Experimental Determinations of the High-Pressure Crystal Structures of Ca3N2

Using synchrotron angle-dispersive X-ray diffraction and Raman spectroscopy techniques in a diamond anvil cell, we have examined the structural behaviors of the cubic (anti-C) Ca 3 N 2 at high pressures and room temperature. Two first-order phase transformations were observed at 9.6 and 19.5 GPa accompanied by large volume collapses of 7 and 9%, respectively. The two high pressure phases were identified as monoclinic (anti-B) and hexagonal (anti-A) structures by Rietveld refinement. With the pressure derivatives fixed at 4, the bulk modulus of anti-C, anti-B, and anti-A type structures were determined as 53.1(2), 61.6(4), and 83.9(2) GPa, respectively, indicating the increased incompressibility of Ca 3 N 2 under high pressure. The phase sequence observed in Ca 3 N 2 is in accordance with the systematic behavior of C → B → A phase transitions occurring in most sesquioxides.