Phase transitions crystallographic study

The interest in the structures of simple R2Si(OH)2 compounds lies in the fact that one of them, Bu 2Si(OH)2, forms a discotic liquid crystalline phase (308,309). Despite many attempts, it has not proved possible to obtain crystals of Bu 2Si(OH)2 suitable for a crystallographic study, the material obtained from various solvents usually being of a fine fibrous nature. The discotic phase of Bu 2Si(OH)2 has been proposed (309) to be due to the formation of dimeric disks of molecules which remain on breaking the interdimer hydrogen bonds in a structure of type 65 at the transition between crystal and mesophase. As has been described, structure type 65 is found for several diols similar to Bu 2Si(OH)2, and it is thus quite likely that Bu 2Si(OH)2 does indeed have the proposed structure. 

Abstract This chapter comments on the motivations and the methods of crystallographic studies at low temperature. Cry-crystallography is a brunch of Crystallography, a science that is too often confused with a technique. On the other hand, the scientific background to study crystal phases at low temperature is here provided, together with a survey of many possible techniques that provide complementary or supplementary information. Several apphcations are discussed, in particular in relation with highly accurate studies like electron density determination or phase transition mechanisms. 

The number of cryo-crystallographic studies on phase transitions is quite large nowadays. It became common with the availability of single crystal diffractometers equipped with digital area detectors that allow very rapid data collections. They are optimal for repeated measurements at variable temperatures of the same crystal in a reasonable amount of time. Thus, the accurate and detailed structural information typically available from a single crystal X-ray diffraction experiment could be... 

The foregoing sections have been devoted primarily to structural crystallography and phase-transition studies hereafter we discuss briefly the use of other crystallographic techniques in the study of low-dimensional organic conductors. Our goal is not an exhaustive review but simply to give some leads toward a different approach to these materials. For example, since crystallography is the science of crystals, two of the sections below are on sample size and quality. 

A Cs NMR study of CsTc04 has revealed the presence of 2 crystallographically inequivalent caesium sites of which the relative populations vary with temperature. The results provide information about changes in the crystal field potential in the vicinity of the cations accompanying a first-order phase transition from an orthorhombic to a tetragonal form at 389 K (Tarasov et al. 1992). 

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