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.

©2007 The American Physical Society