High resolution electron microscopy study of the cationic disorder in Al 2 TiO 5
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As part of a research program devoted to the microstructural characterization of Al2TiO5-based compounds, high resolution electron microscopy (HREM) has been undertaken in order to study the crystallographic arrangement, especially ordering possibilities, of Al and Ti cations in the metallic sublattice of aluminum titanate. It is seen that adequate experimental conditions, mainly defocus setting, for a resolution of at least 2.5 A point-to-point, enable the disordered model to be directly and unambiguously proved on [100]-oriented micrographs.
I. INTRODUCTION A. Generalities
Aluminum titanate Al2TiO5 (tialite) is a synthetic ceramic material of potential interest for many structural applications, owing to its high melting point, low thermal conductivity, and excellent thermal shock resistance. However, a critical feature, which greatly limits the mechanical properties of polycrystalline Al2TiO5, is considerable intergranular microcracking, which occurs due to the high thermal anisotropy of individual grains (orthorhombic /3-Al2TiO5 with aa = -2.9, ab = 10.3, and ac = 20.1 10"6 K"1.1 Another problem arises from the thermodynamical decomposition of Al2TiOs into alumina (A12O3) and rutile (TiO2) below about 1300 °C.12 The latter disadvantage can be overcome by substituting Al, or Al and Ti, by small quantities of Si or Mg ions to form solid solutions which allow the thermal decomposition to be controlled.11 In the course of a general study of various Al2TiO5based compounds,19 a structural investigation of pure and Mg-doped Al2TiO5 polycrystals has been performed by means of High Resolution Electron Microscopy (HREM). The aim of this work was mainly to check the crystallographic arrangement of Al and Ti ions on the metallic sublattice of these oxide materials. During this research, it was also possible to characterize the dislocations.4
work, x-ray diffraction data were interpreted assuming the space group Bbmm, and assigning Ti ions on fourfold sites and Al ions on eightfold sites (see Fig. 1 and Table I). However, the relatively large error factor value (R « 17%) motivated the latter authors to redo similar experiments at 20° and 600 °C, but assuming a disordered distribution of both types of cations on the same atomic positions; acceptable R values of —6% were obtained under this condition, supporting the hypothesis of complete cationic disorder. Since both structures (i.e., ordered and disordered, hereafter labeled O and D) belong to the same space group, with identical atomic positions (leading to the same extinction rules), no superstructure reflection is
B. Crystallographic background
Basic structural studies of Al 2 Ti0 5 have been done by Hamelin7 and Morosin and Lynch.14 In the former
"'Permanent address: INSA, GEMPPM, Bat. 502, u.a. CNRS 341, 69621 Villeurbanne Cedex, France. ""'Permanent address: Instituto de Ceramica y Vidrio (CSIC), Arganda del Rey, Madrid, Spain. 138 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 1, Jan 1991 Downloaded: 14 Mar 2015
0.5 nm FIG. 1. [100] projection of Al
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