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Figure 2
In this geometric construction, two of the three unit-cell axes, [{\bf a}] and [{\bf b}], are the central axes for two cones of half-angles [\beta] and [\alpha], respectively. When the cones intersect in two straight lines, then two specularly equivalent unit cells are constructed, and the third unit-cell axis, [{\bf c}], coincides with any of the two intersections. If the cones' apertures are progressively made to shrink, the two intersections will get closer and closer, until they coincide with a single straight line lying on the a–b plane; at this point the unit cell will have collapsed to a plane, and its volume is null. Further shrinking will have the effect of making even that single intersection disappear; in this case we have no unit cell and, consequently, its volume is an imaginary number.

Journal logoFOUNDATIONS
ADVANCES
ISSN: 2053-2733
Volume 67| Part 1| November 2011| Pages 93-95
doi:10.1107/S0108767310044296
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