High resolution powder diffraction data will be used to obtain lattice parameter and atomic coordinate variations as a function of temperature for Pr0.6Ca0.4MnO3, which has a Jahn-Teller related Pnma - Pnm21 transition at ~232 K, an antiferromagnetic ordering transition at ~160 K and, possibly, spin canting below ~40 K. The results will be analysed in terms of the variations of symmetry-adapted spontaneous strains and cooperative Jahn-Teller distortions of MnO6 octahedra. The specific objectives are to resolve (a) the thermodynamic character of the 232 K transition, expected to be first order due to the Lifshitz condition, (b) order parameter evolution due to strain/order parameter coupling, (c) crossover between two different Jahn-Teller ordering schemes, (d) strain coupling with antiferromagnetic order and spin canting and (e) thickening of twin walls due to octahedral tilting.