A Structural Study on the Rochelle Salt

The crystal structure determination of Rochelle salt [KNa(C4H4O6)·4 H2O] at 213, 274, and 323 K has been carried out. Crystals at 213 K are orthorhombic,P21212, witha=11.880(3),b=14.298(3),c=6.216(2)A,Z=4,R(F)=0.040. At 274 K crystals are monoclinic,P21, witha=11.869(3),b=14.316(8),c=6.223(3)A,α=89.26(4)°,Z=4,R(F)=0.072. At 323 K they are orthorhombic,P21212, witha=11.924(3),b=14.298(3),c=6.230(2)A.Z=4,R(F)=0.073. Two kinds of polarized chain in the ferroelectric phase (274 K) are observed, one similar to those found at the high-temperature paraelectric phase (323 K) and the other similar to those found at the low-temperature paraelectric phase (213 K). The paraelectric crystal structures do not show disorder or anisotropic thermal parameters which justify the hypothesis that the paraelectric–ferroelectric transition was of the order–disorder type. A study of powder diffraction, the momentum of inertia of tartrate ions, and the librational and translation tensor confirms that the transition is second order, and the independence of the two kinds of chains. A structural and thermodynamic model is proposed from these results. The ferroelectric phase is an “ordered” solid solution of two kinds of chains, which is explained from a regular model, according to which the ΔHbetween the two paraelectric phases is 485 cm−1andwfor the solid solution is 152 cm−1.