Sn2P2S6 ferroelectrics customization by post-growth solid-state diffusion doping
For the first time the elevated temperature was used as a post-synthesis mechanism to incorporate metal dopants into a host structure of Sn2P2S6 widely known as a granddaddy of dichalcogenide ferroelectrics with a formula M2P2X6 (M = metal and X = chalcogen). On the example of Cu, it was demonstrated that the integration of dopant atoms into the bulk of already grown crystal could be easily tuned up to 0.5 at% affecting structural, optical, vibrational, electrical and ferroelectric properties. Thermally diffused copper atoms are found to be in a metallic state, inducing a multiaxial expansion of 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 partial substitution of Sn by Cu in the Sn2P2S6 crystal lattice. In a similar manner to hydrostatic pressure, the structurally bonded copper initiates a small downward shift in the critical temperature TC. The presence of copper has a strong 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 0.5 % < x < 0 %, showing a ∼ 20 % increment in respect to the parent structure. The incorporated metal atoms provoke a monotonous expansion of the ferroelectric P-E loops, in the direction of increased dopant content. An increase in the electrical conductivity at higher copper concentration reveals a trend for inducing a metal-like behaviour.