Metallic lattice

Lead has only one form, a cubic metallic lattice. Thus we can see the change from non-metal to metal in the physical structure of these elements, occurring with increasing atomic weight of the elements carbon, silicon, germanium, tin and lead. 

It is often difficult to represent inorganic compounds with the usual structure models because these structures are based on complex crystals space groups), aggregates, or metal lattices. Therefore, these compounds are represented by individual polyhedral coordination of the ligands such as the octahedron or tetrahedron Figure 2-124d). 

In the spring of 1989, it was announced that electrochemists at the University of Utah had produced a sustained nuclear fusion reaction at room temperature, using simple equipment available in any high school laboratory. The process, referred to as cold fusion, consists of loading deuterium into pieces of palladium metal by electrolysis of heavy water, E)20, thereby developing a sufficiently large density of deuterium nuclei in the metal lattice to cause fusion between these nuclei to occur. These results have proven extremely difficult to confirm (20,21). Neutrons usually have not been detected in cold fusion experiments, so that the D-D fusion reaction familiar to nuclear physicists does not seem to be the explanation for the experimental results, which typically involve the release of heat and sometimes gamma rays. 

Hydrides are compounds that contain hydrogen (qv) in a reduced or electron-rich state. Hydrides may be either simple binary compounds or complex ones. In the former, the negative hydrogen is bonded ionicaHy or covalendy to a metal, or is present as a soHd solution in the metal lattice. In the latter, which comprise a large group of chemical compounds, complex hydridic anions such as BH, A1H, and derivatives of these, exist. 

The most extensive group of nitrides are the metallic nitrides of general formulae MN, M2N, and M4N in which N atoms occupy some or all of the interstices in cubic or hep metal lattices (examples are in Table 11.1, p. 413). These compounds are usually opaque, very hard, chemically inert, refractory materials with metallic lustre and conductivity and sometimes having variable composition. Similarities with borides (p. 145) and carbides (p. 297) are notable. Typical mps (°C) are ... 

Finally, oxygen is soluble in metallic titanium up to a composition of TiOo.s with the oxygen atoms occupying octahedral sites in the hep metal lattice distinct phases that have been crystallographically characterized are TieO, TisO and Ti20. It seems likely that in all these reduced oxide phases there is extensive metal-metal bonding. 

The known halides of vanadium, niobium and tantalum, are listed in Table 22.6. These are illustrative of the trends within this group which have already been alluded to. Vanadium(V) is only represented at present by the fluoride, and even vanadium(IV) does not form the iodide, though all the halides of vanadium(III) and vanadium(II) are known. Niobium and tantalum, on the other hand, form all the halides in the high oxidation state, and are in fact unique (apart only from protactinium) in forming pentaiodides. However in the -t-4 state, tantalum fails to form a fluoride and neither metal produces a trifluoride. In still lower oxidation states, niobium and tantalum give a number of (frequently nonstoichiometric) cluster compounds which can be considered to involve fragments of the metal lattice. 

However, like the mp, bp and enthalpy of atomization, it also reflects the weaker cohesive forces in the metallic lattice since for Tc and Re, which have much stronger metallic bonding, the -t-2 state is of little importance and the occurrence of cluster compounds with M-M bonds is a dominant feature of rhenium(III) chemistry. The almost uniform slope of the plot for Tc presages the facile interconversion between oxidation states, observed for this element. 

For simplicity a cell consisting of two identical electrodes of silver immersed in silver nitrate solution will be considered first (Fig. 1.20a), i.e. Agi/AgNOj/Ag,. On open circuit each electrode will be at equilibrium, and the rate of transfer of silver ions from the metal lattice to the solution and from the solution to the metal lattice will be equal, i.e. the electrodes will be in a state of dynamic equilibrium. The rate of charge transfer, which may be regarded as either the rate of transfer of silver cations (positive charge) in one direction, or the transfer of electrons (negative charge) in the opposite direction, in an electrochemical reaction is the current I, so that for the equilibrium at electrode I... 

