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Ruthenium doped TiO2 fibers by electrospinning
Inorganic Chemistry Communications 7 (2004) 679–682 www.elsevier.com/locate/inoche
Ruthenium doped TiO2 fibers by electrospinning P. Viswanathamurthi
a,b,*
, N. Bhattarai c, C.K. Kim b, H.Y. Kim
b,*
, D.R. Lee
b
a Department of Chemistry, Kongunadu Arts and Science College, Coimbatore 641 029, India Department of Textile Engineering, Chonbuk National University, Chonju 561 756, Republic of Korea Department of Advanced Organic Materials Engineering, Chonbuk National University, Chonju 561 756, Republic of Korea b
c
Received 5 February 2004; accepted 13 March 2004 Available online 9 April 2004
Abstract Nano to sub-micron fibers of ruthenium doped titanium dioxide/poly(vinylacetate) hybrid have been prepared by electrospinning method. Pure ceramic metal oxide fibers were obtained by high temperature calcination of the organic–inorganic hybrid fibers. It is observed that the surface morphology and crystallinity of the fibers depend on the calcination temperature and ruthenium content. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Sol–gel; Electrospinning; Calcinations; Metal oxide fibers
Titanium dioxide is an important material for a variety of applications such as catalytic devices, solar cells, and other optoelectronic devices [1–3]. The properties of titanium dioxide, especially the catalytic properties, have proved to be strongly related to its crystal structure and grain size. Titanium dioxide in the anatase phase appears to be the most active species for most substrates [4,5]. Nanocrystalline titanium dioxide undergoes phase transformation and grain growth at relatively lower temperatures, which limits its applications in many fields such as high temperature gas separation and membrane reactors [6,7]. In the several studies related to the thermal stability of membranes, Burgfraaf and co-workers [8] and Cot and co-workers [9] have determined the pore size of some membrane top layers at different temperatures. This high thermal stability of the membranes allows it to be used for gas separation at high temperatures, especially in combination with a chemical reaction where the membrane is used as a catalyst as well as a selective barrier to remove one of the components which has been formed [7,10–12]. In order to improve * Corresponding authors. Tel.: +82-63-270-2351; fax: +82-63-2702348 (P. Viswanathamurthi). E-mail addresses: [email protected] (P. Viswanathamurthi), [email protected] (H.Y. Kim).
1387-7003/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2004.03.013
the thermal and physical properties of titania fibers, the doped titania composite membranes were prepared. Solar energy is believed to be an essential energy source of the next century. A pn type solar cell with all solid-state TiO2 based materials seems to have many benefits for effective utilization of the solar energy because the ultra violet and visible light of solar light would be adsorbed by n-type TiO2 and p-type TiO2 , respectively. Here, transparent semiconductor electrodes with high conductivity are required. Transparent semiconductive n-type TiO2 materials were prepared by codoping ruthenium [13]. Most of the previous work in synthesis of ruthenium doped titanium dioxide has focused on the preparation of films and membranes [13,14]. In this work, we have prepared TiO2 doped with ruthenium in the form of nano to sub-micron fibers by an electrospinning method. Electrospinning technique [15–18] has been found to be unique and cost effective approach for fabricating large surface area membranes for a variety of applications. Electrospinning is a process by which high static voltages are used to produce an interconnected membrane like web of small fibers, with the fiber diameter in the range of 50–1000 nm. This technique can be used with a variety of polymers to produce nanoscale fibrous membranes. Electrospun nanofibers can have
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P. Viswanathamurthi et al. / Inorganic Chemistry Communications 7 (2004) 679–682
