Mn₂P₂S₃Se₃: a two-dimensional Janus room-temperature antiferromagnetic semiconductor with a large out-of-plane piezoelectricity

Structural symmetry breaking in two-dimensional materials plays a vital role in determining their electronic, valleytronic, and magnetic properties. Motivated by the recently synthesized 2D antiferromagnetic semiconductor MnPS₃ and Janus system MoSSe, by first-principles calculations we here propose a new stable antiferromagnetic Mn₂P₂S₃Se₃ Janus monolayer semiconductor with a direct band gap of about 1.75 eV. It is found that 2D Mn₂P₂S₃Se₃ shows a high Néel temperature of up to 315 K and a sizable magnetocrystalline anisotropy with easy in-plane magnetization. Interestingly, spontaneous valley polarization is observed because of the coexistent broken space- and time-inversion symmetries. Meanwhile, Mn₂P₂S₃Se₃ exhibits a large out-of-plane piezoelectricity due to the mirror asymmetry. Moreover, the magnetic transition temperature can be significantly increased under biaxial in-plane compressive strain due to the enhanced magnetic exchange interaction. However, applying the strain does not affect the magnetic order and easy in-plane magnetocrystalline anisotropy of the predicted system. These results demonstrate that the 2D Janus Mn₂P₂S₃Se₃ monolayer is a very promising candidate for designing intriguing antiferromagnet-based valleytronic devices.