Phenakite-Type BeP2N4-A Possible Precursor for a New Hard Spinel-Type Material

BeP2N4 was synthesized in a multi-anvil apparatus starting from Be3N2 and P3N5 at 5 GPa and 1500 degrees C. The compound crystallizes in the phenakite structure type (space group R ($) over bar, no. 148) with a=1269.45(2) pm, c=834.86(2) pm, V=1165.13(4) x 10(6) pm(3) and Z=18. As isostructural and isovalence-electronic alpha-Si3N4 transforms into beta-Si3N4 at high pressure and temperature, we studied the phase transition of BeP2N4 into the spinel structure type by using density functional theory calculations. The predicted transition pressure of 24 GPa is within the reach of today's state of the art high-pressure experimental setups. Calculations of inverse spinel-type BeP2N4 revealed this polymorph to be always higher in enthalpy than either phenakite-type or spinel-type BeP2N4. The predicted bulk modulus of spinel-type BeP2N4 is in the range of corundum and gamma-Si3N4 and about 40 GPa higher than that of phenakite-type BeP2N4. This finding implies an increase in hardness in analogy to that occurring for the beta- to gamma-Si3N4 transition. In hypothetical spinel-type BeP2N4 the coordination number of phosphorus is increased from 4 to 6. So far only coordination numbers up to 5 have been experimentally realized (gamma-P3N5), though a sixfold coordination for P has been predicted for hypothetic delta-P3N5. We believe, our findings provide a strong incentive for further high-pressure experiments in the quest for novel hard materials with yet unprecedented structural motives.