a wide wavelength range 200–1100 nm at room temperature. The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) experiments were carried out using Perkin-Elmer Diamond TG/DTA instrument with a heating rate of 10 1C min 1 from 35 1C to 500 1C. The FT-IR spectra recorded for pure and doped crystals were obtained from KBr pellets on a JASCO FT-IR 410 spectrometer by the KBr pellet method to study the functional groups in sample in the region 4000–400 cm 1. Identically Cut and polished (100) crystal samples were subjected to indentation along their most developed plane surface using MATSUZAWA model MMT-X series micro hardness tester fitted with diamond indenter and load was varied from 1 to 50 g with dwell time of 5 s. A multicrystal XRD developed at NPL [14] has been used to record high-resolution diffraction curves (DCs). The piezoelectric charge coefficient d33
3.2. EDS analysis The incorporation of Cu (II) into the crystalline matrix was confirmed by EDS performed on LSMH crystal (Fig. 3). It appears that the accommodating capability of host crystal is limited and only a small quantity is incorporated into the LSMH crystalline matrix. Further, analysis of surface at different sites indicates that the incorporation is non-uniform over the surface, connected with adsorption mechanisms at ledges of steps.
Fig. 2. Morphology of the crystal.
Fig. 1. Photograph of grown crystals (a) pure lithium sulfate monohydrate and (b) Cu (II) doped crystal.
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K. Boopathi et al. / Journal of Crystal Growth 386 (2014) 32–37
Cu-LSMH
10
20
(305)
(20-1) (031)
0
(020)
(012) (1-1 2)
(010) (101) (110)
200
(100)
400
(102)
600
(10-2)
(10 -1)
800
(001)
Intensity (Cps)
1000
30
40
50
60
10
20
(305)
LSMH
30
(031)
(020)
(1-1 2) (10 2) (20-1)
200
(100)
400
(012)
600
0
Fig. 3. EDS spectrum of Cu (II) doped Lithium sulfate monohydrate.
(10-1)
800
(001)
Intensity (Cps)
1000
(010) (101) (10-2) (110)
2 degree
40
50
60
2 degree Fig. 4. Powder XRD pattern of pure and Cu doped lithium sulfate monohydrate crystal. Table 1 Crystal lattice parameters of the grown crystal. Parameters
Single crystal XRD (present work)
Single crystal XRD [2]
a (Ǻ) b (Ǻ) c (Ǻ) α (1) ß (1) γ (1) Volume (Ǻ3) Crystal system Space group
5.49 4.88 8.18 90.00 107.56 90.00 845.00 Monoclinic P21
5.45 (2) 4.85 (2) 8.16 (3) 90.00 107.37 90.00 831.3 (2) monoclinic P21
3.3. Single crystal and powder X-ray diffraction The calculated parameter values are tabulated in Table 1. It is observed that both the pure and the doped LSMH crystallize into monoclinic system and belong to the P21 space group. However, there are slight variations in the lattice parameters as well as in the cell volume values. It is believed that the incorporation of metal ions in LSMH lattice account for these variations. The powder XRD pattern is recorded by REICH SEIFERT X-ray diffractometer employing Cu Kα (1.54058 Å) with a scan speed of 1 deg/ min and analyzed by using ‘TWO THETA’ refinement software and Fig. 4(a) and (b) shows powder X-ray diffraction pattern of pure and doped crystal. It shows single phase with a slight reduction in peak intensities accompanied by a small shift in their positions and it could be due to lattice strain as a result of metal doping. 3.4. UV–vis–NIR analysis It is evident from the spectrum (Fig. 5) that the percentage of transmission is high for doped LSMH crystals, which is a desirable property for the crystals used for NLO applications. In addition, the light absorbing properties of doped ones are less that the parent LSMH. In the transmission spectrum, the absorption at 968 nm is due to the presence of Cu ions. Normally Cu containing compounds have light absorption in the range between 800 nm and 1000 nm [16]. According to the newly proposed electronegativity scale for crystallized ions [17], we can conveniently select proper metal ions as dopants in crystal lattice to meet our demands for various property modifications. However, by doping with a variety
Fig. 5. UV–vis–NIR transmission spectra of pure and Cu (II) doped crystals.
