Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 177–183
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Spectroscopic, thermal and mechanical studies on 4-methylanilinium p-toluenesulfonate – a new organic NLO single crystal G. Shanmugam, S. Brahadeeswaran ⇑ Department of Physics, Anna University of Technology Tiruchirappalli, Tiruchirappalli 620 024, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Systematic studies have resulted in
the growth of high quality PMPT crystals. " PMPT crystals exhibited good transparency in the useful UV– Visible region. " Spectroscopic, thermal and mechanical studies were reported for the first time. " Physicochemical properties of PMPT suggest its suitability for the NLO applications.
a r t i c l e
i n f o
Article history: Received 9 February 2012 Received in revised form 19 April 2012 Accepted 28 April 2012 Available online 9 May 2012 Keywords: Crystal growth Powder X-ray diffraction Spectroscopic studies Non-linear optical materials
a b s t r a c t Bulk crystals of newly identified organic nonlinear optical material 4-methylanilinium p-toluenesulfonate (PMPT) were grown by slow evaporation solution growth method using ethanol as a solvent. It crystallizes in monoclinic system with a noncentrosymmetric space group P21. The formation of the title compound was confirmed through microanalysis, X-ray diffraction and density measurements. The proton positions and functional groups have been identified and confirmed through nuclear magnetic resonance and Fourier transform infrared spectrums respectively. Optical properties are determined by UV–Visible and photoluminescence spectroscopic studies to explore its efficacy towards device fabrications. Thermal studies exhibited that the newly obtained PMPT crystals are stable up to 199 °C. Its mechanical strength was studied by Vickers micro hardness studies. Ó 2012 Elsevier B.V. All rights reserved.
Introduction Methods of inventing of novel and cost-effective organic materials with efficient nonlinear optical (NLO) coefficient has been the subject of intense research as these materials find their applications in the fields of electro-optic modulation, frequency mixing, signal processing, photonic and opto-electronic technologies, etc. [1,2]. Besides exhibiting strong NLO response, the NLO materials must also accomplish some other technological requirements for practical applications such as wide transparency extending down to UV–Visible region, fast response and thermal and mechanical ⇑ Corresponding author. Tel.: +91 09442317559; fax: +91 0431 2407999. E-mail address:
[email protected] (S. Brahadeeswaran). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.04.100
stabilities. For instance, extensive research has been carried out on 4-N,N-dimethylamino-40 -N0 -methyl stilbazolium tosylate (DAST) single crystals to use their excellent NLO and electro-optical properties efficiently for the generation and detection of terahertz (THz) frequencies [3,4]. However, the growth of bulk or thin films of DAST crystals still remains a big challenge. Therefore, novel materials with DAST constituents with better growth possibilities are greatly desired [5–7]. In this direction, Xu has recently reported on crystal structure of a new compound, namely 4-methylanilinium p-toluenesulfonate (C7H10N+.C7H7O3S) wherein p-toluene sulfonic acid (one of the DAST components) forms a complex with p-methyl aniline [8]. As this molecule crystallizes with non-centrosymmetric crystal structure, we focused our interest towards the systematic studies on PMPT and the results obtained are reported
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here with experimental details. The studies here are reported for the first time in literature, to the best of our knowledge. Experimental details
1%. The experimental density of the grown PMPT crystal was 1.347 g/cm3 and it agreed well with the theoretical density (1.341 g/cm3) calculated by the formula,
qt ¼
Material synthesis PMPT was synthesized by the addition of high purity p-toluenesulfonic acid monohydrate (C7H8SO3.H2O) with p-methylaniline or p-toluidine (C7H9N) according to the following scheme [8]. (See Scheme 1) The prepared solution was then allowed to dry at about 45 °C and the salts were obtained by slow evaporation technique. The final product was purified by recrystallization process several times and then it was used for crystal growth. As Carbon–Hydrogen– Nitrogen–Sulfur (CHNS) analysis (or microanalysis) is an important tool for the certification of synthesized organic compounds at intermediate and final stages, the final product of PMPT was subjected to this analysis using Vario EL III CHNS elemental analyzer by employing helium as a carrier gas. The accuracy of the analyzer was 0.05%. The lower detection limit of the analyzer for Nitrogen was 0.1 lg and for Carbon, Hydrogen, Sulfur it was 0.5 lg. The results of CHNS analysis revealed that the percentage