Thermal, anisotropic microhardness and laser induced surface damage studies on certain metal complexes of thiourea

Journal of Crystal Growth 330 (2011) 43–48

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Thermal, anisotropic microhardness and laser induced surface damage studies on certain metal complexes of thiourea S. Dhanuskodi a,n, T.C. Sabari Girisun a,b a b

School of Physics, Bharathidasan University, Tiruchirappalli 620024, Tamil Nadu, India PG and Research Department of Physics, Bishop Heber College, Tiruchirappalli 620017, India

a r t i c l e i n f o

abstract

Article history: Received 27 May 2010 Received in revised form 2 June 2011 Accepted 12 June 2011 Communicated by M. Roth Available online 17 June 2011

Single crystals of thiourea metal complexes with selected group II metal ions, zinc and cadmium, have been grown by solvent evaporation technique and characterized by XRD studies. The thermal, mechanical and laser induced surface damage properties of thiourea and its metal complexes in (1 0 0) plane were studied. From the improved photopyroelectric technique the thermal properties of the metal complexes were evaluated. Due to larger heat capacity ZTS (382.4 J kg  1 K  1) has better thermal stability than BTCC (304.09 J kg  1 K  1), TTCS (293.5 J kg  1 K  1) and BTZC (255.24 J kg  1 K  1). Vickers hardness studies reveal that the materials have reverse indentation size effect and belong to soft material type. Elastic stiffness was found to be very large for ZTS (8.05) than TTCS (5.38), BTCC (1.57 GPa) and BTZC (0.76 GPa). Multi-shot laser damage studies reveal that ZTS (40 GW/cm2) has higher laser damage threshold and the roles of the group II metal ions in improving the mechanical and thermal stability of the metal complexes are discussed. & 2011 Elsevier B.V. All rights reserved.

Keywords: A1. Thermal conductivity A2. Growth from solutions B2. Nonlinear optical materials

1. Introduction Nonlinear optics (NLO) has emerged as one of the most attractive fields of current research in view of its vital applications in the area of optical modulation, optical switching, optical logic, frequency shifting, high-speed information processing, optical communications and optical data storage for developing technologies in telecommunication and signal processing and different optoelectronic applications [1]. Metal complexes of polarizable organic ligands are currently explored for their NLO properties as they share the advantages of both inorganics and organics, which include extended transparency (down to UV), high optical nonlinearity, amenable crystal growth, good mechanical hardness and chemical inertness. Another remarkable advantage of this class is the high resistance to laser induced surface damage [2]. In general the presence of p-electron delocalization in the molecular structure is found to enhance NLO properties. Thiourea is capable of forming coordinate bonds through sulfur, and more than 700 structures in this group have been reported so far. One such class includes the metal complexes of thiourea, and extensive research has already been focused on these materials. Among these, metal complexes of thiourea with group II metals are interesting to be discussed in the molecular level, as they have a wide variety of

n

Corresponding author. Tel.: þ91 431 2407057; fax: þ91 431 2407045. E-mail address: [email protected] (S. Dhanuskodi).

0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.06.022

NLO applications. Zinc bisthiourea chloride Zn[CS(NH2)2]2Cl2, cadmium bisthiourea chloride Cd[CS(NH2)2]2Cl2 and zinc tristhiourea sulfate (ZTS), Zn[CS(NH2)2]3SO4 with wide transparency are already identified as better alternatives for KDP crystals in frequencydoubling and laser fusion applications [3]. Cadmium tristhiourea sulfate (TTCS) has been recently identified to be an excellent high power optical limiter in photonics industry [4]. In order to realize the practical applications of these materials, it is very necessary to have detailed investigations on the mechanical and thermal properties of the materials. It is well known that microhardness is not only a mechanical characteristic to be routinely measured, but also has been considered to be used in the microstructural investigation method. Of all the wellknown techniques to assess this property of a material, indentation hardness testing is one with the most widespread use [5]. The measurement of hardness is very important as far as the fabrication and failures of devices are concerned. In continuation of our earlier work [5,6] on dielectric and optical analysis of thiourea metal complexes, this article reports the recent results and interpretation of microhardness studies on solution grown single crystals of BTZC, BTCC, ZTS and TTCS. To the best of our knowledge, no work on indepth mechanical analysis of these crystals has been reported till date. Also the thermal parameters of these crystals were also estimated based on the improved photopyroelectric technique. The results of the investigations on laser induced damage in the two important metal-organic crystals are discussed in detail.

