Tetragonal-orthorhombic phase transformation and sintering behavior of KSbOSiO 4 (isomorphous derivative of KTP)
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Fine KSbOSiO 4 (KSS) powders have been prepared by a sol-gel method starting from the system of TEOS, KSb(OH) 6 , H 2 O, C 2 H 5 OH, or mechanochemical mixing method with a stoichiometric mixture of amorphous silica and KSb(OH) 6 . The KSS crystallized sluggishly into a tetragonal symmetry in low temperature and transformed into an orthorhombic one in higher temperature, indicating the amorphous-like powder patterns in the intermediate step. This amorphization process could be related to some redistribution of the polysilicate network structures. The mechanochemical activation energy stored in the powder surface, as well as the coexisting matrix (KSbO 3 ), retards the transformation. It was concluded that the KSS powders composed of high molecular weight siloxane polymers, which were formed by hydrolysis in the presence of a small amount of NH 3 aqueous solution, showed higher sinterability. The sintering, where the calcining process was omitted, enabled the dense tetragonal-type ceramics stabilized at lower temperature, rather than higher temperature, to fabricate.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
It is well known that the properties of single crystal KTiOPO4 (KTP), consisting of chains of alternating TiO6 octahedra and PO 4 tetrahedra parallel to both a and b axes,1 with large nonlinear coefficients, are suited for the second harmonic generation.2-3 Rietveld analysis3 revealed the atomic coordinates of KSbOSiO 4 orthorhombic phase (in short, KSS ortho.), belonging to the same crystal structure as KTP, produced by the replacement of Ti(iv) —> Sb(v), P(v) —3, ( • ) KSS tetra., ( • ) KSS ortho., and 0.6Si(OEt)4 + KSb(OH) 6 - J ; ^ .
spectra (Nicolet FT-IR) of SG-derived powders were measured by the KBr method. III. RESULTS AND DISCUSSION It is often observed that SiO2 components possessing lower reactivity tend to remain as unreactants in the combination reaction of equimolar M 0 x - S i 0 2 (M = metal ion) systems8'9 even in the SG process. This means that it is possible to form a SiO 2 deficient compound, KSbSi0.6O42 ( = K 5 Sb 5 Si 3 0 21 ), which is a candidate of another framework structure except tetragonal phase (low-temperature stabilized type) and orthorhombic phase (high-temperature stabilized type) in the intermediate step of KSS synthesis. Therefore, it is valuable to watch for the following reaction in the lower temperature region. Si(OEt)4 + KSb(OH) 6
ortho. by volatilizing a large part of KSbO 3 when heated at 1473 K. At 1523 K, KSS ortho. slightly existed with complete disappearance of KSbO 3 . Partial melting of the sample was observable when heated at 1573 K. Figure 1 indicates that the next reaction advances without forming KSbSi0.6O42 (SiO 2 deficient compound), because of the shortage of dosed amounts of TEOS.
KSbSio.604.2 + 0.4SiO2 (amorphous)
Therefore we try to investigate the next SG reaction in the presence of a small amount of NH 3 aqueous solution (this experiment is called TEOS shortage reaction). Namely, 0.6Si(OEt)4 + KSb(OH) 6 — KSbSi0.6O4.2 Figure 1 shows the evolutio
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