Theoretical Confirmation of the High Pressure Simple Cubic Phase in Calcium

The remarkable experimental discovery of the simple cubic (sc) structure in compressed calcium is investigated theoretically by means of first-principles calculations. The calculated transition pressure for the occurrence of the simple cubic phase as well as the associated volume collapse agree very well with experiments. Thus, with increasing pressure calcium shows a unique decrease in the number of nearest neighbors (NN), starting from 12 NN (face centered cubic), via a structure with 8 NN (body centered cubic), into a phase with only 6 NN (simple cubic). We present an explanation for this sequence of structural transitions and predict a similar high pressure behavior for Na and Mg.