CdP2N4 and MnP2N4 - Ternary Transition-Metal Nitridophosphates

Abstract

The ternary transition-metal nitridophosphates CdP2N4 and MnP2N4 have been synthesized under high-pressure higherature conditions (5-8 GPa, 1000-1300°C) by using the multianvil technique. Cd and Mn azides can be used as the starting materials, however, with respect to safety considerations, it is much more advantageous to start from metal powders and phosphorus nitride imide, HPN2. Both nitridophosphates crystallize in a structure closely related to the megacalsilite structure type. As a result of the known issues concerning superstructures with this type of structure, TEM investigations were performed on CdP2N4, which revealed that the megacalsilite superstructure is not equally pronounced in all crystallites. By adding NH4Cl as mineralizer, single crystals were obtained that exhibit unequally pronounced superstructure reflections. Consequently, an averaged structural model was used and refined by the Rietveld method [P6322, a = 16.7197(3), c = 7.6428(2) Å, V = 1850.3(2) Å3, Rp = 0.0671, wRp = 0.0869 for CdP2N4 and P6322, a = 16.5543(2), c = 7.5058(2) Å, V = 1781.3(1) Å3, Rp = 0.0526, wRp = 0.0697 for MnP2N4]. The 31P NMR spectra exhibit four signal groups at (6.4, 4.8), 0.8, and -9.7 ppm with pronounced shoulders belonging to the same phase in an approximate area ratio of 4.8:1.1:2.0, thereby proving at least eight P sites.