Homogeneity ranges

The stoichiometry of crystals grown by directional solidification from Eu-rich melts, which should correspond to the Eu-rich limit of the EuSe homogeneity region, is correct within 1 wt% as determined by atomic absorption spectrometry, Stubb [1]. Weight changes accompanying 

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The heavier metal tantalum is distinctly less inclined than niobium to form oxides in lower oxidation states. The rutile phase TaOz is known but has not been studied, and a cubic rock-salt-type phase TaO with a narrow homogeneity range has also been reported but not yet fully characterized. TazOs has two well-established polymorphs which have a reversible transition temperature at 1355°C but the detailed structure of these phases is too complex to be discussed here. 

The homogeneity ranges and defect structures of the hexaborides lead to deviations from stoichiometry through the cation defects (see Table 1). 

Table 1. Homogeneity Ranges and Lattice Constant Data for the Cubic Hexaborides...

Little information is available on homogeneity ranges and defect structures in the dodecaborides. The only variation from stoichiometry in these borides is for YB,2i the limiting phase determined by density measurements is Yq92B,2. This result can be attributed to the size of Y which is the maximum for metals that form the dodecaborides. Attempts to prepare DyB,2 with a nonstoichiometric composition are conclusive. ... 

Only fragmentary information is available concerning the homogeneity ranges in MBgg-phases, mostly for DyBs and YBgj. The observed compositional dependence of... 

In Fig. 2.19, on the contrary, we observe that intermediate solid phases with a variable composition are formed (non-stoichiometric phases). In the diagrams shown here we see therefore examples both of terminal and intermediate phases. (For instance, the Hf-Ru diagram shows the terminal solid solutions of Ru in a and (3Hf and of Hf in Ru and the intermediate compound containing about 50 at.% Ru). These phases are characterized by homogeneity ranges (solid solubility ranges), which, in the case of the terminal phases, include the pure components and which, generally, have a variable temperature-dependent extension. 

Several phase diagrams of binary alloy systems have been shown (see for instance Fig. 2.18) in which the existence of intermediate phases may be noticed. In these systems we have seen the formation of AmB phases, which generally crystallize with structures other than those of the constituent elements, and which have negligible homogeneity ranges. Thermodynamically, the composition of any such phase is variable. In a number of cases, as those exemplified in Fig. 2.19, the possible variation in composition is very small (invariant composition phases or... 

Ti-C In this system one intermediate phase is formed with a homogeneity range extending, depending on temperature, between 32 and 50 at.% C, in short named TiC (or, better, TiC x) and having a NaCl-type structure. 

As for the thermodynamic modelling of Laves phases, this has been successfully performed for several alloy systems. Especially using the sublattice model (see 2.4.2.2) a number of phase diagrams containing Laves phases have been calculated taking into account homogeneity ranges and polytypism. 

An important class of intermetallic phases (generally showing rather wide homogeneity ranges) are the Hume-Rothery phases, which are included within the so-called electron compounds . These are phases whose stable crystal structures may be supposed to be mainly controlled by the number of valence electrons per atom, that is, by the previously defined VEC. 

Extended homogeneity ranges of intermetallic phases have been studied by Singh et al. (2003) in the Mg-Al system. Powders of the two metals (325 mesh size) were mixed and the mechanical alloying performed using a hardened steel vial and... 

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