In the Laboratory
Structural Order–Disorder Transformations Monitored by X-ray Diffraction and Photoluminescence
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R. C. Lima,* E. C. Paris, and E. R. Leite Departamento de Química, Universidade Federal de São Carlos, São Carlos, São Paulo, Brazil; *
[email protected] J. W. M. Espinosa and A. G. Souza CCEN, Departamento de Química, Universidade Federal da Paraíba, João Pessoa, Paraíba, Brazil E. Longo Instituto de Química, Universidade Estadual Paulista, Araraquara, São Paulo, Brazil
The development of new materials with photoluminescent, ferroelectric, optoelectronic, and other properties are attractive for science and modern technology (1, 2). These properties relate to the structural “order–disorder” transformations of materials, which can be promoted by controlled heat treatment. It is necessary for chemistry students to understand the structure of materials, since this is what determines their final properties. Unfortunately, the inorganic chemistry of solid-state materials is insufficiently investigated in undergraduate laboratories. However, this area is generating much interest owing to the growing demand for new materials for specific technological applications. In materials chemistry, the term order–disorder can be discussed from the standpoint of kinetics, thermodynamics, magnetism, and so forth (3). Our discussion is based on structural order–disorder changes in solid materials. These concepts are also indispensable in other courses, including physics and engineering. We have been studying titanates of the ATiO3 type (A = Sr and Pb) in disordered and ordered (crystalline) states (4). Experimental results obtained by XANES (X-ray absorption near edge structure) for the structurally disordered phase have revealed the coexistence of two types of titanium coordination in lead titanate (PbTiO3 or PbT) (5), namely, fivefold oxygen Ti coordination (TiO5, square-pyramidal configuration) and sixfold oxygen Ti coordination (TiO6, octahedral configuration). We have also observed that TiO5 cluster is absent from well crystallized titanates. Photoluminescence in PbT is related to the coexistence of the TiO5 and TiO6 clusters, which cause the structure to become randomly disor-
dered. Lead ions modify the titanate host lattice, producing point defects that contribute to the characteristic inhomogeneous broadening of the photoluminescence on PbT. It is observed that a charge transfer occurs from the TiO5 cluster to the TiO6 cluster, revealing the intrinsic presence of trapped holes and trapped electrons, in the disordered phase of PbT powders (6). The presence of those holes and electrons, existing before the excitation, causes radiative emission when light radiation of suitable wavelength falls on a sample, also contributing to the inhomogeneous broadening, as illustrated in Figure 1. In contrast, the Sm3+ rare earth ions are characterized by narrow bands in the photoluminescence spectrum (7). Long-range order gives rise to sharp diffractions peaks that can be analyzed to determine the crystalline structure with great precision. It is known that even structurally disordered (amorphous) materials possess some degree of order. The structure of glass, for example, presents an intermediate-range order (8–9); that is, it has order over length scales longer than those associated with short-range order, but not so extensive as to constitute long-range order. Although the X-ray diffraction technique (XRD) is widely used in the structural characterization of materials (10), it is not sufficient to precisely identify the short- and intermediate-range orders. On the other hand, photoluminescence spectroscopy (PL), a method of probing the electronic structure of materials, is also used for the analysis of solid-state materials (11) and allows one to check the beginning of ordering when a marker ion such as Sm3+ is present inside the host of the material. Thus, PL complements the XRD technique. Experimental Overview
Figure 1. Illustration of inhomogeneous broadening. The individual optical bands vary slightly from site to site in the host lattice. The dashed line indicates the experimentally observed photoluminescence spectrum.
