The phase diagrams of the ZrO2HfO2--MgO [i] and the ZrO2--HfO2--CaO systems have many common features. These are eutectoid-type diagrams. In both systems, there is a narrow (up to 1%* MgO or CaO) region of monoclinic solid solutions at room temperature and a wide range of solid solutions having a fluorite-type structure; solid solutions based on MgO and CaO have not been established [2].The presence of chemical compounds is of great significance when plotting the phase diagrams of the aforementioned systems. Increasing basicity in the zr+Hi and Mg+Ca series (order) and the difference in the ionic radii of the indicated cations lead to the appearance of one compound each in the ZrO2--MgO and the HfOH--MgO systems: Mg2Zr~012 and Mg2HfsOl~, respectively [3, 4], and in the ZrO2--HfO2--MgO system, there is an unlimited (continuous) series of solid solutions based on these compounds that form fluorite-type rhombic crystals having lattice distortion in the [IIi] direction and belonging to the R3 space group; on the other hand, in the analogous Ca-containing system, there are four compounds, viz., CaHfO~ (CaZrOa) [5], CaHf~09 (CaZr~Og) [6], Ca2Hf?O16 (Ca2ZrT016), CasHfl~044 (Ca6Zr19044) [7]. Among these compounds, calcium tetrazirconate and tetrahafnate and calciumzirconate and hafnate are the most important ones. The latter compounds of the series (Ca2HfT016 and Ca6HflgO44) are metastable [7] and are present only partially in the investigated region of the phase diagram.An analysis of the published literature showed that the structure of the phases present in the ternary systems based on ZrO= has not been studied in detail. This paper deals with a study of the structure of the phases existing in the ZrO~--HfO2--MgO(CaO) systems using the methods of x-ray phase analysis, high-temperature diffraction, and electron microscopy. The obtained results and the new (modified) form of the phase diagram of the ZrO~--HfO2--CaO system are presented.The monoclinic solid solutions of the ZrO2--HfO2--MgO(CaO) systems were studied in order to understand the effect of the additives on the temperature and the nature of the polymorphic transformation. X-ray studies estaSlished that the addition of MgO and CaO leads to a decrease in the temperatures of the monoclinic--tetragonal transformation and that an increase in the concentration of HfO 2 shifts this transformation towards the region of higher temperatures [2].As concerns zirconium and hafnium dioxides, the ~ ~ ~ modification is a martensitetype diffusionless reversible transformation. In this case, the linearity of the temperature dependence of the interplanar distances (lattice parameters) of the aforementioned oxides is retained up to and in the region of the transformation temperatures [8]. Heating the specimens of the ternary systems containing 1% MgO or Ca0 in the furnace of the diffractometer showed that the linear nature of this relation is retained right up to the beginning of the + 8 transformation.In the region of transformation, we observes significant deviations from the ...