Control of Thermal Expansion in a Low-density Framework Modification of Silicon

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The low-density clathrate-II modification of silicon, Si136, contains two distinct cage-like voids large enough to accommodate various types of guest atoms which influence both the host structure and its properties. Although the linear coefficient of thermal expansion of Si136 (293 K < T < 423 K) is only about 20% larger than that of the ground state α-Si (diamond structure), the coefficient of thermal expansion monotonically increases by more than 150% upon filling the framework cages with Na atoms in NaxSi136 (0 < x < 24), ranging from α = 2.6 × 10−6 K−1 (x = 0) to 6.8 × 10−6 K−1 (extrapolated to x = 24) by only varying the Na content, x. Taken together with the available heat capacity and bulk modulus data, the dramatic increase in thermal expansion can be attributed to an increase in the mode-averaged Grüneisen parameter by a factor of nearly 3 from x = 0 to x = 24. These results highlight a potential mechanism for tuning thermal expansion, whereby guest atoms are incorporated into the voids of rigid, covalently bonded inorganic frameworks to influence the lattice dynamics. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. M.B. acknowledges support from Cal Poly in the form of start-up funding. G.S.N. acknowledges support from the National Science Foundation Grant No. DMR-1400957. The authors thank Lynn Ribaud and Saul H. Lapidus at the APS for their assistance in the collection of the synchrotron XRD data.

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Applied Physics Letters, v. 112, issue 18, art. 181901