Geometric frustration in compositionally modulated ferroelectrics

摘要

Geometric frustration is a broad phenomenon that results from an intrinsicincompatibility between some fundamental interactions and the underlyinglattice geometry1-7. Geometric frustration gives rise to new fundamentalphenomena and is known to yield intriguing effects, such as the formation ofexotic states like spin ice, spin liquids and spin glasses1-7. It has also ledto interesting findings of fractional charge quantization and magneticmonopoles5,6. Geometric frustration related mechanisms have been proposed tounderstand the origins of relaxor behavior in some multiferroics, colossalmagnetocapacitive coupling and unusual and novel mechanisms of high Tcsuperconductivity1-5. Although geometric frustration has been particularly wellstudied in magnetic systems in the last 20 years or so, its manifestation inthe important class formed by ferroelectric materials (that are compoundsexhibiting electric rather than magnetic dipoles) is basically unknown. Here,we show, via the use of a first-principles-based technique, thatcompositionally graded ferroelectrics possess the characteristic "fingerprints"associated with geometric frustration. These systems have a highly degenerateenergy surface and exhibit original critical phenomena. They further revealexotic orderings with novel stripe phases involving complex spatialorganization. These stripes display spiral states, topological defects andcurvature. Compositionally graded ferroelectrics can thus be considered as the"missing" link that brings ferroelectrics into the broad category of materialsable to exhibit geometric frustration. Our ab-initio calculations allow a deepmicroscopic insight into this novel geometrically frustrated system.