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In this experiment, we examine structural order–disorder transformations promoted by controlled heat treatment using XRD and PL techniques as tools to monitor the degree of structural order. To this end, Sm3+ ions were added to a PbT lattice via soft chemical synthesis—the so-called polymeric precursor method. These rare earth ions act as markers of the degree of order in the PbT system. The XRD results allowed us to identify the long-range order (crystalline or ordered structure) and PL results allowed us to identify the shortrange (disordered structure), the intermediate-range, and the long-range orders. The goals of this lab experiment are achieved when the students: (i) demonstrate their ability to prepare the materials; (ii) interpret the XRD and PL results in terms of the shortrange, intermediate-range, and long-range orders; and (iii)
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present their work in a report. This experiment is suitable for an advanced undergraduate laboratory course, introducing students to the fundamentals of solid-state materials chemistry, including synthesis, processing, and analysis of XRD and PL results. The synthesis method (polymeric precursor method) offers the advantage of tailor-making the chemistry, structure, and microstructure of materials to achieve specific optical and electrical properties (12–14). Furthermore, with this method it is possible to obtain solid compounds at reduced temperatures, which is more applicable to undergraduate chemistry laboratories. Materials and Methods
Synthesis PbTSm3+ (Sm3+-doped PbT) powders were prepared by the polymeric precursor method (15, 16). Lead(II) acetate trihydrate , Pb(CH3COO)2⭈3H2O; samarium oxide, Sm2O3; and titanium(IV) isopropoxide, Ti[OCH(CH3)2]4, were used as starting materials. Ethylene glycol and citric acid were used as polymerization–complexation agents. Ammonium hydroxide was used to adjust the pH and to prevent lead citrate precipitation. Titanium citrate was formed by the dissolution of Ti[OCH(CH3)2]4 in an aqueous solution of citric acid in a beaker with stirring and heating at 60–70 ⬚C. After the Ticitrate solution was homogenized, a stoichiometric amount of Pb(CH3COO)2⭈3H2O was dissolved in water and then added to the titanium citrate solution, which was kept under slow agitation until a clear solution was obtained. In the preparation of the samarium solution, Sm2O3 was first dissolved in nitric acid (14 M) and gradually added to the Pb兾titanium citrate solution. Aqueous ammonia was added until the pH reached 6– 7. After homogenization of the solution containing the Pb2+ and Sm3+ cations, ethylene glycol was added. Upon continued stirring and heating at 80–90 ⬚C, the solution became more viscous, forming a polymeric resin with no visible phase separation. The molar ratio of lead兾samarium兾titanium cations was 0.96:0.04:1, respectively, the citric acid兾metal molar ratio was fixed at 3:1, and the citric acid兾ethylene glycol mass ratio was fixed at 60:40. Processing The polymeric resin (gel formed inside the beaker) was heated in a muffle furnace EDG FI-1 model programmed in two temperatures: first at 250 ⬚C for 3 hours and second at 300 ⬚C for 1 hour, both at a heating rate of 10 ⬚C min᎑1. In this stage polymer pyrolysis occurs. The polymeric precursor obtained was removed from the beaker, de-agglomerated in porcelain mortar with 100-mesh sieve. This powder was heated at 300 ⬚C for 16 hours, at a heating rate of 10 ⬚C min᎑1 (in alumina crucible) in a muffle furnace to promote the elimination most of the organic matter without crystallization of the material. The resulting material was then divided into seven samples, placed in alumina crucibles, and each sample was heat treated in a muffle furnace for 2 hours at a heating rate of 10 ⬚C min᎑1 under O2 atmosphere at different temperatures: 370, 380, 390, 400, 405, 410, and 450 ⬚C. This last stage was performed to promote the structural order of the material until complete crystallization. www.JCE.DivCHED.org
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Figure 2. X-ray patterns of the calcined PbTSm3+ powders.
