Abnormally soft acoustic phonons in the Mg3Sb2 allomerisms

Softening modes in phonon dispersion are often indications of intriguing physical phenomena such as phase transition and exotic thermal conductance. The promising thermoelectric material Mg3Sb2 and its allomerisms including CaZn2Sb2, SrZn2Sb2, etc. are investigated for phonon dispersion variation in this study using density functional theory (DFT) calculations. Two different soft transverse acoustic (TA) modes are present in Mg3Sb2, one at the A point along the out-of-plane direction and the other at the in-plane M point with a lower frequency. In contrast, the in-plane TA modes of CaZn2Sb2 and SrZn2Sb2 exhibit much higher in energy, but they both have extremely soft TA modes at the A point instead. Here, we construct an oscillator model based on the projected interatomic force constant tensor along vibrational directions. Our results suggest that the disappearing shear interaction of the near-neighbor bond, tilted Mg–Sb plays a large role in the abnormally soft in-plane TA mode in Mg3Sb2. In contrast, the covalent vertical Zn–Sb bonds in CaZn2Sb2 and SrZn2Sb2 have also negligible shear interactions. In all cases, the “missing” nearest-neighbor interaction, combined with the weak next-neighbor interaction and repulsive long-range interaction result in the observed low-energy TA modes. Soft acoustic phonon is associated with lower thermal conductivity frequently. In general, we expect such in-depth analyses on the origin of these abnormally soft phonon modes in the Mg3Sb2 allomerisms would provide insights into seeking high-performance thermoelectric materials in the rich family of the Zintl phase compounds.