Higher manganese silicides: A Nowotny chimney ladder phase for thermoelectric applications

Abstract

Nowotny chimney ladder (NCL) phases are intermetallic binary compounds that typically crystallize in a tetragonal crystal structure and constitute of two separate subsystems. The rich solid-state chemistry of NCL phases inherits fascinating lattice dynamics with unique abilities for structural modifications. As an extensively studied energy material for the thermoelectric application, we overview the emerging aspects for structural interpretation in higher manganese silicides (MnSiγ), a prominently explored example of NCL phase. The progress in understanding the incommensurate composite crystals of MnSiγ is discussed to highlight its functional crystallography for proposing the effective strategies to attain favorable modification of transport properties of charge carriers (concentration, mobility, effective mass), and phonons (lattice thermal conductivity). The application potential and prospective strategies for enabling the rational optimization of the dimensionless thermoelectric figure of merit (zT) are examined, and the possibilities of chemical modification in MnSiγ and related NCL phases are presented.

Odd‐Stoichiometry and Secondary Phase Relations in Higher Manganese Silicides

“…It was inferred that the TE transport properties and microstructure of MnSi x can vary depending on the stoichiometry, wherein the stoichiometric alteration leads to metallic MnSi and semiconducting Si phases within the polycrystals of NCL phase of MnSi 𝛾 for the Si-deficient and Si-excess compositions, respectively. Although the segregation of secondary phases is inevitable as observed previously 17,27 for silicon-rich manganese silicides-based compositions, a nominal composition with x ≈ 1.74 in MnSi x favors the formation of the NCL phase. This can be related to the atomic packing factor and distinct stability of the c-axis length ratio, that is, 𝛾 = c Mn /c Si of [Mn] and [Si] subsystems, as labile Si atoms easily move along the c-axis, the NCL phases can adjust the Si concentration in a quasi-continuous manner to optimize its bonding energy.…”

“…If the equilibrium Si concentration changes, it can only be balanced by a shift of the Si ladder along the chimney. 17 The structural stability and electrical properties of MnSi 𝛾 adhere well to the 14-electron rule, which states that a compound containing a transition metal will have greater structural stability when it has 14 valence electrons (VEC) in its outermost shell as it enables them to form stronger covalent bonds. [43][44][45] The (3+1) dimensional superspace approach is particularly useful for understanding the behavior of the incommensurate NCL phase, which exhibits multiple phases and structural transformations.…”

“…Moreover, a variation in the 3D arrangement of consecutive Si subsystems relative to Mn subsystems is prone to orientational anomalies. 17,22 These structural anomalies typically result in a periodic difference with an inherent distortion in the order of stacking planes. The interstitial diffusion within the crystal lattice facilitates the movement of Si atoms along the c-axis, which can be particularly relevant for the assumption that Si atoms are confined in chimneys.…”

“…[10] Several compounds with structural and stoichiometric variations have been identified with silicon-rich and pseudo-binary compositions having an unusual odd-stoichiometry (including Mn 4 Si 7 , [11,12] Mn 11 Si 19 , [13] Mn 15 Si 26 , [14] and Mn 27 Si 47 [15] ) which has garnered extensive interest in thermoelectric (TE) materials research, owing to their remarkable electrical and thermal transport properties. [16][17][18][19][20] The odd-stoichiometry (x), that is, Si/Mn ratio in MnSi x refers to the empirical observations wherein the ratio of Mn:Si atoms upon synthesis is generally found to be an irrational number that ranges between 1.72 and 1.75. [10,18,21] The inevitable and incommensurate presence of multiple phases in the synthesized MnSi x crystals with varying stoichiometry has primarily led to an obscure interpretation of structure and bonding in MnSi x .…”