ARTICLE IN PRESS
Journal of Crystal Growth 275 (2005) e657–e661 www.elsevier.com/locate/jcrysgro
Single crystal growth and characterization of strontium tartrate S.K. Arora, Vipul Patel, Bhupendra Chudasama, Brijesh Amin Department of Physics, Sardar Patel University, Vallabh Vidyanagar 388120, Gujarat, India Available online 15 December 2004
Abstract Conditions for the growth of large and perfect single crystals of strontium tartrate trihydrate by controlled ionic diffusion of reacting ions through silica hydrogel have been optimized. The TGA/DTA thermograms reveal specific decomposition reactions. The values of several structural and physical parameters have been determined. The anisotropy in Vicker’s microhardness leads to a study of fracture toughness, brittleness index and yield strength. The measurement of DC conductivity reveals hopping and band-type conduction mechanism with specific activation energy values in different temperature domains. Optical absorption study shows the existence of indirect transition at 5.20 eV and direct gap at 5.49 eV. The phonons invoked have been identified as internal vibration of strontium ions at 643 cm1. r 2004 Elsevier B.V. All rights reserved. PACS: 77.84.Bw; 81.10.Dn Keywords: A1. Characterization; A2. Growth from solutions; A2. Single crystal growth; B1. Perovskites; B2. Dielectric materials
1. Introduction Divalent tartrates [1–3] are ferroelectric and piezoelectric compounds, exhibiting nonlinear optical and spectral characteristics. They are used in transducers and several linear and nonlinear mechanical devices [4,5]. Strontium tartrate is an important member of the family with device applications [6]. It has been identified as a strategic material with global business potential [7]. It is one Corresponding author. Fax: +91 2692 236475.
E-mail address:
[email protected] (S.K. Arora).
such material which attracted attention of researchers to bring the gel technique on a firm footing [8,9]. There is, however, scanty information in the literature on its crystal growth and pertinent characteristics, and these are precisely presented here.
2. Experimental procedure Strontium tartrate (SrC4H4O6) is sparingly soluble (1.1 103 g ml1) in water, it decomposes before melting, and it does not vapourize or
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.047
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S.K. Arora et al. / Journal of Crystal Growth 275 (2005) e657–e661
sublime. Consequently, its single crystal growth was carried out via gel technique as the only alternative. Pure sodium metasilicate solution of a particular specific gravity (1.03–1.08 g cm3) was added drop-by-drop to aqueous solution of tartaric acid of a particular molarity (0.25–2.5 M) in a glass tube of length 15 cm and diameter 2.5 cm, agitating the mixture constantly so as to avoid excessive local ion concentration which may otherwise cause premature local gelling. The gel was found to set in 15 min to 15 days, depending upon its pH and the environmental temperature. After ensuring firm gel setting in the pH range 2–6, aqueous solution of strontium chloride of a particular strength (0.25–2.5 M) was poured above the set gel, with the help of a pipette, so as to allow the solution to fall steadily along the wall of the tube, to prevent the gelled surface from cracking. The open end of the tube was closed with cotton to prevent evaporation and contamination of the exposed surface. Such tubes were stored at 35 1C. Crystals were visible within about a week and wellshaped crystals grew in about a month. These were removed from the gel, washed free of adhering gel particles, and dried at room temperature.
3. Results and discussions The supernatant solution, SrCl2, diffuses into the gel column and reacts with the inner electrolyte, tartaric acid, producing the necessary supersaturation for the slow precipitation of SrC4H4O6. At some stage in the gel, when concentration of the diffusant is optimum, a few critical sized nuclei are formed which act as sinks and result in the establishment of radial diffusion channels through which flows the necessary supersaturation for the crystals to grow larger. Subsequent diffusion of SrCl2 enhances the crystal size, with hardly a few new nucleation sites. The best crop of crystals was obtained with the optimized parameters: growth temperature 35 1C; gel pH 3.5; density 1.04 g cm3; ageing 24 h; outer electrolyte 1 M SrCl2; inner electrolyte 1.5 M D-tartaric acid. We obtained crystals of size between 3 2 0.5 and 22 12 5 mm3 (Fig. 1). Kinetic study of growth of these crystals revealed that the nuclea-
Fig. 1. Typical crystals having different morphological forms (scale mm).
