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 Ru1−xMnxSiy (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 Ru1−xMnxSiy 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.