For such a condition of equilibrium to be reached, the atoms must acquire sufficient energy to permit their displacement at an appreciable rate. In the case of metal lattices, this energy can be provided by a suitable rise in temperature. In the application of coatings the diffusion process is arrested at a suitable stage when there is a considerable solute concentration gradient between the surface and the required depth of penetration. 

The anodic reaction consists of the passage of iron ions from the metallic lattice into solution, with the liberation of electrons, which are consumed at the cathode by reaction with water and oxygen. 

If the electrode potential of iron is made sufficiently negative, positively charged iron ions will not be able to leave the metallic lattice, i.e. cathodic protection. 

The depth of this potential minimum will play a part similar to that of the depth of the minimum in Fig. 8a. The energy represented by the vertical arrow in Fig. 9a is the work required to detach a positive atomic core from the surface of the metallic lattice and to leave it at rest in a vacuum. No name for this quantity has come into general use. We shall denote it by Y, c, corresponding to the D of Fig. 8a. 

There have been numerous studies with the objective of gaining an understanding of the factors that influence the stability, stoichiometry, and H-site occupation in hydride phases. Stability has been correlated with cell volume [7] or the size of the interstitial hole in the metal lattice [8] and the free energy of the a p phase conversion. This has been widely exploited to modulate hydride phase stability, as discussed in Sec. 7.2.2.1. 

If the metal atoms are not mobile (as is the case in low—temperature reactions) only hydride phases can result in which the metal lattice is structurally very similar to the starting intermetallic compound because the metal atoms are essentially frozen in place. In effect the system may be considered to be pseudo-binary as the metal atoms behave as a single component. 

LaNis has the CaCu, structure, space group P6/mmm [26] the hexagonal metal lattice is shown in Fig. 4. The crystal structure of LaNi5D7 has been determined 127, 28] and is illustrated in Fig. 5. There are three types of interstitial D sites La2Ni4 tetrahedra, and NiA tetrahedra. The unit cell is doubled along the oaxis because of the formation of a superlattice which is a consequence of long-range correlations between occupied and unoccupied... 

As has been shown by the X-ray diffraction method the parent metals (i.e. Pd or Ni), the a-phase, and /3-phase all have the same type of crystal lattice, namely face centered cubic of the NaCl type. However, the /9-phase exhibits a significant expansion of the lattice in comparison with the metal itself. Extensive X-ray structural studies of the Pd-H system have been carried out by Owen and Williams (14), and on the Ni-H system by Janko (8), Majchrzak (15), and Janko and Pielaszek (16). The relevant details arc to be found in the references cited. It should be emphasized here, however, that at moderate temperatures palladium and nickel hydrides have lattices of the NaCl type with parameters respectively 3.6% and 6% larger than those of the parent metals. Within the limits of the solid solution the metal lattice expands also with increased hydrogen concentration, but the lattice parameter does not depart significantly from that of the pure metal (for palladium at least up to about 100°C). 

Neutron diffraction studies have shown that in both systems Pd-H (17) and Ni-H (18) the hydrogen atoms during the process of hydride phase formation occupy octahedral positions inside the metal lattice. It is a process of ordering of the dissolved hydrogen in the a-solid solution leading to a hydride precipitation. In common with all other transition metal hydrides these also are of nonstoichiometric composition. As the respective atomic ratios of the components amount to approximately H/Me = 0.6, the hydrogen atoms thus occupy only some of the crystallographic positions available to them. 

Crystal Structure and Lattice Parameters (nm) Orthorhombic, a = 0.283, b = 0.554, c = 1.1470 Cr3C2 is an intermediate carbide having carbon chains with C-C distance approximately 0.165 nm running through distorted metal lattice where the Cr atoms are at the corners of trigonal prisms and the carbon atoms in the center of the prisms.i li" ... 

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