of Carbon, Hydrogen, Nitrogen and Sulfur were 60.24% (60.19%), 6.10% (6.13%), 5.07% (5.01%) and 11.40% (11.48%), respectively and they agreed well with those obtained by theoretical calculation which are provided in parenthesis. Crystal growth The saturated solutions were prepared with ethanol at various temperatures to study the solubility and the metastable zone width in order to explore the possibilities of growing PMPT crystals by seeded method. A saturated solution of PMPT was prepared at room temperature (34 °C) based on the solubility curve and the filtered solution was allowed to evaporate at room temperature using a temperature controlled growth chamber. Subsequently, the optically transparent, inclusion free single crystals grown by spontaneous nucleation were used as seed crystals for bulk growth [9]. The saturated solution of about 200 ml was taken in a beaker and it was kept in a constant temperature bath equipped with a programmable Eurotherm Temperature Controller (3216) having a controlled accuracy (±0.01 °C) for seeded growth. Initially the bath temperature was increased by 5 °C above the saturation temperature to dissolve any spurious nucleation present in the solution. The selected seed was introduced in the metastable regime (discussed in the later part of this article) and the seeded growth of PMPT single crystal was carried out by the slow cooling technique [10]. In our growth runs, the initial cooling rate was 0.06 °C/h until the seed introduction and was later reduced to 0.02 °C/h for the growth. The good size and transparent single crystals of PMPT was harvested in the size 28 123 mm3 after a period of 15 days. The density was determined experimentally by floatation technique using Xylene and carbon tetrachloride as various density mediums to confirm the crystallization of the title compound. This method gave fairly accurate results, with a standard error of about
MZ NV
where M, the molar mass, Z, the number of molecules in a single unit cell and V, the volume of the unit cell. Characterization details In order to confirm the phase formation of PMPT crystals the powder X-ray diffraction pattern was recorded using a microprocessor controlled X-ray diffractometer (XPERT-PRO) using Cu-Ka radiation (40 kV, 30 mA). Data were recorded over a 2h range 10.004–79.976° using step scan of 0.017° for a time interval of 12.28s. Single crystal data was collected at room temperature using an Enraf–Nonius CAD-4 Diffractometer with MoKa (0.71073 Å) radiation using carefully selected single crystal of dimension 1 0.5 0.5mm3. Cell parameters were obtained from leastsquares refinement of the setting angles of 25. The theoretical BFDH morphology of the PMPT crystal was determined by the Mercury software using the unit cell and positional coordinates of the grown crystal as a tool. In order to identify the functional groups and proton positions the powder of the grown crystal of PMPT was subjected to FTIR and 1H NMR analysis. The FTIR spectrum was recorded using Jasco spectrometer FTIR (model 1410) for the wavelength range 400–4000 cm1 by KBr pellet technique with a resolution and scanning speed of 4 cm1 and 2 mm/s, respectively. NMR technique was used to detect the presence of particular nuclei in a compound for a given nuclear species. The 400 MHz proton (1H) NMR spectrum was recorded using Bruker NMR spectrometer by dissolving the powder of PMPT crystals in DMSO solvent at 292.9 K. The UV–Vis transmittance spectrum was recorded in the range 190–1100 nm using UV–Visible-Perkin Elmer (model: Lambda 35) spectrophotometer with a wavelength accuracy of ±0.2 nm. The emission spectrum of as grown PMPT single crystal was measured at ambient temperature using Varian Cary Eclipse Fluorescence Spectrophotometer. The emission spectrum was recorded by exciting the sample at 280 nm (kexc) with Xenon lamp and the emission was fed into a monochromator where the emission intensity was recorded as a function of the wavelength. The melting point of the grown crystals was found directly using a melting point apparatus (INLAB Equipments (Madras) Pvt. Ltd). Thermogravimetry and differential thermal analysis (TG-DTA) were carried out using a Simultaneous Thermal Analyzer SDT Q600 V8.3 Build 101 in Nitrogen atmosphere with a heating rate of 20 °C/min from 30 to 500 °C by taking Alumina as crucible and reference material. The balance sensitivity, temperature sensitivity and calorimetric accuracy of the analyzer were 0.1 lg, 0.001 °C and ±2% respectively. Microhardness study was performed using a Shimadzu microhardness tester (HMV-2) fitted with a diamond pyramidal indenter with a face angle of 136°. The resolution of the tester was 0.01 lm. To evaluate the Vickers hardness number, static indentations were performed at room temperature on selected smooth surface (0 0 1) of the PMPT crystal. The applied load was varied from 5 to
Scheme 1. Material synthesis of p-methylanilinium p-toluenesulfonate.
