Crystal structures at low temperature

Evidence for charge ordering can be observed in crystal structures at low temperatures. Thus, in Lao.jCao.sMnC, the Mn4+ environment is nearly isotropic, with all the Mn-0 distances being nearly equal ( 1.92 A). In the Mn3+C>6 octahedra, one of the Mn—O bonds is much longer ( 2.07 A) than the other bonds ( 1.92 A), consistent with orbital ordering. It must be noted that the Mn-O distances in the ab plane of the manganates are much longer than in the c-direction,... 

Figure 7.31 (a) solid-state NMR lineshape of p-f-butylcalix[4]arene with included toluene-dg as a function of temperature. The dotted lines are the theoretical fit for static aromatic rings (129 K), twofold flips of the aromatic ring (179 K), and fourfold rotation (337 K). (b) Improved, ordered X-ray crystal structure at low temperature showing the different toluene orientations, pinched cavity shape and different t-butyl group orientations (reproduced by permission of The Royal Society of Chemistry). 

The problem arises with the atoms which scatter x-rays more strongly than water molecules. If La atoms account for the electron density peak at V in La-faujasite, there are only 3.3 atoms in a 32-fold site. Such low occupancy is hard to believe for a crystal structure at low temperature in the presence of water molecules. Is it possible that in faujasite and Y, polyvalent cations prefer to be hydrated and occupy the supercage, while Na cations occupy both the sodalite unit and the supercage Detailed x-ray studies of other cation forms than those of Table V would be valu-... 

Sm exhibits a rhombohedral crystal structure at low temperatures. In the nine layer sequence of close packed planes, two layers of atoms with a locally hexagonal environment alternate with one atomic layer in a locally cubic environment, so that the stacking pattern is of the form hhchhchhc. Hund s rules predict a ground state of the Sm ion with L = 5, 5 = and / = i with a small theoretical saturation moment of f / B/atom. 

In the case of lithium orthoniobate, Li3Nb04, no meta-stable phase was found that had a rock-salt crystal structure with disordered cation distribution [268]. Nevertheless, solid solutions Li2+xTii-4xNb3x03, where 0 < x < 0.22, have a monoclinic structure at low temperatures and undergo transformation to a disordered NaCl type structure at high temperatures [274]. 

The systematic principles of boron hydride structures ahd chemistry are the principal subjects of the present review. There are several reasons why these principles became clear such a long time after the discovery of these compounds (a) most of the compounds must be handled in grease-free vacuum line systems (b) some of the boron hydrides are unstable at ordinary temperatures, explosive on contact with air, and toxic (c) the structures are based on principles, still incompletely developed, of electron-deficient compounds and (d) location of the hydrogen atoms is a crucial part of the structure determinations, unlike the situation in hydrocarbons, and had to be done for the most part in X-ray diffraction studies of single crystals grown at low temperatures. 

Austenite has a mnch higher solubility for carbon than other forms of steel. Heating the steel to an anstenitizing temperatnre canses any carbides present to dissolve. Alloys capable of forming anstenite at high temperatnres, bnt that transform to other crystal structures at lower temperatures, are said to be hardenable by heat treatment. Martensitic steels are an example. Most carbon and low-alloy steels are hardenable by heat treatment. 

The commonest polymorphic changes of this type are those associated with the onset of free rotation in a crystal structure. Many simple molecular compounds (HC1, HBr and CH4 are examples) show a transition with rising temperature from a complex structure to a simple close-packed arrangement in which the molecules effectively acquire spherical symmetry by free rotation. Similar effects are displayed by a number of ionic crystals containing complex ions of un-symmetrical shape. Thus NaCN, KCN and RbCN have complex structures at low temperatures but transform at higher temperatures to the sodium chloride structure in which the CN ions behave as spherical entities. In some cases rotation may take place only about one axis, so that the molecule or group acquires cylindrical rather than spherical symmetry, and in other cases rotations about different axes may be excited successively at different temperatures. An extreme example of this is ammonium nitrate, in which both cation and anion are capable of... 

The compound C8H8Ru(CO)3 has the structure (1-LV) in the crystal, and at low temperatures (< —130°) in solution its nmr spectrum indicates the same structure since it implies the existence of four different environments for protons. At room temperature, however, the proton resonance spectrum consists of a single sharp line. A detailed study29 of the way in which the low-temperature spectrum collapses led to the conclusion that the rearrangement involves an infinite, reversible sequence of shifts of the type shown schematically in (1-LVI). 

For large negative crystal field at low temperatures the stable stmcture is the ideal two-sublattice in-plane herringbone phase—that is, the 2-in phase in Fig. 4a. The two sublattices which can be oriented in three different ways relative to the triangular lattice lead to six equivalent ground states. The excitation spectrum of this phase in general has a gap. In this phase the molecular wave functions are localized in the substrate plane, and classically the molecular axes are parallel to the surface see Appendix A of Ref. 141 for an interpretation of the order parameters. Thus, the orientational degeneracy of the pararotational phase is broken by the quadrupolar interactions. A closely related structure was already proposed based on atomistic Lennard-... 

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