Experimental Determinations of the High-Pressure Crystal Structures of Ca₃N₂

Abstract

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) Ca3N2 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 Ca3N2 under high pressure. The phase sequence observed in Ca3N2 is in accordance with the systematic behavior of C → B → A phase transitions occurring in most sesquioxides.