Ab initio study of the one-dimensional H-bonded ferroelectric CsH sub(2) PO super(4)

We studied the microscopic mechanism of the paraelectric-ferroelectric (PE-FE) phase transition of CsH sub(2) PO sub(4)(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 P sub()sobtained with the Berry phase formalism for CDP are in good agreement with experiments. The main contribution to P sub()soriginates from a large yx component of the calculated Born effective-charge tensor for the disordered protons. Moreover, this component is [asymtotically =] 2.7 times larger than the zx component of the proton effective-charge tensor relevant to the polarization in the H-bonded FE KH sub(2) PO sub(4)(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 P sub()s 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.