Entropy change for molecular nitrogen

Calculate specific entropy change for $N_2$ from 250 K and 1000 kPa to 1300 K and 60 kPa using $Delta s=phi_2-phi_1-Rln (frac{p_2}{p_1})$ given that $phi_1=6.6587 kJ/kg/K$ and $phi_2=8.4604 kJ/kg/K$.

My question is what the physical meaning of $phi_1$ and $phi_2$ is.