Synthesis, Crystal Structure, and Physical Properties of the Type-I Clathrate Ba_8−δNi_x□_ySi_46–x–y

Abstract

Type-I clathrate phase Ba(8)Ni(x)□(y)Si(46-x-y) (□ = vacancy) was obtained from the elements at 1000 °C with the homogeneity range 2.4 ≤ x ≤ 3.8 and 0 ≤ y ≤ 0.9. In addition, samples with low Ni content (x = 1.4 and 1.6; y = 0) and small Ba deficiency were prepared from the melt by steel-quenching. Compositions were established by microprobe analysis and crystal structure determination. Ba(8-δ)Ni(x)□(y)Si(46-x-y) crystallizes in the space group Pm ̅3n (No. 223) with lattice parameter ranging from a = 10.3088(1) Å for Ba(7.9(1))Ni(1.4(1))Si(44.6(1)) to a = 10.2896(1) Å for Ba(8.00(3))Ni(3.82(4))Si(41.33(6)). Single-crystal X-ray diffraction data together with microprobe analysis indicate an increasing number of framework vacancies toward compositions with higher Ni content. For all compositions investigated, Ni K-edge X-ray absorption spectroscopy measurements showed an electronic state close to that of elemental Ni. All samples exhibit metallic-like behavior with moderate thermopower and low thermal conductivity in the temperature range 300-773 K. Samples with compositions Ba(7.9(1))Ni(1.4(1))Si(44.6(1)) and Ba(7.9(1))Ni(1.6(1))Si(44.4(1)) are superconducting with T(c) values of 6.0 and 5.5 K, respectively.

Rapid crystal growth of type-II clathrates A8Na16Si136 (A = K, Rb, Cs) by spark plasma sintering

“…Pulsed DC current (with a pulse-on and -off time of 36 and 2 ms, respectively) was sourced through the precursor mixture and die assembly while under a uniaxial pressure of 100 MPa in a vacuum of 10 mTorr. The dwell time at the desired temperature (600°C for K 5.8IJ1) Na 16 Si 136 , 565°C Rb 8 Na 16 Si 136 , and 500°C for Cs 8 Na 16 Si 136 ) for each reaction was 1.5 h. The product of each reaction was separated from any unreacted NaSi and ACl by washing with ethanol and distilled water. 28 Fig.…”

“…Single crystal structure refinements indicate that K atoms at the 8b crystallographic site are partially occupied IJK 5.8IJ1) Na 16 Si 136 ) while Rb in Rb 8 Na 16 Si 136 and Cs in Cs 8 Na 16 Si 136 are fully occupied. Our results indicate that SPS can be employed in topochemical ion exchange reactions, thus allowing for the rapid synthesis of single crystals of multinary intermetallic phases that cannot be accessible by traditional crystal growth techniques.…”

“…[6][7][8] As a result, they are of interest for a variety of potential technological applications, including solid state energy conversion, [1][2][3]9 photovoltaics, 10,11 magnetocalorics, [12][13][14] superconductivity, [15][16][17] and as the anode for Li-ion batteries. [18][19][20] Due to this scientific interest and technological importance an understanding of the fundamental structural and physical properties of inorganic clathrates is essential.…”

“…Single crystals of three type-II clathrates, K 5.8IJ1) Na 16 Si 136 , Rb 8 Na 16 Si 136 , and Cs 8 Na 16 Si 136 , were synthesized by SPS. The NaSi precursor along with each alkali-metal chloride were ground together in a 1 : 1 mass ratio and the resulting mixture was loaded in a graphite die assembly.…”

Unraveling the role of vacancies in the potentially promising thermoelectric clathratesBa8ZnxGe46−x−y□y

“…In fact, a figure of merit (zT ) of ∼ 1.4 has been achieved in Ba 8 Ga 16 Ge 30 [9][10][11], which is to date reported to be the highest among intermetallic clathrates [12]. However, the presence of guests or framework substitution also leads to unexpected structural changes, such as the formation of vacancies in the host framework of the clathrates [13][14][15][16][17][18][19][20][21]. Simple concepts such as the Zintl's rule have been useful to some extent to rationalize these effects [13,14]; however, the exact understanding of the mechanisms and their impact on the electronic and transport properties are largely unknown.…”