Semiorganic Crystal Mixed Crystal Hexakis(Urea)Cobalt(II) Sulfate Monohydrate - Optical, Thermal and Mechanical

Abstract

Single crystals of hexakis(urea)cobalt(II) sulfate monohydrate have been grown by slow evaporation solution growth technique at room temperature. The single crystal X-ray diffraction study reveals that the crystal belongs to orthorhombic system with noncentrosymmetric space group Pca21 and the cell parameters are, a = 15.0549(6) A? , b = 7.2105(3) A? , c = 20.0284(9) A? , V = 2174.15 (16) A? 3 and Z = 4. The functional groups of the metal–urea complex were con?rmed by Fourier transform infrared spectroscopy. The powder X-ray diffraction study reveals the crystallinity of the as-grown material. The band-gap energy of the specimen is estimated by Kubelka–Munk algorithm. The dielectric results indicate an increase in dielectric and conductivity parameters with an increase of temperature at all frequencies. The crystal is further characterized by mechanical and thermal analysis.

Introduction

I. INTRODUCTION

Urea crystal is the ?rst organic crystal successfully used in an optical parametric oscillator [1]. It has large nonlinear optical coef?cients, high birefringence, high laser damage threshold and nonlinear optical applications in UV because of its transparency range extending to 200 nm in the short wavelength limit [2] and it also serves as a better crystal compared to KDP for its nonlinear optical coef?cient [3]. Derivatives of urea offer a wide choice of novel materials, expected to play an important role in the development of nonlinear optical (NLO) devices [4] and its derivatives are potential candidates for nonlinear optics [5–7]. The structural and spectroscopic properties of metal–urea com- plexes have been reviewed [8]. The metal–urea complexes are interesting group of compounds with which to investigate the metal–oxygen and metal–nitrogen chemical bonds and the effect of metal interaction on the structure and properties of urea. Metal–amide complexes are of great signi?cant to the pharma- ceutical and medical sciences, for instance antimicrobial and antivirial properties, and could be effective against diseases [8].

Number of cobalt(II) urea complexes have been reported [9–15]. In all cases the urea ligand is bound to the metal atom through an oxygen–metal bond, except for [Co(urea)4](NO3)2, where an urea molecule binds to two metal atoms via two coordination sites viz., one from the oxygen atom and the other from the nitrogen atom [14]. However, all these studies mainly focused on the structural determination of cobalt(II) urea com- plexes. As far as we know, there is no reference in literature regarding the growth and characterization of metal–urea com- plexes. In the present study, we report the growth, structure and characterization of hexakis(urea)cobalt(II) sulfate monohydrate (HUCS) crystals.

II. EXPERIMENTAL

A. Synthesis and Crystal Growth

HUCS was synthesized using AR grade cobalt(II) sulfate hap- tahydrate (E-Merck) and urea (E-Merck) in a stoichiometric ratio of 1:6. The reactants were dissolved in triply distilled water, thoroughly mixed using a magnetic stirrer (3 h). The solution was tightly covered with a perforated polythene sheet and kept at a constant temperature ( ~ 35 1C). The crystallization took place in a period of 30–35 days and the crystals were harvested when they attained suitable size and shape. Photograph of the as-grown HUCS crystal is shown in Fig. 1.

B. Characterization

The Fourier transform IR (FT-IR) spectrum was recorded using AVATAR 330 FT-IR by the KBr pellet technique. The structural analysis of HUCS was carried out for a selected tablet of approxi- mately 0.30 ~ 0.25 ~ 0.20 mm3 using Bruker AXS (Kappa APEXII) X-ray diffractometer. Single crystal XRD data were collected on a diffraction system which employs graphite monochromated Mo Ka   radiation   (l    0.71073 A? ).   The   structures   were   solved   and re?ned by full  matrix  least  squares  on  F2  with  WinGx  soft- ware package utilizing SHELXS97 and SHELXL97 modules. The powder XRD pattern is recorded by PANalytical X’pert PRO X-ray diffractometer. UV–vis spectrum was recorded using CARY 5E UV–vis spectrophotometer in the spectral  range  200–900 nm. DSC analysis was carried out using SDT Q600 (TA instrument) thermal analyzer. The Vickers microhardness was evaluated for the well polished grown crystal using Reichert 4000E Ultrami- crohardness tester. Dielectric measurements were carried  out  by the parallel plate capacitor method as a function of temperature for various frequencies using a Precision LCR meter (AGILENT 4284 A model).

Fig. 1.  Photograph of the as-grown HUCS crystal.

