Magnetic, Optical, and Electronic Properties of the Phosphide Oxides REZnPO (RE = Y, La–Nd, Sm, Gd, Dy, Ho)^†

Well-shaped yellow to red transparent single crystals of the phosphide oxides REZnPO (RE = Y, La–Nd, Sm, Gd, Dy, Ho) were synthesized from the elements and ZnO in NaCl/KCl fluxes in sealed silica ampoules. Four structures (NdZnPO type, R3m) were refined from single crystal X-ray diffractometer data: a = 388.5(2), c = 3032(1) pm, wR2 = 0.0380, 360 F² values for YZnPO, a = 394.6(2), c = 3071(1) pm, wR2 = 0.0587, 226 F² values for SmZnPO, a = 392.2(2), c = 3056(1) pm, wR2 = 0.0262, 462 F² values for GdZnPO, and a = 389.33(6), c = 3030.5(4) pm, wR2 = 0.0453, 217 F² values for DyZnPO each with 14 variables per refinement. The structures are composed of alternate stacks of (RE³⁺O²⁻) and (Zn²⁺P³⁻) layers with covalent RE–O and ZñP bonding within and weak ionic bonding between the layers. The zinc and oxygen atoms have slightly distorted tetrahedral coordination by atoms of phosphorus and the rare earth element, respectively. According to the electron precise formulation RE³⁺Zn²⁺P³⁻O²⁻, these pnictide oxides are transparent in visible light. Susceptibility measurements on β-CeZnPO, β-PrZnPO, and GdZnPO reveal Curie-Weiss paramagnetism with experimental magnetic moments of 2.31, 3.60, and 7.72 μ_B/RE atoms, respectively. β-CeZnPO and β-PrZnPO show antiferromagnetic order with Néel temperatures of 7.4 (Ce) and 2.2 (Pr) K. GdZnPO shows no magnetic ordering down to 2 K. Single crystal absorption spectra measured for REZnPO (RE = Y, La, Pr, Nd, Sm, Dy) in the NIR-Vis region reveal unexpected variations for the optical band gap of these phosphide oxides. For RE = Pr, Nd, Sm, Dy, Ho f-f electronic transitions with nicely resolved ligand-field splittings are observed in the range 6000–20000 cm⁻¹. DFT band structure calculations show similarity between the valence bands of tetragonal and rhombohedral REZnPO as they possess mainly P-3p character. In both cases, the conduction bands have mainly Zn-4s character, but a significant contribution of RE-5d occurs in rhombohedral REZnPO, which is responsible for a smaller optical band gap for the latter compounds. Variations of the energy gaps of tetragonal REZnPO can be explained by hybridization of Zn-4s + RE-5d + RE-4f orbitals for the conduction band. DFT volume optimizations of α- and β-PrZnPO show β-PrZnPO to be more stable by 10.7 kJ mol⁻¹.