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 inter-iors. 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, Co(12)P(7), dodeca-cobalt hepta-phosphide, synthesized at 27 GPa and 1740 K, and at 48 GPa and 1790 K. Co(12)P(7) adopts a structure initially proposed for Cr(12)P(7) (space-group type P , Z =1), consisting of chains of edge-sharing CoP(5) square pyramids and chains of corner-sharing CoP(4) tetra-hedra. This arrangement leaves space for trigonal-prismatic channels running parallel to the c axis. Coupled disordering of metal and phospho-rus atoms has been observed in this structure for related M (12)P(7) (M = Cr, V) compounds, but all Co and P sites are ordered in Co(12)P(7). 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.