Question

What is meant by:
In ionic crystals, some of the atoms may be held together by covalent bonds to form ions having definite positions and orientations in crystal lattice.

Answer 1

This case you can see with partial ionic character, in which compounds have both ionic and covalent character.
Some of the ions in these compounds are connected by covalent compounds. For ex Copper sulphate in which the crystals have a regular position and orientation. The sulphate ion has sulphur atom which is bonded to oxygen through the covalent bond, leading to give proper orientation for the crystal.