International Journal of Optics

Volume 2014 (2014), Article ID 418459, 6 pages

http://dx.doi.org/10.1155/2014/418459

Research Article

Synthesis and Photoluminescence Study of Bi³⁺ and Pb²⁺ Activated Ca₃(BO₃)₂

A. B. Gawande,¹ R. P. Sonekar,² and S. K. Omanwar¹

¹Department of Physics, SGB Amravati University, Amravati 444602, India

²Department of Physics, G.S. College, Khamgaon, Buldhana 444312, India

Received 6 October 2013; Revised 11 December 2013; Accepted 30 December 2013; Published 13 February 2014

Academic Editor: Wojtek J. Bock

Copyright © 2014 A. B. Gawande et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The photoluminescence properties of Pb²⁺ and Bi³⁺ doped Ca₃(BO₃)₂ prepared by solution combustion synthesis technique are discussed. The structure of the prepared phosphor is characterized and conformed by XRD and FTIR. SEM images of the prepared materials show irregular grains with agglomerate phenomena. Prepared phosphors achieved the band emissions, respectively, at 365 nm and 335 nm corresponding to the transition . Optimum concentration, critical transfer distance, and Stokes shift of the synthesized materials were measured. These phosphors may provide an efficient kind of luminescent materials for various applications in medical and industry.

1. Introduction

Several researchers studied the luminescence properties of Bi³⁺ doped phosphors [1–6]. Bismuth can exist in materials in different valence states, such as 0, +1, +2, +3, and +5, or even mixed valence states of +1 and +5. In all of these valence states only Bi³⁺ is normally most stable in most host materials. Usually, the emission peaks of Bi³⁺ occur in the ultraviolet, blue, green, or even red wavelength regions with variation of host materials. For example, GaBO₃ : Bi³⁺ [7] and YBO₃ : Bi³⁺ [8] show UV emission, LnNbO₄ : Bi³⁺ [9] gives blue emission, CaWO₄ : Bi³⁺ [10] and Ca₃Al₂O₆ : Bi³⁺ give green emission [11], and Bi₄Ge₃O₁₂ [12] displays red emission at low temperature. Pb²⁺ doped phosphors have been the interest of many researchers. Many attempts have been made to synthesize Pb²⁺ activated phosphors which emit in varying range from 290 to 470 nm when excited by UV light [13–22]. In recent years, great efforts have been taken by many researchers to discover and develop new rare-earth and transition-metal ion-doped material systems as luminescent materials with high absorption in the UV spectral region [23]. Inorganic materials containing metal ions with s² (Pb²⁺, Tl⁺, Sn²⁺, Sb³⁺, Bi³⁺) configuration can be used in X-ray imaging devices, low pressure lamps, and high-energy physics. For example, Pb²⁺ doped BaSi₂O₅, which emits a broad band around 350 nm under UV excitation, is one of the earliest known phosphors for photocopying lamps [24].

In the present work, two inorganic phosphors Ca₃(BO₃)₂ : Bi³⁺ and Ca₃(BO₃)₂ : Pb²⁺, were prepared using solution combustion synthesis technique. The synthesized phosphors were characterized by using the powder X-Ray Diffractometer (XRD), Fourier Transform InfraRed (FTIR), and Scanning Electron Microscope (SEM). The photoluminescence properties of these phosphors at room temperature were studied using a Spectrofluorometer.

2. Experimental

The phosphors were prepared by a novel technique which is slight variation of solution combustion synthesis method. Detailed description of the method is given in our previous work [25–30]. Bi³⁺ and Pb²⁺ doped Ca₃(BO₃)₃ phosphors were obtained by the combustion of aqueous solution containing stoichiometric amounts (using oxidizer/fuel ratio) of calcium nitrate (AR), lead nitrate (AR), bismuth nitrate (AR), ammonium nitrate (AR), urea (AR), and boric acid (AR) as boron source (Table 1). All the precursors were dissolved in a china dish using little amount of double distilled water. The dish containing the solution was introduced into a muffle furnace maintained at °C. The solution undergoes dehydration followed by decomposition with the evolution of large amount of gases (oxides of nitrogen and ammonia) and ignited to burn with a flame yielding voluminous powder of Ca₃(BO₃)₃ : Bi³⁺ and Ca₃(BO₃)₃ : Pb²⁺. These raw powders were sintered for 3 h at °C and quenched to room temperature on aluminum plate and crushed into a fine powder. The prepared powder samples were then subjected to the powder XRD analysis. PL measurements at room temperature were performed on Hitachi F-7000 Spectrofluorometer in the range 200–500 nm.