approximately 1–2 orders of magnitude of surface area than that found in continuous thin films [19]. The PVAC (13.5 wt%) solution was prepared using DMF/THF (6:4 by wt ratio) as solvents. The titanium dioxide sol solution was obtained from titanium isopropoxide molecular precursor based on sol–gel procedures. 2.84 g of titanium isopropoxide was dissolved in 2.40 g of acetic acid and solution was stirred for 6 h. The electrospinning solution was prepared by mixing the PVAC (13.5 wt%) solution with titanium dioxide sol solution (1.5:1.0 by wt ratio) under stirring for 5 h. Then, 0.1 g ruthenium trichloride trihydrate was added. The resulting solution was stirred for 20 h. In order to study the effect of ruthenium on the morphology of fibers, different amount of ruthenium trichloride (0.1, 0.125 and 0.15 g) was added. The electrospinning solution was taken in a syringe and delivered at a constant flow rate using a capillary. The positive (anode) terminal of a variable high voltage transformer capable of delivering 30 kV is attached to a copper wire inserted into the solution in the syringe and the negative terminal being attached to a aluminium foil covered collector (cathode). Upon applying a high voltage (15 kV) into the solution with the distance between the capillary tip and the target surface being 17 cm, a fluid jet was ejected from the capillary. As the jet accelerated towards the cathode, the solvent evaporated and a charged fiber was deposited on the collector cathode. The as-prepared composite fibers were subjected to high temperature calcination to obtain pure metal oxide fibers. Surface morphology of the electrospinning fibers has been studied by scanning electron microscopy (SEM). Fig. 1 shows the morphology of as-synthesised titania and ruthenium doped titania fibers. It can be seen that the diameter of the fibers is slightly increased by the addition of ruthenium. But there is no change in fiber diameter with increase in ruthenium content. It can also be seen that the surface of the fibers is smooth and
uniform. Fig. 2 showed the morphology of titania fibers with different content of ruthenium calcined at 600 °C. The fibers appear no longer straight when the ruthenium content is 0.1 g and some of the fibers are broken if the ruthenium content is increased. The fibers obtained after calcination at 800 °C showed different morphology (Fig. 3) with respect to ruthenium content. The fiber surface showed shrinkage and roughness for addition of 0.1 g ruthenium. It can be seen that the fiber surface appears smooth and have porous surfaces for higher ruthenium content. The porous nature increases with increase in ruthenium content. The fibers retained their polymorphic structures even at higher calcination temperature (1000 °C) (Fig. 4). At this stage no change in fiber morphology with respect to ruthenium content was observed. It can be seen that the fibers appear to consist of linked particles or crystallites. It is shown that these changes are related to a dramatic change in crystal structure. Fig. 5 gives the XRD patterns of fibers calcined at different temperature. The crystalline peaks appeared correspond to titanium dioxide and ruthenium [20]. The sample calcined at 600 °C (Fig. 5(a)) showed only anatase phase of titanium dioxide and metallic ruthenium. The sample when calcined at 800 °C (Fig. 5(b)) showed presence of rutile and traces of anatase phase of titanium dioxide in addition to ruthenium peak. The sample
Fig. 2. SEM images of ruthenium doped titanium dioxide fibers calcined at 600 °C: (a) 0.1 g ruthenium; (b) 0.125 g ruthenium; (c) 0.15 g ruthenium.
Fig. 1. SEM images of ruthenium doped titanium dioxide fibers: (a) without ruthenium; (b) 0.1 g ruthenium; (c) 0.15 g ruthenium.
Fig. 3. SEM images of ruthenium doped titanium dioxide fibers calcined at 800 °C: (a) 0.1 g ruthenium; (b) 0.15 g ruthenium.
P. Viswanathamurthi et al. / Inorganic Chemistry Communications 7 (2004) 679–682
681
Transmittance
a
b
Fig. 4. SEM images of ruthenium doped titanium dioxide fibers calcined at 1000 °C: (a) 0.1 g ruthenium; (b) 0.125 g ruthenium; (c) 0.15 g ruthenium. 500
1000
1500
2000
2500
Wave number (cm-1)
Intensityns
Fig. 6. IR spectra of ruthenium doped titanium dioxide fibers: (a) as-synthesised hybrid material; (b) after calcination at 800 °C.
c
b
a 30
40 50 2 θ (deg.)
