Pressure-coordination number rule

Generally, the following rules apply for pressure-induced phase transitions Pressure-coordination rule by A. Neuhaus with increasing pressure an increase of the coordination number takes place. 

Pressure-distance paradox by W. Kleber When the coordination number increases according to the previous rule, the interatomic distances also increase. 

Further examples where these rules are observed are as follows. Under pressure, some compounds with zinc blende structure, such as AlSb and GaSb, transform to modifications that correspond to the J3-Sn structure. Others, such as InAs, CdS, and CdSe, adopt the NaCl structure when compressed, and their atoms thus also attain coordination number 6. Graphite (c.n. 3, C-C distance 141.5 pm, density 2.26 gem-3) pr Te diamond (c.n. 4, C-C 154 pm, 3.51 gem-3). 

High-temperature modifications generally have lower coordination numbers, just as low-pressure modifications do. Structures of certain metals such as Zn, Cd and Hg do not conform to this pattern. Mercury has a rhombohedral structure in which each atom is six-coordinated following the (8 — N) rule indicating the presence of the covalent... 

While radius ratio rules generally predict correct structure types for the alkali halides in only about 50% of the cases, the MEG calculations (not even shell stabilized) correctly predict the NaCl structure to be more stable for 15 of the 16 compounds studied. Calculated structural parameters were also in good agreement with experiment, and calculated pressures for the NaCl CsCl transition were in fair agreement with the limited experimental data available. These results indicate that preferred coordination numbers and structural types in alkali halides can be accu-... 

The most useful rule in describing the effect of pressure on solids is the so-called Pressure-Coordination Rule (i, 2) according to which the coordination number is increased with pressure. In Table 1 and 2 examples are listed for various crystal structure transformations which follow this qualitative mle at different pressures and temperatures. An exception to this mle is known, however, for ytterbium (2) the cubic face-centered modification (coordination number = 12) of the metal is transformed at 40 kbar into a cubic space-centered stmcture (coordination number = 8). 

The changes in volume, electrical conductivity, and optical properties that accompany these polymorphic transitions of silica are discussed by Eitel [1952], who also critically reviews the influence of such features as heat pretreatment, pressure, and contaminants on the characteristics of these inversions. The polymorphic transitions of silica may be very slow or may occur without appreciable evidence of inhibition. An empirical rule that crystal phases with lower coordination numbers are more stable at high temperatures than those with higher coordination numbers was postulated by Goldschmidt [1926]. 

In the case of a unary or one-component system, only temperature and pressure may be varied, so the coordinates of unary phase diagrams are pressure and temperature. In a typical unary diagram, as shown in Figure 3.11, the temperature is chosen as the horizontal axis by convention, although in binary diagrams temperature is chosen as the vertical axis. However, for a one-component system, the phase rule becomes F=l-P+2 = 3-P. This means that the maximum number of phases in equilibrium is three when F equals zero. This is illustrated in Figure 3.11 which has three areas, i.e., solid, liquid, and vapour In any... 

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