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50 g for a constant indentation period of 15s for all indentations. The indented impressions were approximately square when viewed under optical microscope. In order to avoid the mutual influence of any previous indentation the distance between any two indentations was maintained to be greater than five times that of the diagonal length of the indented square. Results and discussion Solubility and metastable zone width studies Solubility and Metastable Zone Width curves are the critical parameters for the successful development, optimization and scale-up of a crystallization process. The solubility measurement of a material in any solvent gives indication about the nucleation and availability of the solute material for crystal growth and to decide the cooling rate during the crystal growth. Super saturation is the driving force for the crystallization and it also affects the crystal quality; for growing good quality crystals the solubility of the material should be moderate. A solute will remain in solution until a sufficiently high level of supersaturation has been developed to induce spontaneous nucleation. The difference between the saturated temperature and the nucleation temperature was taken to be the metastable zone width of the system [11]. It will typically be influenced by a variety of process parameters including saturation temperature, rate of supersaturation generation, impurity level, mixing, and solution history [12]. The solubility and metastable zone width details of PMPT in ethanol are shown in Fig. 1. The solubility increases almost linearly with the temperature and hence it suggests slow cooling or slow evaporation solution growth could be a better method to grow good quality single crystals of PMPT. It could also be understood from the figure that the PMPT exhibits desired wide zone width for the ethanol solvent which could be suitable for the growth of large crystals with high growth rate [13]. Spectroscopic studies X-ray diffraction and morphology studies From the powder X-ray diffraction data, various planes of reflections were indexed using mercury software [14]. The indexed powder X-ray diffraction pattern (Fig. 2) was compared with the XRD pattern simulated by the Mercury software and was found to agree with each other. The lattice parameters from single crystal X-ray diffraction are: a = 5.767(1) Å, b = 9.016(4) Å, c = 13.319(2) Å, b = 96.23(2)°. The volume of the system is 691.6(4) Å3 and the
Fig. 1. Solubility and Meta stable zone studies of PMPT in ethanol.
Fig. 2. Powder X-ray diffractogram of PMPT crystal.
number of molecules per unit cell (Z) is two. These results agreed well with the reported values [8]. The compound crystallizes in a noncentrosymmetric space group (P21–monoclinic) which is a basic precondition for a potential second harmonic generation (SHG) material. The molecular structure, morphology and typical picture of the grown PMPT crystal are shown in Fig. 3. The PMPT crystals have well developed morphology with several habit faces. For each face, its parallel Friedel plane is also present in the grown crystal. planes were the The result indicated that the (0 0 1) and ð0 0 1Þ most prominent planes over other planes. NMR and FTIR spectral studies Proton NMR spectroscopy, often known as Proton Magnetic Resonance (PMR), plays a vital role in the structural confirmation of the grown material as provided in the following point: (a) a relation between the number of signals in the spectrum and the number of different kinds of hydrogen atoms in the molecules, (b) same ratio for the areas underneath each signal with the number of hydrogen atoms causing that corresponding signals and (c) splitting of the principal signal (doublet, triplet and quartet, etc.) according to the number of neighboring nonequivalent protons [15,16]. For many simple compounds, one can predict the molecular structure through the spin–spin splitting pattern by the n + 1 rule, where n is the number of protons present in the neighboring carbon. The separation in the position of the spectral signals of hydrogen atoms in the different chemical environments from that of some arbitrarily chosen standard is called the Chemical Shift (d).The chemical shifts of atoms in a molecular chain depend on the electronic and molecular environment of the nuclei [17]. The 1 H NMR spectrum of PMPT with its proton position indications was shown in Fig. 4. The two singlet at d = 2.3 was assigned to the two methyl protons in positions 1 and 10 . The doublets at d = 7.22 ppm and d = 7.11 ppm were attributed to the protons in positions 3, 7 of the p-toluenesulfonate and 30 , 70 of the p-toluidinium ring, respectively. Similarly the neighboring doublets at d = 7.28 ppm and d = 7.48 ppm were attributed to the protons in positions 4, 6 and 40 , 60 of the same molecules respectively. The broad peak at d = 9.81 was attributed to the protons in the amino functional group of the anilinium part of the complex [15–17]. The abovementioned results of PMR have confirmed the molecular structure of PMPT material. The middle infrared (IR) spectrum of the PMPT crystal is shown in Fig. 5. The frequency assignment of FTIR spectrum using the standard spectra of the functional groups [17,18] is provided in
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Fig. 3. (a and b) Molecular and packing diagram of PMPT; (c) typical PMPT crystal with well developed (0 0 1) plane.
