(a) Describe the structure of lithium nitride and explain how it is able to function as a lithium ion conductor. The structures of Li₃P and Li₃As are analogous to that of the nitride. How do you expect the degree of ionic character in these compounds to vary?

(b) Epitaxial MgB₂ films can be grown from B₂H₆ and Mg vapour at temperatures up to 1030 K. Explain the meaning of ‘epitaxial’ and state what particular properties the films possess.