Table 1: Molar ratio of ingredients used for material preparation.

Material
Molar ratio

Ca(NO₃)₂ : Bi(NO₃)₃ : H₃BO₃ : CO(NH₂)₂ : NH₄NO₃ 

 :  : 2 : 6 : 3 

()

Ca(NO₃)₂ : Pb(NO₃)₂ : H₃BO₃ : CO(NH₂)₂ : NH₄NO₃ 

 :  : 2 : 6 : 3 

(,  )

3. Results and Discussion

3.1. X-Ray Diffraction Pattern of Synthesized Materials

The powder XRD of synthesized material, analyzed on Rikagu Miniflex II X-ray Diffractometer, is shown in Figure 1 and found in good agreement with the ICDD file (01-070-0868). The crystal structure of the prepared material Ca₃(BO₃)₂ can be refined to be rhombohedral, space group R-3c with lattice parameters  Å and  Å.

Figure 1: X-ray powder diffraction patterns of Ca₃(BO₃)₂ synthesized by solution combustion synthesis.

3.2. FTIR Spectra of Synthesized Materials

The FTIR spectra measured at room temperature are shown in Figure 2. Generally, the FTIR analysis of the studied borate material shows four distinct frequency regions, from 1200 to 1600 cm⁻¹ and from 800 to 1200 cm⁻¹, which are assigned to the stretching vibrations of both triangular BO₃ and tetrahedral BO₄ units, respectively. As usual, frequencies of B–O vibrations depend on the boron coordination. Stretching frequencies of a coordinated groups decrease as the coordination number increases. Thus, the experimental results indicate that B–O bond asymmetric stretching of BO₃ groups is located in the region from 1150 to 1500 cm⁻¹. Moreover, the condensation of BO₃ groups (either chains or rings) leads to two types of B–O bands. One is short B–O bands (oxygen bonded to a single B atom or nonbridging oxygen). The other is longer B–O–B bonds. The responding B–O stretching frequencies are accordingly distributed into two bands or groups of bands, centered near 1450 and 1200 cm⁻¹ [31]. As seen in Figure 2, the strong bands observed above 1200 cm⁻¹ should be assigned to the B–O stretching mode of the triangular [BO₃]³⁻ groups, while the bands with maxima at about 700–800 cm⁻¹ should be attributed to the B–O out of plane bending, which confirms the existence of the [BO₃]³⁻ groups [32, 33].

Figure 2: FTIR spectra of Ca₃(BO₃)₂ synthesized by solution combustion synthesis.

3.3. SEM Images of Synthesized Phosphors

Figure 3 shows the Scanning Electron Microscope (SEM) images of powders prepared at 800°C. It was observed that the microstructure of the phosphors consisted of irregular grains with agglomerate phenomena. The average size of the powders is about 2–12 μm. The results show that phosphors have a good crystallinity and a relatively low sinter temperature.

(a)

(b)

Figure 3: SEM images of (a) Ca₃(BO₃)₂ : Bi³⁺ and (b) Ca₃(BO₃)₂ : Pb²⁺.

