CRYSTAL GROWTH & DESIGN
Flux Growth and Optoelectronic Study of PbWO4 Single Crystals
2007 VOL. 7, NO. 2 296-299
S. K. Arora* and Bhupendra Chudasama Department of Physics, Sardar Patel UniVersity, Vallabh Vidyanagar - 388120, Gujarat, India ReceiVed June 17, 2006; ReVised Manuscript ReceiVed NoVember 19, 2006
ABSTRACT: Lead tungstate (PbWO4) single crystals have been obtained by the flux growth technique using strontium chloride (SrCl2) as flux. Optimum growth conditions are described for obtaining well-faceted transparent crystals. A thermogravimetric study shows that the compound is stable in the temperature range 25-1000 °C. Analysis of optical absorption normal to the ab-plane recorded at room temperature in the spectral range of 200-800 nm reveals the absorption edge, the type of transition being the allowed indirect one at 4.52 eV. The crystal has been characterized by determining useful optical and dielectric parameters. Introduction
Experimental Procedures
Lead tungstate (PbWO4) has attracted more attention as a promising candidate1 for studying scintillation characteristics, as compared to other well-known scintillators, such as NAI:TI and BGO.2-4 It is the most attractive material for high-energy physics applications because of its high density (8.3 g/cm3), short decay time (less than 10 ns for the most part of light output), and high irradiation damage resistance (107 rad for undoped and 108 rad for La-doped PbWO4).5,6 It occurs in nature as tetragonal stolzite with scheelite-type structure (space group I41/a) and as monoclinic respite with wolframite structure (space group P21/c),7 the latter phase transforming irreversibly to the former at about 400 °C. Although its luminescence properties were first recognized as early as 1940s,8 there is renewed interest in its optical spectra and scintillation properties to understand the excited-state dynamics of emission centers.9 This is because PbWO4 has been approved for use in experiments to construct a high precision electromagnetic calorimeter at Large Hadron Collider.10 In addition, rare-earth doped PbWO4 is also found to exhibit appreciable ionic conductivity.11,12 A little effort has been made for its nano- and microcrystallization using a mild hydrothermal autoclave13 and sodium metasilicate gel,14 but the crystals were sub-millimeter size and imperfect. Literature survey reveals that the compound generally has been crystallized by Czochralski15-18 and Bridgmann19-21 techniques. However, thermal strains on the growing crystals due to high temperatures involved adversely affect their utility. The hardware required are expensive too. Also, WO3 group related green emission was never observed in Czhochralski grown crystals.22 Moreover, the temperature gradients involved and solid-liquid interface in Bridgmann growth do not prevent preferential spurious nucleation sites along the translating ampule. No attempts have been made so far to grow this compound by flux technique.23 Therefore, it was thought worthwhile to pursue its flux growth in the laboratory. Flux growth is preferable to Czochralski and Bridgmann growth. This is because equilibrium defects incorporated in the former are at the lowest, and the growth initiates and proceeds at a lower temperature than the melting point of the compound itself. In flux growth, the growing crystal is not exposed to steep temperature gradients and the crystal can grow in an unconstrained fashion, and so it can develop facets. In this paper, the first attempt of flux growth of single crystals of lead tungstate is reported, and their useful optical and dielectric characteristics are described.
