(i) Schottky defect
(ii) Density of the crystal decreases.
(iii) Crystal having
(a) high coordination number and
(b) ions (cations and anions)of almost similar sizes.
For example: KCI, KBr. 

When crystals of alkali metal halides such as NaCI are heated in the atmosphere of sodium vapour, the  excess of sodium atoms are deposited on the surface of the crystal. The CI^- ions diffuse to the surface and combine with sodium atoms to form NaCI. This happens by the loss electrons by Na atoms to form the visible light falls on the crystal and occupy anionic sites. As a result, the structure has an excess of sodium.
The crystals acquired yellow colour because when visible light falls on the crystals, they absorb energy from the visible region to excite the electrons. Hence, they are coloured. Similarly, excess of Li makes LiCI crystals pink and excess of K makes KCI crystals viloet.

Non-stoichiometric compounds: Non-stoichiometric compounds are those in which the ratio of the number of positive ions and the negative ions does not correspond to the simple whole number as suggested by the formula. There is a large number of non-stoichiometric ionic solids which contain an excess or deficiency of one of the components. As a result, the crystal structure becomes irregular and this leads to the presence of a defect. Such defects are also called non-stoichiometric defects.
For example, vanadium oxide has the formula VO_x where x lies between 0.6 and 1.3. Similarly, iron (II) oxide samples contain more oxygen atoms than iron atoms while samples contain more oxygen atoms than iron atoms while samples of zinc oxides, ZnO usually has excess of zinc atoms than oxygen atoms. However, it may be noted that in each case, the electrical neutrality of the crystal is maintained.

When imperfections or defects in a crystal are caused by the departure from the definite arrangement in the vicinity of an atom or a group of atoms, the imperfections are called point defects. The imperfections are caused either by missing or dislocations of a constituent particle to a position meant for another particle or shifting to an interstitial position.
Point defects are of two common types:
(i) Schottky defect
(ii) Frenkel defect.
For Schottky and Frenkel defects.

Structure of alkali metal halides: At ordinary temperature and pressure,halides of alkali metals (iodides, bromides and chlorides of lithium, sodium, potassium and rubidium) posses NacI type structure with 6:6 coordination.
When high pressure is applied, the coordination number increases and the alkali metal halides transform to CsCI structure with coordination number 8:8.

(i) Dislocation: Lattice imperfections which are extended along lines are called dislocations or line defects.
(ii) F-centre: The electrons trapped in anion vacancies are called F-centres.
(iii) Anti-ferroelectric: If in a piezoelectric crystal, the dipoles in alternative polyhedra pointing up and down so that there is no net dipole moment, the crystal is reffered to as anti-ferroelectric. For example, PbZrO₃ (lead zirconate).

(i) When there is spontaneous alignment of magnetic moments of domains in the same direction, we get ferromagnetism. These have strong magnetic effects and ordering of domains persists even when magnetic field is removed and ferromagnetic substance becomes permanent magnet.
(ii) When the magnetic moments of the domains are aligned in parallel or anti-parallel directions in unequal numbers resulting in net magnetic moment, we get ferromagnetism.

Physical properties such as electrical conductivity, thermal conductivity, refractivity index etc. show different values when measured along different directions in the same crystal. This phenomenon is called anisotropy. It arises because of different arrangement of particles in different directions. quartz shows this property because it is crystalline.

(i) Ferrites are magnetic material. 
(ii) They are used in telephones and memory loops in computers.

(i) Piezoelectric crystals behaves as mechanical electrical transducer. 
(ii) Piezoelectric crystals are used as pickups in record players where they produce electric signals by application of pressure. 
(iii) We generally become tense and pessimistic under stress. But we should face every situation, accept every worst situation as challenge and maintain our mental balance. It will definitely result in our performance just like in piezoelectric crystals. We can learn from these non-living crystals to be optimistic as worst situations can be converted into an opportunity.