60
70
Fig. 5. X-ray diffraction patterns of ruthenium doped titanium dioxide fibers: (a) after calcination at 600 °C; (b) after calcination at 800 °C; (c) after calcination at 1000 °C.
calcined at 1000 °C (Fig. 5(c)) showed only rutile phase of titanium dioxide and metallic ruthenium. These results indicated that the organic contents were removed entirely from PVAC/TiO2 –Ru composite fibers when the calcination temperature was above 600 °C and anatase– rutile transformation of titanium dioxide took place when the calcination temperature was increased from 600 to 1000 °C. The formation of pure metal oxide fiber is further supported by IR spectra (Fig. 6). In as-synthesised form, strong bands are absorbed in the range 1000 and 2000 cm 1 which can be assigned to bending and stretching frequencies of the PVAC. The band associated with the vibrational mode of the skeletal O–Ti–O bonds of anatase phase appeared at 470 cm 1 [21]. After calcination at 800 °C, all these strong features are removed. No sign
of adsorbed water or hydroxy, carbonate or hydrocarbon impurity can be observed. Instead, there is a shift of the main anatase band from 470 to 700 cm 1 occurred. This is due to formation of titanium dioxide rutile particles [21]. In conclusion, nano to sub-micron fibers of ruthenium-doped titanium dioxide have been successfully prepared using an electrospinning method. It has been observed that the calcination temperature and the ruthenium content determine the morphology and crystallinity of the fibers. The fibres have porous surfaces at particular range of temperature and then changed into polycrystalline structure when temperature is increased. The porous and polycrystalline structure of the electrospun fibres provide a surface area to volume ratio roughly 1–2 orders of magnitude higher than that known for continuous thin films. We believe these nanofibres could be used in solar cells and sensors applications.
Acknowledgements This work was supported by the Grant of Post-Doc. Program, Chonbuk National University (2002).
References [1] M.R. Hoffman, S.T. Martin, W. Choi, D.W. Bahneman, Chem. Rev. 95 (1995) 6. [2] M. Taramasso, G. Perego, B. Notari, US Patent 4, 410 (1983) 501. [3] O.B. Regan, M. Gratzel, Nature 35 (1991) 737. [4] M.V. Rao, K. Rajeshwar, V.R. Ververker, J. Dubow, J. Phys. Chem. 84 (1980) 1987.
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P. Viswanathamurthi et al. / Inorganic Chemistry Communications 7 (2004) 679–682
[5] S. Nishimoto, B. Ohtani, H. Kajiwara, T. Kagiya, J. Chem. Soc. Faraday Trans. 81 (1985) 61. [6] M.A. Anderson, M.J. Geiselmann, Q.J. Xu, Membr. Sci. 39 (1988) 243. [7] R.R. Bhave, Inorganic Membranes Synthesis, Characteristics and Applications, Van Nostrand Reihold, New York, 1991. [8] A.F.M. Leenaars, K. Keizer, A.J. Burgfraaf, J. Mater. Sci. 19 (1984) 1077. [9] A. Larbot, J.P. Fabre, C. Guizard, L. Cot, J. Am. Ceram. Soc. 72 (1989) 257. [10] C.J. Brink, G.W. Scherer, Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, San Diego, 1990. [11] E.M. Ezzo, T. Einabarawy, A.M. Youssef, Surf. Technol. 9 (1979) 111. [12] A. Makishima, M. Asami, K. Wada, J. Non-Cryst. Sol. 121 (1990) 310.
[13] H. Lin, T. Uchino, H. Kozuka, T. Yoko, ICR Ann. Rep. 4 (1997) 22. [14] D.S. Bae, K.S. Han, S.H. Choi, Mater. Lett. 33 (1997) 101. [15] J. Doshi, D.H. Reneker, J. Electrostat. 35 (1995) 151. [16] D.H. Reneker, A.L. Yarin, H. Fong, S. Koombhonge, J. Appl. Phys. 87 (2000) 4531. [17] K.H. Lee, H.Y. Kim, M.S. Khil, Y.M. La, D.R. Lee, Polymer 44 (2003) 1287. [18] P. Viswanathamurthi, N. Bhattarai, H.Y. Kim, D.R. Lee, S.R. Kim, M.A. Morris, Chem. Phys. Lett. 374 (2003) 79. [19] P. Gibon, H. Schreuder-Gibson, D. Riven, Colloids Surf. A (2001) 187. [20] G. Busca, G. Ramis, J.M. Gallardo Amores, B.S. Escribano, P. Piaggio, J. Chem. Soc. Faraday Trans. 90 (1994) 3181. [21] M. Ocana, V. Fornes, F.V. Garcia Ramos, C.J. Serna, J. Solid State Chem. 75 (1988) 364.
