The novel high-pressure/high-temperature compound Co12P7 determined from synchrotron data

Abstract

The structural properties of cobalt phosphides were investigated at high pressures and temperatures to better understand the behavior of metal-rich phosphides in Earth and planetary interiors. Using single-crystal X-ray diffraction synchrotron data and a laser-heated diamond anvil cell, we discovered a new high pressure–temperature (HP–HT) cobalt phosphide, Co12P7, dodecacobalt heptaphosphide, synthesized at 27 GPa and 1740 K, and at 48 GPa and 1790 K. Co12P7 adopts a structure initially proposed for Cr12P7 (space-group type P\overline{6}, Z =1), consisting of chains of edge-sharing CoP5 square pyramids and chains of corner-sharing CoP4 tetrahedra. This arrangement leaves space for trigonal–prismatic channels running parallel to the c axis. Coupled disordering of metal and phosphorus atoms has been observed in this structure for related M 12P7 (M = Cr, V) compounds, but all Co and P sites are ordered in Co12P7. All atomic sites in this crystal structure are situated on special positions. Upon decompression to ambient conditions, peak broadening and loss of reflections at high angles was observed, suggesting phase instability.