Neue Mischoxyde mit Spinellstruktur. (Mit einem kristallchemischen Beitrag zum Problem des Ferro‐magnetismus von Fe‐haltigen Spinellen)

Abstract

Es werden von verschiedenwertigen Kationen zahlreiche Mischoxyde hergestellt, deren Bruttoformel der von Spinellen entspricht. Eine Anzahl dieser Mischoxyde hat kubische oder schwach deformierte Spinellstruktur. In Analogie zu den bereits bekannten Spinellen LiAI5O8 und LiFe 3+5O8 liefert auch Cu1+Fe5O8 einen kubischen Spinell, der mit dem tetragonal deformierten Spinell Cu2+Fe2O4 Mischkristalle gibt. Auch eine Anzahl Spinelle der allgemeinen Formel M1+M3+M4+O4 und M2+M 3+aM 3+bO4 wurde erhalten. Bei den Fe3+‐haltigen Spinellen wurde die Auffassung anderer Forscher, daß das Vorhandensein von Fe3+‐Atomen in den Tetraederlücken des Sauerstoffgerüsts Voraussetzung für das Auftreten von Ferromagnetismus sei, durch neue Beispiele bestätigt. Die Nichteisenatome der Fe‐Spinelle haben jedoch, sofern diese Atome selbst ein magnetisches Gesamtmoment haben, auf die magnetischen Eigenschaften ebenfalls einen gewissen Einfluß.

Crystallite-growth, phase transition, magnetic properties, and sintering behaviour of nano-CuFe2O4 powders prepared by a combustion-like process

“…2 at a calcination temperature of 1000 °C weak reflections of CuO were detected, indicating that the resulting spinel phase has a copper deficit, as also observed by other authors [36,39]. In addition, during the calcination process a partial reduction of Cu 2+ to Cu + in the spinel phase has been found [36,40,41]. The amount of Cu + depends on the calcination temperature and cooling procedure.…”

“…The observed weight loss during the heating process is due to the loss of oxygen and the partial reduction of Cu 2+ to Cu + [36,39]. As seen, the reoxidation during the cooling phase is not complete, because it is a slow process as described in [36,41]. Parfenov and Nazipov [40] found in CuFe 2 O 4 samples sintered above 1000 °C and cooled to room temperature, that Cu + occupies the tetrahedral sites in the spinel structure.…”

On the Synthesis of Ramsdellite LiTiMO4 (M = Ti, V, Cr, Mn, Fe): An Experimental and Computational Study of the Spinel–Ramsdellite Transformation

“…LiTiCrO 4 (S) has been reported by several authors to adopt a normal spinel structure (space group Fd3m) with all Li ions at the (tetrahedral) 8a site and a random distribution of Ti and Cr over the (octahedral) 16d site, corresponding to the formulation [20,[33][34][35][36] although a slight inversion (12 %) in cationic site distribution regarding Li and Ti has been found by neutron diffraction experiments. [15] This, in addition to the higher enthalpy calculated for the transformation, would account for a more hindered phase transition and mean that the full transformation requires a much higher temperature than in the case of Ti or V. The situation is different for Mn and Fe, however, which present an important degree of inversion, with the proposed cationic distributions being [ [17,22] We have already discussed that the spinel ↔ ramsdellite transformation is entropically controlled therefore, in view of the proposed mechanism, the phase transition for the Mn and Fe systems will also be impeded due to enhanced electrostatic repulsions between higher valent cations in neighboring face-sharing polyhedra.…”

Self-propagating high-temperature synthesis of lithium-chromium ferrites Li0.5Fe2.5-xCrxO4

“…They include the hexagonal ferrites BaFe 12 O 19 and SrFe 12 O 19 used as permanent magnets and as the magnetic stripes on credit and debit cards, the cubic ferrite Li 0.5 Fe 2.5 O 4 which is relevant to microwave and memory-core applications and the normal ferrites Mg 0.5 Zn 0.5 Fe 2 O 4 used in television sets' focusing systems and in high-frequency cellular telephones. Li 0.5 Fe 2.5 O 4 has been the subject of extensive technical and fundamental study both in its pure form and in its substituted form, in which various isovalent and nonisovalent metals are made to replace the Fe 3+ ions [1][2][3][4][5][6][7][8][9][10][11]. The impetus behind these studies has firstly been that the cubic crystal structure and presence solely of Fe 3+ ions in the host material has allowed detailed modelling of the exchange interactions giving rise to its ferrimagnetic order and secondly that subsequent substitution either with magnetic or with nonmagnetic cations has allowed studies of a variety of different magnetic states arising from the perturbed magnetic exchange interactions between ions.…”