3.4. Photoluminescence of Ca₃(BO₃)₂ : Bi³⁺

Figure 4 displays the excitation and emission spectra of Ca₃(BO₃)₂ : Bi³⁺. The ground state of Bi³⁺ ion with 6s² configuration is the ¹S₀ spin-orbit singlet state and the excited states of the 6s6p configuration are ³P₀, ³P₁, ³P₂, and ¹P₁ states in sequence of energy increase. The transition between ¹S₀ and ¹P₁ is parity and spin-allowed, while the transition between ¹S₀ and ³P₁ is largely allowed due to spin-orbit coupling. Other transitions are strictly forbidden. Typically at room temperature, emission is observed from the ³P₁ → ¹S₀ transition. As an activator, excitation usually occurs from the ¹S₀ ground state to the ³P₁ or ¹P₁ excited state because the ¹S₀ → ³P₀ and ¹S₀ → ³P₂ transitions are strongly forbidden. The emission of Bi³⁺ ions originates from the ³P₀ state at low temperatures [34], while at higher temperatures the emission occurs mainly from the ³P₁ level, in which transition is allowed by spin-orbit mixing of the ³P₁ and ¹P₁ states. In Figure 4, broad excitation bands are in the region from 260 nm to 310 nm with maximum at 290 nm which can be ascribed to ¹S₀ → ³P₁ transition of Bi³⁺. The emission band is observed at 365 nm from the ³P₁ excited state level to the ¹S₀ ground state upon excitation with 290 nm. Additionally, we observed no splitting or multiple bands in the emission spectra, which indicate that Bi³⁺ here is supposed to occupy the position of Ca²⁺ ions in the lattice. In view of the extensive literature on the luminescence of Bi³⁺ in inorganic host, it can be seen that Bi³⁺ in inorganic hosts absorbs light of wavelengths mainly in the 200–310 nm range and emits in the 330–450 nm range.

Figure 4: PL and PLE spectra of (BO₃)₂ ( = 0.001, 0.002, 0.003, 0.005, 0.01), excitation spectra monitored at 365 nm emission, and emission spectra monitored at 290 nm excitation.

Further, the photoluminescence spectra of Ca₃(BO₃)₂ : Bi³⁺ with different Bi³⁺ doping concentrations were investigated. The variation of emission intensity of Bi³⁺ with different Bi³⁺ doping concentration is shown in Figure 5. It is observed that, with increasing Bi³⁺ doping concentration, the emission intensity of Ca₃(BO₃)₂ : Bi³⁺ increases and reaches a maximum at 0.003 mol. After this concentration, emission intensity decreases due to concentration quenching phenomenon. In this case, the energy transfer occurs from one activator to another until an energy sink in the lattice is reached. Finally, the Stokes shift of the synthesized phosphor was calculated to be 7086 cm⁻¹ using the excitation band at 290 nm and the emission band at 365 nm.

Figure 5: Variation in emission intensity of (BO₃)₂ ( = 0.001, 0.002, 0.003, 0.005, 0.01).

3.5. Photoluminescence of Ca₃(BO₃)₂ : Pb²⁺

The photoluminescence spectrum of Pb²⁺ doped in Ca₃(BO₃)₂ host material is shown in Figure 6. It can be described by the ¹S₀ → ³P0,1 transition, which originates from the 6s² → 6s¹6p² interconfigurational transition. Typically at room temperature, emission is observed from the ³P₁ → ¹S₀ transition [35], although at low temperatures the highly forbidden ³P₀ → ¹S₀ emission is also observed [36]. The excitation band for the synthesized phosphor Ca₃(BO₃)₂ : Pb²⁺ was observed at 270 nm, which is assigned to the ¹S₀ → ³P₁ transition and the emission band was observed at 335 nm which can be ascribed to transition from ³P₁ excited state to the ¹S₀ ground state. No splitting or multiple bands in the emission spectra for prepared phosphor are observed. Hence, we believed that the Pb²⁺ ions are incorporated at only one site (Ca²⁺ ion site) in the crystal lattice. In many inorganic hosts, the emission band of Pb²⁺ ion is in the UV region. It is also known that, in some hosts, Pb²⁺ ion emits in visible region. This diversity is depending strongly on the site occupied by Pb²⁺ ions, electronegativity of the ligand, crystal structure of the host lattice, and temperature [14, 37].

Figure 6: PL and PLE spectra of (BO₃)₂ ( = 0.001, 0.003, 0.005, 0.01, 0.02, 0.03, 0.04), excitation spectra monitored at 335 nm emission, and emission spectra monitored at 270 nm excitation.

The luminescence intensities of Pb²⁺ doped phosphors always depend on the doped Pb²⁺ ions concentration [38–40]. Therefore, the photoluminescence spectra of Ca₃(BO₃)₂ : Pb²⁺ with different Pb²⁺ doping concentrations were investigated. The variation of emission intensity of Pb²⁺ with different Pb²⁺ doping concentration is shown in Figure 7. It is observed that the position and shape of the photoluminescence bands have exhibited no obvious changes with Pb²⁺ concentration. The emission intensity of Ca₃(BO₃)₂ : Pb²⁺ increases and reaches a maximum at 0.005 mol. The Stokes shift was calculated to be 7186 cm⁻¹ using the excitation peak at 270 nm and the emission peak at 335 nm.