Growth experiments were carried out in air atmosphere, using a 50 mL alumina crucible. The starting composition consists of 99.999% pure PbWO4 (Sigma) and 99.99% pure SrCl2. A total of 3.0 g of PbWO4 and 6-12 g of SrCl2 weighed with an accuracy of 0.1 mg were thoroughly mixed and then loaded into the crucible placed appropriately into the furnace. The crucible was covered with a loosely fitting alumina lid. This helped to lessen evaporation and to control saturation in the growth charge. Temperature of the furnace was programmed by a Eurotherm temperature controller model 2416, which can control the temperature with an accuracy of 0.1 °C, being measured by a Chromel/ Alumel thermocouple. The charge was heated to 975 °C at a steady rate of 75 °C/h and kept there for 2 h. This entailed complete dissolution of the solute. Then, it was slowly cooled to 820 °C at a rate of 3 °C/h. During this period, the solvent evaporated and resulted in supersaturation appropriate to form a critical size nuclei at the bottom of the crucible. After the evolving period, the furnace power was shut off. The residual flux was dissolved in hot distilled water, and thus the crystals were washed and retrieved. Crystallinity of the grown compound was verified by X-ray diffraction (XRD) recorded on a Philips’ Xpert MPD’ (Holland) powder diffractometer using Cu KR (λ ) 1.54056 Å) radiation at room temperature. The thermogram was recorded from ambient temperature to 1000 °C on Perkin-Elmer TGA-7. Optical spectrum along the abplane was taken on a Perkin-Elmer Lambda 19 double beam UVVIS-NIR spectrophotometer. For the purpose, meticulously selected thin flakes of less than 1 mm thickness were employed. These were optically flat and smooth surfaces devoid of any micotopographical steps, corners, and kinks.
* Phone: +91-2692-226844. Fax: +91-2692-236475. E-mail: sarorak@ yahoo.com.
Crystal Growth The high-temperature mixture (PbWO4 + SrCl2) is essentially not complicated by formation of any binary or ternary compounds or solid solution. The general principles of equilibria as well as the law of mass action, etc. are applicable to this high-temperature low viscous solution. The solvent SrCl2 brings the solute PbWO4 into a single (liquid) phase at 975 °C. Cooling the charge down to 820 °C gives rise to the necessary supersaturation. This leads to desired slow precipitation of PbWO4. The size and quality of the resulting crystal product were sensitive to the cooling rate, with fewer and larger ones obtained from the more slowly cooled melt. Figure 1 shows some typical as-grown lead tungstate (PbWO4) crystals, the maximum size being 7 × 6 × 2 mm. The crystals are slightly yellowish in color with dipyramidal morphology having (110) habit faces. The optimum condition for the best quality and sized crystals is a solute-solvent weight ratio (PbWO4/SrCl2) 1:3; growth temperature 975 °C; soak period 2 h; cooling rate 3 °C/h. Figure 2 reveals that the maximum crystal yield (43.12%) is obtained
10.1021/cg060368t CCC: $37.00 © 2007 American Chemical Society Published on Web 01/18/2007
Optoelectronic Study of PbWO4 Single Crystals
Figure 1. Some typical as-grown crystals of PbWO4. The inset shows a small piece of PbWO4 used for recording optical absorption spectra.
Crystal Growth & Design, Vol. 7, No. 2, 2007 297
Figure 3. Powder X-ray spectrum of PbWO4 single-crystal recorded at room temperature.
Figure 2. PbWO4 crystal yield at different flux concentration.
when the solvent weighs 3 times the solute. Adjustment of solute-solvent ratio facilitates nucleation control and prevents inclusions, hopper as well as dendritic growth. In all the growth runs, there is always some amount of residual flux, SrCl2, remaining unutilized at the end of the crystallization process. This compound is highly water-soluble, having a moderately large positive temperature coefficient of solubility at 25 °C.24 Hence, the grown crop of crystals was easily harvested by leaching out the crucible contents with hot distilled water. The EDAX spectrum taken of the grown sample indicated stoichiometric presence of the elements. The trace uptake of Sr, if it occurs at all in the growing matrix, does not hamper the material characteristics against the background that iron, fluorine, (and probably chlorine) incorporation into the material lattice has been found to substantially influence the luminescence characteristics.25-27 Figure 3 shows the powder X-ray spectrum in which the reflections are indexed as tetragonal scheelite class. The computed unit cell parameters are a ) b ) 5.465 Å, c ) 12.045 Å; R ) β ) γ ) 90°; cell volume V ) 359.73 × 10-24 cm3 and X-ray density ) 8.401 g/cc. The TGA thermogram (not shown here) runs parallel to the temperature axis without any valleys or humps indicating compositional stability of the crystal in the temperature range of 25-1000 °C. Optical and Dielectric Characterization A typical transmission spectrum of a PbWO4 single crystal is shown in Figure 4. Evidently, the crystal is transparent (>65%) in the visible to infrared region. This is in close agreement with the observation by Baccaro et al.28 Also, the crystal is seen to absorb heavily in the ultraviolet region (240270 nm).
