We present a new interatomic potential capable of describing the cubic and tetragonal phases of zirconia. From an analysis of molecular-dynamics simulations of the structural fluctuations in the cubic phase close to the transition temperature, we show that the cubic-to-tetragonal transition is displacive. In addition, the discontinuous change in volume and the latent heat at the transition demonstrate clearly that the transition is first order. The model correctly predicts that doping with yttria tends to stabilize the cubic phase at lower temperatures. A description of the first-order nature of dopant-driven transitions is given.