Characterization The powders were analyzed by XRD, using a RIGAKU DMAX 2500 PC diffractometer in a θ–2θ configuration ranging from 5⬚ to 75⬚, λ = 1.5406 Å, with a graphite monochromator, and by photoluminescence emission using a U1000 Jobin–Yvon spectrometer double monochromator coupled to a cooled GaAs photomultiplier and a conventional photon counting system. The 488.0 nm exciting wavelength of an argon ion laser was used, with the laser’s maximum output power kept within 200 mW. All the measurements were taken at room temperature. Hazards Titanium(IV) isopropoxide is an eye irritant and is flammable. Citric acid is a skin and respiratory irritant and a severe eye irritant. Lead(II) acetate trihydrate is toxic and a cancer-suspect agent. The students must avoid contact with skin and inhalation. Aqueous ammonia has a strong noxious odor, is corrosive, and should be contained in a fume hood. Samarium oxide is radioactive. The students should avoid contact with the skin and eyes. Nitric acid is harmful through inhalation and in contact with skin. It should be contained in a fume hood. Ethylene glycol is poisonous, an irritant, and may cause an allergic skin reaction. All these reagents should be handled with care. The students are required to wear suitable protective clothing and gloves. Results and Discussion The XRD patterns of the PbTSm3+ powders calcined at 370, 380, 390, 400, 405, 410, and 450 ⬚C are shown in Figure 2. The PbTSm3+ samples treated from 370 to 405 °C display a diffuse pattern centered at 2θ = 28.9°, characteristic of the noncrystalline (disordered) structure. The beginning of structural order (long-range periodicity) starts at 410 ⬚C and a pattern of crystalline material (completely ordered structure) corresponding to the PbT tetragonal perovskite phase
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Figure 3. Photoluminescence spectra of the calcined PbTSm3+ powders.
Figure 4. Magnified photoluminescence spectra of the calcined PbTSm3+ powders.
is observed at 450 ⬚C. All of the samples, disordered and ordered, were investigated by PL. The photoluminescence spectra of PbTSm3+ powders heat-treated at 370, 380, 390, 400, 405, 410, and 450 ⬚C are shown in Figure 3. A broad band corresponding to the photoluminescence of PbT (5, 17) in the powders annealed from 370 ⬚C to 405 ⬚C is observed. The broad PbT band occurs because the disordered PbT is composed of a random mixture of TiO5 and TiO6 clusters (4, 5), which are responsible for the photoluminescence. Two narrow bands are observed at 380 ⬚C corresponding to the photoluminescence of the Sm3+ ion (18). At this temperature, a certain degree of structural order already exists in the short and intermediate ranges of the materials structure. An XRD analysis (Figure 2) of the material obtained at this temperature shows a completely disordered pattern, indicating that the beginning of the short- and intermediaterange order in the material could not be characterized by this technique. The bands corresponding to Sm3+ ion are clearly visible at 390 ⬚C. In the photoluminescence spectrum for the sample calcined at 400 ⬚C a third narrow band ascribed to Sm3+ is observed. At 405 ⬚C, an increase in the intensity of these narrow bands is observed, as well as a shift of the broad PbT band to a lower wavelength (higher energy) with a decline in the intensity, indicating the beginning of a long-range order. The magnified spectra of the powders obtained at 410 and 450 ⬚C are shown in Figure 4. The powder treated at 410 ⬚C exhibits a low intensity broad band corresponding to PbT photoluminescence and at 450 ⬚C (completely crystalline structure) only high intensity narrow bands corresponding to Sm3+ ions are seen. At the latter temperature, the PbT structure presents a long-range periodic order (only the TiO6 clusters were present) and no photoluminescence is observed since this phenomenon depends on a certain degree of structural disorder in the material. A limit of structural order–disorder is required for photoluminescence to occur, so
completely ordered and completely disordered PbT materials will not show photoluminescence. However, characteristic narrow bands of Sm3+ photoluminescence are observed at 450 ⬚C.