Figure 4. Transmission spectrum of PbWO4 single crystals.
The absorption coefficient is known to be related to photon energy hν by the expression
Rhν ) (hν - Eg)1/n
(1)
where the exponent n can take values of 2, 1/2, 2/3, and 1/3 for the allowed direct, indirect, and the forbidden direct, indirect transitions, respectively. In our case, the best fit is obtained with n ) 1/2, implying that lead tungstate is an indirect band gap material where the wave vector difference between the electrons in valence band and conduction band is supplied by lattice phonon. If the phonon energy and wave vector are denoted by Ep and Q respectively, the energy - and wave vector conservation conditions in the optical process are
pw ) Ecv ( Ep, Kc - Kv ) (Q
(2)
From the graph of (Rhν)1/2 f hν shown in Figure 5, the fundamental optical absorption edge is quite obvious. The indirect allowed optical band gap comes out as 4.52 eV. This value matches excellently with that derived from electron-hole separation in thermoluminescence excitation spectra studied by Murk et al.29 and from the band structure calculation by Zhang et al.30 The extinction coefficient (K) and the refractive index (n) have been calculated via the relations31
K)
(n - 1)2 + K2 Rλ ;R) 4π (n + 1)2 + K2
(3)
and are shown as plotted in Figure 6. Furthermore, complex
298 Crystal Growth & Design, Vol. 7, No. 2, 2007
Arora and Chudasama
Figure 5. (Rhν)/1/2 vs photon energy (eV) fitted to zero absorption. The inset shows optical density and transmission loss as a function of wavelength.
Figure 7. Dielectric constant (r) and dielectric loss factor (i) as a function of wavelength. The inset shows the relation between (n21)-1 and E2 in the energy range 4.0-4.5 eV. Table 1. Evaluated Optical Parameters of PbWO4 Single Crystals N0
a1
E0, eV
Ed, eV
M-1
M-3
σ × 10-2
1.11
0.01
1.15
0.41
0.36
0.27
0.44
of parameters, as defined by Wemple and Di Domenico,33
Ed ) F/E0
(7)
eqs 6 and 7 can be coupled, neglecting the values of K in the region 4.0 - 4.5 eV, to give
Figure 6. Graphical variation of refractive index (n) and extinction coefficient (K) as a function of incident wavelength along the ab-plane. The inset shows the relation between (n) and E2 in the energy range 4.0-4.5 eV.
