The phase transition from seven-fold coordinated Zr in the monoclinic polymorph to eight-fold coordinated Zr in the tetragonal polymorph is understood to be a reconstructive phase transition of first order . In contrast, the transition between tetragonal and cubic polymorphs is thought to be a displacive phase transition of second order, which is allowed by the group-subgroup relation between the phases . However, a latent heat has been measured for this transition, suggesting that weak firstorder behaviour could be present . Furthermore, there are conflicting reports from computational studies with some suggesting it to be second-order and others suggesting it to be first-order .