The ninefold superstructure of SC(NHz) 2 was determined by single-crystal X-ray diffractometry at 170 K; orthorhombic, Pbnm, a = 5.467 (1), b = 7.545 (1), c=76.867(I1) A, V=3171.1A 3, Z=36, Dx= 1.435 Mg m -3, Cu K~t, 2 = 1.5418 A, ~ = 5.89 mm-', R=0.048 for 1510 independent reflections. The commensurate phase is stable between the ferroelectric phase (I) and the incommensurate phase (II), in a temperature range of about 2 K. The structure is characterized by a rotation of the SC(NH2) 2 molecule along the c axis coupled with a displacement of the center of mass in a plane perpendicular to the axis. Around mirror planes (z = ¼ or 2) the local structure is isostructural with phase I. Therefore the superstructure is constructed of alternately polarized layers which are sandwiched by domain walls (or discommensurations) whose local structure is that of the paraelectric room-temperature phase (V) around z = 0 and ½. AbstractTwo recent attempts to derive quantitative energy relationships from statistical analysis of structural parameters observed in different crystal environments are examined. Both are based on the assumption of a Boltzmann-like distribution for the probability of a 0108-7681/88/040445-04503.00 structure being observed in a deformed state. This assumption is shown to be untenable. An alternative model taking explicit account of the perturbing forces responsible for structural deformation is then considered. Although low-energy regions of molecular potential energy surfaces can certainly be recognized and mapped from distributions of observed structures,

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kind of molecular properties of water: one is to disturb crystallization, while other is to promote crystallization. The competition between them can generate a pure amorphous state. Furthermore, lattice relaxation occurs around the boundary between crystal domains and the amorphous solid, since sharp Bragg peaks are observed at any phases.

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