Lattice dynamics and local structure in AMg2Sb2 thermoelectric Zintl phases

The layered compounds AMg2Sb2 are interesting Zintl phases with a rather simple trigonal structure that hosts only five atoms per unit cell. An outstanding thermoelectric figure of merit of zT~1.5 was reported for n-type Mg3Sb2. This performance can be ascribed to multi-valley conduction but also to its low thermal conductivity and anomalous lattice dynamics. To understand the relation between structure and functionality, we have studied phonons in Mg3Sb2 and Mg3Bi2 by inelastic neutron scattering, which reveals sharp modes that contrast with the low thermal conductivity. Complementary 121Sb Mössbauer spectroscopy yields precise Sb thermal factors. A lattice hardening is observed as the mass of A increases in the series A=Mg, Ca, Sr. In parallel, the local environment of Sb distorts, as revealed by the electric field gradient. We will discuss our results in relation with phonon calculations and elastic constants measurements by resonant ultrasound spectroscopy.