Editing Borate (section)

== Structures ==
Borate anions (and functional groups) consist of [[trigonal planar molecular geometry|trigonal planar]] {{chem2|BO3}} and/or [[tetrahedral molecular geometry|tetrahedral]] {{chem2|BO4}} structural units, joined together via shared oxygen atoms (corners) or atom pairs (edges) into larger clusters so as to construct various ions such as {{chem2|[B2O5](4-)}}, {{chem2|[B3O8](7-)}}, {{chem2|[B4O12](12-)}}, {{chem2|[B5O6(OH)5](2-)}}, {{chem2|[B6O13](8-)}}, etc. These anions may be cyclic or linear in structure, and can further polymerize into infinite chains, layers, and tridimensional frameworks.<ref name=wibe2001/><ref name=muta2016/> The terminal (unshared) oxygen atoms in the borate anions may be capped with hydrogen atoms ({{chem2|\sOH}}) or may carry a negative charge ({{chem2|\sO−}}).

The planar {{chem2|BO3}} units may be stacked in the crystal lattice to have [[pi bond|π-conjugated]] [[molecular orbital]]s, which often results in useful optical properties such as strong [[harmonic]]s generation, [[birefringence]], and [[ultraviolet light|UV transmission]].<ref name=muta2016/>

Polymeric borate anions may have linear chains of 2, 3 or 4 trigonal {{chem2|BO3}} structural units, each sharing oxygen atoms with adjacent unit(s).<ref name=wibe2001/> as in [[lithium metaborate|{{chem2|LiBO2}}]], contain chains of trigonal {{chem2|BO3}} structural units. Other anions contain cycles; for instance, [[Sodium metaborate|{{chem2|NaBO2}}]] and {{chem2|KBO2}} contain the cyclic {{chem2|[B3O6](3−)}} ion,<ref name=GandE.P205/> 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 {{chem2|BO3}} and {{chem2|BO4}} 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.
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