Crystal structure and thermoelectric properties of Sr-Mo substituted CaMnO₃: a combined experimental and computational study

A combination of experimental and computational techniques has been employed to study doping effects in perovskite CaMnO₃. High quality Sr-Mo co-substituted CaMnO₃ ceramics were prepared by the conventional mixed oxide route. Crystallographic data from X-ray and electron diffraction showed an orthorhombic to tetragonal symmetry change on increasing the Sr content, suggesting that Sr widens the transition temperature in CaMnO₃ preventing phase transformation-cracking on cooling after sintering, enabling the fabrication of high density ceramics. Atomically resolved imaging and analysis showed a random distribution of Sr in the A-site of the perovskite structure and revealed a boundary structure of 90° rotational twin boundaries across {101}_orthorhombic; the latter are predominant phonon scattering sources to lower the thermal conductivity as suggested by molecular dynamics calculations. The effect of doping on the thermoelectric properties was evaluated. Increasing Sr substitution reduces the Seebeck coefficient but the power factor remains high due to improved densification by Sr substitution. Mo doping generates additional charge carriers due to the presence of Mn³⁺ in the Mn⁴⁺ matrix, reducing electrical resistivity. The major impact of Sr on thermoelectric behaviour is the reduction of the thermal conductivity as shown experimentally and by modelling. Strontium containing ceramics showed thermoelectric figure of merit (ZT) values higher than 0.1 at temperatures above 850 K. Ca_0.7Sr_0.3Mn_0.96Mo_0.04O₃ ceramics exhibit enhanced properties with S_1000K = -180 μV K^-1, ρ_1000K = 5 × 10^-5 Ωm, k_1000K = 1.8 W m^-1 K^-1 and ZT ≈ 0.11 at 1000 K.