An analysis of the extensive information on the interaction of transition metals of group IV with high-melting platinum metals in binary [1][2][3][4][5][6][7][8][9][10][11] and ternary [12][13][14][15][16][17][18][19][20][21][22][23][24][25] systems accumulated at the Institute of Materials Science of the Academy of Sciences of Ukraine by V. N. Eremenko's school and also data of Western authors [26][27][28] showed that the platinum metals as well as titanium, zirconium, hafnium, which are in the same group of the periodic table, are metallochemically not complete analogs. For instance, upon transition from ruthenium to osmium, rhodium, and, in particular, iridium, the number of compounds forming in binary systems greatly increases. The replacement of titanium by zirconium leads, on the one hand, to an increase of the number of compounds, on the other hand to the appearance of a different crystal structure in compounds with the same stoichiometry ( Fig. 1, Table 1). Yet neither the dimensional factor nor the factor of electron concentration expressed in terms of the sum of (s + d)-electrons per atom is completely determining.Nevertheless a number of regularities were discovered in the structure of the phase diagrams of binary systems formed by transitions metals of group IV and high-melting platinum metals. In all the systems phases with the equiatomic composition AB form. It can be seen from Fig. 1 that they all (except ZrOs) melt congruently and are high-melting compounds. Their melting point is sometimes as high as 2600°C. Such high thermal stability is well correlated with the published data on the high thermodynamic stability of these phases [29, 30]. The crystal lattice of the high-temperature modification of the mentioned phases belongs to the structural type CsCI. In systems with ruthenium and osmium it is stable down to room temperature, in systems with rhodium and iridium a lowering of the temperature entails polymorphic transformation in consequence of the lattice being distorted into a tetragonal one and then into a more complex one, viz., a monoclinic or rhombic one. In all systems with rhodium and iridium there forms a phase with composition AB 3 and with a crystal lattice of type AuCuy A characteristic trait of the phase diagrams under consideration is that there is no trace at all of Laves phases in systems with titanium whereas in systems with zirconium all the platinoids except rhodium form a Laves phase, and in systems with hafnium only osmium does, although the criteria of the formation of Laves phases (the dimensional factor ~md the electron concentration) are favorable in all the systems under consideration. Consequently the mentioned criteria are, as repeatedly pointed out a necessary but not a sufficient condition of the formation of Laves phases.In the range of compositions rich in a transition metal of group IV the number of intermediate phases increases in the series Ti ---Hf--, Zr. In the series of platinoids their number increases from osmium to rhodium and then to iridium; ruthenium, ...
