Multiformity and fluctuation of Cu ordering in Cu2Se thermoelectric materials†
Cuprous selenide (Cu2Se) has recently shown a very high dimensionless thermoelectric figure of merit zT as well as a dramatic increase in thermoelectric performance during the critical second-order phase transition. The present study indicates that the ultrahigh thermoelectric performance arises from its specific structural features involving multiformity of Cu ordering and drastic structural fluctuation during phase transition. The Cu2Se sample consists of domains of different ordered lamellar structures of Cu atoms which are coherently immersed in the long-range ordered Se pseudo-fcc framework. The specific self-independent binary-sublattice structures have been found to enhance phonon scattering while still guaranteeing good carrier mobilities. Upon increasing the temperature to near phase transition, the multiple structures undergo intense and stepwise changes including appearance of new ordered structures for copper, disordering and diffusion of Cu atoms across the interlayers, and finally random distribution of Cu in the Se cubic sublattice which alters a little during the phase transition. Such extreme structural fluctuation results in critical electron and phonon scatterings that expedite an exceptional enhancement of thermoelectric performance.