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The subsolidus phase relations of the system La2O3-CaO-B2O3 are investigated by means of X-ray diffraction (XRD) analyses. There are seven binary compounds and five ternary compounds in this system. The phase diagram comprises eighteen three-phase regions. A new ternary compound Ca4LaO(BO3)(3) (CLOB) is identified. It has a monoclinic structure with cell parameters a=8.1732(1) Angstrom, b=16.0860(3) Angstrom, c=3.6268(1) Angstrom, beta =101.40(1)degrees and space group Cm. At low temperature, the XRD patterns of the CLOB compound indicate that a phase transition in the low temperature range is absent. The expansion coefficients in the three major directions are different, the value of alpha (c) being the largest. (C) 2001 Elsevier Science B.V. All rights reserved.