Interionic potentials for the cubic perovskite crystals KMnF₃ and KZnF₃ are derived and used in calculations of the configurational energies of intrinsic and substitutional defects. The pressure dependences of soft-mode frequencies and elastic constants are investigated as well as the temperature dependence of the former. Structural, elastic and vibrational properties in the tetragonal phase are estimated. Some results are obtained which probe the effects of impurities on displacive phase transitions.