Crystalline system Trigonal

Primitive three-dimensional lattices have been classified into seven crystalline systems triclinic, monoclinic, orthorombic, tetragonal, cubic, trigonal, and hexagonal. They are different in the relative lengths of the basis vectors as well as in the angles they form. An additional seven nonprimitive lattices, belonging to the same crystalline systems, are added to the seven primitive lattices, which thus completes the set of all conceivable lattices in ordinary space. These 14 different types of lattices are known as Bravais lattices (Figure 3). 

Binary Systems and Related Compounds.—Halides. The thermodynamics of gas-phase equilibria in the W-F2 and W-F2-H2 systems at high temperatures have been described.The Raman spectrum of solid MoF exhibits Mo—F stretching bands at 746, 722, and 690 cm These results suggest that the compound has a similar structure to NbF4, with each molybdenum co-ordinated to six fluorine atoms.The Raman spectrum of crystalline M0F5 has also been reported and interpreted in terms of the crystal structure.The electronic spectrum of liquid M0F5 has been determined and shown to be consistent with a trigonal-bipyramidal molecular unit. ... 

Crystalline form,—Lithium nitrate crystallizes in rhombohedra (trigonal system).15 P. W. Bridgman observed no new form of lithium nitrate between 20° and 200°, and press, between 1 and 12,000 kgrms. per sp. cm. The older authorities—e.g. P. Kremers—supposed this salt to be trimorphic, but the supposed polymorphism is probably due to their mistaking hydrates for polymers of the anhydrous salt. It is doubtful if lithium nitrate is isomorphous with silver or sodium nitrate, although J. W. Retgers says that sodium and lithium nitrates are isomorphous. 

Crystalline Tellurium. — Molten tellurium solidifies to a brittle, silvery, crystalline mass, which is easily powdered. The crystalline modification can also be obtained by sublimation of the element or by its slow formation, for example in the gradual decomposition of hydrogen telluride5 or in the slow atmospheric oxidation of an aqueous solution of an alkali telluride.6 When obtained of appreciable size the crystals are generally found to be prismatic, of the trigonal system, and isomorphous with metallic selenium (a c=l 1-3298 a=86-8°).7... 

One example of a concrete system where one observes optical spectrum caused by the Ai-E electronic transition is the N-V center in diamond. This center consists of a substitutional N atom and three nearest C atoms (one of the nearest C atoms is replaced by the vacancy) and it has a trigonal symmetry. The ZPL line at 637 nm of this center corresponds to the electronic transition between the triplet 3A and the 3E electronic states. In the standard model of this center the electronic states of the center come from the occupation and the splitting of the aj and t2 levels arising from three C radicals. The crystalline field of a trigonal symmetry splits the t2 level into a number of states including the ground (Aj) state and the first excited E-state (see, e.g. Refs. [17-25]). Our experimental study of the optical transition between the E and the Aj electronic states indeed showed the 7 3 dependence of the ZPL width at low temperatures. 

Porter, M. W. and Spiller, R. C. (1951). Crystals of the tetragonal, hexagonal, trigonal and orthorhombic systems. In The Barker index of crystals. A method for the identification of crystalline substances, Vol. 1, W. Heffer Sons, Cambridge. [14, 95] Porter, M. W. and Spiller, R. C. (1956). Crystals of monoclinic system. In The Barker index of crystals. A method for the identification of crystalline substances, Vol. 2, W. Heffer Sons, Cambridge. [14]... 

Amorphous chromium sesquioxide is a green powder, the tint of which depends upon the method of preparation the colour becomes brownish on heating. The crystalline oxide forms very dark green hard crystals belonging to the trigonal system (ditrigonal scaleno-hedral) ... 

The only reference to a tris-bidentate species is the compound (221), which is reported to be a green crystalline compound with /ies = 4.85 BM and g = 2.08. Many other [MnNj] compounds should therefore be accessible. One interesting recent example is the Mn" compound of the ligand (223). This molecule lies on a crystallographic three-fold axis is nearly octahedral [MnN ], but with a significant twist towards the eclipsed, trigonal prism, with Mn—N (pyrrole) 2.054 and (azometh-ine) 2.148(2) A and is the first example of Mn " or any d system, with a crossover from the high spin S = 2 state to the low spin S = 1 state at 40-50 K. ... 

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