Figure 1
Phys. Rev. B 82, 134126 (2010)

Squares of carbon. In bct-carbon, 4-atom rings are joined together in a three-dimensional network (above; “short” bonds come out of the plane of the screen). A perpendicular view (top) shows the vertical arrangement of the squares (shown edge-on as horizontal bonds). This form of pure carbon may appear when graphite is placed under high pressure.