Constant pressure molecular dynamics simulations have been made for a simple ionic model for crystalline potassium nitrate at several pressures and temperatures which span the known phase diagram. The nitrate ion was modeled as a rigid fourhyphen;site entity with distributed charges and a quaternion algorithm was used to solve the rotational equations of motion. Several structural features of KNO3are reproduced in the simulations. The 300 K, 0.1 GPa aragonitehyphen;like phase II of KNO3has been reproduced although in the model it appears at a pressure between 1 and 2 GPa. The disordering of the nitrate orientations which occurs at high temperatures has also been observed. At 3 GPa the stable phase of the model is very similar to the recently characterized phase IV of KNO3. Spontaneous phase transitions in the simulations did not always produce well ordered structures and this appears to be a problem inherent in the method.
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