Critical cation-anion radius ratio and two-dimensional antiferromagnetism in van der Waals TMPS₃(TM = Mn, Fe, Ni)

Two-dimensional TMPS3 antiferromagnets, transition metal (TM) = Mn, Fe, Ni, are studied by high-energy x-ray diffraction and atomic pair distribution analysis over a broad temperature range. Results show that the compounds exhibit common average but distinct local atomic structure, including distinct distortions of the constituent TM-S octahedra, magnitude and direction of atomic displacements, TM-TM distances and TM-S-TM bond angles. The differences in the local structure may be rationalized in terms of the Pauling's rule for the critical ratio of TM2+ cation and S2- anion radii for octahedral coordination. We argue that the observed differences in the local structure are behind the differences in the antiferromagnetic properties of TMPS3 compounds, including different magnetic anisotropy and Neel temperature.