Figure 7: Variation in emission intensity of (BO₃)₂ ( = 0.001, 0.003, 0.005, 0.01, 0.02, 0.03, 0.04).

According to Blasse [41], for the critical concentration the average shortest distance between nearest activator ions is equal to the critical transfer distance . is, in fact, the critical separation between donor (activator ion) and acceptor (quenching ion), at which the nonradiative rate equals that of the internal single ion relaxation. The value can be practically calculated using the following equation:

where is the critical concentration, the number of Ca²⁺ ions in the Ca₃(BO₃)₂ unit cell, and the volume of the unit cell. By taking the values of (for Bi³⁺) and 0.005 (for Pb²⁺), and  Å³, respectively; the critical transfer distance for Ca₃(BO₃)₂ : Bi³⁺ and Ca₃(BO₃)₂ : Pb²⁺ was measured to be about 27 Å and 23 Å, respectively.

4. Conclusion

Two inorganic phosphors were prepared by a solution combustion synthesis method followed by heating of the precursor combustion ash at 800°C for 3 h in air. SEM images of the synthesized phosphors show irregular grains with agglomerate phenomena. Synthesized phosphors Ca₃(BO₃)₂ : Bi³⁺ and Ca₃(BO₃)₂ : Pb²⁺ achieved the band emissions, respectively, at 365 nm and 335 nm corresponding to the transition ³P₁ → ¹S₀. The optimum concentration of Bi³⁺ and Pb²⁺ in Ca₃(BO₃)₂ is measured to be 0.003 and 0.005 mol, respectively. For these values of optimum concentrations the critical transfer distance, , was measured to be about 27 Å and 23 Å, respectively. Finally, the Stokes shifts of Ca₃(BO₃)₂ : Bi³⁺ and Ca₃(BO₃)₂ : Pb²⁺ were measured to be 7086 cm⁻¹ and 7186 cm⁻¹, respectively. The emission bands of both the phosphors are in the UV region and the phosphors can be potential candidates for application in UV lamps.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

One of the authors (A. B. Gawande) wishes to thank The Chairman, FIST project, SGB Amravati, University, Amravati for providing powder X-ray diffraction facility for this work.

References

F. Kellendonk, T. van den Belt, and G. Blasse, “On the luminescence of bismuth, cerium, and chromium and yttrium aluminium borate,” The Journal of Chemical Physics, vol. 76, no. 3, pp. 1194–1201, 1982.

M. Ilmer, B. C. Grabmaier, and G. Blasse, “Luminescence of Bi³⁺ in gallate garnets,” Chemistry of Materials, vol. 6, no. 2, pp. 204–206, 1994.

A. Srivastava, “On the luminescence of Bi³⁺ in the pyrochlore Y₂Sn₂O₇,” Materials Research Bulletin, vol. 37, no. 4, pp. 745–751, 2002.

E. W. J. L. Oomen and G. Blasse, “Luminescence of Bi³⁺ in the metaphosphates LnP₃O₉ (Ln = Sc, Lu, Y, Gd, La),” Journal of Solid State Chemistry, vol. 75, no. 1, pp. 201–204, 1988.

A. Wolfert, E. W. J. L. Oomen, and G. Blasse, “Host lattice dependence of the Bi³⁺ luminescence in orthoborates LnBO₃ (with Ln = Sc, Y, La, Gd, or Lu),” Journal of Solid State Chemistry, vol. 59, no. 3, pp. 280–290, 1985.

F. Kellendonk, M. A. van Os, and G. Blasse, “Luminescence of bismuth in yttrium aluminum borate,” Chemical Physics Letters, vol. 61, no. 2, pp. 239–241, 1979.

V. P. Dotsenko, N. P. Efryushina, and I. V. Berezovskaya, “Luminescence properties of GaBO₃:Bi³⁺,” Materials Letters, vol. 28, no. 4–6, pp. 517–520, 1996.

L. Chen, H. Zheng, J. Cheng et al., “Site-selective luminescence of Bi³⁺ in the YBO₃ host under vacuum ultraviolet excitation at low temperature,” Journal of Luminescence, vol. 128, no. 12, pp. 2027–2030, 2008.

