Cr and Ru substituted defect manganese silicides MnSi$_{\delta }$ ($\delta \sim$ 1.72-1.74) as low thermal conductivity thermoelectrics*

Defect manganese silicides MnSi$_{\delta }$ ($\delta \sim$ 1.72-1.74) belong to a large family of compounds known as Nowotny chimney-ladder (NCL) phases and are closely related to an orthorhombic NCL compound TiSi$_{2}$. One interesting feature is the low lattice thermal conductivity ($\kappa_{L} \sim$ 2.5 W/m K) which may be due to several reasons:$_{\, }$ Since $\delta $ doesn't exceed 1.75 in MnSi$_{\delta }$, a considerable concentration of random vacancies exists on the Si-sublattice and can give rise to a low thermal conductivity. In addition, as synthesized MnSi$_{\delta }$ is a mixture of many phases including Mn$_{4}$Si$_{7}$, Mn$_{11}$Si$_{19}$, Mn$_{15}$Si$_{26}$ and Mn$_{27}$Si$_{47}$ and in all these phases, while a-lattice parameter is closely matched, the c-lattice parameter substantially varies with $\delta $. Such a closely matched a-lattice parameter can cause lattice strain and potentially reduce $\kappa_{L}$. Ru$_{2}$Si$_{3}$ forms solid solutions and Cr can be substituted as much as 20{\%} in MnSi$_{\delta }$. These substitutions can favorably modify the lattice strain and reduce the thermal conductivity further. Hence manganese silicides substituted with small amounts of Cr and Ru have been synthesized. Thermoelectric properties including resistivity, Seebeck and Hall coefficients and thermal conductivity will be studied and presented. *This work was supported as part of the Center for Revolutionary Materials for Solid State Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001054.