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 Nb₃As₂C, 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 Nb₃P₂C and 75.4(5)% in Nb₂PC. In contrast, MSS performed poorly on As-containing MAX phases, only 8.9(4)% for Nb₃As₂C and 64(2)% for Nb₂AsC, as opposed to the pure phases obtained by SSS. The experimental analyses combined with first-principles calculations reveal that the dominant formation route of Nb₃P₂C is through NbP + Nb + C → Nb₃P₂C. 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 Nb₂PC + NbP → Nb₃P₂C. 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.

321相是非典型系列的MAX相，其中A = As/P，具有优异的弹性，在晶体结构中具有MA-三角棱柱双层结构。迄今为止，除Nb ₃ As ₂ C外，其他321种化合物的纯相尚未实现，阻碍了对其固有性质的研究。在这里，熔盐烧结（MSS）和固态合成（SSS）被用于合成含As/P的321相和211相。通过多相 Rietveld 精修分析最终产物的相组成，我们发现 MSS 可以有效提高含 P MAX 相的纯度，Nb ₃ P ₂ C 和 75.4(5 )% Nb ₂个人电脑。相比之下，MSS 在含 As 的 MAX 相上表现不佳，与SSS 获得的纯相相比， Nb ₃ As _{2 C 仅为 8.9(4)%，Nb 2} AsC为 64(2)% 。实验分析结合第一性原理计算表明，Nb ₃ P ₂ C的主要形成途径是NbP + Nb + C → Nb ₃ P ₂ C。此外，我们发现MSS对含P MAX相的优势正在促进三种考虑的化学反应路线，尤其是在 Nb ₂ PC + NbP → Nb ₃ P _2上C. 此外，从理论上和实验上揭示了由电子、空穴和开轨道组成的两个 321 相的固有物理性质和费米表面拓扑结构，其中电子载流子在电传输中占主导地位。可行的合成方法和形成机理对于获得高纯度的含As/P的MAX相和探索新的MAX相具有指导意义。同时，固有的物理特性将为未来在 321 相上的应用提供很大的支持。