Quarter glide planes

The onl other symmetry element involving translation which remains to be meniiqued is the glide plane having a translation of one-quarter of a eell-faee diagonal this type of glide, symbolized d, is found only in a few space-groups. 

In a similar way a glide plane normal to the c axis of a crystal having a translation of one-quarter of the ab diagonal (symbolized d) nullifies all hkO reflections except those for which A-j-A is divisible by 4. 

The effect of cation vacancies in alumina-rich samples is to change the slip plane from 111 to 110, with glide still in the (110) direction [140,147-149]). Dissociation into quarter partial dislocations is again observed but with a larger separation [147]. 

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