tion density, as measured by the total crystal count in an experiment, decreased with increasing gel pH, gel density, its ageing period and the neutral gel thickness. But increase in concentration as well as the amount of SrCl2 resulted in increased nucleation density. Further, the following observations are noteworthy: (i) With gel pH greater than 6.0, turbid crystals grew, decreasing the pH to 4.5 resulted in dendritic crystals. (ii) Decreasing the concentration of supernatant liquid from 2.5 to 1.5 diminished the chances of the dendritic growth. (iii) Gels of density more than 1.06 g cm3 were translucent to opaque, hence not suitable for single crystal growth. (iv) The neutral gel [8,9] influences the reaction velocity between the two ionic reactants, and hence it effectively suppresses the nucleation count, but it does not favour the quality of the product. The crystals obtained are nonhygroscopic structures and exhibit morphological forms of monoclinic perovskite type [9], their pycnometric and Xray densities are found to be 2.054 and 2.053 g cm3, respectively. The valence electron plasma energy, _op ; is 1=2 _op ¼ 28:8 ZR=M ; (1)
ARTICLE IN PRESS S.K. Arora et al. / Journal of Crystal Growth 275 (2005) e657–e661 Table 1 Some fundamental data on the as-grown crystals
10
Plasma energy (eV) Penn gap (eV) Fermi energy (eV)
16.446 5.679 12.329
3
Polarizability (cm ) Penn analysis Clausious–Mossotti
0.398 1022 0.412 1022
433
0
(a) DTA
Ep ¼
_op ð1 1Þ
1=2
;
E F ¼ 0:2948ð_op Þ4=3 :
(2)
Following Ravindra and Srivastava [11], the polarizability is " # ð_op Þ2 S0 M 0:396 1024 cm3 ; a¼ (3) 2 2 ð_op Þ S 0 þ 3E p r where S0 is a constant for a particular material which is given by [11] Ep 1 Ep 2 S0 ¼ 1 ; (4) þ 3 4E F 4E F while the Clausious–Mossotti relation gives 3M 1 1 a¼ ; 4pN a r 1 þ 2
(5)
where the symbols have their usual significance. All these data, as computed for the crystal are displayed in Table 1. The thermogram (Fig. 2) was obtained with the crystal, employing a heating rate of 10 1C min1. The first stage of decomposition is the dehydration that begins at 393 K and terminates at 513 K. The net weight loss of 18.75% corresponds to the loss of three water molecules, suggesting that the grown crystals were initially trihydrated. The second stage of decomposition sets in at 513 K and is completed at 538 K, resulting in the
-20
353
-30 -40
TGA
313
-50 273 273
where Z ¼ ðð1 Z Sr Þ þ ð4 ZC Þ þ ð10 Z H Þ þ ð9 Z O ÞÞ ¼ 46 is the total number of valence electrons, r the density and M the molecular weight of SrC4H4O6 3H2O crystal. Explicitly dependent on _op are the Penn gap and the Fermi energy [10], given by
-10
(b)
393
Weight (%)
Values
Temperature Difference ( K/mg)
Parameters
e659
-60 473
673
873
1073
Fig. 2. Simultaneous TGA and DTA thermograms.
formation of strontium oxalate–carbon monoxide complex, corresponding to 12% of total weight loss. The reaction stoichiometry also confirms it. In the third stage (598–618 K), the complex is decomplexed to form strontium oxalate as the second intermediate product, which between 643 and 678 K is turned into the basic carbonate as the final stable product, as confirmed by 8.25% mass lost. The final product SrCO3 remains stable up to 1073 K. Out of so many models employed to understand kinetics of decomposition [12], the Coats–Redfern relation [13] seems to be the best preferred approach. Micromechanical hardness has been determined using Vicker’s diamond indenter. The indentation hardness (Fig. 3) is divided into three regions. The initial region (AB) at low loads is linear, loaddependent. The intermediate region (BC) at moderate loads is prominently nonlinear, loaddependent. The final region (CD) at higher loads is load-independent. The general behaviour is that the microhardness Hv increases linearly with load, reaching a peak value Hp, beyond which it decreases faster, almost exponentially until it becomes load-independent, giving a saturated value Hs. The nature of curves for the as-grown (0 1 0) and (1 0 0) surfaces is found similar. Using the exponential region, the plot of ln Hv vs. P1 (inset of Fig. 3) gives (see Table 2) the deforming force DF ; estimated using the relation, H v ¼ H v0 expðDF =PÞ: This force DF may be interpreted as the critical force required to form an entangled mesh of dislocations. The fracture toughness Kc
ARTICLE IN PRESS S.K. Arora et al. / Journal of Crystal Growth 275 (2005) e657–e661
e660
1/T
180 5.2
0.0014 -10
Sample 1
5
ln Hv
160 B
0.0026
0.003
0.0034
-14 Sample 3
y = 0.0408x + 4.601
4.4 5
10
-18
15
ln σdc
0
Hv
0.0022
Sample 2
4.8 4.6
140
0.0018
1/P
120
-22
A
IV I
100
-26
D
C
III II
80 0
0.5
1 P
1.5
2
Fig. 3. Variation of Vicker’s microhardness (GN m2) with applied load (N) for the as-grown {0 1 0} plane.