Fig. 4. 1H NMR spectrum of PMPT.
Table 1. The peaks at 3489 and 3413 cm1 in the higher frequency region are associated with asymmetric and symmetric stretching of N–H in the amino ðNHþ 3 Þ group respectively. The presence of methyl groups are confirmed through the symmetric and asymmetric stretching vibrations of C–H at 2919 and 1438 cm1 respectively. The sharp peak observed at 1714 cm1 was due to the stretching of C@C in the aromatic ring and at 1022 and 809 cm1 were attributed to inplane and out of plane bending of aromatic ring C–H, respectively [16–18]. The sharp intense peaks at 490 and 560 were assigned to the symmetric and asymmetric stretching of the sulfonate ðSO 3 Þ group.
laser frequency conversion applications. As can be seen in the Fig. 6, the as grown PMPT is optically transparent in the entire UV region (with 89% transmittance). It is also seen that a strong absorption band occurs at 280 nm and that this absorption is due to the promotion of an electron from a ‘non-bonding’ (lone-pair) n orbital to an ‘antibonding’ p orbital designated as p⁄ (n ? p⁄) and no characteristic absorption was observed in the entire visible region. This transmittance window (300–1100 nm) is suitable for the generation of second harmonic (k = 532 nm) as well as third harmonic lights (k = 354.6 nm) from the Nd:YAG laser (k = 1064 nm) [19]. In addition, studies on optical-absorption properties provide a method of understanding optically induced electronic transitions and furnish ideas about the band structure as well as the energy gap in crystalline materials. In general, the energy gap (Eg) can be determined by electrical and optical processes through conductive and absorption methods respectively. Herein, the energy band gap was calculated from the UV–Vis absorbance data by extrapolating the plot of (ahm)2 vs. (hm), (where a is the absorption coefficient). The computation of optical absorption coefficient for fundamental absorption employs quantum manipulations. Essentially, these manipulations consist of treating the incident radiation as a perturbation which couples the electron state in valance band to its counterpart in conduction band using the technique of quantum perturbation theory. The optical absorption coefficient (a) of PMPT was calculated using the relation
1 d
a ¼ log Transmission spectroscopic studies The optical transmission range, transparency cut-off and the absorbance band are the most important optical parameters for
1 T
where d is the thickness of the crystal and T is the transmittance. The absorption obey a relation of the form
ahv ¼ Aðhv Eg Þn
Fig. 5. FTIR spectrum of PMPT.
which is valid for allowable direct transitions between the simple parabolic bands [20]. For a direct transition, n = 1/2 or 3/2 depending on whether the transition is allowed or forbidden in quantum mechanical sense. Similarly, n = 2 or 3 for indirect allowed or forbidden transitions respectively [21]. The band gap is determined from the Tauc’s plot of hm vs. (ahm)1/n, for which the value of n gives the best linear graph in the band edge region. There will be a single linear region in direct transition and two linear portions in indirect transition. By taking the solitary linear behavior as an evidence for the direct transition between valence and conduction bands, the optical energy band gap (Eg) of PMPT was estimated by extrapolating the linear portion of the curve to a point (ahm)2 = 0. Using this method, the optical band gap has been estimated to be about 4.4 eV (inset of Fig. 6). As a consequence of this wide band gap, the grown crystal could be expected to possess high damage threshold and large transmittance in the visible region [22,23].
