`Li^(o+)` ion has very small size and has an exceptionally high charge to radius ratio and therefore salts of `Li^(o+)` ion with small anions viz. `H^(Θ)` , `F^(Θ)` and `N^(3-)` are highly stable due to their high lattice energy.
`U prop (1)/(r^(o+)+r_(Θ))`
Thus LiH, LiF and `Li_(3)N` show exceptionally high thermal stability.