Synthesis, formation mechanism, and intrinsic physical properties of several As/P-containing MAX phases

Abstract

• Molten-salt sintering is applicable to P-based MAX phases • Solid-state sintering is suitable to As-based MAX phases • Route 1: NbP+Nb+C → Nb 3 P 2 C is most favorable to obtain pure Nb 3 P 2 C • Molten-salt sintering do well on all routes, especially for Nb 2 PC+NbP→Nb 3 P 2 C • Various intrinsic physical properties of 321 phases are firstly reported 321 phases are an atypical series of MAX phases, in which A = As/P, with superior elastic properties, featuring in the MA-triangular-prism bilayers in the crystal structure. Until now, besides Nb 3 As 2 C, the pure phases of the other 321 compounds have not been realized, hampering the study of their intrinsic properties. Here, molten-salt sintering (MSS) and solid-state synthesis (SSS) were applied to synthesize As/P-containing 321 phases and 211 phases. Analyzing the phase composition of the end-product via multiple-phase Rietveld refinement, we found that MSS can effectively improve the purity of P-containing MAX phases, with the phase content up to 99% in Nb 3 P 2 C and 75.4(5)% in Nb 2 PC. In contrast, MSS performed poorly on As-containing MAX phases, only 8.9(4)% for Nb 3 As 2 C and 64(2)% for Nb 2 AsC, as opposed to the pure phases obtained by SSS. The experimental analyses combined with first-principles calculations reveal that the dominant formation route of Nb 3 P 2 C is through NbP + Nb + C → Nb 3 P 2 C. Moreover, we found that the benefits of MSS on P-containing MAX phases are on the facilitation of three considered chemical reaction routes, especially on Nb 2 PC + NbP → Nb 3 P 2 C. Furthermore, the intrinsic physical properties and Fermi surface topology of two 321 phases consisting of electron, hole, and open orbits are revealed theoretically and experimentally, in which the electron carriers are dominant in electrical transport. The feasible synthesis methods and the formation mechanism are instructive to obtain high-purity As/P-containing MAX phases and explore new MAX phases. Meanwhile, the intrinsic physical properties will give great support for future applications on 321 phases.