Ruthenium doped TiO2 fibers by electrospinning P. Viswanathamurthi
a,b,*
, N. Bhattarai c, C.K. Kim b, H.Y. Kim
b,*
, D.R. Lee
b
a Department of Chemistry, Kongunadu Arts and Science College, Coimbatore 641 029, India Department of Textile Engineering, Chonbuk National University, Chonju 561 756, Republic of Korea Department of Advanced Organic Materials Engineering, Chonbuk National University, Chonju 561 756, Republic of Korea b
c
Received 5 February 2004; accepted 13 March 2004 Available online 9 April 2004
Abstract Nano to sub-micron fibers of ruthenium doped titanium dioxide/poly(vinylacetate) hybrid have been prepared by electrospinning method. Pure ceramic metal oxide fibers were obtained by high temperature calcination of the organic–inorganic hybrid fibers. It is observed that the surface morphology and crystallinity of the fibers depend on the calcination temperature and ruthenium content. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Sol–gel; Electrospinning; Calcinations; Metal oxide fibers
Titanium dioxide is an important material for a variety of applications such as catalytic devices, solar cells, and other optoelectronic devices [1–3]. The properties of titanium dioxide, especially the catalytic properties, have proved to be strongly related to its crystal structure and grain size. Titanium dioxide in the anatase phase appears to be the most active species for most substrates [4,5]. Nanocrystalline titanium dioxide undergoes phase transformation and grain growth at relatively lower temperatures, which limits its applications in many fields such as high temperature gas separation and membrane reactors [6,7]. In the several studies related to the thermal stability of membranes, Burgfraaf and co-workers [8] and Cot and co-workers [9] have determined the pore size of some membrane top layers at different temperatures. This high thermal stability of the membranes allows it to be used for gas separation at high temperatures, especially in combination with a chemical reaction where the membrane is used as a catalyst as well as a selective barrier to remove one of the components which has been formed [7,10–12]. In order to improve * Corresponding authors. Tel.: +82-63-270-2351; fax: +82-63-2702348 (P. Viswanathamurthi). E-mail addresses: [email protected] (P. Viswanathamurthi), [email protected] (H.Y. Kim).
1387-7003/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2004.03.013
the thermal and physical properties of titania fibers, the doped titania composite membranes were prepared. Solar energy is believed to be an essential energy source of the next century. A pn type solar cell with all solid-state TiO2 based materials seems to have many benefits for effective utilization of the solar energy because the ultra violet and visible light of solar light would be adsorbed by n-type TiO2 and p-type TiO2 , respectively. Here, transparent semiconductor electrodes with high conductivity are required. Transparent semiconductive n-type TiO2 materials were prepared by codoping ruthenium [13]. Most of the previous work in synthesis of ruthenium doped titanium dioxide has focused on the preparation of films and membranes [13,14]. In this work, we have prepared TiO2 doped with ruthenium in the form of nano to sub-micron fibers by an electrospinning method. Electrospinning technique [15–18] has been found to be unique and cost effective approach for fabricating large surface area membranes for a variety of applications. Electrospinning is a process by which high static voltages are used to produce an interconnected membrane like web of small fibers, with the fiber diameter in the range of 50–1000 nm. This technique can be used with a variety of polymers to produce nanoscale fibrous membranes. Electrospun nanofibers can have
680
P. Viswanathamurthi et al. / Inorganic Chemistry Communications 7 (2004) 679–682
