Isostructural transition

Fig. 3.3 shows a similar ionic-model construction of the electronic energies for the isostructural transition-metal oxide MnO. In this case, the Madelung energy lifts the Mn 3d level above the top of the... 

This type usually called K2NiF4-structure 15) was investigated for the first time by Winkler and Brehler 337) at (KaMgF4). During the last years many oxides and fluorides of this type have been reported. Meanwhile isostructural transition metal fluorides containing the larger A-ions K, Rb, Cs, NH4 and T1 are known. 

When the itinerant state is formed, a volume collapse AV/V is always encountered, as predicted by the theory of the preceding sections. In one of the lanthanides, cerium, this volume collapse is particularly accentuated for its isostructural transition from the y to the a form, possibly associated with a change in metallic valence from three to four (both oxidation numbers are stable in cerium chemistry) (see Fig. 1 of Chap. A),... 

The much weaker collapse encountered experimentally in the non-isostructural transition of americium might perhaps be associated with the fact that the 5f are already (slightly) bonding in this metal in its low pressure modification. As seen in Fig. 2, AV/V increases the more the 5f are localized. 

The variation of hardness with multilayer wavelength in a range of different types of structures. These include multilayers of (a) isostructural transition metal nitrides and carbides, which show the greatest hardening (b) nonisostructural multilayer materials, where slip cannot occur by the movement of dislocations across the planes of the composition modulation, because the slip systems are different in the two materials and (c) materials where different crystal structures are stabilized at small layer thicknesses, such as AIN deposited onto TiN. 

The Zintl concept has been extended to include transition metal compounds, with some success.Although many main group metal Zintl compounds are known, there are relatively few examples of isostructural transition... 

Fig. 4.10 a V/Vq plot for BagSi4 as a function of pressure (points) along with a third-order Birch-Mumaghan equation of state (line). The macroscopic order parameter corresponds to spontaneous i c., the Variation of the volume cmrected from the compressibility, b Aspo ,a eous as a function of pressure fits with the theoretical analysis based on the Landau theory of phase transition null before the transition pressure, a jump at the transition and a square-root evolution after the transition. Such a behaviOT is also observed for Si atomic displacement parameters that can be used as microscopic order parameter [82]. The correlation between these physical quantities underlines the relationships between the isostructural transition and disordering of the Si sub-lattice... 

Cerium metal is discussed in ch. 4 and only a brief mention of its high pressure behavior will be made here (for references see the list in ch. 4). Cerium can exist at atmospheric pressure in the fee (y) or dhep (iS) form and undergoes an isostructural transition near 100 K to another fcc-form referred to as o-Ce. The y-a Ce transition occurs at 7 kbar at room temperature and this transition is accompanied by about 8% volume decrease. This is one of the most widely studied transitions as a function of pressure and temperature and is believed to involve a valence change from 3 towards a higher valence state (3.7 ). The y to a transition line terminates at a critical point the very first example in which a solid - solid transition was shown to exhibit a liquid-vapor-like critical point. A pressure-induced phase transition near 50 kbar, initially reported to be yet another isostructural transition has been shown to be from fee (a-Ce) to an orthorhombic phase with the a-U structure. Stager and Drickamer (1964) have reported a pronounced resistance anomaly near 120 kbar indicative of a phase transition, but the nature of this transition is unknown. The fusion behavior of Ce is again unique in that it exhibits a minimum. 

In CeAl2, Croft and Jayaraman (1979) and Bartholin etal. (1980) observed an isostructural transition around 7 GPa, accompanied by a volume collapse of about 4%. This observation was interpreted, like in the case of the lanthanide chalcogenides, as a valence change from - - 3 to -I- 4 of the Ce ion. This interpretation was supported by the fact that the corresponding compound of the non-4f metal La, LaAl2, did not show any phase transition up to 3 GPa, the highest pressure reached for both... 

Isostructural volume collapses observed under pressure in lanthanide pnictides and chalcogenides LnX. For each compound the table lists, from top to bottom the pressure (range) of collapse (GPa), the volume decrease on collapse (%), and references. (x means compound unknown, - studied under pressure, but no isostructural transition observed, and -I- compound exists, but no HPXRD study known.)... 

Lawrence et al. (1981) review the effects of different NaCl-type additives, forming solid solutions with SmS, on the isostructural-transition pressure. 

In the solid state all three elements have typically metallic structures. Technetium and Re are isostructural with hep lattices, but there are 4 allotropes of Mn of which the o-fomi is the one stable at room temperature. This has a bcc structure in which, for reasons which are not clear, there are 4 distinct types of Mn atom. It is hard and brittle, and noticeably less refractory than its predecessors in the first transition series. 

The structural chemistry of the actinides is often similar to that of lighter transition metals, such as Zr and Hf, and to that of the lanthanides however, the diffuse nature of the 5/ orbitals leads to some differences and specifically to interesting magnetic and electrical properties. The actinide sulfides are generally isostructural with the selenides, but not with the analogous tellurides. The binary chalcogenides of uranium and thorium have been discussed in detail [66], but the structural... 

Coordination chemistry of ER The monomeric fragments E-R are isolobal to carbon monoxide, and many complexes analogous to transition metal carbonyls have been synthesized (41 to 43, see Figure 2.3-7) [68], In most cases these reactions started with those clusters which have a high tendency to dissociate and to form monomers, such as pentamethylcyclopentadienylaluminum(I) or the alkylgal-lium(I) or alkylindium(I) derivatives. Often the products are isostructural to the respective metal carbonyls, but exceptions are the gallium compounds 44 and 45. 

The only other known transition metal terphenyl derivative is the blue, crystalline cobalt species Co(ju.-Br)C6H3-2,6-Mes2(THF) 239 (Fig. 12). It is isostructural to the corresponding magnesium derivative discussed earlier. It was synthesized via the reaction of CoCl2 with the Grignard reagent,... 

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