The occurrence of various types of calcium oxalate crystals was studied in 251 species and subspecies of Cactaceae to determine whether they are useful characters for Cactaceae systematics. Crystal hydration states were identified by X-ray powder diffraction and polarizing microscopy as monoclinic calcium oxalate monohydrate (COM) and tetragonal calcium oxalate dihydrate (COD). Ninety-eight percent of taxa studied contained either COM or COD crystals, or both. Different morphologies of crystals were further defined by light microscopy and scanning electron microscopy as druses, raphides, styloids (prisms), and crystal sand. In particular, the preponderance of one of the hydration states (COM or COD) was characteristic for certain Cactus subfamilies. Data showed that in Pereskioideae, Maihuenioideae, and Opuntioideae COM is predominant, while in Cactoideae COD prevails. In the remainder of Cactoideae, the crystals were quite variable. In tribe Hylocereeae, many species form both COM and COD as well. In the genera Hylocereus , Epiphyllum , Selenicereus , and Weberocereus , COM forms were almost exclusively represented by raphides together with different crystal forms in their epidermal cells. In the remainder of the Cactoideae, crystals did not follow any observable patterns. Development of crystallographic standards for identifying crystal forms microscopically is proposed for future crystal studies.
Benzophenone was the first organic molecular material to be identified as polymorphic. It is well known that benzophenone crystallizes in a stable orthorhombic alpha-form (m.p. 321 K) with space group P2(1)2(1)2(1) and a = 10.28, b = 12.12, c = 7.99 A, [Girdwood (1998). Ph.D. thesis. Strathclyde University, Glasgow, Scotland]. Here we report two separate structure determinations of the metastable beta-form (m.p. 297-299 K). Crystalline material of the metastable polymorph was obtained from a melt supercooled to approximately 243 K. The structure was determined from X-ray powder diffraction data by employing a novel, computational systematic search procedure to identify trial packing arrangements for subsequent refinement. Unit-cell and space-group information, determined from indexing the powder diffraction data, was used to define the search space. The structure was also determined from single-crystal diffraction data at room temperature and at 223 K. The metastable phase is monoclinic with space group C2/c and a = 16.22, b = 8.15, c = 16.33 A, beta = 112.91 degrees (at 223 K). The structures derived from the individual techniques are qualitatively the same. They are compared both with each other and with the stable polymorph and other benzophenone derivatives.
The scintillation properties of trans-stilbene crystals grown for the first time by application of the solution growth technique to the scale of 10 cm are reported. Measurements of the scintillation light output, pulse shape discrimination, and neutron detection efficiency were made with sets of crystals cut as 50 cm diameter cylinders of different lengths from 0.3 to 10 cm. Comparison to liquid scintillators and traditional melt-grown stilbene showed that at increasing sizes new solution-grown crystals exhibit better scintillation performance that makes them promising for use in large scale neutron detectors. Results are discussed in relation to structural imperfections attributed to different methods of growth.
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