Metastable ternary compounds

Fig. 18. Comparison of the temperature dependence of the magnetization of the metastable ternary compounds Nd2Fe23B3 and NdFe12B6 (dashed curves) with that of the corresponding amorphous alloys (full curves). (After Buschow et al. 1986b.)...

Chlorine solubility is essentially restricted by the formation of silicon tetrachloride and, in the presence of hydrogen, of trichlorosilane. The suppression of these reactions, according to our calculations, must lead to the metastable ternary compounds nickel silicide chlorides . This thesis is supported by earlier theoretical calculations [11], which indicated a possible existence of calcium silicide bromides [12], and furthermore by the preparation of rare earth silicide iodides , rare earth chlorid silicides and rare earth bromids silicides [13]. 

A number of metastable ternary compounds with different compositions and crystal stmcture has been referred in the fiterature orthorhombic [1959Kon, 1975Sch, 1977Dub, 1977Edn, 2006Yel], triclinic [1969Bun,... 

Amorphous networks, 4-5 nAmorphous solid synthesis via ultrathin-film multilayer composites analysis of solid-state reaction mechanisms, 357,358/ application to synthesis of metastable ternary compounds, 366 control of crystallization of amorphous aUoy, 360,363,365-367/ control of formation of homogeneous amorphous alloy, 360,361-36 differential scanning calorimetric procedure, 359-360 grazing measurement procedure, 359 lugb-angje XRD procedure, 359 length sddes vs. course of solid-state reactions, 360,361-362/363 quantitative analysis of interdiffiision reaction, 356-357... 

No ternary compounds are known at room temperature. However hydrides (Fei (Cr c)Hy (x = 0.05,0.25 and 0.50 yunder hydrogen pressure up to 7 GPa. They are metastable at room temperature and present a hexagonal lattice structure like CrH [2002Ant]. The solid phases are presented in Table 2. The H solubihty in sohd alloys was measured by [1965Sch] between 400 and 1000°C and evaluated at 1200 and 1400°C by [1966Bur]. 

There is no ternary compound in the C-Cu-Fe system. The unary phases and die metastable phases discussed in this assessment are listed in Table 2. 

Despite the occurrence of binary AIB2 borides (see also Fig. 2), no ternary representatives are known (Mn, Mo)B2 found from isothermal sections is a stabilized high-T phase by conversion to lower T by a statistical ( ) metal-metal substitution. Both MnB2 and M0B2 are high-T compounds stable above 1075°C and 1517°C respectively WB2 is claimed but is either metastable or impurity stabilized. Similar examples are observed with (W, Pd>2B5 (M02B5 type) as well as (Mo, Rh),, (B3 and (W, Ni), B3 (Mo,., 83 type). The phase relations in the B-rich section of the Mo(W)-B binaries, however, are not known precisely. 

Compounds made by insertion at room temperature are often metastable - if heated, they change their structure or decompose into other compounds. That does not rule out using thermodynamics it just means that processes happening slowly compared to the duration of an experiment are assumed to be frozen. At room temperature, the ratio of Mo to Se in a host like Mo Seg is fixed. From the point of view of thermodynamics, the constraint that the host remain Mo Seg means that we can regard an intercalation compound like Li -MogSeg as a pseudo-binary compound instead of a ternary one. 

The application of solid compounds with cluster units as sources for new molecular compounds, as well as for new polymers opens up an attractive field in chemistry. In view of the wealth of binary and ternary solid compounds, there is no end in sight. The wide range of materials should encourage many chemical groups to work much more in this direction. Especially fascinating is the possibility to form new modifications of the elements which as metastable systems could also have interesting chemical and physical properties. 

J The concept of counter-phases. When a stable compound penetrates from a binary into a ternary system, it may extend right across the system or exhibit only limited solubility for the third element. In the latter case, any characterisation also requires thermodynamic parameters to be available for the equivalent metastable compound in one of the other binaries. These are known as counter-phases. Figure 6.16 shows an isothermal section across the Fe-Mo-B system (Pan 1992) which involves such extensions for the binary borides. In the absence of any other guide-... 

The ternary systems display a variety of structural chemistry depending on the sizes of the alkaline and lanthanide metals (Scheme 3 Fig. 3 [43, 45-57]). The smaller alkali cations determine the expected coordination structures as found in salt-like compounds, e.g., Na3Y (NH2)6 or KY(NH2)4. Layer structures are observed in alkali metal poor systems like MLa2(NH2)7 while cesium derivatives, apart from the lanthanum compounds, form perowskit-like arrangements as in CsEu(NH2)3 and Cs3Ln2(NH2)9. The mono ammoniates of some Cs-systems are probably metastable. Preparation of analogous ternary systems with Li were unsuccessful in contrast to, e.g., LiAl(NH2)4 [58]. 

Since the initial discovery of metastable quasicrystals, many ternary inter-metallic compounds have been produced in the quasicrystalline state, which are thermodynamically stable at room temperature. These have been obtained... 

Structural data are available (Table 30) for a range of binary, ternary and quaternary sulfides of manganese, almost invariably Mn", and these set the scene for the structures to be expected in the compounds with the more discrete polyhedra.319 Indeed, the structural pattern is established in the simple binary compound MnS. Whereas, the stable modification of this (a-MnS) is green and has the cubic rock salt structure with [MnS6] octahedra, the well-known flesh-coloured precipitates of the qualitative analysis system are metastable / - and y-modifications, which have [MnS4] tetrahedra with respectively the zinc blende or diamond (cubic) and wurtzite (hexagonal) structures. And so, in the rest of the known solids, there are almost equal numbers of four-coordinate tetrahedra and six-coordinate octahedra with no other polyhedra having been detected. 

The vertical section of the Nd-Fe-B ternary system which passes through the Fe comer and the phase Nd2Fe14B is shown in fig. 4a. Schneider et al. emphasize that this is not a pseudo-binary section. The dominant feature of this vertical section is the peritectic reaction L + Fe —> at 1180° C. It also follows from the results shown in fig. 4a that cooling of a liquid whose composition corresponds to leads to the formation of primary crystallized Fe. The concentration limit beyond which no primary Fe crystals are formed is at 77 at.% Fe. This is very close to the overal composition of commercial magnets, as will be discussed in more detail in section 3.2. Schneider et al. note that the vertical section of fig. 4a represents the stable situation which applies only to melts that were kept near the liquidus temperatures for a sufficiently long time. For superheated alloys the vertical section is quite different and corresponds to a metastable situation (fig. 4b). A comparison of the two vertical sections reveals that the liquidus temperatures, and the temperature at which the univariant reaction L - + tj begins, are unaltered, but that the temperature at which the 4> phase forms is lower in fig. 4b than in fig. 4a. Furthermore, one notices a new phase in fig. 4b (x) which is formed peritectically at 1130 °C. The latter temperature is below the temperature of the stable reaction L + Fe - 4> (1180 °C). Schneider et al. note that the primary crystallization of is suppressed in the metastable sequence (fig. 4b), in favour of Fe. In the microstructure one now observes that primary Fe is surrounded by a shell of Fe + which is the decomposition product of x- The x phase was identified by Grieb et al. (1987), as a compound of the 2 17 structure type. 

Crystallographic and magnetic data of some selected ternary R-Fe-B compounds. Top part stable compounds. Bottom part metastable compounds. For more details, see main text. The lattice constant c of R1+EFe4B4 refers to the R-sublattice, the corresponding value of the Fe sublattice is given between parentheses, the quantity e for each compound can be obtained by using the relation e = cFe/cR — 1. 

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