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Single crystals of lithium diborate, Li20.2B~Oa, have been synthesized and the crystal structure determined. The crystals belong to the space group I41cd, with unit-cell dimensions:a=9.47 A, c=10.26 A.An approximate set of atomic parameters was determined by trial-and-error methods, and the parameters were refined by calculating electron-density projections. The result shows that the borate anion consists of two three-dimensional interlocking networks, extending throughout the crystal. The basic unit of the networks is a group consisting of two boron atoms tetrahedrally coordinated by oxygen and two boron atoms triangularly coordinated by oxygen. The atoms in the group are arranged as a twisted double ring. This group is similar to one found as an isolated ion in borax, Na20.2B~O a. 10H20. Half of the boron atoms in lithium diborate are four-fold coordinated. This is shown to be consistent with a relationship valid for most borate structures. Two exceptions to this relationship are discussed.

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