G. Shanmugam, S. Brahadeeswaran / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 177–183 Table 1 FTIR spectral data of PMPT. Wave number (cm1)
Tentative assignments
3489.54 3413.60 2919.18 1922.00 1714.75 1506.94 1438.55 1371.72 1237.31 1177.44 1022.84 809.02 682.95 560.97 490.53
Amino N–H asymmetric stretching (NHþ 3) Amino N-H symmetric stretching (NHþ 3) Methyl C–H symmetric stretching (CH3) C@N stretching vibration Aromatic ring C@C stretch Symmetric N–H bending (NHþ 3) Methyl C–H asymmetric bending Asymmetric S(=O)2 stretching N-H deformation vibration Symmetric S(=O)2 stretching Aromatic ring Inplane C–H bend Aromatic ring C–H out of plane bending C-S stretching vibration C-N out of plane bending , SO 3 asymmetric stretching Out of plane ring C@C bend , SO 3 symmetric stretching
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absorption bands could be explained by a strong inhomogeneous broadening of the optical transition [31]. The strength of the electron–phonon interaction could be ascribed to the difference between the excitation and the emission maximum (Stokes shift) [32]. The UV band was attributed to the exciton-related emission near the band-edge, usually reported as the recombination of free excitons [33]. As the energies of emission peaks are lower than the band gap energy, the observed PL is not related to a direct electronic transition between the valence band and the conduction band. Hence, it could be understood that the emission may be associated with the radiative recombination between trapped electrons and trapped holes which occur between localized states situated inside the band gap [34]. In general, the shift in the emission peak of the sample from shorter to longer wavelengths depends on the strength of their intermolecular interaction which is lower for solution and higher for solids [35]. Moreover, the occurrence of little change in emission peak is related to the physical form of the sample such as concentration for solution or thickness for crystal [36]. The broad PL spectra are optical phenomena caused by diverse electronic transitions occurring in different energy levels (deep or shallow holes) within the band gap [32]. The violet and blue emission can thus be attributed to shallow defects in the band-gap and to a more ordered structure, while the green, yellow and red emission are linked to defects deeply inserted in the band-gap and to a greater disorder in the lattice. In this sense, each color represents a different type of electronic transition and is linked to a specific structural arrangement [32,37]. Therefore, it is indicative that the charge transfer process as well as the trapping of electrons occurs due to greater contribution of the shallow holes than the deep holes [38,39]. SHG studies
Fig. 6. Optical transmission spectrum of PMPT single crystal along with its Tauc plot (as inset).
Photoluminescence studies The photoluminescence (PL) spectroscopy is one of the effective tools to provide relatively direct information about the physical properties of materials at the molecular level, including shallow and deep level defects and gap-states [24]. These studies have lead to newer materials for fluorescent lamps, X-ray image plates, dosimetry materials, lasing media, etc. [25–27]. Here, the necessitated non-equilibrium carrier concentration in the electronic bands or in the electronic states of a defect structure is obtained by optical irradiation and the emission was due to the radiative recombination of electron and hole pair, which is particularly important for laser processes in the visible spectral range and for chromophores in organic crystals [28]. Fig 7 shows the room temperature PL spectrum of PMPT single crystal when it was excited at 280 nm (which is shown as inset of Fig. 7). It was a broad spectrum and showed two peaks centered at about 2.92 eV (425 nm) and 3.15 eV (395 nm) and no other visible emission was observed. This covers the intact violet and blue regions of the visible spectrum which may be attributed to the p⁄ ? n transition [29,30]. The spectra found here resemble the general behavior of fluorescing organic molecules but there is no strict mirror symmetry between the absorption and the emission spectrum. The different width of emission and
In order to find NLO efficiency, the powdered sample derived from grown crystal was subjected to SHG studies by employing Kurtz powder technique. For our experiment, a Q-switched Nd: YAG laser delivering energy of 6.05 mJ/pulse at 1064 nm with a repetition rate of 10 Hz and pulse width of 10 ns was used as source of fundamental laser light. The standard potassium dihydrogen phosphate (KDP) crystals, powdered to have particle size comparable to that of PMPT sample, was used as the reference. The reference and PMPT were filled into separate micro capillary tubes of diameter about 1.8 mm. A tight packing of powder samples was ensured with the aid of mechanical vibrator. The fundamental laser light was focused using a lens of focal length of 20 cm. The capillary tubes were placed 3 cm away from the focal point to avoid laser-induced damage in samples. The output, could be seen as a bright green flash emission, was congregated by a monochromator towards the photomultiplier tube. The SHG output was converted into electrical signal and was displayed on a digital storage oscilloscope. The SHG results revealed that the PMPT sample had the output intensity of 71.4 mV whereas that of KDP was 25.5 mV. The SHG efficiency of the sample was compared with KDP and was found to be nearly 2.8 times greater than that of KDP. Higher values of conversion efficiency than those presented here are expected to be achieved for PMPT crystals by optimizing the phase-matching orientations [40]. Thermal studies Thermogravimetric/differential thermal analysis (TG/DTA) measurements were used to examine the thermal stability of the crystalline samples and to define the conditions for the thermal treatment on them. From the melting point apparatus, the melting point of the grown crystals was found to be about 204(±1) °C. The thermograms observed from simultaneous TG/DTA were shown in Fig. 8. The TG curve revealed a single stage weight loss (>85%)
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Fig. 7. (a) Emission and (b) excitation curves of PMPT single crystal.
