Crystallographic modification

Uranium exhibits three crystallographic modifications as follows alpha —(688C)—> beta —(776C)—> gamma. Uranium is a heavy, silvery-white metal which is pyrophoric when finely divided. 

White crystalline solid occurs in five different crystallographic modifications as follows ... 

Fig. 10. The (r ) /R (R interatomic distance) ratio for different crystallographic modifications of actinide metals is plotted vs atomic number Z. The ratio is a rough measure of f-f overlapping. In the figure, the value for a 3 d-metal (Fe) is given for comparison (from )...

The monoclinic structure encountered in Am at high pressures is a low symmetry crystallographic form this is a clear indication that the 5 f orbitals have become bonding in the solid. The formation in compression of lower symmetry crystallographic modifications occurs also in the heavier actinides, as we may see from Figs. 1 and 2. 

Figure 28. Topochemical rules are based on the photochemical behavior of cinnamic adds in three crystallographic modifications.

The first metal-xenon compound with direct Au—Xe bonds has been reported by Seidel and Seppelt1027-1029 [Eq. (4.252)]. The salt crystallizes in two crystallographic modifications differing in cation-anion interactions. In each form the cation is a regular square. In the triclinic modification the Au—Xe bond lengths are between 2.7330 A and 2.7779 A and there are three weak interionic contacts (Au—F... 

FIGURE 51. Crystallographic modification of the Cu3(p,3-I)(p,-I)2 core units after single-crystal to single-crystal 41 >41 transformation induced by evaporation of coordinated MeCN. 

The variation of the enthalpies of formation of the trichlorides, tribromides and triiodides are shown in fig. 24. The general patterns is the same as observed for the trifluorides. Also for the trichlorides, tribromides and triiodides the variation of the quantity AfH°(LnX3, cr, 298.15 K) - AfH°(Ln3+, aq, 298.15 K) with the ionic radius has been examined, as shown in figs. 25 to 27. The results for the trichlorides clearly show a difference between the two crystallographic modifications, but for the tribromides and triiodides a difference is not very evident. The trends shown in figs. 25 to 27 have been used to estimate the enthalpies of formation of those compounds for which no or no reliable experimental data are available. 

Berkelium metal exhibits two stable crystallographic modifications, double hexagonal closest packed (dhcp) and face-centered cubic (fee). Thus it is isostructural with the two preceding elements, all of which exhibit the fee structure at high temperature. The room-temperature lattice constants of the dhcp form are ao = 0.3416 0.0003 nm and c0 = 1.1069 0.0007 nm, yielding a calculated density of 1.478 x 104 kg/m3 and a metallic radius (CN = 12) of 0.170 nm (119). The room-temperature fee lattice parameter is a0 = 0.4997 0.0004 nm from which the... 

Active aluminas are much more widely used in catalytic applications than silicas, since they are not only excellent support materials, but also very active as catalysts in their own right for several applications. However, the chemistry of aluminas is much more complicated than that of silica, since many crystallographic modifications exist, only few of which are useful as catalysts. 

In contrast to Haber and co-workers [4-6, 38], Centi et al. [98, 99] observed the spreading of V2O5 not only on anatase but also on the rutile modification. Based on their results, Haber and co-workers [4-6, 38] proposed that spreading was dependent on the crystallographic modification of the support and on the type of crystal plane. 

When the CO disproportionation is catalyzed by cobalt, some ordered metastable structures are detected inside the active metal nanoparticles after the reaction. These structures are regular thin (approximately 5 atoms in thickness) alternating cobalt layers of different crystallographic modifications (Figure 4.17). Note that the appearance of such structures at thermodynamically equilibrium states of the catalyst substance is contrary to the Gibbs phase rule for the phase equilibria in solids. Thus, the metastable layered structures may be considered an analogue of spatial dissipative structures. 

Tris(acetylacetonato)gallium(III) exists in two crystallographic modifications, differing slightly in density. The X-ray crystal-structure determination shows the central GaOe core to be close to octahedral, with an average Ga-0 distance of 1.95 A. 

Copper(n) formate forms tetra- and dihydrates, each of which loses water to yield different crystallographic modifications of the anhydrous salt. Some thermodynamic [143] and structural [144] properties of these (and other related) compounds have been given. The dihydrate was not formed [144] by the partial removal of water fi om the tetrahydrate. 

Three different crystallographic modifications of the anhydrous salt can be prepared. The decompositions of these reactants (430 to 506 K) exhibit sigmoid a-time curves which have been attributed [11] to a chain-type process through the formation of the volatile and unstable copper(I) salt as intermediate. This was indicated by the formation of a copper mirror on the containing vessel. Schuffenecker et al. [11] identified the rate limiting step as electron transfer. Erofeev et al. [10,12] showed that the kinetic behaviours of two forms of the salt were different. This was ascribed to variations in the dispositions of copper ions in the crystal structures of the reactants. 

In this case one of the components exists in two or more crystallographic modifications and the temperature of the highest polymorphic transformation lies above the eutectic temperature of the system. 

Solids The most stable crystallographic modification at a pressure of 101 325 Pa... 

Enthalpy of polymorphic transformation is the heat connected with the transition of the compound from one crystallographic modification to the other. 

For further exploring the possible occurence of a crystallographic modification in the mechanical mixture during the reaction, in particular the possible formation of mixed Bi-Mo-O or Bi-Mo-P-0 phases, a systematical critical investigation was carried out on the high resolution XRD pattern (noise level = 0.1% of the intensity of the... 

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