Lattice collapse and the magnetic phase diagram of Sr1-x Cax Co2 P2

We report that the 122-type Sr1-x Cax Co2 P2 solid solution undergoes an anomalous structural transition from the uncollapsed to the collapsed ThCr2 Si2 structure at a distinct onset composition near x=0.5. Correlated with the structural changes, the electronic system evolves from a nearly ferromagnetic Fermi liquid to an antiferromagnetic metal, through a complex crossover regime. The structural collapse, driven by P-P bonding across the (Sr,Ca) layers, is much more pronounced in this system than it is in the analogous Fe-based system, indicating a strong sensitivity of structure to total electron count in the transition-metal pnictide 122 family.