Herein nine tetragonal lithium garnets were synthesised; A3B2Li7O12 (A =La, Pr, Nd) (B = Zr, Hf), La3Zr1.75Ce0.25Li7O12 and LaSr2B2 Li7O12 (B = Nb, Ta). Both A and B site doping was undertaken to assess the relative importance of each site on the transition temperature. These materials were studied by variable temperature XRD analysis to ascertain if any new insights could be gained regarding the phase transition. Interestingly, we determine that, irrespective of A site substitution it is the B site which is the predominant factor in determining the phase transition temperature. It is shown that a direct relation solely to cell volume is too simplistic, rather it is suggested the sites which dictate the degree of tetragonality, which correspond to the framework polyhedra, are of higher importance than A site cations which reside within. Furthermore, the transition temperatures identified enabled regression analysis to predict the ideal octahedral B site radius for a room temperature stable cubic Li7 phase.