Electromechanical Properties of the Ferroelectric Crystal NaH₃(SeO₃)₂ near the Curie Point

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Elastic compliance constants (at constant electric field) s₁₁^E, s₂₂^E and s₃₃^E and electromechanical couplins factors k₁₂ and k₁₃ have been measured in NaH₃(SeO₃)₂ near the Curie point, −79°C. A new theory, which takes account of elastic nonlinearity or the third elastic constants, is proposed to explain the change of s^D (at constant electric displacement) below the Curie point. This theory is valid for most of the elastic anomalies in ferroelectric crystals.