Slip systems rock salt

The rock-salt structure is shown in Fig. 6.13. In crystals having this structure, the smallest spacing between ions of the same type is along <110> and the most widely spaced planes with these closely packed directions are the 100 planes. Experimental observations confirm the slip direction as <110> but the slip planes are usually found to be the 110 planes. The systems for which slip is easiest are termed the primary slip systems and, thus, for rock-salt structures they are usually 110 < li0>. Slip may occur with greater difficulty on other systems and these are termed secondary slip systems. Slip does not occur on 100 planes because of the electrostatic interaction that occurs between the ions in this process. This is depicted in Fig. 6.14, in which the initial (a, c) and mid-shear (b, d) positions of the ions are shown. For slip on 100 planes (a to b), the distance between like ions is increased and between opposite ions, it is decreased. For slip... 

Slip plane behavior is less consistent. In MgO, the separate anion and cation closest-packed planes are 111, but the combined anion and cation closest-packed planes are 100 the observed 110 slip plane is explained in terms of the lower repulsion between like ions during slip on this plane compared with 100 [28]. In more complex structures, it becomes difficult to decide which is the closest-packed plane, and the simple-minded approach loses its value. In MgAl204, for example, stoichiometric crystals prefer the dose-packed 111 anion planes (see Figure 9.1), whereas nonstoichiometric crystals (Mg0 nAl203, > 1) slip on 110 planes, as in rock salt. On the other hand, garnets and rare-earth sesquioxides, in spite of the complexity of their structures, follow the dictates of the underlying bcc lattice and slip on the 110 111) system, as in bcc metals. 

The rock-salt structure is frequently found among the oxides of the alkaline earth metals (MgO, CaO, etc.) and the transition metals (MnO, FeO, CoO, NiO). In the latter case, the oxides are nonstoichiometric and should be written Mi -xO, due to the existence of trivalent ions and their charge-compensating cation vacancies. In all cases, the easy slip system is 110 (110) (as described in Sections 9.2.1 and 9.3.2). Slip on the 001 plane is much more difficult, and slip on 111 has never been reported. This is discussed below for MgO, followed by a discussion of the effect of nonstoichiometry in the transition metal oxides. 

Titanium carbide has had the rock salt structure assigned to it, e.g. LiF, NaCl, MgO, etc. with slip system assumed to be of 110. However, the primary slip system turns out to be the one indicated above, namely 111, rather than 110, which is the characteristic slip system in FCC structures [27]. 

The work in reference (16), demonstrating the hardening effect of radiation, introduces a note of caution in ascribing the same type of hardness anisotropy to crystals having the same slip systems, because there is reported data for all, even nonceramic, rock-salt materials NaCl and KCl, which show the hard direction on (001) to be [100], not [110]. These anomalous data emerge from very soft crystals, NaCl (1.91 GPa) and KCl (0.92 GPa), which do have IIOKIIO) slip systems some attempt at interpretation is made in Section 3.6.1. 

Figure 3.21. Reciprocal mean effective resolved shear stress curves for Knoop indentation calculamd for (001) planes of a rock-salt cubic crystal using (a) llOKllO), (b) OOlKlTO), (c) (IIIKIIO) slip systems.

Unlike the main minerals with the rock salt structure, fluorite cleaves on 111 planes and slips on 100 in the (0 11 direction [189]. Above 200 °C, slip on jl 10 (11 0) is also possible. Above 320 °C, polycrystalline plasticity is observed with the [10 0 planes providing three independent slip systems and the 110 planes giving two more [190]. Dislocations in fluorite have been studied by etch-pitting methods, and by TEM [191] but the latter is not without difficulty, since the mineral rapidly damages under electron and ion irradiation. A cubic symmetry void superlattice can be formed in TEM if the radiation flux is not restricted [192]. 

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