Abstract
Single crystals of (Na/Sr)–(Ga/Si) quaternary type-I clathrates, Na8−ySryGaxSi46−x, were synthesized by evaporating Na from a mixture of Na–Sr–Ga–Si–Sn in a 6 : 0.5 : 1 : 2 : 1 molar ratio at 773 K for 12 h in an Ar atmosphere. Electron-probe microanalysis and single-crystal X-ray diffraction revealed that three crystals from the same product were Na8−ySryGaxSi46−x with x and y values of 7.6, 2.96; 8.4, 3.80; and 9.1, 4.08. It was also shown that increasing the Sr and Ga contents increased the electrical resistivity of the crystal from 0.34 to 1.05 mΩ cm at 300 K.
Highlights
Silicon (Si) clathrate compounds are composed of host Si atoms organized in three-dimensional frameworks and guest atoms enclosed in the Si cages of the frameworks
The framework Si atoms can be partially replaced by Ga atoms in some type-I clathrate compounds, such as A8Ga8Si38 (A 1⁄4 K, Rb, Cs), as described by Sui et al.[5]
The compositions of the obtained single crystals were analyzed with an electron-probe microanalyzer (EPMA, JEOL, JXA-8200)
Summary
Silicon (Si) clathrate compounds are composed of host Si atoms organized in three-dimensional frameworks and guest atoms enclosed in the Si cages of the frameworks. Kasper et al rst synthesized binary Si clathrate, Na8Si46, in 1965.1 Since many researchers have studied Si clathrate compounds,[2] altering their physical properties by partial or full substitution of different elements for the host and guest atoms. Kawaji et al synthesized a type-I clathrate, (Na,Ba)8Si46;3 this compound was the rst Si clathrate superconductor with a TC value of 4 K derived from the partial substitution of Ba for Na in the Na8Si46 cages. Another clathrate composed of Ga/Si cages and Ba atoms, Ba7.94Ga15.33Si30.67, was shown to have a relatively high thermoelectric dimensionless gure of merit, ZT, of 0.87 at 870 K.6,7 Sr8Ga11Si35 8 and Sr8Ga13.6Si32.4,9 which contain Sr guest atoms in Ga/Si cages, exhibit electrical resistivities of approximately 0.2 and 0.26 mU cm, respectively, at 280 K
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