The phase relationship between langatate (LTG, La3Ta0.5Ga5.5O14) and associated phases was studied. Equilibrium solid phase tie lines were obtained for the ternary system La2O3-Ga2O3-Ta2O5 by phase identification of sintered samples with various compositions through X-ray diffraction analysis. The solid solution range of LTG was determined in the temperature range of 1300-1350 degrees C. A new compound was identified that was stable above 1485 degrees C up to its melting point. Liquidus phases were investigated by in-situ observation of the crystallization process from melts having various compositions. Differential scanning calorimetry analysis was carried out to study the phase transitions associated with eutectic and peritectic reactions. On the basis of the results of these experiments, a primary phase diagram was established around LTG. It was concluded that LTG is an incongruent-melting compound and has a peritectic relation to LaGaO3. (c) 2006 Elsevier B.V. All rights reserved.