Table 2 Microhardness data obtained on the as-grown crystals Plane Parameters 2
Hardness (GN m ) Pm (N) Ps (N) DF (N) Kc (MN m3/2) Bp (m1/2) sp (GN m2)
Hp Hs
As-grown (0 1 0)
As-grown (1 0 0)
166 102 0.07 1.0 0.041 0.166 3.029 108 55.33
331 82 0.15 1.0 0.323 0.167 12.730 108 110.33
and the brittleness index B too have been calculated (see Table 2) using Eq. (6) K c ¼ P=bc3=2 ;
B ¼ H v =K c ;
(6)
where P is the load applied, c is the crack length as measured from the corner of the indentation mark to the crack tip, and b is a numerical constant that depends upon the indenter geometry. In our case, b ¼ 7: Further, following Wytt [14], the yield strength sn has been calculated (Table 2). The (1 0 0) face has the yield strength twice as large as that of (0 1 0) face, and this supports the observation that Hp of (1 0 0) face is roughly two times that of (0 1 0) face.
Fig. 4. Plot of ln sDC ! 1=T (K1) for the {0 1 0} plane.
The influence of reciprocal temperature on logarithmic DC conductivity is shown in Fig. 4. Although the sample above 363 K exhibits nearly semiconducting nature, the ln sDC ! 1=T curve is, however, not perfect linear throughout. Apparently, a hopping conduction in part with a small polaron model [15] may be operative. We are inclined to suspect in our crystals a ferroelectric to paraelectric transition corresponding to the conductivity anomaly, manifested by a peak, indicating relatively large polarization related with bound molecular currents occurring at around 315 K. This conforms to the feeble peak on an otherwise smooth DTA curve. In the region marked I in Fig. 4 electron hopping influenced by scattering mechanism is operative. The inflexion at 363 K manifests change over of the conduction mechanism from electron-hopping to proton-hopping type on account of liberation of water molecules. The anomaly at 623 K indicates chemical/structural phase change in the sample, caused by anhydrous SrC4H4O6 decomposing into the carbonate. The Arrhenius relation sDC ¼ s0 expðE a =kT Þ has been used for calculation of the activation energy. Region I (315–363 K) where electron-hopping mechanism operates has the activation energy 0.5 eV. Region II (363–393 K) where protonhopping mechanism operates needs 0.5 eV for proton conduction. For regions III (393–423 K) and IV (423–503 K) we obtained the activation energy of water translation as 0.89 and 0.91 eV, respectively.
ARTICLE IN PRESS S.K. Arora et al. / Journal of Crystal Growth 275 (2005) e657–e661
2.E+4 55 45
α2
α2/3
1.E+4
35 25 15 5
5.E+4
5
5.4
5.2 hν(eV)
5.49 eV
0.E+4 5
5.2
5.4 hν
Fig. 5. Plot of a2 vs. hn (eV) for the direct allowed transition.
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indirect forbidden transition at 5.202 eV which is less than the first knee point E 1 ¼ 5:319 eV: Evidently, indirect forbidden transition prevails below 5.319 eV, while indirect allowed transition occurs above this value. Since the direct allowed band gap is 5.49 eV, direct allowed transition prevails in the region of higher photon energy. Also, referring to Fig. 6, each of the threshold energy intervals E4–E1 and E3–E2 is twice the phonon energy, 2Ep. Consequently, phonon energies involved in indirect transition are found to be 0.08 and 0.02 eV. The energy 0.08 eV is to be considered as lattice phonons, corresponding to 643.93 cm1 which, it has been conjectured, represents the internal vibration of Sr–O bond.
20
References 6 α1/3
15
IV
4
α1/2
2
III
E4=5.47
5.202 eV
10
0 0
5.2
5.4 hν(eV)
II I
E3=5.41 E2=5.37
5
E1=5.31 5.378 eV
0 0
5.2
5.4 hν
Fig. 6. Plot of a1=2 vs. hn (eV), representating zones and knee points in the spectrum.
We have also determined [16] the fundamental absorption edge for prominent electronic transitions, obeying the power law behaviour, an ¼ Ak ðhn E g Þ: The graph of a2 vs. hn (Fig. 5) yields direct allowed band gap as 5.49 eV. But the plot of a2=3 vs. hn (inset in Fig. 5) does not contain the sharp edge, ruling out the existence of direct forbidden transition in the crystal. Interestingly, the absorption coefficient a1=2 vs. hn (Fig. 6) does not change noticeably up to 5.3 eV, but one discerns rapid increase thereafter. This typical dependence is suggestive of interband transitions that require phonons for momentum conservation. Using n ¼ 13; the graph (inset in Fig. 6) gives the
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