THEORETICAL CONFIRMATION OF THE HIGH-PRESSURE SIMPLE CUBIC PHASE IN CALCIUM

The remarkable experimental discovery of the simple cubic (sc) structu re in compressed calcium is investigated theoretically by means of fir st-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 pres sure calcium shows a unique decrease in the number of nearest neighbor s (NN), starting from 12 NN (face centered cubic), via a structure wit h 8 NN (body centered cubic), into a phase with only 6 NN (simple cubi c). We present an explanation for this sequence of structural transiti ons and predict a similar high pressure behavior for Na and Mg.