A Phenomenological Theory of the Phase Transition in Ba_2NaNb_5O_<15>

概要

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A phenomenological theory of the successive phase transitions at 560℃ and 300℃ in Ba_2-NaNb_5O_<15> is presented. The irreducible representation τ^^^^4 at k=0 and the representation T_2 or T_3 at k=(b_1+b_2)/2 are associated with the modes responsible for the transitions D^5_<4h>→C^2_<4v> and D^5_<4h>→D^1_<2h> or D^<13>_<2h>, respectively. The space group C^<11>_<2v> is composed of the symmetry elements common to C^2_<4v> and D^1_<2h> or D^<13>_<2h>. 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, rolarization and stress. When the coefficients of P^2_3 term and ξ^2+ξ^2 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 s^p_<66(T)> in the C_<4v> phase.
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1974-07-05