Synthesis of anorthite by the Pechini process and structural investigation of the hexagonal phase
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Synthesis of anorthite by the Pechini process and structural investigation of the hexagonal phase Seong-Hyeon Hong Center for Cement Composite Materials, University of Illinois at Urbana – Champaign, Urbana, Illinois 61801
J. Francis Young and Ping Yu Center for Cement Composite Materials and Department of Materials Science and Engineering, University of Illinois at Urbana – Champaign, Urbana, Illinois 61801
R. James Kirkpatrick Center for Cement Composite Materials and Department of Geology, University of Illinois at Urbana – Champaign, Urbana, Illinois 61801 (Received 17 August 1998; accepted 31 December 1998)
Stoichiometric CaAl2 Si2 O8 gels prepared by the Pechini process crystallize to triclinic anorthite via an intermediate, previously unknown pseudohexagonal phase. Hydrothermal treatment of this pseudohexagonal phase at 350 ±C transforms it to hexagonal anorthite. These combined processes produce the hexagonal phase via a low-temperature, chemical route in contrast to the conventional high-temperature melting and crystallization method. Powder x-ray diffraction data and nuclear magnetic resonance (NMR) spectra of the hexagonal anorthite agree well with the published structure. They indicate a high degree of tetrahedral SiyAl ordering within the layers, but long-range disorder in the average structure due to (001) stacking faults present between the layers.
I. INTRODUCTION
Anorthite, CaAl2 Si2 O8 , is a promising glass-ceramic material for high temperature structural applications such as ceramic substrates.1,2 It is the Ca end member of the feldspar family, and there has been significant interest of its AlySi ordering, as a basis for understanding thermochemistry and structure of more complex, intermediate plagioclase feldspars.3–5 Anorthite has several reported polymorphic forms, but only three forms are considered to be distinct room temperature phases: triclinic, orthorhombic, and hexagonal.6–9 Previous claims for pseudo-orthorhombic and pseudohexagonal forms8 have not been substantiated.9 The triclinic phase occurs in nature and is considered to be the thermodynamically stable phase at all temperatures. The hexagonal phase is unknown in nature, and the orthorhombic phase has been found naturally only once.10 All the phases can be synthesized in the laboratories. Hexagonal anorthite was first reported by Davis and Tuttle,6 and its crystal structure has been determined by Takeuchi and Donnay.11 Its phase stability, metastable crystallization, and phase transformations have been investigated by Abe et al.8,9 For these studies, a glass of anorthite composition was melted above 1400 ±C and crystallization monitored on cooling to about 1100 ±C. Hexagonal anorthite has a layer structure topologically similar to that of hexacelsian (Fig. 1). It consists of sheets of apex-sharing AlySi tetrahedra held together by interlayer Ca.11 Thus, although it has the stoichiometry 1828
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 5, May 1999
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