X. Xiao and B. Yan, “Hybrid precursors synthesis and optical properties of LnNbO₄:Bi³⁺ blue phosphors and Bi3+ sensitizing of on Dy³⁺'s luminescence in YNbO₄ matrix,” Journal of Alloys and Compounds, vol. 421, no. 1-2, pp. 252–257, 2006.

Y. Jin, Y. Hu, L. Chen, X. Wang, G. Ju, and Z. Mu, “Persistent luminescence in Bi³⁺ doped CaWO₄ matrix,” Radiation Measurements, vol. 1536, pp. 18–24, 2013.

H. Ju, W. Deng, B. Wang, J. Liu, X. Tao, and S. Xu, “The structure and luminescence properties of green Ca₃Al₂O₆:Bi³⁺ phosphors,” Journal of Alloys and Compounds, vol. 516, pp. 153–156, 2012.

G. Blasse and C. W. M. Timmermans, “The luminescence of some oxidic bismuth and lead compounds,” Journal of Solid State Chemistry, vol. 52, no. 3, pp. 222–232, 1984.

I. Pekgozlu, E. Erdoğmuşa, S. Çubukb, and A. S. Başakb, “Synthesis and photoluminescence of LiCaBO₃: M (M: Pb²⁺ and Bi³⁺) phosphor,” Journal of Luminescence, vol. 132, no. 6, pp. 1394–1399, 2012.

I. Pekgözlü, E. Erdoğmuşa, B. Demirelb, M. S. Gökb, H. Karabulutc, and A. S. Başak, “A novel UV-emitting phosphor: Li₆CaB₃O8.5:Pb²⁺,” Journal of Luminescence, vol. 131, no. 11, pp. 2290–2293, 2011.

A. Mergen and I. Pekgozlu, “Photoluminescence properties of Pb²⁺ doped M₂Mg(BO₃)₂ (M = Sr, Ba),” Journal of Luminescence, vol. 134, pp. 220–223, 2013.

W. H. M. M. van de Spijker and W. L. Konijnendijk, “Crystallographic and Pb²⁺ luminescence data of SrLaBO₄ and CaLaBO₄,” Inorganic and Nuclear Chemistry Letters, vol. 14, no. 11, pp. 389–392, 1978.

I. Pekgözlü, S. Seyyidoğlu, and S. Taşcıoğlu, “A novel blue-emitting phosphor: BaAl₂B₂O₇: Pb²⁺,” Journal of Luminescence, vol. 128, no. 9, pp. 1541–1543, 2008.

S. Taşcıoğlu, I. Pekgözlü, and A. Mergen, “Synthesis and photoluminescence properties of Pb²⁺ doped SrAl₂B₂O₇,” Materials Chemistry and Physics, vol. 112, no. 1, pp. 78–82, 2008.

I. Pekgözlü and H. Karabulut, “Synthesis and photoluminescence of Pb²⁺ doped SrB₂O₄,” Inorganic Materials, vol. 45, no. 1, pp. 61–64, 2009.

R. Sankar and G. V. S. Rao, “Luminescence studies on doped borates, A₆MM′(BO₃)₆,” Journal of Alloys and Compounds, vol. 281, no. 2, pp. 126–136, 1998.

G. Blasse, S. J. M. Sas, W. M. A. Smit, and W. L. Konijnendijk, “Luminescent materials with dolomite structure,” Materials Chemistry and Physics, vol. 14, no. 3, pp. 253–258, 1986.

W. M. Yen and M. J. Weber, Inorganic Phosphors, CRC Press, New York, NY, USA, 2004.

J. A. Groenink and G. Blasse, “Some new observations on the luminescence of PbMoO₄ and PbWO₄,” Journal of Solid State Chemistry, vol. 32, no. 1, pp. 9–20, 1980.

C. K. Lin, M. Yu, M. L. Pang, and J. Lin, “Photoluminescent properties of sol-gel derived (La, Gd)MgB₅O₁₀:Ce³⁺/Tb³⁺ nanocrystalline thin films,” Optical Materials, vol. 28, no. 8-9, pp. 913–918, 2006.

