Following the determination of the structures of several anhydrous cesium borates and especially two new compounds with original formula, Cs-3 B7O12 and CS3B13O21, a new form of the Cs2O-B2O3 phase diagram is presented. Thermal analyses using DTA and temperature-resolved X-ray powder diffraction were performed to determine the thermal properties of some compounds. CsBO2, CsB3O5, CsB5O8 (variety IV) and CsB9O4 melt congruently at 720degreesC, 835degreesC, 670degreesC and 597degreesC, respectively. The other borates melt incongruently at 620degreesC for Cs2B4O7, 685degreesC for Cs3B7O12 (B/Cs = 2.33) and 663degrees C for Cs3B13O21 (B/Cs = 4.33). These two latter replace Cs4B10O17 (B/Cs = 2.5) and Cs2B8O13 (B/Cs = 4) in the phase diagram, respectively. CsB9O14 and CsB3O5 crystallize in a non-centrosymmetric space group that leads to non-linear optical properties, which have already been studied for the latter. (C) 2003 Elsevier B.V. All rights reserved.