Structure of Li4B2O5: High-temperature monoclinic and low-temperature orthorhombic forms

Crystal structures of both polymorphs of Li4B2O5 were determined. The high temperature phase crystallized in a monoclinic space group I2/a with lattice constants a=10.2269(18) Å, b=4.6988(5) Å, c=8.7862(16) Å, and β=93.562(14)°, while the low temperature one crystallized in an orthorhombic space group Pca21 with lattice constants a=10.1497(8) Å, b=4.7365(5) Å, c=17.5880(14) Å. Though the lattice of the low temperature phase is of higher symmetry than that of the high temperature one, the structure itself loses symmetry elements when it transforms from the high temperature phase to the low temperature polymorph. The bond strain associated with the high temperature structure is supposed to be the reason for the phase transition. A slight increase in volume per formula unit was observed when Li4B2O5 transforms from the high temperature polymorph to low temperature structure, which is quite unusual and has to be studied further.