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Conclusions This experiment provides an interesting laboratory exercise for advanced undergraduate students. The experiment serves not only as an introduction to materials chemistry, but also illustrates the importance of using complementary techniques in the analysis of structural order–disorder transformations promoted by controlled heat treatment. We have demonstrated that X-ray diffraction, a characterization technique commonly employed in solid-state chemistry, is not sufficient to identify short- and intermediate-range structural ordering (non-periodic structure or disordered or noncrystalline), which can be verified by photoluminescence spectroscopy when a marker ion such as Sm 3+ is added in the structure, in this case in the PbT structure. We have used the X-ray diffraction and photoluminescence techniques as tools to monitor the structural order–disorder transformations promoted by controlled heat treatment. Our synthesis is easily achieved with low cost, and the experiment is versatile because certain experimental details can be varied to produce distinct, but comparable, results among groups. For example, varying the concentration and type (Sm3+ or Er3+) of doping ions, the matrix (PbTiO3 or CaTiO3), and the calcination temperatures can generate important discussions about order–disorder transformations, which are connected with the material properties. Acknowledgments The authors acknowledge the Brazilian financing agencies FAPESP/CEPID, CNPq/PRONEX, CAPES, and FAPESQ/PRODOC/DCR.
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Supplemental Material
Detailed information about the experiment and notes for the instructor and students are available in this issue of JCE Online. Literature Cited 1. Bolstad, D. B.; Diaz, A. L. J. Chem. Educ. 2002, 79, 1101– 1104. 2. Chandler, C. D.; Roger, C.; Hampden-Smith, M. J. Chem. Rev. 1993, 93, 1205–1241. 3. Sima, V. J. Alloys Comp. 2004, 378, 44–51. 4. Pontes, F. M.; Longo, E.; Leite, E. R.; Lee, E. J. H.; Varela, J. A.; Pizani, P. S.; Campos, C. E. M.; Lanciotti, F., Jr.; Mastellaro, V.; Pinheiro, C. D. Mater. Chem. Phys. 2002, 77, 598–602. 5. Leite, E. R.; Pontes, F. M.; Paris, E. C.; Paskocimas, C. A.; Lee, E. J. H.; Longo, E.; Pizani, P. S.; Varela, J. A.; Mastelaro, V. Adv. Mat. Opt. Elect. 2000, 10, 235–240. 6. Orhan, E.; Varela, J. A.; Zenatti, A.; Gurgel M. F. C.; Pontes, F. M.; Leite, E. R.; Longo, E.; Pizani, P. S.; Beltran, A.; Andres, J. Phys. Rev. B 2005, 71, 085113-1–08113-7. 7. Blasse, G.; Grabmaier, B. C. Luminescent Materials, 1st ed.; Springer-Verlag: Berlin, 1994; pp 1–19.
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8. Salmon, P. S. Nature Mater. 2002, 1, 87–88. 9. Hufnagel, T. C. Nature Mater. 2004, 3, 666–667. 10. Butera R. A.; Waldeck, D. H. J. Chem. Educ. 1997, 74, 115– 119. 11. Lasher, D. P.; De Graff, B. A.; Augustine, B. H. J. Chem. Educ. 2000, 77, 1201–1203. 12. Rovai, R.; Lehmann, C. W.; Brandley, J. S. Angew. Chem., Int. Ed. 1999, 38, 2036–2038. 13. Martin, J. D.; Goettler, S. J.; Fossé, N.; Iton, L. Nature 2002, 419, 381–383. 14. Eckert, T.; Barsh, E. Phys. Rev. Lett. 2002, 89, 125701-1– 125701-4. 15. Pontes, F. M.; Leite, E. R.; Pontes, D. S. L.; Longo, E.; Santos, E. M. S.; Mergulhao, S.; Pizani, P. S.; Lanciotti, F.; Boschi, T. M.; Varela, J. A. J. Appl. Phys. 2002, 91, 5972–5978. 16. Kakihana, M.; Okubo, A. T.; Uchiyama, M.; Yashima, O.; Yoshimura, M. M.; Nakamura, Y. Chem. Mater. 1997, 9, 451– 456. 17. Longo, E.; Orhan, E.; Pontes, F. M.; Pinheiro, C. D.; Leite, E. R.; Varela, J. A.; Pizani, P. S.; Boshi, T. M.; Lanciotti, F.; Beltrán, A., Jr; Andrés, J. Phys. Rev. B 2004, 69, 125115-1– 121551-7. 18. Hussain, N. S.; Aruna V.; Buddhudu S. Mat. Res. Bull. 2000, 35, 703–709.
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