of metal ions it is easy to generate suitable defects in a crystal structure in order to increase a range for NLO applications [18]. 3.5. FT-IR spectral analysis Infrared spectrum is an important record, which provides more information about the functional group of the compound. In this technique, almost all functional groups in a molecule absorb characteristically a definite range of frequency [19]. The peak appearing at 3494 cm 1 is attributed to the presence of water molecule in the title compound. The characteristic stretching vibrations of SO4 group appear at 1118, 887 and 648 cm 1. The FT-IR spectrum of Cu (II) (0.1 mol%) doped LSMH crystal is shown in Fig. 6. Although it provides similar features as that of pure LSMH spectrum, there is a peak suggesting wide range of interactions for the groupings. 3.6. Thermal studies The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out using Perkin-Elmer Diamond TG/ DTA instrument in the presence of nitrogen atmosphere. A trace of TG/DTA curve is shown in Fig. 7(a) and (b). For pure LSMH, the
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decomposition temperature. As a result of Cu (II) doping, the peak maximum in DTA corresponding to first stage of decomposition is shifted to 79.16 1C that indicates loss of water molecules which is present in the dopant and second stage decomposition values is shifted to higher values by 11 1C. Recently, interests in the influence of dopants on the thermal behavior of optical materials have attracted many researchers to study the mechanisms between them. 3.7. Microhardness studies The structure and nature of bonding of the crystalline solids have influence on their mechanical hardness [20]. Microhardness testing is one of the methods of understanding the mechanical properties of materials such as fracture behavior, yield strength, brittleness index, and temperature of cracking [21]. The (100) crystal slices are well polished with a thickness 3 mm to avoid the surface defects which may influence the hardness value strongly. Fig. 8 shows the variation of P versus Vickers hardness number (Hv). Vickers microhardness was calculated from the relation Hv ¼ 1:8544P=d Fig. 6. FT-IR spectrum of (a) pure and (b) Cu (II) doped crystals.
2
where P is the applied load in gram and d is the average diagonal length of the indented impressions in mm. It is evident from the plot that the Vickers hardness number increases with the applied load. The hardness experiment on pure and doped crystals suggests that the sample has a VHN of 107.4 and 136.5 kg mm2 for an applied load of 50 g on the [100] plane. In solution grown crystal, it has been observed in all the cases [22,23] that a more prefect crystal has higher hardness than a crystal with less perfection. In the case of solution grown crystal the microparticles of solution that get into the crystal are responsible for crystal defects and the inclusion of solution micro-particles into the crystal results in decrease of hardness. 3.8. HRXRD studies Fig. 9(a) shows the diffraction curve (DC) recorded for a typical SEST grown lithium sulfate monohydrate (LSMH) single crystal specimen using (100) diffracting planes in symmetrical Bragg geometry. As seen in the figure, the DC contains a single sharp peak and indicates that the specimen is free from structural grain boundaries. The FWHM (full width at half maximum) of the curve is 18″ which is somewhat more than that expected from the plane wave theory of dynamical X-ray diffraction [24] for an ideally
Fig. 7. (a) TG and DTA curve of pure crystal. (b) TG and DTA curve of Cu(II) doped crystal.
endothermic peak appears at 142 1C. The residue in terms of percentage after release of water molecule is calculated for pure sample as 85.6%. The Cu doped LSMH crystals show same features as that of pure LSMH, but there is a distinct shift in the
Fig. 8. Variation of microhardness with load of pure and doped crystal.
