[en] X-ray phase analysis, X-ray dilatometry, infrared-spectroscopy, crystal optics and measuring of melting points have been used for studying solid solutions of the system La₂Zr₂O₇-La₂Hf₂O₇. The phase diagram of the system has been plotted. It has been established that solid solutions of the system are crystallized in cubic lattice of the pyrochlor type. Increase of lanthanum hafnate content in the solid solution changes the form of crystals, grain sizes and rises the amount of closed pores in the sample. The lattice parameters and refractive indices of solid solutions decrease linearly with the increase of Hf⁴⁺ concentration. Simultaneously coefficients of thermal expansion of solid solutions decrease in the temperature range 20-450 deg