III. RESULTS AND DISCUSSION

A. FT-IR Analysis

Urea has three coordination sites viz., the carbonyl oxygen and the two nitrogen atoms. Among the three  coordinating  sites, urea usually acts as a monodentate ligand except in [Co(H2N- CONH2)4](NO3)2 [14] complex in which it acts as a bidentate ligand. The formation of unidentate oxygen–metal bonds between urea molecules and metal ions causes only minor changes in the infrared spectrum of urea molecule [8]. The FT-IR spectrum of HUCS crystal is shown in Fig. 2. The infrared spectrum con?rmed that the coordination of the metal ions with the urea molecules is through oxygen rather than the nitrogen atom. A decrease in the N–H stretching frequency of the complex compared to the free ligand indicates N-coordination, while a decrease in the C–O stretching frequency and an increase in C–N stretching frequency indicate O-coordination [16]. The C–N stretching frequencies at 1471 and 1001 cm—1 of urea are shifted to higher frequencies (1486 and 1024 cm—1) and the N–H stretching vibrations at 3460 and 3350 cm—1 of urea [17] are shifted to lower frequencies ( ~ 3424 and ~ 3320 cm—1). The C–O stretching  ( ~ 1680 cm—1) and N–H bending ( ~1630 cm—1) vibrations of urea  [17]  are shifted to lower frequencies ( ~ 1649 and ~1616 cm—1). The char- acteristic vibrational patterns of some metal urea complexes along with HUCS crystal are given in Table 1.

Fig. 2. FT-IR spectrum of HUCS.

B. Single crystal XRD analysis

The structural analysis of HUCS crystal was carried out by single crystal XRD analysis. The ORTEP and packing diagrams of HUCS crystal are given in Figs. 3 and 4. It crystallizes in the orthorhombic system with noncentrosymmetric space group Pca21 and crystal data are given in Table 2. These lattice parameters closely match the previous study [18] of structure determination of HUCS. The central cobalt atom is coordinated by six oxygen atoms of urea molecule with Co–O–C  bond  angles  ranging from 128.28(18)1 to 135.61, O–Co–O bond angles widely ranging from 85.85(7)1 to 178.57(8)1 and Co–O bond lengths lying in the  range  from  2.0730(19)  to  2.1431(18) A? in a distorted octahedral geometry. The crystal contains [Co(CO(NH ) ]2þ cations, stacks of SO2— anions and free water molecule. It has been reported that these crystals are isostructural to the Ni(II) [19] and Zn(II) [20] hexakis(urea) complexes. Crystal system, space group and cell parameters of some cobalt(II)–urea complexes are given in Table 3.

C. Powder XRD Analysis

The indexed powder XRD pattern of the as-grown HUCS crystal pattern is shown in Fig. 5. It shows that the sample is of single phase without any detectable impurity. Narrow peaks  indicate the good crystallinity of the material. The observed peaks are in good agreement with the JCPDS ?le [18]. The simulated XRD pattern using single crystal XRD data (Fig. 5a) closely resembles the experimental pattern.

D. Optical Studies

The optical absorption spectrum of the HUCS crystal (Fig. 6) shows two bands peaking at ~ 235 nm and ~535 nm. The absorption spectrum shows a band at ~ 535 nm, typical of octahedral geometry of cobalt(II) [21]. The Kubelka–Munk theory [22] provides a correlation between re?ectance and concentra- tion. The concentration of an absorbing species can be determined using the Kubelka–Munk formula,

FðRÞ¼ ð12RÞ2=2R ¼ a=s ¼ Ac=s

where F(R) is the Kubelka–Munk function, R is the re?ectance of the crystal and s is scattering coef?cient, A is the absorbance and c is concentration of the absorbing species. The measurement of [F(R)hv] as a function of hv provides the band gap energy Eg of the material. The direct and indirect band gap energies  can  be obtained from the intercept of the resulting straight lines with

Table 1  FT-IR frequencies of some metal urea complexes (cm—1).

Fig. 3. ORTEP diagram of HUCS.

Fig. 4. Packing diagram of HUCS.

the energy axis at [F(R)hv]2¼ 0 and [F(R)hv]1/2¼ 0 respectively. The band gap energies of the specimens are deduced as 5.02 eV (direct) and 4.90 eV (indirect) (Fig. 7), which is smaller than the reported value of urea (6.18 eV, ~ 200 nm) [23]. First-principle calculations predict the direct band gap of 4.27 eV for urea [23].

E. DSC Analysis

The recorded DSC spectrum of HUCS is shown in Fig. 8. The decomposition was recorded from 0 to 300 1C. In the DSC curve, sharp endothermic peaks at 91 and 130 1C  are  due  to  the liberation of water molecules. The endothermic peak at  143  and 158 1C could be due to the decomposition of HUCS crystals into fragments and their subsequent volatilization.