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S. Dhanuskodi, T.C. Sabari Girisun / Journal of Crystal Growth 330 (2011) 43–48

2. Experimental

2.2. Analysis techniques

2.1. Synthesis and crystal growth

The grown crystals were confirmed by single crystal X-ray diffraction analysis and also the morphology was studied using ENRAF NONIUS CAD-4 diffractometer. The powder X-ray diffraction analysis (XRD) was also carried out using Rigaku diffractometer ˚ radiation over the range 10–801. The thermal with Cu Ka (1.5406 A) parameters of the grown crystals were determined by an improved photopyroelectric (PPE) technique. A 120 mW He–Cd laser (442 nm) modulated by a mechanical chopper is used as the optical heating source. The photopyroelectric signal phase and amplitude have been measured normal to the prominent (1 0 0) plane. The mechanical characterization of the crystals has been done by microhardness testing at ambient temperature (303 K). Transparent crystals free from cracks with flat and smooth faces are chosen for the static indentation tests. No crystallographic face other than (1 0 0) face is well developed to do the indentation tests. The selected face has been indented gently by the loads varying from 10 to 75 g for a dwell period of 10 s using Vickers diamond pyramid indenter. The indented impressions are approximately square in shape. The shape of the impression depends on material, structure and face. The length of the two diagonals has been measured by a calibrated micrometer attached to the eyepiece of the microscope after unloading and the average is found out. For a particular load at least five welldefined impressions have been considered and the average of all the diagonals (d) has been taken. The Vickers hardness number

The materials were synthesized according to the following reactions: 2CS (NH2)2 þZnCl2-Zn [CS (NH2)2]2Cl2 2CS (NH2)2 þCdCl2-Cd [CS (NH2)2]2Cl2 3CS(NH2)2 þZnSO4-Zn[CS(NH2)2]3SO4 3CS(NH2)2 þCdSO4-Cd[CS(NH2)2]3SO4

Since thiourea has the coordination capacity to form different phases of metal–thiourea complexes, the mixtures of the reactants have to be stirred well to avoid co-precipitation of multiple phases. The product was purified by repeated recrystallization before it is used for the crystal growth. Single crystals were obtained from aqueous solution by slow evaporation technique after a period of 30 days at a pH of 3.00. Optical quality crystals of pure thiourea (6  4  3 mm3), BTZC (12  5  5 mm3), BTCC (11  10  3 mm3), ZTS (12  6  4 mm3) and TTCS (10  4  2 mm3) were obtained (Fig. 1).

800

BTZC (100) Plane MoK α1 (+,−,−,+) 48"

400 0 -200

-100

0

100

Glancing angle [arc s]

200

600

400

BTCC ( 100) Plane MoKα1 (+,−,−,+) 94"

200

0 -200

0

200

TTCS

Diffracted X-ray intensity [c/s]

1200

ZTS

BTCC

Diffracted X-ray intensity [c/s]

Diffracted X-ray intensity [c/s]

BTZC

300

ZTS (100) Plane MoKα1 (+,−,−,+)

Vac

200 37" 100 0 -200

Glancing angle [arc s] Fig. 1. Single crystals and morphology of metal complexes of thiourea.

