Why is the difference between Cp and Cv often neglected for solids?

Specific heat at constant pressure signifies that external pressure is kept constant during whole heating process.The heat capacity at constant pressure C_p has more magnitude than that of heat capacity at constant volume. The Cp involves expansion of gas and increment of average temperature of gas. Thus, the supplied heat is diverted to some fraction in expansion work.
However, the heat capacity at constant volume reflects that total supplied heat is actually used in increasing the temperature of gas. Thus, no expansion wok is possible in heating a gas at constant volume. Thus the numerical value of C_p is greater than that of C_v which is shown below.
C_p – C_v = nR
Where n is the number of moles and R is the gas constant.
But in the case of solid, the intermolecular attractive forces between the molecules are very strong. Therefore no expansion in volume of solid is possible. Thus C_P and C_v becomes almost similar. That is why the difference between C_p and C_v often neglected for solids.