[en] Full text: In accordance with x-ray diffraction measurements hot pressed lanthanum modified lead zirconate titanate (Pb_yLa_x)(Zr_yTi_1-y)O₃ ceramics with y = 85 and x = 1, 2, 3, 4, 4.5, 5, 6, 8, 10 mole percents (PLZT x/85/15) have a homogeneous perovskite structure, where the size of the elementary cell at room temperature monotonously decrease with increase of the La concentration. The measurements of the dielectric permittivity ε, polarisation and thermal expansion has been done for the given samples. HP4284A precision LRC meters was used to measure the dielectric properties at temperature range 20⁰C - 300⁰C in frequencies 1 kHz - 100 kHz. Ferroelectric properties of the ceramics were evaluated by polarisation-electric field hysteresis loops measured on a modified Sawyer-Tower circuit at frequency 15 Hz. The thermal expansion dl(t) in the temperature range 25⁰C - 400⁰C for PLZT x/85/15 ceramics have been made in the unpolarized and in the polarized states with Perkin Elmer Dynamic Mechanical Analyser DMA7 to determine the field induced deformation effects. The phase transition for the temperature dependence of the dielectric permittivity is between paraelectric (PE) and ferroelectric (FE) or antiferroelectric (AFE) phase which shifts to the lowest temperatures, maximum of ε become lower and diffuse (without showing characteristic dispersion of the diffuse phase) with increase of the La concentration. Thermal expansion results shows phase transition between PE and rhombohedral symmetry of the ferroelectric phase for PLZT with La concentration until 3%. With La concentration from 4% phase transition is between PE and AFE phase and, in addition to increase of the La, characteristic thermal expansion jump of AFE phase become lower and practically disappear at the La 6%. The given results confirm that La stabilise AFE phase and give a possibility to establish diffuse AFE phase. The reason of the diffuse phase is depress of AFE property with increasing of La concentration but not because of the fluctuation of the constitution