-100

0

100

Glancing angle [arc s]

200

S. Dhanuskodi, T.C. Sabari Girisun / Journal of Crystal Growth 330 (2011) 43–48

(Hv) has been calculated using the following standard formula [7]: Hv ¼ 1:8544P=d

2

ð1Þ

where P is the applied load in kg, d in mm and Hv is in kg/mm2. Crack initiation and materials chipping become significant beyond 50 g of the applied load; so hardness test cannot be carried out above this load. As indentation initiates plastic deformation in crystals, which is highly directional in nature, any anisotropic effect shown by the size of the indentation mark will be reflected in hardness number. In order to study the hardness anisotropy present in crystals, the crystal is initially mounted on the stage of the microscope properly and indented. The initial position (01) of the index line has been set when one of the diagonals of the indented impression has been parallel to the (1 0 0) direction. The stage of the microscope has then been rotated keeping the indenter fixed and Hv has been measured at every 301 interval. Laser induced surface damage studies were carried out using a Q-switched Nd:YAG laser (1064 nm, 10 ns, 10 Hz). The (1 0 0) plane was irradiated with a laser beam and the energy required to cause the catastrophic damage was measured using a power meter.

3. Results and discussions From the XRD analysis, the cell parameters (Table 1) and morphology (Fig. 1) of metal complexes were obtained. From the recorded pattern of the samples, it was found that BTZC, BTCC and ZTS turn non-centrosymmetric while TTCS remains centrosymmetric as original. But the architecture of introducing an inorganic component into the organic crystal to break the centrosymmetry is noticeable and yields BTZC, BTCC and ZTS alone of noncentrosymmetric nature. While in TTCS, the metal complex could not turn non-centrosymmetric because of excess presence of sulfur around the metal complexes, which does not promote delocalization [6]. The HRXRD diffraction curve (DC) recorded for BTZC, BTCC and ZTS single crystal on (1 0 0) diffracting plane (Fig. 1) shows that the DC contains a single sharp peak and indicates that the specimen is free from structural grain boundaries. The full width at half maximums (FWHM) of the curve are 48, 94 and 37 arcsec, which are not very close to that expected from the plane wave theory of dynamical X-ray diffraction, for an ideally perfect crystal but such magnitude is very common to the real life crystals. It is interesting to see the asymmetry of the DC in ZTS and BTZC. For a particular angular deviation (Dy) 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 the interstitial defects. This can be well understood by the fact that due to vacancy defects,

45

which may be due to fast growth, as shown schematically in the inset, the lattice around these defects undergoes 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 (yB) as d and sin yB are inversely proportional to each other in the Bragg equation. However, these point defects with much lesser density as in the present case hardly affect the performances of the devices based on such crystals. If the concentration is high, the FWHM would be much higher and often leads to structural grain boundaries. Point defects up to some extent are unavoidable due to thermodynamical considerations. It may be mentioned here that though the unit cell volume (and hence the lattice parameter), which contains the vacancy, reduces as the number of unit cells, which undergo tensile stress around the defect core and cause X-ray scattering along the negative side of the peak, are much more than that of the unit cells, which contain the vacancy defects. Effectively, the d-value around the defect core increases and hence one expects more scattered intensity on the negative side of the peak when the crystal contains predominantly the vacancies. In the HRXRD pattern of BTCC, the much broadness with good scattered intensity along the wings/tails of the DC indicates that the crystal contains both vacancy and interstitial type of defects with considerable density. It is worth to mention here that the observed scattering due to point defects is of short range order as the strain due to such minute defects is limited to the very defect core and the long range order could not be expected. Due to such short range scattering, o scan with wide opening for the slit in front of the detector is good enough to collect the scattered intensity. The thermal diffusivity (ad) and the thermal effusivity (e) were determined from the measured PPE signal phase and amplitude. Using the values of ad and e, thermal conductivity (k) and heat capacity (Cp) were calculated and tabulated in Table 2. It is inferred from the table that the heat capacity of ZTS is higher than TTCS. Also in the laser damage threshold analysis, the spread of heat induced (L) by the laser interaction with the material depends on diffusivity [8]: L ¼ 4X t

ð2Þ

Table 2 Thermal parameters of metal complexes of thiourea in (1 0 0) plane. Thermal Heat capacity, Thermal Compound Thermal Cp (J kg  1 K  1) conductivity, diffusivity, ad effusivity, k (W m  1 K  1) (10–7 m2 s  1) e (J m  2 K  1 s  1/2) ZTS TTCS BTZC BTCC

