Structural and thermoelectric properties of Mn(Si1−xAlx)1.75: a commensurate phase mixture approximation

Abstract

Higher manganese silicide (HMS) compounds with the nominal stoichiometry of Mn(Si_1−xAl_x)_1.75 (0 ≤ x ≤ 0.04) are synthesized using the arc melting method. Structural investigations are carried out executing powder XRD Rietveld refinements on different models from the Nowotny chimney ladder (NCL) phases. It is revealed that the commensurate two-phase mixture model, based on the I2d Mn₁₅Si₂₆ and Pn2 Mn₂₇Si₄₇ superstructures, approximates quite well the modulated structure of the prepared materials with the majority phase as Mn₂₇Si₄₇ for x = 0. Le Bail analysis enhances the validity of the commensurate approximation, comparing the two-phase mixture with the (3 + 1)D incommensurate structural model with the superspace group I4₁/amd(00γ)00ss. Al substitution causes expansion of both tetragonal supercells across the z axis, while the a lattice constant is affected marginally. Interestingly, the evolution of the phase fraction across the Mn(Si_1−xAl_x)_1.75 (0 ≤ x ≤ 0.04) series favours the formation of Mn₁₅Si₂₆, and a noticeable change occurs at x = 0.025, which seems to be correlated with an abrupt variation in the electrical transport properties. Electrical conductivity and Seebeck coefficient measurements show that Al doping induces an effective increase in hole concentration, resulting in a significant increase in σ for x = 0.025. An improvement in ZT is achieved with Mn(Si_0.975Al_0.025)_1.75 phase demonstrating a maximum value close to 0.6 at 773 K. In addition, the replacement of high-purity Si with two types of recycled Si kerf from the PV industry is attempted here to develop HMS phases. Powder XRD and SEM analyses show the existence of an appreciable amount of MnSi in both kerf-based products, strongly affecting their thermoelectric (TE) properties. Although the two kerf-based phases present lower efficiencies, this is the first time p-type silicide thermoelectrics are synthesized via the arc melting method using recycled Si, which is quite important for future research.