Description:
<jats:p>The electron density distribution in lithium triborate LiB<jats:sub>3</jats:sub>O<jats:sub>5</jats:sub> has been studied at room temperature by X-ray diffraction using Ag <jats:italic>K</jats:italic>
<jats:italic>\alpha</jats:italic> radiation up to 1.02 Å<jats:sup>−1</jats:sup> [1439 unique reflections with <jats:italic>I</jats:italic> > 3\sigma(<jats:italic>I</jats:italic>)]. Conventional refinements with a free-atom model yield <jats:italic>R</jats:italic>(<jats:italic>F</jats:italic>) = 0.0223, <jats:italic>wR</jats:italic>(<jats:italic>F</jats:italic>) = 0.0299, <jats:italic>S</jats:italic> = 1.632. Atom charge refinements show that the lithium should be considered a monovalent ion. Multipolar refinements were undertaken up to fourth order, imposing local non-crystallographic symmetry constraints in order to avoid phase problems leading to meaningless multipole populations due to the non-centrosymmetry of the structure (space group: <jats:italic>Pn</jats:italic>
<jats:italic>a</jats:italic>2<jats:sub>1</jats:sub>). The residual indices decreased to: <jats:italic>R</jats:italic>(<jats:italic>F</jats:italic>) = 0.0147, <jats:italic>wR</jats:italic>(<jats:italic>F</jats:italic>) = 0.0193, <jats:italic>S</jats:italic> = 1.106. The net charges are in good agreement with what can be expected in borate chemistry. Deformation density maps are analysed in terms of \sigma and \pi bonding. The experimental electron distribution in the <jats:italic>p</jats:italic>
<jats:sub>
<jats:italic>z</jats:italic>
</jats:sub> orbitals of triangular B atoms and surrounding O atoms has been analysed by introducing idealized hybridized states. In parallel, the electron density has been determined from <jats:italic>ab initio</jats:italic> Hartree–Fock calculations on fragments of the structure. Agreement with the X-ray determination is very good and confirms the nature of bonding in the crystal. The amount of transfer of \pi electrons from the oxygen to the triangular B atoms is estimated to be 0.22 electrons by theory.</jats:p>