64.38 82.56 96.51 67.30

1295.2 1442.11 1559.70 1364.00

382.4 293.5 255.24 304.09

3.28 4.14 4.84 3.54

Table 1 Cell parameters of metal complexes of thiourea. Sample

˚ a (A)

˚ b (A)

˚ c (A)

Crystal system and space group

Volume (A˚ 3)

TU BTZC BTZC [6] BTCC BTCC [6] ZTS ZTS [6] TTCS TTCS [6]

7.86 12.978(3) [12.852(2)] 13.065 6.505(9) [6.485(0)] 6.485(6) 7.978(4) [7.802(2)] 7.773(4) 8.751(4) [8.732(2)] 8.77(2)

8.9014 12.751(4) [12.661(2)] 12.722 13.235(0) [13.098(9)] 13.106(1) 11.434(5) [11.203(5)] 11.126(5) 9.032(2) [9.012(3)] 9.05(2)

14.8293 5.901(7) [5.889(2)] 5.860 5.948(8) [5.831(9)] 5.812(2) 15.875(7) [15.629(9)] 15.491(5) 9.793(2) [9.654(6)] 9.83(1)

Orthorhombic (Pbnm) Orthorhombic (Pna21) Orthorhombic (Pna21) Orthorhombic (Pna21) Orthorhombic (Pna21) Orthorhombic (Pca21) Orthorhombic (Pca21) a ¼91.7(4) b ¼ 119.9(3) g ¼ 95.7(1) (P1¯) a ¼91.3(2) b ¼ 119.9(1) g ¼ 95.5(2) (P1¯)

1037.84 976.52 [971.2] 978.99 512.197(4) [495.3] 494.09 1410.05 [1359.6] 1339.70 716.23(3) [714.15(6)] 718.91(3)

Z

4 2 2 4 4 2 2

46

S. Dhanuskodi, T.C. Sabari Girisun / Journal of Crystal Growth 330 (2011) 43–48

with X¼k/rCp, t is the laser pulse duration and r is the density of the material. In Table 2 one can notice that BTZC has higher thermal conductivity and thermal diffusivity. However, ZTS and BTCC have a higher specific heat capacity, but a lower thermal effusivity than TTCS and BTZC. These results indicate that TTCS and BTZC conduct heat away faster than ZTS and BTCC. At the same time ZTS and BTCC have a lower thermal impedance (or thermal effusivity) meaning that they adjust more rapidly to the temperature of its surroundings than TTCS and BTZC, or ZTS and BTCC can exchange thermal energy with their surroundings more easily. This accounts for the possibility of a larger laser damage threshold for ZTS than BTCC, TTCS and BTZC. Fig. 2 shows the variation in Hv as a function of applied load in (1 0 0) plane of thiourea TU, BTZC, BTCC, ZTS and TTCS. In all the samples, the hardness increases with the increase in applied load (P) and hence refers to Reverse Indentation Size Effect (RISE). This is attributed to the existence of distorted zone, near crystal– medium interface, effect of vibrations, specimen chipping, radial cracks, etc. [9]. The increase in Hv shows that the introduction of inorganic polyhedra has greatly improved the mechanical stability. The variation in the hardness is purely due to the change in the p electron cloud movement between the donor and the acceptor of the complex. As stated in the structural part, excess presence of sulfur in TTCS has greatly reduced the delocalization. By Meyer’s law the size of indentation and the load are related as P ¼ k1 dn

ð3Þ

where k1 is the material constant and n is Meyer’s index. log P ¼ logk1 þ n logd

ð4Þ

100 TU ZTS BTCC BTZC TTCS

Hv (Kg/mm2)

