Rare-Earth Metal(III) Oxide Selenides M₄O₄Se[Se₂] (M = La, Ce, Pr, Nd, Sm) with Discrete Diselenide Units: Crystal Structures, Magnetic Frustration and Other Properties

The rare-earth metal(III) oxide selenides of the formula La 4O4Se[Se2], Ce4O 4Se[Se2], Pr4O4Se[Se2], Nd4O4Se[Se2], and Sm4O 4Se[Se2] were synthesized from a mixture of the elements with selenium dioxide as the oxygen source at 750°C. Single crystal X-ray diffraction was used to determine their crystal structures. The isostructural compounds M4O4Se[Se2] (M = La, Ce, Pr, Nd, Sm) crystallize in the orthorhombic space group Amm2 with cell dimensions a = 857.94(7), b = 409.44(4), c = 1316.49(8) pm for M = La; a = 851.37(6), b = 404.82(3), c = 1296.83(9) pm for M = Ce; a = 849.92(6), b = 402.78(3), c = 1292.57(9) pm for M = Pr; a = 845.68(4), b = 398.83(2), c = 1282.45(7) pm for M = Nd; and a = 840.08(5), b = 394.04(3), c = 1263.83(6) pm for M = Sm (Z = 2). In their crystal structures, Se2- anions as well as [Se-Se] 2- dumbbells interconnect ∞2{[M 4O4]4+} layers. These layers are composed of three crystallographically different, distorted [OM4]10+ tetrahedra, which are linked via four common edges. The compounds exhibit strong Raman active modes at around 215 cm-1, which can be assigned to the Se-Se stretching vibration. Optical band gaps for La4O 4Se[Se2], Ce4O4Se[Se2], Pr4O4Se[Se2], Nd4O 4Se[Se2], and Sm4O4Se[Se 2] were derived from diffuse reflectance spectra. The energy values at which absorption takes place are typical for semiconducting materials. For the compounds M4O4Se[Se2] (M = La, Pr, Nd, Sm) the fundamental band gaps, caused by transitions from the valence band to the conduction band (VB-CB), lie around 1.9 eV, while for M = Ce an absorption edge occurs at around 1.7 eV, which can be assigned to f-d transitions of Ce 3+. Magnetic susceptibility measurements of Ce4O 4Se[Se2] and Nd4O4Se[Se2] show Curie-Weiss behavior above 150 K with derived experimental magnetic moments of 2.5 μB/Ce and 3.7 μB/Nd and Weiss constants of θp = -64.9 K and θp = -27.8 K for the cerium and neodymium compounds, respectively. Down to 1.8 K no long-range magnetic ordering could be detected. Thus, the large negative values for θp indicate the presence of strong magnetic frustration within the compounds, which is due to the geometric arrangement of the magnetic sublattice in form of [OM4]10+ tetrahedra