occurred between 206 °C and 322 °C. The shape of the TG curve indicated that the melting which is immediately followed by decomposition with the formation of the volatile reaction products of the sample [41]. The disintegration process continues with the confiscation of almost all fragments as gaseous products, leading to the bulk decomposition of the compound before 490 °C since the initial mass of the sample was 2.45 mg and at a temperature of about 490 °C, only 6.1% (0.1549 mg) of the initial mass was obtained as residue. The absence of any weight loss or phase transition around or before its melting point, confirmed the nonexistence of any lattice entrapped solvent or moisture on the grown material. The corresponding DTA curve shows two notable thermal events. It implies that the material undergoes an irreversible endothermic transition at 199 °C, which is the onset of melting process. The peak of the endothermic, which represents the temperature 203 °C at which the melting terminates, is corresponding to its melting point [42]. The second endothermic event represents the evaporation of the volatile reaction products since the sample absorbs energy in order to melt and also to evaporate [43]. The results obtained from TG/DTA studies showed that the PMPT crystals are stable up to 199 °C, the onset of its melting point.
Mechanical studies The structure and nature of bonding of the crystalline solids have influence on their mechanical hardness. 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 [44]. The Vickers Hardness Number (VHN) of PMPT was calculated using the expression
Hv ¼ 1:854
P 2
d
where Hv is the Vickers hardness number in kg/mm2, P is the applied load in kg, and d is the average diagonal length of the indentation in mm. The typical Vickers hardness values of the grown crystal for various loads on the prominent plane (0 0 1) were plotted in Fig. 9. Each data point represents an average value obtained from several indentations. The hardness increased up to a load of 100 g, above which cracks started developing due to the release of internal stress generated locally by indentation. The increase of hardness may be attributed to the work hardening of the surface layers. From
Fig. 8. TG/DTA thermograms of PMPT.
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Fig. 9. Dependence of microhardness Hv on applied indentation load P for (0 0 1) sample.
the microhardness study it is understood that the PMPT crystal exhibits reverse indentation size effect (ISE) where the apparent microhardness increases with increasing applied test load in contrast to normal ISE [45]. Conclusions Single crystals of PMPT have been grown from ethanol by solution growth technique and its lattice parameters were confirmed by single-crystal XRD. It crystallizes in a monoclinic system with a space group P21. The electronic spectrum shows that the as-grown PMPT crystal had nearly 89% transparency in the entire UV–Vis region with a lower cut off around 280 nm. The crystal’s band gap energy was estimated to be 4.4 eV. The luminescence curve suggests that PMPT could be used as a violet or blue emission material by employing appropriate filters depending on the applications. TG/ DTA thermograms revealed the thermal stability of PMPT. The SHG efficiency of the grown material was found to be 2.8 times that of standard KDP. The Vickers microhardness test clearly revealed that the grown crystal obeys the reverse indentation size effect for the applied loads. The good transparency, higher SHG efficiency, luminescence behavior, wide band gap and metastable zone width along with high melting point suggest that the PMPT could be a promising material for the NLO applications and its device fabrications. Studies such as phase matching, laser damage threshold etc. would further enhance the knowledge on this crystal. Acknowledgments This work is supported by the Department of Science and Technology (DST), New Delhi, India, through the SERC FAST TRACK YOUNG SCIENTIST SCHEME (SR/FTP/PS-53/2007 Dt. 22-08-08) and its financial support is hereby gratefully acknowledged. We are thankful to Dr. K. Nehru, Assistant Professor, Dept. of Chemistry, Anna University of Technology- Tiruchirappalli, Tiruchirappalli, India for the discussions on NMR spectra. References [1] X. Zhang, M. Li, Z. Shi, Z. Cui, Mater. Lett. 65 (2011) 1404–1406.
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