80

60

The plot of log P vs. log d fitting data before cracking (after least square fitting) gives a straight line (Supplementary data), which are in good agreement with Meyer’s law. The slope of the graph gives the value of ‘n’. For a material, the hardness increases with load, n42, and decreases with the same for no2. In the present case, all the values of n are greater than 2 and this result agrees very well with the argument. According to Onitsch and Hanneman, for hard materials, n should be less than 1.6 and for the soft one, n should be greater than 1.6. Hence both thiourea and its complexes belong to soft materials category. The intercept of the straight line (Supplementary data) gives us the value of log k1 and hence k1, the standard hardness, can be evaluated. The values of k1 can also be obtained from the slope of the plot dn vs. P (not shown here) and the values match very well in both the cases. The experimental observations can also be explained by Hays and Kendall’s theory of resistance pressure. According to this law, a relationship between indentation test load and indentation size by modifying Kick’s law is PW ¼ k2 d2

ð5Þ

where k2 is an another hardness constant and (P  W) is the effective indentation test load considered in the microhardness calculations. According to this law, there is a minimum level of indentation load (W), also known as resistance pressure, below which no plastic deformation occurs. Combining Eqs. (3) and (5), we get   W k2 2 dn ¼ þ ð6Þ d k1 k1 The plot of d2 vs. dn is a straight line with a slope k2/k1 and intercept W/k1 (Supplementary data). Knowing already the value of k1, the values of the other hardness constant (k2) and resistance pressure (W) are estimated. The value of W in (1 0 0) plane is the material resistance to the initiation of plastic flow and it clearly shows that ZTS has larger resistance than BTZC, BTCC, TTCS and TU. It is well known that the material takes some time to revert to elastic mode after the applied load is removed. Hence a correction (x) has to be applied to the observed ‘d’ value. Hence Kick’s law can be modified as P ¼ k2 ðd þ xÞ2

ð7Þ

Thus by plotting dn/2 vs. d (Supplementary data), the values of ‘x’ are determined from the intercepts of straight lines. The striking factor is that x is positive only when n o2 and negative for n42. The value of ‘x’ gives an idea about the nature of the dislocations, in particular the orientation of the crystal [10]. Also the elastic stiffness constant is calculated using Wooster’s empirical formula:

40

20

10

20

30 Load (gm)

40

50

7=4

C11 ¼ Hv

Fig. 2. Vicker’s hardness (Hv) vs. applied load (p) of metal complexes of thiourea.

ð8Þ

which gives an idea about the tightness of bonding between the neighboring atoms and is given in Table 3. From the hardness

Table 3 Mechanical parameters of TU and its metal complexes in (1 0 0) plane. Parameter 2

Hv (kg/mm ) at 10 g Meyer’s index, n k1 (kg/m)  106 k2 (kg/m)  107 Material resistance, W (gm) Elastic stiffness constant, C11 (GPa) Yield Strength, sv (MPa) Correction factor, x (mm)

TU

BTZC

ZTS

BTCC

TTCS

5.89 3.61 5.1 0.36 0.4 0.22 199.08  1.72

11.99 2.39 1.2 0.74 2.7 0.76 405.41  0.864

46.27 3.28 5.23 6.25 57.8 8.05 156.46  0.998

18.19 2.59 1.0 1.3 5.8 1.57 615.1  1.001

36.77 2.77 1.71 3.78 3.17 5.38 124.31  0.835

S. Dhanuskodi, T.C. Sabari Girisun / Journal of Crystal Growth 330 (2011) 43–48

value, the yield strength (sv) is calculated using the following relation [11]:

sv ¼ Hv =2:9f1ðn2Þ½ð12:5ðn2ÞÞ=ð1ðn2ÞÞn2 g

ð9Þ

The various hardness parameters for the samples evaluated are given in Table 3. The microhardness studies clearly picturize the improved mechanical stability of the metal complexes than its parent molecule. Thus the idea of incorporating the central metal ion to form metal complex has not only broken the centrosymmetric behavior but also made the complexes mechanically stable. This is because in the molecular structure level, coordination around the metal atom involves three sulfur atoms from three thiourea molecules and one oxygen atom from a sulfate group, which make the complexes more stable than the parent molecule. Hence it is concluded that the increase in hardness is mainly due to the inorganic polyhedra. ZTS are BTCC are more stable than TTCS and BTZC, respectively, which is due to the variation in the bond length, crystal system and space group of

