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octahedral sites are occupied in an ordered manner with the sequences Li, M, vacancy,
Li, M, vacancy, etc. for adjacent face-sharing octahedral sites along the c-axis of the
rhomohedral unit cell. The M±O±M bond angles are close to 1578 [5]. In an ideal
perovskite structure the B±O±B angles are 1808 and there is one large interstice per
BO3 unit, which is occupied by A ions with 12-fold coordination. The HMO3
structure is built of NbO6 corner sharing octahedra as in the ReO3 structure (A-site
de®cient perovskite-like ABO3 structure). The H atoms are found to statistically
distribute on two types of positions at room temperature. The doubling of the edge of
the primitive cell arises due to the tilts of the octahedra from the cube axis by 15.88
[7]. Table 1 lists the hexagonal lattice constants of LiMO3 prepared from HMO3 using
the LiNO3 melt together with LiMO3 prepared by solid state reaction and precursor
HMO3 as well as literature data [11]. TGA data (Fig. 4) and FTIR spectra (Fig. 5)
reveal that LiMO3 obtained from HMO3 is anhydrous, similar to the compound
obtained by solid state reaction.
SEM images of LiMO3 prepared from HMO3 are shown together with those of
HMO3 and LiMO3 prepared by solid state reactions in Figs. 6 and 7. We see LiMO3
derived from HMO3 (Figs. 6c and 7c) are crystallites of rather uniform size, which
differs from that of the solid state reaction products (Figs. 6a and 7a). LiMO3 prepared
by solid state reaction shows crystals of irregular size and small agglomerated
particles at the surface of the crystals. For the purpose of comparison the intermediate
HMO3 SEM images are shown in Figs. 6b and 7b. We see that HMO3 phases are
uniform is size, which leads to the formation of rather uniform sized LiMO3. This is
remarkable since the larger grain growth during the conventional solid state reaction
lowers the surface area of the material, which is disadvantageous for many catalytic
applications. LiMO3 prepared from HMO3 is quite uniform in size and shows clean
surfaces without any second phase. Accordingly, LiMO3 obtained from HMO3 by ion
exchange is considered to be phase-pure.
4. Conclusion
In summary, we have shown the reversible H /Li ion exchange reaction between
HMO3 and LiMO3 (M Nb, Ta) using molten LiNO3 at 3208C for 5 days. The
present method yields nearly uniformly sized crystallites of LiMO3 in contrast to
those prepared by conventional solid state synthesis. The present method is very
convenient and easily produces high purity ferroelectric LiMO3. It is also relatively
simple and inexpensive compared to other methods of preparation of high purity
LiMO3 including the sol±gel and Pechini methods.
Acknowledgments
We thank Mrs. Marlies Schwitzke for the SEM measurements, Mr. Thomas
Metzing for assisting in the FTIR measurements and Dr. W.F. Chu for the TGA