Cationic disordering

The low-temperature (/1-)AE3(BN2)2 phases exhibit two distinct structures for AE = Ca and Sr that can be derived from the cation disordering in their respective high-temperature phases. For / -Ca3(BN2)2 an orthorhombic (Cmca) superstructure of the cubic cell with fi-a bo a, Cq ly l a was obtained, in which the former 8f sites are occupied by seven calcium ions in an ordered fashion. In contrast, the structure of / -Sr3(BN2)2 is simply the result of a transition from a cubic body-centered (Im3m) into a primitive structure (Pm3m), in which the former 2 a position (0, 0, 0 1/2, 1/2, 1/2) is split into two independent positions, of which only one is occupied by strontium (Fig. 8.6). 

The rearrangement of the Fe atoms during the transformation was inferred from differential broadening of the XRD peaks of hematite all reflections except 110, 113 and 300, for which the structure factor depends on the position of the Fe atom, are broadened. This suggests that the peak broadening results from cation disorder ( Po-mies et al. 1998 1999). 

Besancon J.R. (1981) Rate of cation disordering in orthopyroxenes. Am. Mineral. 66, 965-973. 

Annersten, H., Ericsson, T. Filippidis, A. (1982) Cation disordering in Ni-Fe olivines. Amer. Mineral., 67,1212-17 [see Ribbe Lumpkin (1984)]. 

After Oostra, the condition Jx J2 may not be fulfilled in the case of TEA(TCNQ)2 however, the most likely explanation for the reported behavior of x(7) is that the cation disorder plays an important role. Oostra observed, in fact, that a noticeable excess magnetic susceptibility could be produced below 210 K, on rapid cooling (see Fig. 11) and he attributed this excess to an effect of the TEA cations, which did not have the time to order completely in the cristal [40]. 

An appropriate analysis of the dynamics of the orientational disorder of the cations can be performed by means of proton NMR second-moment studies [53-55]. Such studies have established in the case of TEA(TCNQ)2 that the cation disorder is dynamic above 270 K while it is static below 205 K, with some transitional regime in between [53,54]. The wide transition of TEA(TCNQ)2 at about 210 K may then be considered to be a continuous structural transition from a state of dynamic disorder at high T to a state... 

A weak 2kF diffuse x-ray scattering [58], seen only at low temperature, confirms that p = in this salt and indicates that the Fermi wave vector is rather well defined despite structural cation disorder. A stronger 4kF scattering [58] observed from 25 K to 300 K is an indication of large Coulomb effects in this salt, with U t, in agreement with other known magnetic and thermopower data [57,58]. 

This idealistic description had to be tempered by the observation of some cationic disorder between Ca2+ and Mn2+ (Co2+), similar to the one already encountered in the Ba2MnMnAl2Fi4 compounds. In Ba2CaMnFe2F]4, this disorder was evidenced... 

The Boltzmann distribution (Eq. 6) relates the dependence of temperature on the amount of cation disorder, where A, is the activation energy required to exchange interstitial sites of and cations (i.e., between octahedral and tetrahedral sites). Energy values for these interchanges generally fall in the range 0.11-0.14 eV ... 

Kudoh Y., Kuribayashi T., Mizobata H., and Ohtani E. (2000) Structure and cation disorder of hydrous ringwoodite, y-Mgi,89Sio,98Ho.3o04. Phys. Chem. Mineral. 27, 474—479. 

Thus, polymorphism with the formation of monoclinic and tetragonal modifications is typical for U- and Th-compounds with general formula BM(X04)2 (X = P, As, V). These modifications differ in the packing of the XO4 anions, the orientation of anions around a cation, and the form of the (B,M)On coordination polyhedra (n = 8 or 9). The crystal chemical formula of the BM(P04)2 phosphates and their arsenate and vanadate analogues can be written as B0.5M0.5XO4, owing to the cation disorder. 

A key mineralogical problem with aluminates is the question of cation disordering in spinels. Numerous Al MAS NMR studies have been carried out on MgAl204. One O MAS NMR study of spinels quenched from between 700 and 1400°C showed an increase in disorder with temperature. Both the and Al MAS NMR spectra showed a similar increase in disorder but the value determined from O NMR was systematically lower (Millard et al. 1992). O MAS has been used to monitor the solid state NMR reaction of sol-gel formed LaAlOs (Figure 6.22). Heating to 450°C produced O signals from LaO(OH) and an aluminium oxyhydroxide, but peaks from... 

Recent studies by Xu et al. (1999a,b 2000) have demonstrated that two structural transitions occur within the Lii cAli-xSii+ c04 system at room temperature. When x i > -0.3, the Ar and Si cations disorder over the tetrahedral sites, and when xsi > 0.65, the... 

Gobbi GC, Christofferesen R, Otten MT, Miner B, Buseck PR, Kennedy GJ, Fyfe CA (1985) Direct determination of cation disorder in MgAl204 spinel by high-resolution Al magic-angle-spinning NMR spectroscopy. ChemLett, 771-774... 

Sickafus KE, Larson AG, Yu N, Nastasi M, Hollenburg GW, Gamer FA, Bradt RC (1995) Cation disordering in high-dose neutron-irradiated spinel. J Nucl Mater 219 128-134 Sickafus KE, Matzke Hj, Hartmaim Th, Yasuda K, Valdez JA, Chodak P III, Nastasi M, Verrall RA (1999) Radiation damage effects in zirconia. J Nucl Mater 274 66-77. 

The vAl-O-Al IR band was used to study the effects of the caustic ratio on the structure of sodium aluminate solutions.118 Variable-temperature Raman spectra of the spinel MgAl204 showed a band at 727 cm" 1 for samples annealed above 800°C. This was ascribed to cation disorder in these samples.119 The angular force constant method was used to analyse vibrational modes for MA1204, where M = Mg or Zn.120 Ab initio calculations have been made of vibrational wavenumbers involving the OH group in topaz, Al2Si04(0H)2.121... 

The pyrochlore-type compounds, where the crystal structure is usually considered as a cation-ordered fluorite derivative with % vacant oxygen site per fluorite formula unit, constitute another large family of oxygen anion conductors [9, 33, 41—43, 84—88]. The unoccupied sites provide pathways for oxygen migration furthermore, the pyrochlore structure may tolerate formation of cation and anion vacancies, doping in both cation sublattices, and antistructural cation disorder. Regardless of these factors. 

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