Olivine Structured Oxide

AB2X4 Compounds (Spinel and Olivine Structures). The AB2X4 spinels, based on the mineral MgAl204, and the inverse spinels, B[AB]04, are predominately ionic mixed oxides, containing a CCP-like array of X anions (Fig. 3.17). Most... 

In the olivine structure there are six kinds of atom ABCXYZi (Mg2Si04 is a well known example). The connectivity matrix for the case of an oxide (in which A is at the centers of symmetry and C is four-coordinated) is ... 

Another aspect of tuning the redox potential of an electrode material has been demonstrated by Goodenough et al. [3, 29]. They have shown that the use of polyanions (X04) such as (804) , (P04), (As04), or even (W04) lowers 3if-metals redox energy to useful levels compared to the Fermi level of the Li anode. Thus, the most attractive key point of the polyanion frameworks can be seen in the strong X-O covalency, which results in a decrease of the Fe-O covalency. This inductive effect is responsible for a decrease of the redox potential in comparison to the oxides [29, 30]. The polyanion P04 unit stabilizes the olivine structure of LiFeP04 and lowers the Fermi level of the Fe redox couple through... 

Specific examples in which results from ALCHEMI are compared with X-ray or neutron diffraction studies on the same sample have been presented for olivine [51], clinopyroxene [57] and feldspar [54] To date, all published planar ALCHEMI studies on mineral samples other than oxides containing high-Z elements (e.g. ceramic nuclear wasteforms, [58]) have been internally consistent and tractable. For example, a combined X-ray diffraction and ALCHEMI study on a complex, nonstoichiometric clinopyroxene [52] has been used to determine the location of vacancies in the structure. In... 

Transition metal ions most susceptible to large Jahn-Teller distortions in octahedral coordination in oxide structures are those with 3d4, 3d9 and low-spin 3(f configurations, in which one or three electrons occupy eg orbitals. Thus, the Cr2+ and Mn3+, Cu2+, and Ni3+ ions, respectively, are stabilized in distorted environments, with the result that compounds containing these cations are frequently distorted from type-structures. Conversely, these cations may be stabilized in distorted sites already existing in mineral structures. Examples include Cr2+ in olivine ( 8.6.4) and Mn3+ in epidote, andalusite and alkali amphiboles ( 4.4.2). These features are discussed further in chapter 6. 

The value of A, and hence the calculated CFSE, for Ni2+ ions in the olivine Ml sites appears to be anomalously high compared to values acquired in other octahedral sites in oxide structures, including bunsenite and MgO (table 5.1), Ni2Si04 spinel ( 5.4.3), and other phases listed later (table 5.19). The dis-crepency results from incorrect assignments of bands i and j listed in table 5.4... 

The Ni2+, Cr3+ and low-spin Co3+ ions do not acquire additional stabilization in distorted octahedral sites. They are expected to favour smaller sites that more closely approximate octahedral symmetry than other available sites in the crystal structures. As noted in 6.8.2, the high octahedral CFSE s acquired by these three cations in small octahedral sites in silicate and oxide structures accounts for the observed relative enrichments of Ni2+ in the olivine Ml and orthopyroxene Ml sites, the sole occupancy by Cr3+ of pyroxene Ml sites, and the occurrence and stability of low-spin Co3+ in Mn(IV) oxides. 

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