F. Mechanical Studies

Transparent crystals free from cracks were selected for micro- hardness measurements. Before indentations, the crystals were carefully lapped and washed to avoid surface effects. The Vickers hardness indentations were made on the as-grown surface of the HUCS crystal at room temperature with the load ranging from  25 to 100 g, keeping the time of indentations as 10 s  for  all trials. The Vickers hardness number Hv was calculated from the

Table 2

Crystal data and structure re?nement for HUCS crystal.

Chemical formula                                                C6 H26 Co N12 O11 S

Formula weight                                                533.38

Temperature                                                           293 K

Wavelength                                                             0.71073 A?

Crystal system, space group                          Orthorhombic, Pca21

Unit cell dimensions                                           a ¼ 15.0549(6)A? , a ¼  901 b ¼ 7.2105(3)  A? , b¼  901 c ¼ 20.0284(9)  A? , g¼  901

Volume                                                                     2174.15(16) A? 3

Z, Calculated density                                          4, 1.630 Mg/m3

Absorption coef?cient                                       0.960 mm—1

F(000)                                                                       1108

Crystal size                                                             0.30 ~ 0.25 ~ 0.20 mm3

Theta range for data collection                   2.71 — 32.901

Limiting indices                                                    — 18r h r 22,  — 6r k r 10,  — 29 rl r 27

Re?ections collected/unique                          16,634/6897 [R(int) ¼ 0.0331]

Completeness to theta¼ 25.00                      98.1%

Absorption correction                                       Semi-empirical from equivalents

Max. and min. transmission                            0.8311 and 0.7615

Re?nement method                                             Full-matrix least-squares on F2

Data/restraints/parameters                                6897/34/383

Goodness-of-?t on F2                                        1.015

Final  R indices [I 42sigma(I)]                        R1 ¼ 0.0402,  wR2¼ 0.1001

R indices (all data)                                             R1 ¼ 0.530, wR2¼ 0.1075

Absolute structure parameter                         0.071(10)

Largest  diff. peak and hole                              1.027 and  — 0.779 e A? — 3

Table 3

System, space group and cell parameters of some cobalt(II)- urea complexes.

Following equation:

v ¼ 1:8544 ~ P=d  kg=mm ,

where P is the applied load in kg and d is the mean diagonal length of the indentation impression in micrometer. A plot of load (P) versus Vickers hardness number (Hv) reveals that the hardness of the grown crystal increases as the load increases (Fig. 9). Cracks started developing around the indentation mark beyond a load of 100 g. This may be due to the internal stresses released during the indentation. Mayer’s index number (n) or work hardening index is calculated from the slope of  the  straight line plot  between log d and log P and it is equal to 5.3, classifying HUCS crystal to soft material category.

G. Dielectric properties

Fig. 10(a–c) are the plots of dielectric constant (er), dielectric loss  (tan d)  and  AC  conductivity  (sac)  versus  temperature  at different frequencies. It can be seen that dielectric parameters, viz. dielectric constant, dielectric loss and AC conductivity increase with the increase in temperature. er and the tan d values decrease with the increase in frequency while sac  increases with the increase in frequency.

The dielectric constant of the material is generally composed of four different types of contributions like ionic, electronic, orientational and space charge polarizations. The large value of dielectric constant at low frequencies may be due to the contribution of all these polarizations. The decreased dielec- tric constant at higher frequencies could be due to the reduction in the space charge polarization. Space charge polarization is generally active at lower frequencies and high temperatures indicating the purity and perfection of the grown crystal [24]. The increase in conductivity could be attributed to the reduction in the space charge polarization at higher frequencies [25]. In the present study, dielectric constant varying proportionally with temperature is essentially due to the temperature variation of the polarizability [26]. The low er value dielectric materials have potential applications in microelectronic industries.

Conclusion

Transparent pink colored crystals of hexakis(urea)cobalt(II) sulfate monohydrate were grown in de-ionized water by slow condensation of aqueous solution. It belongs to orthorhombic system with noncentrosymmetric space group Pca21. The various functional groups and crystallinity of the as-grown specimens were con?rmed by FT-IR and powder X-ray diffraction analyses. The direct and indirect band gap energies fall in close range and they are lower than the Eg of urea. The low dielectric constant at high frequency is indicative of a less defective good quality optical crystal. The central metal atom cobalt is coordinated by six oxygen atoms of urea molecules in a distorted octahedral geometry.

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