Lattice vacancies are created when an equal number of cations and anions are missing from their correct lattice sites in an ionic crystal. What is this stoichiometric defect called?

Among the various materials related to electronics, the energy band gap is maximum in ________.

A cubic sample of edge length L is maintained at a temperature of 4 K. The speed of sound in the material of the sample is 5 × 103 m/s. The minimum value of L required to excite the lowest frequency phonon mode is closest to: