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Title: Anisotropic Structural Collapse of Mg3Sb2 and Mg3Bi2 at High Pressure

Journal Article · · Chemistry of Materials

Alloys between Mg3Sb2 and Mg3Bi2 have recently been shown to be exceptional thermoelectric materials due in part to their anomalously low thermal conductivity. In the present study, in situ high-pressure synchrotron X-ray diffraction was used to investigate the structure and bonding in Mg3Sb2 and Mg3Bi2 at pressures up to 50 GPa. Our results confirm prior predictions of isotropic in-plane and out-of-plane compressibility but reveal large disparities between the bond strength of the two distinct Mg sites. Using single-crystal diffraction, we show that the octahedral Mg–Sb bonds are significantly more compressible than the tetrahedral Mg–Sb bonds in Mg3Sb2, which lends support to prior arguments that the weaker octahedral Mg bonds are responsible for the anomalous thermal properties of Mg3Sb2 and Mg3Bi2. Further, we report the discovery of a displacive and reversible phase transition in both Mg3Sb2 and Mg3Bi2 above 7.8 and 4.0 GPa, respectively. Furthermore, the transition to the high-pressure structure involves a highly anisotropic volume collapse, in which the out-of-plane axis compresses significantly more than the in-plane axes. Single-crystal diffraction at high pressure was used to solve the monoclinic high-pressure structure (C2/m), which is a distorted variant of the ambient-pressure structure containing four unique Mg coordination environments.

Research Organization:
Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States); Univ. of Illinois, Chicago, IL (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES). Materials Sciences & Engineering Division; National Science Foundation (NSF); USDOE Office of Science (SC), Basic Energy Sciences (BES)
Grant/Contract Number:
AC52-07NA27344; NA0003975; SC0019252; SC0019299; FG02-94ER14466; AC02-06CH11357; EAR-1634415
OSTI ID:
1765294
Alternate ID(s):
OSTI ID: 1774968
Report Number(s):
LLNL-JRNL-814732; 1023098
Journal Information:
Chemistry of Materials, Vol. 33, Issue 2; ISSN 0897-4756
Publisher:
American Chemical Society (ACS)Copyright Statement
Country of Publication:
United States
Language:
English

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