sification at these low temperatures. The major part of densifi- cation is attributed to rearrangement of the grains. The rela- tively small grain coarsening of CGO is due to dissolution and precipitation of CGO from the melt. When precipitated it will contain some CoO in solid solution. In order not to consume the liquid too rapidly, it is advantageous that the solid ceria has a rather low solubility for the dopant, as it is the case for Co. [16] Otherwise the liquid vanishes prior to maximum densi- fication. We summarize and conclude that nanometer-sized CeO 2 (ss) can be sintered at low temperatures to dense ceramics with nanometer-scaled grain size by the additions of transition metal oxides. The rapid densification may be attributed to size-dependent melting of the dopant in the neck region of the particle contacts. The transient nature of the grain bound- ary layer allows to tailor the electrical properties of CeO 2 (ss) from electronic to purely ionic. This material offers new possi- bilities for electrochemical devices. The low sintering temper- atures also open up new preparation sequences for electrode± electrolyte sandwich structures. We are convinced that other nanoscaled oxide powders may show similar low sintering temperatures when properly doped with small MO additions. The usual grain coarsening during the later stage of sintering of ceramics may be kept to a mini- mum and nanoscaled crystallite sizes will be possible by con- ventional sintering. Experimental Ce 0.8 Gd 0.2 O 2±x (denoted CGO; #595 250A, Rhodia GmbH, Frankfurt, Ger- many) with a crystallite size of 40 nm and a specific surface area of about 24 m 2 /g was used as starting powder. The total amount of impurities was less than 0.2 wt.-%. Transition metal nitrates (e.g., Co(NO 3 ) 2 ´6H 2 O, Fluka AG, Buchs, Switzerland) were added to a CGO/ethanol suspension (Fluka, puriss. p.a.). After drying at 120 C for 2 h the powders were isostatically dry-pressed at 840 MPa for 3 min. The samples (6  3  3 mm 3 ) were sintered in a dilat- ometer. Constant heating rate sintering was performed at a rate of 5 C/min. Densities were measured using the Archimedes principle before and after sin- tering. Received: August 23, 2000 Final version: February 25, 2001 ± [1] T. H. Etsell, S. N. Flengas, Chem. Rev. 1970, 70, 339. [2] D. Schneider, M. Gödickemeier, L. J. Gauckler, J. Electroceram. 1997, 1, 165. [3] I. Riess, Solid State Ionics 1992, 52, 127. [4] I. Riess, M. Gödickemeier, L. J. Gauckler, Solid State Ionics 1996, 90, 91. [5] M. Mogensen, T. Lindergaard, U. R. Hansen, J. Electrochem. Soc. 1994, 141, 2122. [6] D. L. Maricle, T. E. Swarr, S. Karavolis, Solid State Ionics 1992, 52, 173. [7] B. G. Pound, Solid State Ionics 1992, 52, 183. [8] Y.-M. Chiang, E. B. Lavik, I. Kosacki, H. L. Tuller, J. Electroceram. 1997, 1, 7. [9] L. Kundakovic, M. Flytzani-Stephanopoulos, J. Catal. 1998, 179, 203. [10] E. Perry Murray, T. Tsai, S. A. Barnett, Nature 1999, 400, 649. [11] Y.-C. Zhou, M. N. Rahaman, J. Mater. Res. 1993, 8, 1680. [12] P.-I. Chen,I.-W. Chen, J. Am. Ceram. Soc. 1994, 77, 2289. [13] O.-H. Kwon, G. L. Messing, Acta Metall. Mater. 1991, 39, 2059. [14] I.-W. Chen, X.