R. P. Sonekar, S. K. Omanwar, S. V. Moharil, P. L. Muthal, S. M. Dhopte, and V. K. Kondawar, “Luminescence in LaBaB₉O₁₆ prepared by combustion synthesis,” Journal of Luminescence, vol. 129, no. 6, pp. 624–628, 2009.

A. B. Gawande, R. P. Sonekar, and S. K. Omanwar, “Combustion synthesis and energy transfer mechanism of Bi³⁺→Gd³⁺ and Pr³⁺→Gd³⁺ in YBO₃,” Combustion Science and Technology, 2014.

A. B. Gawande, R. P. Sonekar, and S. K. Omanwar, “Synthesis & photoluminescence study of UV emitting borate phosphor Ca₃B₂O₆:Pb²⁺,” AIP Conference Proceedings, vol. 1536, p. 601, 2013.

R. P. Sonekar, S. K. Omanwar, S. V. Moharil, S. M. Dhopte, P. L. Muthal, and V. K. Kondawar, “Combustion synthesis of narrow UVB emitting rare earth borate phosphors,” Optical Materials, vol. 30, no. 4, pp. 622–625, 2007.

J. T. Ingle, A. B. Gawande, R. P. Sonekar, S. K. Omanwar, Y. Wang, and L. Zhao, “Combustion synthesis and optical properties of Oxy-borate phosphors YCa₄O(BO₃)₃:RE³⁺ (RE = Eu³⁺, Tb³⁺) under UV, VUV excitation,” Journal of Alloys and Compounds, vol. 585, pp. 633–636, 2014.

A. B. Gawande, R. P. Sonekar, J. T. Ingle, R. S. Palaspagar, and S. K. Omanwar, “Synthesis of narrow band UVB phototherapy phosphor LaB₅O₉:Pr-Gd,” International Journal of Basic and Applied Research, pp. 39–41, 2012.

L. Wu, X. L. Chen, H. Li et al., “Structure determination of a new compound LiCaBO₃,” Journal of Solid State Chemistry, vol. 177, no. 4-5, pp. 1111–1116, 2004.

H. You, X. Wu, X. Zeng et al., “Infrared spectra and VUV excitation properties of BaLnB₉O₁₆:Re (Ln = La, Gd; Re = Eu, Tb),” Materials Science and Engineering B, vol. 86, no. 1, pp. 11–14, 2001.

A. Rulmont and M. Almou, “Vibrational spectra of metaborates with infinite chain structure: LiBO₂, CaB₂O₄, SrB₂O₄,” Spectrochimica Acta A, vol. 45, no. 5, pp. 603–610, 1989.

A. A. Setlur and A. M. Srivastava, “The nature of Bi³⁺ luminescence in garnet hosts,” Optical Materials, vol. 29, no. 4, pp. 410–415, 2006.

H. F. Folkerts and G. Blasse, “Luminescence of Pb²⁺ in several calcium borates,” Journal of Materials Chemistry, vol. 5, no. 2, pp. 273–276, 1995.

G. Blasse and B. C. Grabmaier, Luminescent Materials, Springer, New York, NY, USA, 1994.

I. Pekgözlü, S. Taşcıālu, and A. Menger, “Luminescence of Pb²⁺ in MAl₂B₂O₇ (M = Ca, Sr),” Inorganic Materials, vol. 44, no. 10, pp. 1151–1154, 2008.

A. Manavbasi and J. C. LaCombe, “A new blue-emitting phosphor, SrZnO₂:Pb²⁺, synthesized by the adipic acid templated sol-gel route,” Journal of Luminescence, vol. 128, no. 1, pp. 129–134, 2008.

S. F. Wang, M. K. Lü, F. Gu et al., “Photoluminescence characteristics of Pb²⁺ ion in sol-gel derived ZnTiO₃ nanocrystals,” Inorganic Chemistry Communications, vol. 6, no. 2, pp. 185–188, 2003.

Z. Xiu, S. Liu, F. Xu et al., “Synthesis and optical properties of Pb-doped Sr₅(PO₄)₃Cl nanorods,” Journal of Alloys and Compounds, vol. 441, no. 1-2, pp. 219–221, 2007.

G. Blasse, “Energy transfer in oxidic phosphors,” Philips Research Reports, vol. 24, pp. 131–144, 1969.