approximately 1–2 orders of magnitude of surface area than that found in continuous thin films [19]. The PVAC (13.5 wt%) solution was prepared using DMF/THF (6:4 by wt ratio) as solvents. The titanium dioxide sol solution was obtained from titanium isopropoxide molecular precursor based on sol–gel procedures. 2.84 g of titanium isopropoxide was dissolved in 2.40 g of acetic acid and solution was stirred for 6 h. The electrospinning solution was prepared by mixing the PVAC (13.5 wt%) solution with titanium dioxide sol solution (1.5:1.0 by wt ratio) under stirring for 5 h. Then, 0.1 g ruthenium trichloride trihydrate was added. The resulting solution was stirred for 20 h. In order to study the effect of ruthenium on the morphology of fibers, different amount of ruthenium trichloride (0.1, 0.125 and 0.15 g) was added. The electrospinning solution was taken in a syringe and delivered at a constant flow rate using a capillary. The positive (anode) terminal of a variable high voltage transformer capable of delivering 30 kV is attached to a copper wire inserted into the solution in the syringe and the negative terminal being attached to a aluminium foil covered collector (cathode). Upon applying a high voltage (15 kV) into the solution with the distance between the capillary tip and the target surface being 17 cm, a fluid jet was ejected from the capillary. As the jet accelerated towards the cathode, the solvent evaporated and a charged fiber was deposited on the collector cathode. The as-prepared composite fibers were subjected to high temperature calcination to obtain pure metal oxide fibers. Surface morphology of the electrospinning fibers has been studied by scanning electron microscopy (SEM). Fig. 1 shows the morphology of as-synthesised titania and ruthenium doped titania fibers. It can be seen that the diameter of the fibers is slightly increased by the addition of ruthenium. But there is no change in fiber diameter with increase in ruthenium content. It can also be seen that the surface of the fibers is smooth and
uniform. Fig. 2 showed the morphology of titania fibers with different content of ruthenium calcined at 600 °C. The fibers appear no longer straight when the ruthenium content is 0.1 g and some of the fibers are broken if the ruthenium content is increased. The fibers obtained after calcination at 800 °C showed different morphology (Fig. 3) with respect to ruthenium content. The fiber surface showed shrinkage and roughness for addition of 0.1 g ruthenium. It can be seen that the fiber surface appears smooth and have porous surfaces for higher ruthenium content. The porous nature increases with increase in ruthenium content. The fibers retained their polymorphic structures even at higher calcination temperature (1000 °C) (Fig. 4). At this stage no change in fiber morphology with respect to ruthenium content was observed. It can be seen that the fibers appear to consist of linked particles or crystallites. It is shown that these changes are related to a dramatic change in crystal structure. Fig. 5 gives the XRD patterns of fibers calcined at different temperature. The crystalline peaks appeared correspond to titanium dioxide and ruthenium [20]. The sample calcined at 600 °C (Fig. 5(a)) showed only anatase phase of titanium dioxide and metallic ruthenium. The sample when calcined at 800 °C (Fig. 5(b)) showed presence of rutile and traces of anatase phase of titanium dioxide in addition to ruthenium peak. The sample
Fig. 2. SEM images of ruthenium doped titanium dioxide fibers calcined at 600 °C: (a) 0.1 g ruthenium; (b) 0.125 g ruthenium; (c) 0.15 g ruthenium.
Fig. 1. SEM images of ruthenium doped titanium dioxide fibers: (a) without ruthenium; (b) 0.1 g ruthenium; (c) 0.15 g ruthenium.
Fig. 3. SEM images of ruthenium doped titanium dioxide fibers calcined at 800 °C: (a) 0.1 g ruthenium; (b) 0.15 g ruthenium.
P. Viswanathamurthi et al. / Inorganic Chemistry Communications 7 (2004) 679–682
681
Transmittance
a
b
Fig. 4. SEM images of ruthenium doped titanium dioxide fibers calcined at 1000 °C: (a) 0.1 g ruthenium; (b) 0.125 g ruthenium; (c) 0.15 g ruthenium. 500
1000
1500
2000
2500
Wave number (cm-1)
Intensityns
Fig. 6. IR spectra of ruthenium doped titanium dioxide fibers: (a) as-synthesised hybrid material; (b) after calcination at 800 °C.
c
b
a 30
40 50 2 θ (deg.)
