Na-Ga-Si type-I clathrate single crystals grown via Na evaporation using Na-Ga and Na-Ga-Sn fluxes

Abstract

Single crystals of a Na-Ga-Si clathrate, Na₈Ga_5.70Si_40.30, of size 2.9 mm were grown via the evaporation of Na from a Na-Ga-Si melt with the molar ratio of Na : Ga : Si = 4 : 1 : 2 at 773 K for 21 h under an Ar atmosphere. The crystal structure was analyzed using X-ray diffraction with the model of the type-I clathrate (cubic, a = 10.3266(2) Å, space group Pm3̄n, no. 223). By adding Sn to a Na-Ga-Si melt (Na : Ga : Si : Sn = 6 : 1 : 2 : 1), single crystals of Na₈Ga _x Si_46-x (x = 4.94-5.52, a = 10.3020(2)-10.3210(3) Å), with the maximum size of 3.7 mm, were obtained via Na evaporation at 723-873 K. The electrical resistivities of Na₈Ga_5.70Si_40.30 and Na₈Ga_4.94Si_41.06 were 1.40 and 0.72 mΩ cm, respectively, at 300 K, and metallic temperature dependences of the resistivities were observed. In the Si L_2,3 soft X-ray emission spectrum of Na₈Ga_5.70Si_40.30, a weak peak originating from the lowest conduction band in the undoped Si₄₆ was observed at an emission energy of 98 eV.

Fig. 1. Optical micrographs of the single crystals grown via Na evaporation from the Na–Ga–Si–Sn starting sample at 873 K for 3 h under an Ar atmosphere (∼10⁵ Pa), followed by alcohol and acid treatments.

Fig. 2. Ga content of type-I clathrates Na₈Ga_xSi_46−xversus heating temperatures of Na–Ga–Si mixture (△) and Na–Ga–Si–Sn mixtures (●).

Fig. 3. Plots of a-axis length (a), 6c and 24k site occupancies of Ga (b), and normalized volumes of [Si/Ga]₂₄ cage and [Si/Ga]₂₀ cage (c) versus Ga content x of Na₈Ga_xSi_46−x.

Fig. 4. [Si/Ga]₂₀ and [Si/Ga]₂₄ cages of Na₈Ga_5.70Si_40.30 depicting with spheres of occupancies (a) and 99% probability displacement ellipsoids (b).

Fig. 5. Plot of a-axis length versus ionic radius (coordination number 12) of the guest atoms for Na₈Si₄₆, Na₈Ga_5.7Si_40.3 and A₈(Al/Ga)₈Si₃₈ (A = Na, K, Rb, and Cs). The hypothetical a-axis length of Na₈Ga₈Si₃₈ estimated in Fig. 3(a) is also plotted with ×.

Fig. 6. Si L_2,3 SXE spectra of Na₈Ga_5.70Si_40.30, Na₈Si₄₆, and d-Si.

Fig. 7. Temperature dependence of electrical resistivity for single crystals of Na₈Ga_5.70Si_40.30, Na₈Ga_4.94Si_41.06, and Na₈Si₄₆.