Topologically Linked Crystals
  We discovered a new class of topological crystals, namely linked rings of crystals. Two rings of tantalum triselenide (TaSe3) single crystals were linked to each other while crystal growing. The topology of the crystal form is called a "Hopf link", which is the simplest link involving just two component unknots linked together exactly once. The feature of the crystals is not covered by the conventional crystallography. 
Toru Matsuura, Masanori Yamanaka, Noriyuki Hatakenaka, Toyoki   Matsuyama, and Satoshi Tanda