60
70
Fig. 5. X-ray diffraction patterns of ruthenium doped titanium dioxide fibers: (a) after calcination at 600 °C; (b) after calcination at 800 °C; (c) after calcination at 1000 °C.
calcined at 1000 °C (Fig. 5(c)) showed only rutile phase of titanium dioxide and metallic ruthenium. These results indicated that the organic contents were removed entirely from PVAC/TiO2 –Ru composite fibers when the calcination temperature was above 600 °C and anatase– rutile transformation of titanium dioxide took place when the calcination temperature was increased from 600 to 1000 °C. The formation of pure metal oxide fiber is further supported by IR spectra (Fig. 6). In as-synthesised form, strong bands are absorbed in the range 1000 and 2000 cm 1 which can be assigned to bending and stretching frequencies of the PVAC. The band associated with the vibrational mode of the skeletal O–Ti–O bonds of anatase phase appeared at 470 cm 1 [21]. After calcination at 800 °C, all these strong features are removed. No sign
of adsorbed water or hydroxy, carbonate or hydrocarbon impurity can be observed. Instead, there is a shift of the main anatase band from 470 to 700 cm 1 occurred. This is due to formation of titanium dioxide rutile particles [21]. In conclusion, nano to sub-micron fibers of ruthenium-doped titanium dioxide have been successfully prepared using an electrospinning method. It has been observed that the calcination temperature and the ruthenium content determine the morphology and crystallinity of the fibers. The fibres have porous surfaces at particular range of temperature and then changed into polycrystalline structure when temperature is increased. The porous and polycrystalline structure of the electrospun fibres provide a surface area to volume ratio roughly 1–2 orders of magnitude higher than that known for continuous thin films. We believe these nanofibres could be used in solar cells and sensors applications.
Acknowledgements This work was supported by the Grant of Post-Doc. Program, Chonbuk National University (2002).
References [1] M.R. Hoffman, S.T. Martin, W. Choi, D.W. Bahneman, Chem. Rev. 95 (1995) 6. [2] M. Taramasso, G. Perego, B. Notari, US Patent 4, 410 (1983) 501. [3] O.B. Regan, M. Gratzel, Nature 35 (1991) 737. [4] M.V. Rao, K. Rajeshwar, V.R. Ververker, J. Dubow, J. Phys. Chem. 84 (1980) 1987.
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P. Viswanathamurthi et al. / Inorganic Chemistry Communications 7 (2004) 679–682
[5] S. Nishimoto, B. Ohtani, H. Kajiwara, T. Kagiya, J. Chem. Soc. Faraday Trans. 81 (1985) 61. [6] M.A. Anderson, M.J. Geiselmann, Q.J. Xu, Membr. Sci. 39 (1988) 243. [7] R.R. Bhave, Inorganic Membranes Synthesis, Characteristics and Applications, Van Nostrand Reihold, New York, 1991. [8] A.F.M. Leenaars, K. Keizer, A.J. Burgfraaf, J. Mater. Sci. 19 (1984) 1077. [9] A. Larbot, J.P. Fabre, C. Guizard, L. Cot, J. Am. Ceram. Soc. 72 (1989) 257. [10] C.J. Brink, G.W. Scherer, Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, San Diego, 1990. [11] E.M. Ezzo, T. Einabarawy, A.M. Youssef, Surf. Technol. 9 (1979) 111. [12] A. Makishima, M. Asami, K. Wada, J. Non-Cryst. Sol. 121 (1990) 310.
[13] H. Lin, T. Uchino, H. Kozuka, T. Yoko, ICR Ann. Rep. 4 (1997) 22. [14] D.S. Bae, K.S. Han, S.H. Choi, Mater. Lett. 33 (1997) 101. [15] J. Doshi, D.H. Reneker, J. Electrostat. 35 (1995) 151. [16] D.H. Reneker, A.L. Yarin, H. Fong, S. Koombhonge, J. Appl. Phys. 87 (2000) 4531. [17] K.H. Lee, H.Y. Kim, M.S. Khil, Y.M. La, D.R. Lee, Polymer 44 (2003) 1287. [18] P. Viswanathamurthi, N. Bhattarai, H.Y. Kim, D.R. Lee, S.R. Kim, M.A. Morris, Chem. Phys. Lett. 374 (2003) 79. [19] P. Gibon, H. Schreuder-Gibson, D. Riven, Colloids Surf. A (2001) 187. [20] G. Busca, G. Ramis, J.M. Gallardo Amores, B.S. Escribano, P. Piaggio, J. Chem. Soc. Faraday Trans. 90 (1994) 3181. [21] M. Ocana, V. Fornes, F.V. Garcia Ramos, C.J. Serna, J. Solid State Chem. 75 (1988) 364.