Ab initio vibrational properties and isotope effect in H-bonded ferroelectric materials

G. COLIZZI; J. LASAVE; R. MIGONI; C. JOHNSTON; S. KOVAL; R. MENCHÓN; F. TORRESI; J. KOHANOFF

San Sebastián

Conferencia; Trends in Nanotechnology International Conference (TNT2019); 2019

Conference series: "Trends in Nanotechnology" (TNT)

Motivated by the miniaturization of devices to nano levels, the interest in reducing power consumption andimproving energy efficiency has continuously grown over the last years. Within this scenario, ferroelectricmaterials offer a wide range of useful properties, such as nonzero switchable electric polarization in theabsence of an external field, piezoelectricity and pyroelectricity. This makes them promising candidates forapplications such as capacitors, memories or energy harvesting devices.[1, 2]In this work, we report the ab initio structural, vibrational and isotope effect properties of potassiumdihydrogen phosphate (KDP or KH 2 PO 4 ), a prototype member of the hydrogen-bonded ferroelectriccompounds. A striking manifestation of its phenomenology is the huge isotope effect in its ferroelectric-paraelectric transition temperature T c , which changes from 122K to 229K upon deuteration.Using KDP as a case model, we have developed a novel method for selecting the exchange-correlationfunctional in Density Functional Theory calculations. This method incorporates quantum nuclear effects aposteriori in the description of the system, allowing to take them into account in the selection process. ForKDP, the nonlocal van der Waals functional vdW-DF was selected as it provided the best agreement with H-bond geometries experimental data. With this choice of functional, we computed the phonon modes at theΓ point of the Brillouin zone, the phonon dispersion curves and the phononic contribution to specific heat.We were also able to calculate the electric polarization and to identify the microscopic mechanism thatoriginates it. [3] The devised method might be useful in the study of other H-bonded materials where theisotope effect proves to be of crucial importance. [4] [5]References[1] S. Pandya et al., Nat. Mater. 17, 432 (2018).[2] T. Y. Kim et al., Nano Converg. 5, 30 (2018).[3] R. Menchón et al., Phys. Rev. B 98, 104108 (2018).[4] C. Shi et al., Nat. Commun. 9, 481 (2018).[5] TNT conference (2019), book of abstracts, pag. 87