Crystal field effect on the olivine-spinel transformation
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Electrostatic energies of olivine, modified spinel and spinel with M2SiO4 stoichiometry (M=Ni, Mg, Co, Fe and Mn) and those of their constituent ions were calculated by the method of Bertaut, to obtain better understanding on each polymorph; the stability relations under high pressures and the effect of ionic size on the electrostatic stability.Stability relations of these polymorphs can be explained on the basis of proposed criteria; minerals with high repulsive energy and “simple” energetics only can survive under high pressures, where the “simple” energetics means that the electrostatic energies of constituent ions do not differ greatly from each other. Olivine does not have high repulsive energy, but involves a complex energetics. It cannot, therefore, survive under high pressures. Instability of modified spinel under very high pressures can be explained also in terms of its complex energetics. Spinel has high repulsive energy and does not involve a complex energetics. It is, therefore, stable under high pressures.Electrostatic energies plotted against ionic radius ratio rM⁄rSi reveal the effect of ionic size on crystal energetics as follows : Electrostatic energies of M ion, O ion and the whole crystal become higher, whereas that of Si ion becomes lower with an increase of ionic radius ratio. Electrostatic energy of Mg2SiO4 olivine is slightly different from the trend of those of transition metal olivines. A rapid increase in electrostatic instability of olivine structure is observed at the ionic radius of Mn.
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Rejection properties of positively charged PDM/PSF nanofiltration(NF) membrane to inorganic ions are studied.Ion selectivity is correlated to their type and valence as well as Shannon′s radius.The experimental results show that the effect of valence of anions is insignificant to rejection,but the valence of cations has an effect on the rejection considerably;the rejection of the membrane to divalent cations(and multivalent cations) is higher than to monovalent cations.And the NF membrane has a higher rejection to composite ions than to mono ions.The rejection to anions increases with increasing of Shannon′s radius,but the rejection to cations is decreasing with increasing of Shannon′s radius.
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Mg1−xZnxFe2O4 (with x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) spinel ferrite was synthesized via sol-gel auto combustion method. XRD confirms the formation of spinel phase with a secondary phase of Fe2O3 (in case of x = 1.0). Structural parameters changes as the Zn concentration increases which can be ascribed to the replacement of an ion with lower ionic radius (Mg2+) with that of an ion with higher ionic radius (Zn2+). Variation in the magnetic properties with Zn content are interpreted in terms of migration of Fe3+ between tetrahedral and octahedral sites.
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The effects of mono- and divalent cations on the properties of multicomponent phosphate glasses were investigated. The Tg, Yp and α were increased with increasing ionic radius of the alkali ion. As a result, the water durability of the phosphate glasses was improved. An introduction of divalent cations also resulted in the same tendency. It is evident from the IR measurement that the coordination number of cross-linking cations is increased with increasing ionic radius of the cation, and as a result, the bonding strength between cations and oxygen was weakened. It is considered that the improvement of the water durability is due to the increasing linking density, which is accompanied by increasing steric hindrance of the additional cations against attack by water molecules.
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