Local order in layered NiPS₃ and Ni_0.7Mg _0.3PS₃

The family of two-dimensional magnetic materials M^IIPS ₃ where M =Mn, Fe, Ni, Mg, Zn, etc shows a wide range of fascinating magnetic behaviour. It also shows potentially useful chemical properties including intercalation of nonlinear optical molecules and lithium ions. These properties are due to a crystal structure in which the ab planes are well-ordered in the plane but poorly correlated along c. Here, the short-range ordering is modelled in NiPS₃ and Ni_1-xMg _xPS₃ (x = 0.3). X-ray diffuse scattering from NiPS3 shows pronounced streaking along c*, indicative of stacking faulting in these layered compounds. Electron diffraction from Ni_1-xMg _xPS₃ (x = 0.3) shows substantial diffuse scattering due to short-range order within the ab plane, and this can be modelled by allowing the metal species to cluster. The possibility of clustering has implications for interpretation of the magnetic behaviour of the family, including the glassiness observed in Fe_1-xMn_xPS₃.