Periclase crystal structure

Another factor contributing to the asymmetry and breadth of absorption bands in crystal field spectra of transition metal ions is the dynamic Jahn-Teller effect, particularly for dissolved hexahydrated ions such as [Fe(H20)6]2+ and [Ti(H20)6]3+, which are not subjected to static distortions of a crystal structure. The degeneracies of the excited 5Eg and 2Eg crystal field states of Fe2+ and Ti3+, respectively, are resolved into two levels during the lifetime of the electronic transition. This is too short to induce static distortion of the ligand environment even when the cations occupy regular octahedral sites as in the periclase structure. A dual electronic transition to the resolved energy levels of the Eg excited states causes asymmetry and contributes to the broadened absorption bands in spectra of most Ti(m) and Fe(II) compounds and minerals (cf. figs 3.1,3.2 and 5.2). 

Figure 5.1 Crystal structure of periclase. Note that undistorted octahedra containing Fe-+ ions share all edges with other [Mg06] octahedra throughout the structure. Filled circles cations open circles O2- ions.

In the crystal structure of pyrite, which is modelled on the periclase structure (fig. 5.1), Fe2+ ions occupy Mg24- positions and the mid-points of (S-S)2- dimeric anions are located at the O2- positions. Each Fe2+ ion is in octahedral coordination with one sulphur atom belonging to six different (S-S)2- dimers, and the... 

Crystal Structure Periclase has a cubic face-centered crystal lattice iso-morphous with that of sodium chloride and calcium oxide see Figure 8.1. 

The crystal structure of magnesium oxide is illustrated in Figure 37. This is one of the simplest structures and is very similar to that of rocksalt, NaCl. Each Mg atom is surrounded by six O atoms, and to preserve electrical neutrality, each O atom is surrounded in turn by six Mg atoms. The entire lattice can be considered as built up from cubes, with O and Mg atoms at alternate corners this is the only known crystalline form of MgO and is sometimes called periclase. 

Rapid cooling of the clinker is preferred for many reasons, notably to prevent the reversion of alite to belite and lime in the 1100 1250 °C regime and also the crystallization of periclase (MgO) at temperatures just below 1450 °C. The magnesium content of the cement should not exceed about 5% MgO equivalent because most of the Mg will be in the form of periclase, which has the NaCl structure, and this hydrates slowly to Mg(OH)2 (brucite), which has the Cdl2 layer structure (Section 4.6). Incorporation of further water between the OH- layers in the Mg(OH)2 causes an expansion that can break up the cement. Accordingly, only limestone of low Mg content can be used in cement making dolomite, for example, cannot be used. Excessive amounts of alkali metal ions, sulfates (whether from components of the cement or from percolating solutions), and indeed of free lime itself should also be avoided for similar reasons. 

Crystal field spectral measurements of transition metal ions doped in a variety of silicate glass compositions (e.g., Fox et al., 1982 Nelson et al., 1983 Nelson and White, 1986 Calas and Petiau, 1983 Keppler, 1992) have produced estimates of the crystal field splitting and stabilization energy parameters for several of the transition metal ions, examples of which are summarized in table 8.1. Comparisons with CFSE data for each transition metal ion in octahedral sites in periclase, MgO (divalent cations) and corundum, A1203 (trivalent cations) and hydrated complexes show that CFSE differences between crystal and glass (e.g., basaltic melt) structures,... 

The X-ray diffraction patterns show the presence of the hydrotalcite-like structure in all the dried Mg-Fe and Mg-Al samples. Figure 1 reports the X-ray diffraction patterns of Mg/Fe=2.0, Mg/Al=2.0 and MgO samples after calcination. The patterns of the calcined Mg-Fe and Mg-Al samples correspond to that of a poorly crystallized MgO. The shift of the main reflections in the Mg/Al sample towards higher 20 values with respect to MgO is due to the decrease in the volume of the crystallographic cell of MgO (periclase), in agreement with an isomorphic replacement of Mg " cations with the smaller Af cations (Mg " = 0.65 A Ar =... 

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