Dimorphic CeZnPO and PrZnPO

Well-shaped single crystals of the phosphide oxides CeZnPO and PrZnPO were obtained from the rare earth metals, zinc oxide, and red phosphorous in an equimolar NaCl/KCl flux in sealed quartz ampoules. The high-temperature (β) NdZnPO type modifications were obtained at 1170 K while the low-temperature (α) ZrCuSiAs type modifications crystallized at 970 K. The four compounds were characterized by X-ray diffraction on powders and single crystals: P4/nmm, a = 401.08(9), c = 882.0(4) pm, wR2 = 0.1277, 173 F² values, 9 variables for α-CeZnPO, a = 399.3(2), c = 877.2(7) pm, wR2 = 0.0583, 220 F² values, 12 variables for α-PrZnPO, and R3m, a = 401.2(4), c = 3121(3) pm, wR2 = 0.0221, 509 F² values, 14 variables for β-CeZnPO, a = 399.0(2), c = 3108(1) pm, wR2 = 0.0452, 291 F² values, 14 variables for β-PrZnPO. In both modifications we observe clear separation into rare earth-oxygen and zinc-phosphorous layers without any P-P and Zn-O interactions, leading to electron precise formulations Ce ³⁺Zn²⁺P^3-O^2- and Pr ³⁺Zn²⁺P^3-O^2- for these transparent compounds. The crystal chemical peculiarities are briefly discussed.