Abstract
A phenomenological theory of the successive phase transitions at 560°C and 300°C in Ba2-NaNb5O15 is presented. The irreducible representation \\hatτ4 at k=0 and the representation T2 or T3 at k=( b1+ b2)/2 are associated with the modes responsible for the transitions D4h5→C4v2 and D4h5→D2h1 or D2h13, respectively. The space group C2v11 is composed of the symmetry elements common to C4v2 and D2h1 or D2h13. The bases for the above representations are taken as the transition parameters, and the elastic Gibbs energy is constructed as a function of the transition parameters, polarization and stress. When the coefficients of P32 term and ξ2+ζ2 term are assumed to depend linearly upon temperature, most of the temperature dependence of dielectric, elastic and piezoelectric properties are qualitatively explained, except for the anomaly of s66(T)P in the C4v phase.
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