Detailed EPR and x-ray studies of the static critical behaviour in are reported. All measurements were performed on single crystals. Dilatometric experiments have allowed us to characterize the phase transition. It is accompanied by a contraction of the a and b parameters, which is consistent with a rotation of the octahedra around the c axis, and by an anomaly along the c direction. The first-order character of the structural phase transition has been demonstrated using EPR and diffractometric measurements. DSC experiments have allowed us to conclude that this discontinuity is weak. In addition, the interpretation of the phase transition in the continuous or quasi-continuous approach (i.e. for and ) has allowed us to conclude that this latter is essentially of a 2D character.