**1. Introduction**

Calcium titanate (CaTiO3) or perovskite was found by Rose [1] in the Ural Mountains in 1839 and named after the Russian Statesman Count Lev Perovski. Perovskite is a relatively rare mineral, which is a promising material for use as matrices for safe long-term storage of actinides and their rare earth analogs that are present in radioactive waste [2]. It is of particular interest for petrology and cosmochemical research as a mineral which is a part of refractory Ca-Al inclusions often found in carbonaceous chondrites, which are the earliest objects of the solar system with unusual isotopic characteristics [3–5].

In addition to perovskite, two more calcium titanates, Ca3Ti2O7 [6] and Ca4Ti3O10 [7], melting incongruently, were found in the CaO-TiO2 system. The other calcium titanates, Ca4TiO6 [8], Ca3TiO5 [9], Ca8Ti3O14 [8, 10], Ca2TiO4 [9], Ca5Ti4O13 [11], Ca2Ti3O8 [6], CaTi2O5 [12, 13], CaTi3O7 [14], Ca2Ti5O12 [15], and CaTi4O9 [16], are mentioned in the literature. They also seem to be unstable, which does not exclude their possible existence [17]. Compiled in this paper on the basis of data [7, 18], as well as the results of recent studies by Gong et al. [19], the phase diagram of the CaO-TiO2 system in the high-temperature region is shown in **Figure 1**.
