Critical Behavior in the Spontaneous Polarization of Ferroelectric Tri-Glycine Sulfate

The spontaneous polarization P ( t ) of tri-glycine sulfate was measured in a range 90 K to 323 K by the pyroelectric and the D - E loop method. A non-classical critical-exponent, β=0.45±0.02 was observed in the region -9.3<ln(- t )<-6.8, where t ≡( T - T C )/ T C and T C ≃322 K. Mean field analyses taking account of electroelastic coupling revealed that a distinct deviation from the classical behavior of P 0 ( t ) exists in the region ln (\(-t){}-5\). Normalization [ P ( t )/ P 0 ( t )] 3 can be approximately expressed by the correction term L ·ψ( L ), where L =1+3 b ln t 0 /2| t | and ψ( L ) is the term for free crystals. In the region -8.3<ln (- t )<-6.1, b =0.027and t 0 =0.009 are obtained. The critical amplitude and region, and the correlation lengths were estimated according to Nattermann.