Metal excess defect is due to anionic vacancies. When crystals of an ionic solid like NaCl are heated in an atmosphere of sodium vapours, sodium atoms are deposited on the surface of the crystal. The Cl- ions diffuse to the surface of the crystal and combine with Na+ ions to form NaCl. The Na+ ions are formed by the’ loss of electrons. These release electrons diffuse into the crystals and occupies anionic sites. The anionic sites occupied by the electrons are known as P-centre. They impart yellow colour to the crystal