Ferroelectricity and isotope effects in one-dimensional H-bonded KDP crystals from first principles

S. KOVAL

La Plata

Workshop; 6th Workshop on Novel Methods for Electronic Structure Calculations; 2015

We studied the microscopic mechanism of the paraelectric-ferroelectric (PE-FE) phase transitionof the one-dimensional FE compound CsH2PO4 (CDP) by means of first-principles electronic structure calculations. The calculated structural parameters in the PE and FE phases as well as the total spontaneous  polarization Ps obtained with the Berry phase formalism for CDP are in good agreement with experiments. The main contribution to Ps originates from a large yx-component of the calculated Born effective-charge tensor for the disordered protons. Moreover, this component is ~ 2.7 times larger  than the zx-component of the proton effective-charge relevant to the polarization in the  H-bonded FE KH2PO4 (KDP). This is the main feature that compensates the different number of protons  per formula unit involved in the phase transitions for CDP and KDP to give close values for  their measured Ps. Correlations among protons and heavy atoms along chains in the b direction lead to larger instabilities for the global and local FE distortions in CDP and its deuterated  counterpart DCDP. We conclude that the tunneling particle is a dressed proton(deuteron) in agreement with experiments and with recent ab initio results for KDP.