Rock-salt crystals, strength

Surface cracks. Most solid substances have very numerous small cracks in their surfaces.6 The first evidence for this comes from a comparison of the actual strength of crystals with that deduced from theoretical considerations. In the case of the ionic lattice of sodium chloride the theoretical strength calculated from consideration of the electrostatic forces between the ions is of the order 200 kg. per sq. mm. actually dry crystals of rock salt can be broken at 0 4 kg. per sq. mm. If strained in air the deformation of rock salt is very small, before it breaks. It has long been known, to those who work in salt mines, that rock salt can be bent... 

Color centers in alkali halide crystals are based on a halide ion vacancy in the crystal lattice of rock-salt structure (Fig. 5.76). If a single electron is trapped at such a vacancy, its energy levels result in new absorption lines in the visible spectrum, broadened to bands by the interaction with phonons. Since these visible absorption bands, which are caused by the trapped electrons and which are absent in the spectrum of the ideal crystal lattice, make the crystal appear colored, these imperfections in the lattice are called F-centers (from the German word Farbe for color) [5.138]. These F-centers have very small oscillator strengths for electronic transitions, therefore they are not suited as active laser materials. 

Rochelle salt 48-9, 56, 570 rock(s) 821 analysis 394 crystal — see quartz salt, coloured 602 soap 581 ropes, strength of 61 rosacic acid 644 roselite 716 Rose s metal 659... 

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