Hv (Kg / mm2)

80 ZTS BTCC BTZC TTCS TU

60

40

the complexes. According to the valence shell electron pair repulsion theory [12], there is always repulsion between the atoms that are coordinated in a molecule depending on the lone pair of electrons. Also the presence of hydrogen bond network in both the cases is different. Further the values of elastic stiffness constant clearly show the variation in the tight packing of molecules in the system. This argument is further justified by the value of C11 in Table 3, which clearly shows the tight binding nature of the molecules in ZTS. For studying the crystal anisotropy, the variation in Hv as a function of crystal orientation over a range of 0–3301 was plotted (Fig. 3). No distortion in shape of the indentation marks has been observed with the crystal orientation. Here the variation is periodic, in which the minimum repeats at every 901 change in orientation. For the samples, Hmin is observed at 301, 1201, 2101, 3001 and Hmax is observed at 01, 901, 1801, 2701 . Thus the observations indicate the anisotropic behavior of microhardness in the crystals. The crystal structure and the slip system play an important role in the observed variation of hardness with load and also the variation of hardness with crystal orientation. As the size of the indentation is regarded as the ease of slip, larger impressions appear when the resolved shear stress is high enough and smaller impressions when it is low. For proper orientation of the slip plane, the yield stress will be minimum. Thus, the directional variation in hardness is due to the change in orientation of the slip system of the crystal with respect to the indenter [13]. The laser damage threshold of the grown crystals was evaluated using Power density ðPÞ ¼ E=t0 A

20

0 0

100

200 Angle (°)

300

Fig. 3. Anisotropic behavior of metal complexes of thiourea.

400

47

ð10Þ

where E (mJ) is the energy required to cause damage, t0 (ns) is the pulse width and A is the area of the laser spot. As the samples with high dislocation densities have a lower resistance to laser damage, they were avoided. This was facilitated by etching the cleaved counterparts and ensuring low defect densities. As expected ZTS (40 GW/cm2) has larger laser damage threshold than TTCS (8 GW/cm2), BTCC has larger laser damage threshold (15 GW/cm2) than BTZC (6 GW/cm2). This result agrees very well with the

Fig. 4. Optical micrograph of laser induced damage viewed along the axis of the laser radiation (BTZC, BTCC, ZTS and TTCS).

48

S. Dhanuskodi, T.C. Sabari Girisun / Journal of Crystal Growth 330 (2011) 43–48

predictions reached through PPE and microhardness studies. Thus ZTS was found to be more efficient than other complexes for laser fusion applications. In order to probe the nature of the damage in crystals investigated, the damaged samples were subsequently examined under optical microscope (Carl Zeiss Germany Trinocular Microscope). The surface morphology of the damage pattern reveals the nature and the possible origin of the damage in crystals. From the magnified images (Fig. 4), it is identified that the damage patterns are unique in the sense that they are highly local, without any spreading cracks or fragmentation. Hence the dominant mechanism operating is due to thermal effects.

4. Conclusion The structural characterization shows that BTZC, BTCC and ZTS are noncentro-symmetric and TTCS is centrosymmetric. Both thermal and mechanical stabilities of the metal complexes are improved by the central metal ion. PPE studies show that, with large heat capacity and small thermal conductivity, ZTS (382.4 J kg  1 K  1, 3.28 W m  1 K  1) has better thermal stability than other complexes. Vicker’s microhardness study on solvent evaporation grown thiourea and its metal complexes reveals that the Vicker’s hardness number increases with the increase in load and hence the material belongs to soft material category as Meyer’s index is greater than 1.6. The stiffness constant is quite high for ZTS followed by BTCC, BTZC, TTCS and TU, revealing that the binding forces between ions are quite high. The periodic variation of hardness with orientation

shows the anisotropic nature of the crystals. Laser damage studies reveal that ZTS (40 GW/cm2) is more efficient than other complexes for laser fusion experiments.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jcrysgro.2011.06.022.

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