High temperature X-ray diffraction study on incommensurate composite crystal MnSiγ - (3+1)-dimensional superspace approach

The (3 + 1)-dimensional crystal structure of a higher manganese silicide (MnSi gamma) phase is revealed using in situ high temperature powder X-ray diffraction (XRD) above room temperature. The compound consists of two tetragonal subsystems of [Mn] and [Si] with an irrational c-axis ratio gamma = c(Mn)/c(Si). The in situ XRD results show that the MnSi gamma phase is stable, in a vacuum of similar to 1 Pa, up to 1093 K and partially decomposes into the monosilicide (MnSi) phase with further increase in temperature. Refined a- and c(Mn)-axis lengths increase linearly and the thermal expansion coefficients are comparable with those of typical metallic electrode materials. In contrast, c(Si)-axis length changes its increment against temperature at T-BD similar to 773 K. As a result, the temperature dependence of gamma starts to decrease gradually above T-BD, from 1.7387(1) (at 773 K) to 1.7244(1) (at 1173 K). This finding implies that the MnSi gamma phase consecutively changes its irrational composition above T-BD, a typical temperature where the silicides exhibit a maximum figure-of-merit. An increase in hole carrier concentration is expected according to the valence electron counting concept. Although the nearest Mn-Mn and Mn-Si distances increase monotonically with temperature, the nearest Si-Si distance much increases by similar to 0.05 angstrom from 2.439(4) angstrom at 773 K to 2.493(7) angstrom at 1173 K. The latter increase is considered to be caused by the introduction of excited electron carriers into the anti-bonding orbital, due to the bipolar diffusion (BD). (C) 2014 Elsevier B.V. All rights reserved.