Theoretical study of ferroelectric potassium nitrate

We present a detailed study of the structural behavior and polarization reversal mechanism in phase III of KNO3, an unusual ferroelectric material in which the nitrate groups rotate during polarization reversal. This material was one of several studied in a previous work O. Diéguez and D. Vanderbilt, Phys. Rev. Lett. 96, 056401 2006 where methods were described for computing curves of energy versus electric polarization. In the present work, we extend and systematize the previous first-principles calculations on KNO3 and analyze in detail a two-parameter model in which the energy of the system is written as a low-order expansion in the polarization and the nitrate group orientation. We confirm that this model reproduces the first-principles results for KNO3 very well and construct its parameter-space phase diagram, describing regions where unusual triple-well potentials appear. We also present first-principles calculations of KNO3 under pressure, finding that its energy-versus-polarization curves change character by developing a first-derivative discontinuity at zero polarization.