Infinitely Adaptive Transition-Metal Ordering in Ln₂O₂MSe₂-Type Oxychalcogenides

A number of Ln₂O₂MSe₂ (Ln = La and Ce; M = Fe, Zn, Mn, and Cd) compounds, built from alternating layers of fluorite-like [Ln₂O₂]²⁺ sheets and antifluorite-like [MSe₂]^2- sheets, have recently been reported in the literatures. The available MSe_4/2 tetrahedral sites are half-occupied, and different compositions display different ordering patterns: [MSe₂]^2- layers contain MSe_4/2 tetrahedra that are exclusively edge-sharing (stripe-like), exclusively corner-sharing (checkerboard-like), or mixtures of both. This paper reports 60 new compositions in this family. We reveal that the transition-metal arrangement can be systematically controlled by either Ln or M doping, leading to an "infinitely adaptive" structural family. We show how this is achieved in La₂O₂Fe_1-xZn_xSe₂, La₂O₂Zn_1-xMn_xSe₂, La₂O₂Mn_1-xCd_xSe₂, Ce₂O₂Fe_1-xZn_xSe₂, Ce₂O₂Zn_1-xMn_xSe₂, Ce₂O₂Mn_1-xCd_xSe₂, La_2-yCe_yO₂FeSe₂, La_2-yCe_yO₂ZnSe₂, La_2-yCe_yO₂MnSe₂, and La_2-yCe_yO₂CdSe₂ solid solutions.