Crystal structures differences

The calcium ion is of such a size that it may enter 6-fold coordination to produce the rhombohedral carbonate, calcite, or it may enter 9-fold coordination to form the orthorhombic carbonate, aragonite. Cations larger than Ca2+, e.g., Sr2+, Ba2+, Pb2+, and Ra2 only form orthorhombic carbonates (at earth surface conditions) which are not, of course, isomorphous with calcite. Therefore these cations are incapable of isomorphous substitution in calcite, but may participate in isodimorphous or "forced isomorphous" substitution (21). Isodimorphous substitution occurs when an ion "adapts" to a crystal structure different from its own by occupying the lattice site of the appropriate major ion in that structure. For example, Sr2+ may substitute for Ca2 in the rhombohedral lattice of calcite even though SrC03, strontianite, forms an orthorhombic lattice. Note that the coordination of Sr2 to the carbonate groups in each of these structures is quite different. Very limited miscibility normally characterizes such substitution. 

This polymer has a crystal structure different from the normal hexagonal poly(thioformaldehyde). At 153° C it changes to the hexagonal configuration. Credali and Russo (8) state that this new form ran be heat-stabilized by acetylation of its end groups and that the acetylated polymer does not degrade at 250-260° C, a temperature range at which ordinary poly(thioformaldehyde) is very unstable. 

It should be thermodynamically impossible for one set of Kvst charts to serve both hydrate structures (si and sll), due to different energies of formation. That is, the Kysi at a given temperature for methane in a mixture of si formers cannot be the same as that for methane in a mixture of sll formers because the crystal structures differ dramatically. Different crystal structures result in different xst values that are the denominator of Kvst (= yt/xSi). However, the Katz Kvst charts do not allow for this possibility because they were generated before the two crystal structures were known. The inaccuracy may be lessened because, in addition to the major component methane, most natural gases contain small amounts of components such as ethane, propane, and isobutane, which cause sll to predominate in production/transportation/processing applications. 

Conversions between the modifications quartz, tridymite and cristobalite progress only slowly because their crystal structures differ relatively much. The conversion between the a and the 6 form of the same modification on the other hand takes place rather fast because the differences in structure are relatively small. All in all this means that transitions from one modification to another are only possible when they are slowly heated or cooled. This is the only way in which the building blocks are offered the possibility to regroup to form a new crystal structure. By heating or cooling quickly it is possible to skip certain modifications. As you can see in figure 6.6 the transition from a to B quartz at standard pressure takes place at 573 °C. 

As will be shown later, these crystal structures differ considerably from the V-amyloses. 

Interstitial Solubility. The y phase, as shown in Figure 5, could be observed by x-ray diffraction methods in the pattern of a sample which contained as low as 2 atomic % deuterium, and metallographically in samples which contained still lesser amounts. The fact that a zirconium deuteride with a crystal structure different from that of the metal is formed at a very low concentration of deuterium indicates that the solubility of deuterium in the metal at room temperature is very small, if it exists at all. The reason seems to be this A hexagonal unit cell provides one octahedral and two tetrahedral voids for each interlayer translation, as given in Table V. 

C ahn and Hanneman 11341 and Finn and Gatos 11351 observed that thin annealed wafers of several 111-V compounds, like GaAs, bend spontaneously. This effect can be attributed to a difference in surface stress of the two faces. The III-V compounds have, due to their crystal structure, different opposite faces. If. for instance, the surface stress of the top side is smaller than that of the bottom side, the wafer bends downwards. The degree of bending is related to the difference in surface stress AT by Stoney s formula 136 ... 

With respect to the first requirement, a polycrystalline metal as ordinarily used exposes at die surface many different types of crystal structure (different crystal faces, edges, corners, and boundaries between crystals). Each type of structure has its special chemical properties. Measurements made on ordinary polycrystalline material are a composite quantity which may be useful for technological purposes but which give little information for an understanding of the basic process of oxidation. It is not yet generally appreciated that for thin oxide films die differences in rates and structures on the different crystal faces are under many conditions quite large, as indicated below. 

Solid and volatile organomagnesium compounds can be slowly sublimed in a static high vacuum. For instance, we used this technique recently to obtain crystals of ether-free diphenylzinc. Since only a few organomagnesium compounds can be sublimed, the perspectives of this approach are limited. We attempted the sublimation of dineopentylmag-nesium, which will surely have a crystal structure different from that in the gas phase. Unfortunately, only low quality crystals have been obtained so far. 

The extent of the reactions indicated by Equations 1 and 2 or the molar sulfate-to-sulfur ratio is 2.4 zb 0.2 when rock pyrite is used and 1.4 zb 0.4 for sedimentary pyrite found in the coals used in this work. Although both materials are FeS2 of the same crystal structure, differences in reacivity have been documented which are attributed to impurities and crystal defects peculiar to the various possible modes of formation (7). For coal, no significant variation in this ratio was found with ferric ion concentration, acid concentration, coal, or reaction time. The results for each coal are found in Table II. 

