Crystal structure and thermoelectric properties of chimney-ladder compounds in the Ru₂Si₃-Mn₄Si₇ pseudobinary system

Phase relationships of manganese-substituted ruthenium sesquisilicide alloys have been investigated by using X-ray powder diffraction and scanning and transmission electron microscopy. A series of chimney-ladder phases Ru_1-xMn_xSi_y (0.14 ≤ x ≤ 0.97, 1.584 ≤ y ≤ 1.741) are formed over a wide compositional range between Ru₂Si₃ and Mn₄Si₇. The compositions of these chimney-ladder compounds deviate slightly from the composition line connecting Ru₂Si₃ and Mn₄Si₇, which corresponds to the ideal composition line satisfying VEC (valence electron counting) = 14 rule. The occurrence of this compositional deviation is discussed in terms of the VEC rule and the atomic packing. The thermoelectric properties of the directionally solidified Ru_1-xMn_xSi_y alloys (0.55 ≤ x ≤ 0.90) have also been investigated as a function of the Mn content and temperature. The dimensionless figure of merit (ZT) for those alloys with a high Mn content (x ≥ 0.75) increased with the increase in Mn content. The ZT value for a crystal with x = 0.90 was as high as 0.76 at 874 K.