text: Borate anions (and functional groups) consist of trigonal planar  and/or tetrahedral  structural units, joined together via shared oxygen atoms (corners) or atom pairs (edges) into larger clusters so as to construct various ions such as , , , , , etc. These anions may be cyclic or linear in structure, and can further polymerize into infinite chains, layers, and tridimensional frameworks. The terminal (unshared) oxygen atoms in the borate anions may be capped with hydrogen atoms () or may carry a negative charge ().
  The planar  units may be stacked in the crystal lattice so as to have π-conjugated molecular orbitals, which often results in useful optical properties such as strong harmonics generation, birefringence, and UV transmission.
  Polymeric borate anions may have linear chains of 2, 3 or 4 trigonal  structural units, each sharing oxygen atoms with adjacent unit(s). as in lithium metaborate|, contain chains of trigonal  structural units. Other anions contain cycles; for instance, Sodium metaborate| and  contain the cyclic  ion, consisting of a six-membered ring of alternating boron and oxygen atoms with one extra oxygen atom attached to each boron atom.
  The thermal expansion of crystalline borates is dominated by the fact that  and  polyhedra and rigid groups consisting of these polyhedra practically do not change their configuration and size upon heating, but sometimes rotate like hinges, which results in greatly anisotropic thermal expansion including linear negative expansion.