The observation of compensation behaviour for the decompositions of a group of comparable reactants under similar conditions is ambiguous because such behaviour could be explained either (following the approach used in with heterogeneous catalysis [59]) as the result of relatively small crystal structural differences in the rate determining bond rupture step, or (as above) by differences in the physical conditions existing within each individual reactant sample. 

The radius of gyration of the nucleosome is 4.0 to 4.5 nm as determined by neutron scattering 26.37,38) x-ray scattering The model proposed for the nucleosome structure based on the histone octamer crystal structure differs from that resulting from the crystal structure studies of the nucleosome core particle This controversywill be resolved when higher resolution structures become available. 

Only a limited amount of information is available on the magnetic properties of ternary R-Fe-B or R-Co-B compounds having compositions and crystal structures different from Nd2Fe14B. Compounds of the composition RFe4B were studied... 

Alloying with other metals can modify crystal structure. Different techniques, e.g. low energy ion scatterings and HRTEM have been used to determine segregation in small particles. We show here the information obtained by HRTEM imaging at the level of the interface. 

Both silver and gold form ideal solid solutions with palladium. However, stoichiometric compositions with unique properties, such as in Hunter s preferred membrane composition of PdsAg, [31], might suggest the possibility of intermetallic compounds or ordered structures differing from that of the ideal solutions [35]. Palladium and copper also form ideal solid solutions, but in this system phase diagrams clearly show additional phases with crystal structures differing from the parent fee phase of the solid solutions. 

A mineral species of calcium carbonate (CaC03) with a crystal structure different from that of vaterite and calcite, which are the other two forms of CaC03. It is precipitated from ocean surface waters mainly by organisms (e.g., coral) that use it to make their shells and skeletons. 

Two aspects of the crystal chemistry of natural and synthetic apatites need to be recognized. First, the HA in bone is nonstoichiometric, has a Ca/P ratio of less than 1.67, and contains carbonate ions, sodium, magnesium, fluorine, and chlorine (Posner, 1985a). Second, most synthetic hydroxyapatites actually contain substitutions for the phosphate and/or hydroxyl groups and vary tom die ideal stoichiometry and Ca/P ratios. Oxyhydroxyapatite [Ca,o(P04) 0], a-tricalcium phosphate (a-TCP), )3-tricalcium phosphate O-TCP) or )8-Whitlockite [Caj(P04)J, tetracalcium phosphate (Ca4P209), and octocalcium phosphate [Cag(HP04)2(P04)4 5H20] have all been detected via x-ray diffraction (XRD), Fourier transform into spectroscopy (FITR), and chemical analyses (Kohn and Ducheyne, 1992 Ducheyne et al., 1986, 1990 Koch et al., 1990). These compounds are not apatites per se since the crystal structure differs from that of actual apatite. 

Despite very clear structural motifs based on four-connected atoms within the frameworks, the clathrates seldom have completely ordered crystal structures. Different kinds of crystallographic disorder appear including defects in the framework and in the cages, mixed occupation by different elements, and positional disorder of the so-formed sub-sites. The resulting defect or substitution variants tend to form superstructures by ordering of defects or different atoms on the substitution positions. Most of these effects have already been studied on representatives of the clathrate-I type, thus some such examples are given below. 

The iron is located between helix 9 and 18 at the centre of domain II. The two currently available crystal structures differ in the iron coordination [158] describes... 

The density of a substance depends on its physical state. Solids are usually more dense than liquids (ice and water are obvious exceptions). Thus, the density depends on the type of packing of molecules in a crystal or a liquid. If the density is measured in solution, the relation of the molecules of the solvent to the molecule of solute may influence density. In Table III are recorded densities obtained by several different methods. The variable values reflect not only technical problems of measuring density, but also the differences obtained when crystal structure differs and when the molecule is in solution. 

If polyacetylene is oxidized by either of the methods described in the proceeding section, the reaction product has a crystal structure different from the pristine polymer. This was first observed for the oxidation by treatment with iodine, AsFc-etc. by R. H. Baughman et al. The diffraction patterns of electrochemically oxidized poly(acetylene) containing AsFg, CIO, and SbF as the counterions were investigated by Monkenbusch et al. Further work on the diffraction pattern of iodinated polyacetylene is available from a number of other sources, especially from Shimamura et al. on the diffraction from oriented samples. 

A classic example of an alternating-type copolymer is found in the ethylene-carbon monoxide copolymer [34-39]. This copolymer is polymorphic. The a form is stable at low temperature and transforms to the P form at 140 °C. The melting temperature of this form is about 255 °C. This temperature is much greater than that for linear polyethylene or any of its random copolymers. This again is the result of a crystal structure different from that of polyethylene that accompanies the high extent of alternation. 

The steroids can be analyzed as solutions in solvents such as carbon disulfide and chloroform, or as suspensions in mineral oil. The latter technique suffers from the disadvantage that crystal structure differences may alter the results. The former technique is difficult in the case of the free alcohol form of these compounds because of their particularly low solubility in the solvents mentioned. This disadvantage may be partially overcome by using the so-called double beam infrared spectrometer which tends to cancel out backgroimd absorption due to solvent. The... 

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