Shell Model Study of Local and Global Energy Barriers in KDP

Abstract

We perform a study of the energetics of KH₂PO₄ (KDP) by using a shell model (SM) which was constructed by adjusting the interaction parameters to ab initio calculations, and was fitted to reproduce phonons, polarization-inversion energies and structural properties. We calculate the energy profiles by performing global displacements and local distortions following the ferroelectric (FE) mode pattern in clusters of different sizes embedded in a paraelectric (PE) phase matrix. These properties are expected to be relevant to the PE-FE phase transition. The obtained SM results are compared to corresponding ab initio (AI) data. The global instabilities are found in good agreement for both KDP and DKDP. We also find qualitative good agreement in the KDP structure and even quantitative agreement in the expanded DKDP structure for the local distortions. The SM results reproduce well different trends like increasing instabilities as the cluster sizes grows, as the heavier atoms are included, and as the volume is increased, in accordance with the corresponding data from AI calculations.

Keywords:

• Ferroelectric
• hydrogen bond
• shell model
• KDP

Acknowledgments

This work was supported in part by the Agencia Nacional de Promoción Científica y Tecnológica under Grant PICT99 Nr. 03-07248 and CONICET under Grant PIP 5302. R. M., J. L., and S. K. acknowledge additional support from CONICET.