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Fig. 9(b) shows the high-resolution diffraction curve recorded for a typical Cu (II) doped LSMH single crystal using (100) diffracting planes in symmetrical Bragg geometry by employing the multicrystal X-ray diffractometer with MoKα1 radiation. As seen in the figure, the DC contains a single peak and indicates that the specimen is free from structural grain boundaries. The FWHM of the curve is 13″ which is a bit more than that expected for an ideally perfect crystal from the plane wave theory of dynamical X-ray diffraction [24], but close to that expected for a nearly perfect real life crystal. For a particular angular deviation (Δθ) of glancing angle with respect to the peak position, the scattered intensity is much more in the positive direction in comparison to that of the negative direction. This feature indicates occupancy of bigger size Cu (II) in place of Li þ . According to the electronegativity scale for crystallized ions [17], we cannot rule out the predominant occupancy of interstitial type of point defects than that of vacancy defects which might have come right from the raw material or self-interstitials. This is because of the fact that if the former one alone is the reason, (replacement of Li þ by Cu2 þ ), one K þ vacancy defect is also expected for each substitutional incorporation of Cu2 þ , where the ionic electronegativity values for Li þ , Cu 2 þ , and K þ are 1.009, 1.372, and 0.998, respectively. Li þ and K þ have very similar ability to occupy the same lattice [17]. Due to predominant occupation of defects at interstitial sites or replacement of a lattice atom by a bigger size dopant, the lattice around these defects undergoes compressive stress [26].” However, the single diffraction curve with reasonably low FWHM indicates that the crystalline perfection is fairly good. 3.9. Piezoelectric measurements
Fig. 9. High-resolution X-ray diffraction curves recorded for (100) diffracting planes. In symmetrical Bragg geometry using MoKα1 radiation for lithium sulfate monohydrate single crystals: Pure and (b) Cu (II) doped specimens.
perfect crystal. The broadening of diffraction curve without the presence of any splitting can be attributed to variety of defects like randomly oriented mosaic blocks, dislocations, Frankel defects, implantation induced defects (due to simultaneous existence of vacancies as well as interstitial defects) etc. But depending upon the nature of asymmetry, as investigated in the Refs. [14,25,26], one can expect predominant occupation of vacancy or interstitial defects, which can be realized in the following way. For a particular angular deviation (Δθ) of glancing angle with respect to the peak position, the scattered intensity is much more in the negative direction in comparison to that of the positive direction. This feature clearly indicates that the crystal contains predominantly vacancy type of defects than that of interstitial defects. This can be well understood by the fact that due to vacancy defects, as shown schematically in the inset, the lattice around these defects undergo tensile stress and the lattice parameter d (interplanar spacing) increases and leads to give more scattered (also known as diffuse X-ray scattering) intensity at slightly lower Bragg angles (θB) as d and sin θB are inversely proportional to each other in the Bragg equation (2dsin θB ¼nλ; n and λ being the order of reflection and wavelength respectively which are fixed). However, these point defects with much lesser density as in the present case hardly give any effect on the performance of the devices based on such crystals. If the concentration is high, the FWHM would be much higher and often lead to structural grain boundaries [26]. Point defects up to some extent are unavoidable due to thermodynamical considerations and growth conditions [25].
The crystals were subjected to piezoelectric characterizations. Finely polished basal surface of the crystal was coated with high grade silver paste and dried at 60 1C for various measurements. Piezoelectric charge coefficients (d33 pC/N) is an important parameter associated with piezoelectric material which is defined as the amount of charge developed at the opposite surfaces of the material when one unit force is applied across it. The piezoelectric charge coefficient d33 (pC/N) was measured using piezometer (PM300, PiezoTest, UK). A constant tapping force of 0.25 N at a frequency of 110 Hz was applied on the electroded surfaces of as grown and poled LSMH crystal. After poling under a field of 10 kV/cm for 30 min at room temperature, d33 values of pure LSMH crystal is 48 pC/N and for Cu (II) doped LSMH crystal is 68 pC/N. The piezoelectric properties are largely affected by the dislocations [27]. Any defect irrespective of their origin may cause slowing down of the domain wall mobility and tends to reduce the d33 coefficient [28]. Due to the addition of Cu2 þ the crystalline perfection of the crystal has been enhanced. The greater crystalline perfection may be one reason for the increasing piezoelectric coefficient. 3.10. Laser damage threshold measurements The laser induced breakdown in the crystals caused by various physical processes such as electron avalanche, multiphoton absorption, and photo ionization for the transparent materials whereas in case of high absorbing materials, the damage threshold is mainly due to the temperature rise, which leads to straininduced fracture [29,30]. It also depends upon the specific properties of material, pulse width, and wavelength of laser used. For the long-pulse regime τ 4100 ps, the damage process occurs mainly by the rate of thermal conduction through the atomic lattice and for the short-pulse regime τ r10 ps, the optical breakdown is a nonthermal process and various nonlinear ionization mechanisms multiphoton, avalanche multiplication, and tunneling become important [26].