dielectric constants are determined through the equations,31
r ) n2 - K2; i ) 2nK
(4)
where r (real part) is the dielectric constant and i (imaginary part) is the dielectric loss factor. Figure 7 shows the energy dependence of these dielectric parameters. It may be noteworthy that in both Figures 6 and 7, the maxima occur at 240 nm. Interestingly, for photon energies less than the energy gap (E < Eg), that is typically the range 4.0-4.5 eV representing the Urbach tail in Figure 5, the spectral variation of the refractive index can be expressed by the CauchySellmaier function expanded in even powers of E. The first approximation of this function gives the expression32
n(E) ) n0 + a1E2
(5)
where n0 and a1 are constants. The best fit of n against E2 in the transparent range of E occurs between 4.0 and 4.46 eV. Table 1 contains the evaluated (within accuracy of 0.1%) constants of eq 5. Upon the basis of the validity of Krammers-Kroning relationship, the real part of dielectric constant takes the form33
r ) 1 +
F (E0 - E2) 2
(6)
where the two parameters E0 and F have a straightforward relation with the electric dipole strength and the corresponding transition frequencies of all oscillators. By a special combination
r(E) ) n2(E) ) 1 +
E dE 0 E02 - E2
(8)
Thus, the values of E0 and Ed are estimated by plotting (n2 1)-1 against E2 and fitting the relation to a straight line as shown in the inset of Figure 7. On the basis of the single-effective oscillator model, however, E0 and Ed are connected to r and the moments M-1 and M-3 of the (E) optical spectrum, through the relations33
E02 )
M-1 2 M-13 ,E ) M-3 d M-3
(9)
where rth moment of the optical spectrum is
Mr )
2 π
∫E∞ ri(E)dE
(10)
t
Here Et is absorption threshold energy. Table 1 also gives values of the single-effective oscillator parameters E0, Ed, M-1, and M-3 obtained of the flux-grown PbWO4 single crystals. Finally, optical density and transmission loss (db) of the crystalline PbWO4 are estimated by the relation
(T1)
F(E) ) log10
(11)
Their energy dependence is plotted in the inset of Figure 5 where again the maxima is reached at 240 nm. The optical absorption coefficient R is exponentially related to the sample temperature T′ as34
R ) R0 exp
[
σ (hν - hν0) kBT′
]
(12)
where R0 and hν0 are the material dependent constants,35 and σ, known as the steepness parameter, is temperature (T′) dependent, which characterizes broadening of the absorption
Optoelectronic Study of PbWO4 Single Crystals
Crystal Growth & Design, Vol. 7, No. 2, 2007 299
ments of optical spectrum were estimated, and the steepness parameter σ was determined in the region of Urbach tail at room temperature. Acknowledgment. This work is carried out under Defense Research and Development Organization (DRDO) sponsored Major Research Project No. 0203374/M/01. References
Figure 8. Relation between ln R and E (eV) at room temperature.
edge due to electron-phonon or exciton-phonon interactions in the lattice.36 At constant (room) temperature, the graph representing ln R versus hν in the range of the Urbach tail (4.04.5 eV) yields a straight line (see Figure 8), yielding fairly accurately the value of σ, which also is given in Table 1. Interestingly, in the experimental curve (Figure 5) showing spectral variation of absorption, we notice small yet distinct steps at 310 and 370 nm, which overlap with the tail portion of fundamental absorption. These may be because more than one phonon are participating in the transition and that the phonons in the Urbach region allow connecting states with the same velocity dE/dk at phonon energies greater than those connecting the band edge excitons (where dE/dk ) 0). As a consequence, one expects strong exciton-phonon interaction. Probably the greatest mystery associated with Urbach’s rule (eq 12) is the wide variety of exciton-phonon coupling Hamiltonians, which apparently yield this simple rule in the high-temperature range as in our situation, compared to that published by Itoh.37 The low value of steepness parameter (σ ) 0.44 × 10-2) in our crystal, therefore, does not match with the value reported,37 and it may be ascribed to a large concentration of excitons, resulting in much intense excitonphonon coupling strength. In general, the exciton line will lie on a continuous background of other optical processes to other states. Since the Hamiltonian also couples the excitons to these other states, there are in general interference effects between the ‘exciton transition’ and the background on which it lies. The interaction of excitons and phonons broadens the exciton resonances. At the same time, the greater the exciton-phonon interaction at a given temperature, smaller is the value of steepness parameter,38 as observed in our samples. The values of σ0 and hν0, however, could not be determined here because of the lack of experimental facility of measuring optical absorption at different temperatures. Conclusion (1) Single crystals of lead tungstate have been successfully grown by the direct flux method using strontium chloride as flux. (2) Lead tungstate is an indirect band gap material with Eg ) 4.52 eV. (3) The validity of Cauchy-Sellmaier equation was evaluated in the energy range of 4.0-4.5 eV, and its parameters were calculated. (4) Applying the single-effective oscillator model, the mo-
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CG060368T
