Infinitely adaptive transition metal oxychalcogenides: The modulated structures of Ce₂O₂MnSe₂ and (Ce_0.78La_0.22)₂O₂MnSe₂

This article reports the syntheses, structures, and physical properties of the oxychalcogenides (Ce_1-xLa_x)₂O₂MnSe₂ with x = 0-0.7. These materials have a layered structure related to that of the LaOFeAs-derived superconductors but with the transition metal sites 50% occupied. Ce₂O₂MnSe₂ contains alternating layers of composition: [Ce₂O₂]²⁺ and [MnSe₂]^2-. The size mismatch between the layers leads to an incommensurate structure with a modulation vector of q = αa∗+ 0b∗+0.5c∗ with α = 0.158(1), which can be described with a (3 + 1)D superspace structural model in superspace group Cmme(α,0,¹/₂)0s0 [67.12]. There is a strong modulation of Mn site occupancies, leading to a mixture of corner- and edge-sharing MnSe_4/2 tetrahedra in the [MnSe₂]^2- layers. The modulation vector can be controlled by partial substitution of Ce³⁺ for larger La³⁺, and a simple commensurate case was obtained for (Ce_0.78La_0.22)₂O₂MnSe₂ with α = ¹/₆. The materials respond to the change in relative size of the oxide and chalcogenide blocks by varying the ratio of corner- to edge-sharing tetrahedra. The superspace model lets us unify the structural description of the five different ordering patterns reported to date for different Ln₂O₂MSe₂ (Ln = lanthanide) materials. Mn moments in Ce₂O₂MnSe₂ and (Ce_0.78La_0.22)₂O₂MnSe₂ order antiferromagnetically below T_N = 150 K, and Ce moments order below ∼70 K. The magnetic structures of both materials have been determined using neutron diffraction. Both materials are semiconductors; Ce₂O₂MnSe₂ has σ = 9 × 10^-6 Ω^-1 cm^-1 at room temperature and an activation energy for charge carrier mobility from RT to 170 °C of ∼0.4 eV.