-H. Wang, Nature 2000, 404, 168. [15] C. Herring, J. Appl. Phys. 1950, 21, 301. [16] E. N. Naumovic, V. V. Kharton, A. V. Kovalevsky, V. V. Samokhval, in Proc. of the 17th Riso International Symposium on Materials Science (Eds: F. W. Poulsen, N. Bonanos, S. Linderoth, M. Mogensen, B. Zachau-Chris- tiansen), Risù National Laboratory, Roskilde, Denmark 1996, p. 375. [17] A. I. Beljaev,J. E. Studencov, Legkie Metally 1937, 6(3), 17. [18] C. M. Kleinlogel, L. J. Gauckler, J. Electroceram. 2000, 5, 231. [19] C. Kleinlogel, L. J. Gauckler, unpublished. [20] E. Barrera, T. Viveros, A. Avila, P. Quintana, M. Morales, N. Batina, Thin Solid Films 1999, 346, 138. [21] B. Pejova, T. Kocareva, M. Najdoski, I. Grozdanov, Appl. Surf. Sci. 2000, 165, 271. [22] I. Sebastian, H. Neddermeyer, Surf. Sci. 2000, 454, 771. [23] D. W. Oxtoby, Nature 1990, 347, 725. [24] R. W. Cahn, Nature 1990, 323, 668. [25] S. L. Lai, J. Y. Guo, V. Petrova, G. Ramanath, L. H. Allen, Phys. Rev. Lett. 1996, 77, 99. [26] K. F. Peters, Y.-W. Chung,J. B.Cohen, Appl. Phys. Lett. 1997, 71, 2391. [27] B.Pluis, T. N. Taylor,D. Frenkel, J. F. van der Veen, Phys. Rev. B 1989, 40, 1353. [28] G. A. Allen, R. A. Bayles, W. W. Gile, A. W. Jesser, Thin Solid Films 1986, 144, 297. [29] H. Reiss, I. B. Wilson, J. Colloid Interface Sci. 1948, 3, 551. [30] K. H. Hanszen, Z. Phys. 1960, 157, 523. [31] C. R. M. Wronski, Br. J. Appl. Phys. 1967, 18, 1731. [32] P. Z. Pawlow, Z. Phys. Chem. 1976, 65, 545. [33] P. R.Couchman, W. A. Jesser, Nature 1977, 269, 481. [34] H. Reiss, P. Mirabel, R. L. Whetten, J. Phys. Chem. 1988, 92, 7241. [35] R. R. Vanfleet, J. M. Mochel, Surf. Sci. 1995, 341, 40. [36] D. Beaglehole, J. Cryst. Growth 1991, 112, 663. [37] M. Wautelet, Solid State Commun. 1990, 74, 1237. [38] R. B.Heady, J. W. Cahn, Metall. Trans. 1970, 1, 185. Highly Organized Mesoporous Titania Thin Films Showing Mono-Oriented 2D Hexagonal Channels** By David Grosso, Galo J. de A. A. Soler-Illia, Florence Babonneau, ClØment Sanchez,* Pierre-Antoine Albouy , Aline Brunet-Bruneau, and A. Ruud Balkenende Throughout the last decade, research on mesostructured materials experienced a major burst, following the micellar- template concept initially developed by Beck and co-work- ers. [1] Many mesoporous silica superstructures (2D and 3D hexagonal, cubic, lamellar, rectangular) have been obtained as powders, [1±3] fibers, [4] monoliths, [5] and thin films. [6±8] The Adv. Mater. 2001, 13, No. 14, July 18 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim,2001 0935-9648/01/1407-1085 $ 17.50+.50/0 1085 COMMUNICATIONS ± [*] Dr. C. Sanchez, Dr. D. Grosso, Dr. G. J. de A. A. Soler-Illia, Dr. F. Babonneau Chimie de la Matire CondensØe, UPMC±CNRS 4 place Jussieu, F-75252 Paris (France) E-mail: clems@ccr.jussieu.fr Dr. P.-A. Albouy Laboratoire de Physique des Solides, UniversitØ Paris-Sud F-91405 Orsay (France) Dr. A. Brunet-Bruneau Laboratoire d'Optique des Solides, URA±CNRS n7601 4 place Jussieu, Case 80, F-75252 Paris (France) Dr. A. R. Balkenende Philips Research Laboratories Prof. Holstlaan 4, NL-5656 AA Eindhoven (The Netherlands) [**] Acknowledgements are due to M. Lavergne for the TEM pictures. Finan- cial support from CNRS, Philips, CVC, CONICET, and Fundación Antorchas is gratefully recognized.