Crystalline solids have an ordered internal arrangement of atoms due to which they…

Crystalline solids have an ordered internal arrangement of atoms due to which they exhibit symmetry. The atoms are arranged in a symmetrical fashion in the three-dimensional network known as space lattice. Each point in a crystal lattice is known as lattice point or lattice site.
Each point in a crystal lattice represents a constituent particle which can be an atom, ion or a molecule. The smallest repeating subunit in the lattice is known as unit cell. The unit cells are described as simple (points at all the corners), body-centered (points at all the corners and in the center), face-centered (points at all the corners and center of all faces), and end-centered (points at all the corners and centers of two opposite end faces) unit cells. For the stable ionic crystalline structures, the radius ratio limit for a cation to fit perfectly in the lattice of anions is determined by the radius ratio rule. This also defines the number of nearest neighbours of opposite charges surrounding the ion, which is known as the coordination number. This depends upon the ratio of radii of two types of ions, r⁺/r^–. The radius ratios for coordination numbers 3, 4, 6 and 8 are 0.155 – 0.225–0.414, 0.414–0.732 and 0.732–1, respectively. The coordination number of ionic solids increases with increase in pressure and decreases with increase in temperature.
 
The ionic radii of Na⁺, K⁺ and Br^– are 137, 148 and 195 pm, respectively. The coordinaiton number of cation in KBr and NaBr structures are, respectively,