Pressure-Driven Evolution of the Covalent Network in CaB6

We synthesized and solved an unexpectedly complex crystal structure of CaB6 under high pressures (up to 44 GPa) and temperatures. The only known crystal structure in the large family of metal hexaborides, a simple cubic cP7 type discovered over 80 years ago, is shown here to transform into a tetragonal tI56 configuration comprised of unfamiliar 24-atom boron units. The interpretation of the convoluted x-ray diffraction pattern was accomplished with an ab initio evolutionary search which identified the tI56 structure (28 atoms per primitive unit cell) without any parameter input. The exotic CaB6 phase was successfully quenched down to ambient pressure.