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In the present investigation, Q switched Nd: YAG laser operating at 1064 nm radiation was used. The laser was operated at the repetition rate 10 Hz with pulse width of the laser beam 10 ns. For the LDT measurement diameter of the laser beam is 1 mm, laser beam was focused on the crystal with a 20 cm focal length convex lens and the crystal was placed just at the focal point. LDT values for (100) planes of pure and doped crystals were recorded when the clear visible spot occurred on surface with audible sound. As given in the experimental details, to get the bulk LDT values, the beam was focused inside the crystal, but close to the surface with a predetermined spot size of diameter 0.20 mm in air. The pulse energy of each shot was measured using the combination of phototube and oscilloscope. The surface damage threshold of the crystal was calculated using the expression: Power density ðP d Þ ¼ E=τπ r 2 where E is the input energy (mJ), τ is the pulse width (ns) and r is the radius of the spot (mm). The measured multiple shot laser damage threshold value for pure and Cu (II) added LSMH is 2.8 and 3.1 GW/cm2. The higher crystalline perfection of grown crystal may be responsible for the larger laser damage threshold. Azarov et al. [31] reported that the damage threshold was influenced by the dislocation in the crystal and the crystal with many dislocations presented low damage threshold. 3.11. NLO property studies The NLO property of the crystal was confirmed by the Kurtz Perry powder technique [15]. The crystals are ground to powder and packed between two transparent glass slides. The first harmonic output of 1064 nm from Nd: YAG laser was made to fall normally, passed through the pure and doped LSMH powder sample. The SHG behavior in these crystals was confirmed from the emission of intense green radiation (λ ¼532 nm) by the sample. The Cu2 þ doped crystal has lower intensity of the green radiation than pure LSMH. The depressed SHG efficiency may be due to the disturbance of electronic charge distribution in doped crystal [32]. 4. Conclusions Good quality single crystals of pure, Cu (II) doped LSMH were grown successfully by slow evaporation solution technique. AAS, EDS result reveals that the amount of dopant incorporated into the crystal lattice is less than the concentration of the dopant in the corresponding solution. Single crystal X-ray diffraction study shows that the small variation in lattice parameter values is due to the contribution of metal dopant in the interstitial sites. The powder X-ray diffraction analysis shows small shift in intensity and peak position. The pure and doped LSMH crystals have good transmission in the range 200–1100 nm and the doped crystals possess increased transmission. Using the FT-IR spectroscopy functional group of the compounds has been confirmed. It is seen from thermal analysis that the decomposition temperature of the doped LSMH crystals increases, when compared to pure LSMH.
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The HRXRD result shows that the crystalline perfection of the doped crystal is considerably good compared to pure crystal. The piezoelectric studies show that doped crystals have higher piezoelectric coefficient than pure LSMH crystal. The laser damage threshold value for pure and Cu (II) doped LSMH was found to be 2.8 and 3.1 GW/cm2 at 1064 nm wavelength of Nd: YAG laser radiation. Form the SHG efficiency analysis doped LSMH crystal has less efficiency than that of pure LSMH.
Acknowledgment The authors are thankful to Dr. Binay Kumar, Department of Physics and Astrophysics, University of Delhi, India for the piezoelectric measurements, Dr. R. Gopalakrishnan, Anna University, Chennai for providing microhardness studies and SAIF, IIT Madras,
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