Customization of Sn2P2S6 ferroelectrics by post-growth solid-state diffusion doping

Abstract

For the first time, we demonstrated successful post-synthesis incorporation of metal dopants at elevated temperature into a host structure of Sn₂P₂S₆, known as the grandfather of dichalcogenide ferroelectrics with a formula M₂P₂X₆ (M = metal and X = chalcogen). With the example of Cu, we show that the integration of dopant atoms into the bulk of an already grown crystal could be easily tuned up to 0.5 at% affecting the structural, optical, vibrational, electrical, and ferroelectric properties. Thermally diffused copper atoms in Sn₂P₂S₆ bulk are in the metallic state, inducing a multiaxial expansion of the Sn₂P₂S₆ unit cell for 2–3.4%. The energy reduction between indirect and direct optical transitions was observed, combined by small hardening for acoustic and soft optical vibrational modes originating in the partial substitution of Sn by Cu in the Sn₂P₂S₆ crystal lattice. Similar to hydrostatic pressure, the structurally bonded copper initiates a small downward shift in the critical temperature T_c. The presence of copper has a substantial impact on the shape smearing of the ferroelectric domains as well as on the behaviour of the dielectric permittivity. The real part of the dielectric constant ε′ reaches its maximal value at intermediate concentrations of Cu 0% < x < 0.5%, showing an ∼20% increase with respect to the parent structure. The incorporated metal atoms provoke a monotonous expansion of the ferroelectric P–E loops along the increased dopant content direction. An increase in the electrical conductivity at higher Cu concentration reveals a trend for inducing a metal-like behaviour.