$MA{\mathrm{N}}_3$ ($M$=V, Nb, Ta; $A$=Si, Ge) monolayers, intrinsic Janus materials derived from the $M{A}_2{\mathrm{N}}_4$ family, are known to be either in 1T phase or in 2H phase with polar characteristics. However, the exploration of phase space in two-dimensional (2D) $MA{\mathrm{N}}_3$ monolayers remains incomplete, and the discovery of new phases could further enrich the understanding of its structure-property relationships and expand its functional versatility. In this work, through systematic structural searches and stability evaluations, we have successfully identified a third phase in 2D $MA{\mathrm{N}}_3$ monolayers, hereafter referred to as the L phase, using first-principles calculations, and systematically investigated its structural, electronic, and magnetic properties. The results show that the L phase preserves intrinsic polarity while exhibiting pronounced composition-dependent polarization behavior. Compared with the 1T/2H phases, the polarization strengths of ${\mathrm{VSiN}}_3$ and ${\mathrm{VGeN}}_3$ in the L phase decrease, whereas those in ${\mathrm{NbGeN}}_3$ and ${\mathrm{TaGeN}}_3$ increase by more than a factor of 2. Based on these findings, the classification of low polarization state and high polarization state to describe the relative magnitude of polarization across different structural phases of the same material system is proposed. In addition, the L-$MA{\mathrm{N}}_3$ monolayers feature distinct flat bands near the valence band maximum, leading to a high density of states near the Fermi level. Upon hole doping, the systems exhibit emergent Stoner-type ferromagnetism and undergo a transition from nonmagnetic semiconductors to ferromagnetic half-metals. These findings provide a theoretical foundation and material candidates for polymorphic regulation and the design of spintronic devices in two-dimensional polar materials.

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• Density of states
• Electronic structure
• Magnetic order
• Structural properties
• Ferroelectrics
• Janus materials
• Monolayer films
• Density functional calculations