Structural and dynamic properties of MgAgSb-based thermoelectric materials studied by neutron scattering Xiyang Li 1, Baotian Wang 2, Tatiana Guidi 3, Maxim Avdeev 4,5, Lunhua He 1,2, Huaizhou Zhao 1, and Fangwei Wang 1,2
1Institute of Physics, Chinese Academy of Sciences, China (P.R.C)
2China Spallation Neutron Source, Institute of High Energy Physics, Chinese Academy of Sciences, China (P.R.C)
3ISIS facility, Rutherford Appleton Laboratory, U.K.
4Australian Nuclear Science and Technology Organisation, Australia
5School of Chemistry, The University of Sydney, Australia
A breakthrough on thermoelectric materials was recently reported in MgAgSb-based compounds with high ZT value (ZT ~ 1.4 at 450 - 530 K), stimulating the new prospects on the near room temperature (RT) application of thermoelectric generation and solid-state cooling. Here, high resolution neutron powder diffraction (NPD) and inelastic neutron scattering (INS) experiments were performed using ECHIDNA @ ANSTO and MARI @ ISIS, respectively. Rietveld structure refinement of the NPD data indicates that the space group of MgAg0.97Sb0.99 is I-4c2 both at 3 K and 300 K. Combining magnetic and heat-capacity measurements demonstrated that there are no phase transitions between 3 K to RT, in contrast with rich structural phase transitions in the RT - 700 K range. Considering the remarkable crystal-grain-size effect on the thermoelectric performance, we refined the apparent size of ~ 22 nm, validating the TEM result of 10 - 20 nm. We demonstrated that the phonon density of states (PDOS) deduced from MARI data is roughly in agreement with our ab initio calculations. We also analyzed the resonant bonding and LA-TO coupling which are believed to responsible for the low phonon thermal conductivity. Amazingly only 0.5% Ni doping at Ag site reduces the thermal conductivity by more than 8%. High resolution INS experiments to accurately measure the low frequency PDOS will be performed to study the Ni doping effect on the low thermal conductivity mechanism, combining with ab initio calculation.
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