Each powder was placed in a graphite die (30 mm inner diameter) lined with a graphite foil (0.35mm thick) and sintered under vacuum. The sintering cycle, optimized in previous work , was applied to all the compositions. This cycle consists in heating up to 1450°C at a speed of 100°C/min and under a pressure of 20 MPa, then applying a dwell time of 15 min at 1450°C and under a pressure of 40 MPa or 80 MPa (hereafter referred as HP sample). Finally, the sample was cooled at 100°C/min down to room temperature. The pellets obtained have a diameter of 30 mm and a thickness of about 4-5 mm.