Synthesis, crystal structure, and thermoelectric properties of a new layered carbide (ZrC) 3 [Al 3.56 Si 0.44 ]C 3
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A new quaternary layered carbide, (ZrC)3[Al3.56Si0.44]C3, has been synthesized and characterized by x-ray powder diffraction and thermopower and electrical conductivity measurements. The crystal structure was successfully determined using direct methods and further refined by the Rietveld method. The crystal is trigonal (space group R3m, Z ⳱ 3) with lattice dimensions a ⳱ 0.331389(7), c ⳱ 4.90084(7) nm, and V ⳱ 0.46610(1) nm3. The final reliability indices calculated from the Rietveld refinement were Rwp ⳱ 9.53% (S ⳱ 1.70), Rp ⳱ 7.22%, RB ⳱ 1.81%, and RF ⳱ 0.94%. The crystal structure is composed of the NaCl-type [Zr3C4] slabs separated by the Al4C3-type [Al0.89Si0.11C] layers. This material had thermoelectric properties comparable to the layered carbides (ZrC)2[Al3.56Si0.44]C3 (Zr2[Al3.56Si0.44]C5), (ZrC)2Al3C2, and (ZrC)3Al3C2 in the temperature range of 373–1273 K, with the maximal power-factor value of 6.6 × 10−5 W m−1 K−2 at 545 K. The two quaternary carbides have been found to form a homologous series with the general formula of (ZrC)n[Al3.56Si0.44]C3 (n ⳱ 2 and 3).
I. INTRODUCTION
The ternary carbides (ZrC)2Al3C2 and (ZrC)3Al3C2 form a homologous series, the general formula of which is (ZrC)mAl3C2 (m ⳱ 2 and 3).1–3 These crystal structures, belonging to the same space group P63mc, can be regarded as intergrowth structures where the Al4C3-type [AlC] layers are the same while the NaCl-type [ZrmCm+1] layers increase in thickness with increasing m value. These two types of layers shear the carbon-atom network at their boundaries; the C–C distances are ∼0.335 nm for both carbides with m ⳱ 2 and 3.3 On the other hand, the C–C distance of ZrC crystal is 0.330 nm [⳱ a(ZrC)/√2] and that of Al4C3 is 0.334 nm [⳱ a(Al4C3)], where a(ZrC) and a(Al4C3) represent the a-axis lengths of the ZrC and Al4C3 crystals, respectively. These distances are close to each other and also to those of the carbon-atom networks in (ZrC)mAl3C2. Fukuda et al.3 have therefore concluded that the closeness of the C–C distances between the ZrC and Al4C3 crystals, being expressed by the equation a(ZrC)/√2 ≈ a(Al4C3), is the principal reason for the formation of these layered carbides. Kidwell et al.4 have successfully prepared a new carbide Al8SiC7 and characterized it using an x-ray powder
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0372 2888 J. Mater. Res., Vol. 22, No. 10, Oct 2007 http://journals.cambridge.org Downloaded: 13 Mar 2015
diffraction (XPRD) method. The crystal is hexagonal with a ⳱ 0.33127(7) nm and c ⳱ 1.9242(4) nm. Because the a-axis length is almost equal to a(ZrC)/√2, Fukuda et al.5 expected the formation of layered carbides for the reaction products between ZrC and Al8SiC7 (Al:Si:C ⳱ 3.56:0.44:3.11) and actually prepared the new one, the chemical formula of which is (ZrC)2[Al3.56Si0.44]C3. Crystal data are as follows: crystal trigonal, space group R3m, Z ⳱ 3 (unit-cell content, [6Zr–12(Al+Si)–15C]), and lattice dimensions a ⳱ 0.331059(5), c ⳱ 4.09450(5) nm, and V
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