PRÉCIPITATION DANS LES SPINELLES NON STŒCHIOMÉTRIQUES Mg1-3xGa2+2x□xO4

Dans le système MgGa2O4-Ga2O3 la phase εMg constitue une étape intermédiaire dans la décomposition des spinelles non stœchiométriques (γ), décomposition qui conduit à l'état stable biphasé (MgGa2O4 + Ga2O3β). La phase εMg présente une structure à antiphases périodiques dérivant de la structure spinelle. Les structures des deux précipités (εMg et Ga2O3β) sont caractérisées par une déformation monoclinique du sous-réseau anionique. La transformation γ → εMg est une transformation à plan invariant, plan invariant proche du plan des parois d'antiphase périodiques et du plan d'accolement observé en microscopie électronique. La transformation γ Ga2O3β est une transformation à direction invariante proche d'un cisaillement simple.In the system MgGa2O4-Ga2O3, non-stœchiometric spinels, when decomposed at high temperatures, form an intermediate metastable phase εMg before the final stable state (MgGa2O4 + Ga2O3β). This phase has a one dimensional periodic antiphase domain structure derived from the spinel structure. The structures of both precipitates (εMg and Ga2O3β) are characterized by a monoclinic distortion of their oxygen framework. An invariant plane lattice strain describes the γ → εMg transformation ; this invariant plane is near the antiphase boundary plane and the habit plane observed in electron microscopy. An invariant line lattice strain describes the γ → Ga2O3β transformation ; this strain is not much different from a simple shear

Influence of Mn-site doping on charge and orbital ordering in La 1 / 3 Ca 2 / 3 Mn 1 − y M y O 3 manganites ( M = Ni , Ga )

International audienceThe influence of Ni and Ga doping on orbital and charge ordering (CO) in La1/3 Ca2/3 Mn1−y My O3 (M: Ni, Ga, and 0≤y≤0.1) has been investigated by combining magnetic and transport measurements with observation and analysis in transmission electron microscopy (TEM). Both types of doping cause a gradual decrease of TCO—the temperature of the charge ordering transition—while the temperature dependences of the magnetization and electric resistivity remain typical of the CO state. In the undoped parent sample, direct observations by TEM reveal the extended commensurate striped superstructure with q-vector 1/3a* over the whole temperature range <200 K. In Ni-doped samples, the long-range superstructure is kept but has an incommensurate character with reduced q-vector by 18%–20% for Ni concentration y=0.03. The extent of the superstructure parameter modulation is in agreement with the resulting decrease in the concentration of Mn3+ ions due to their substitution by Ni²+. Unlike doping by Ni, the incorporation of Ga on the Mn site completely suppresses the formation of the extended striped superstructure. Analysis of the obtained data allows us to conclude about the crucial role of the orbital-orbital (super)exchange interaction (rather than the Jahn-Teller interaction) in the stabilization of striped superstructure in the Mn-site-doped La1/3 Ca2/3 MnO3 system

Influence of Mn-site doping on charge and orbital ordering in La 1 / 3 Ca 2 / 3 Mn 1 − y M y O 3 manganites ( M = Ni , Ga )

International audienceThe influence of Ni and Ga doping on orbital and charge ordering (CO) in La1/3 Ca2/3 Mn1−y My O3 (M: Ni, Ga, and 0≤y≤0.1) has been investigated by combining magnetic and transport measurements with observation and analysis in transmission electron microscopy (TEM). Both types of doping cause a gradual decrease of TCO—the temperature of the charge ordering transition—while the temperature dependences of the magnetization and electric resistivity remain typical of the CO state. In the undoped parent sample, direct observations by TEM reveal the extended commensurate striped superstructure with q-vector 1/3a* over the whole temperature range <200 K. In Ni-doped samples, the long-range superstructure is kept but has an incommensurate character with reduced q-vector by 18%–20% for Ni concentration y=0.03. The extent of the superstructure parameter modulation is in agreement with the resulting decrease in the concentration of Mn3+ ions due to their substitution by Ni²+. Unlike doping by Ni, the incorporation of Ga on the Mn site completely suppresses the formation of the extended striped superstructure. Analysis of the obtained data allows us to conclude about the crucial role of the orbital-orbital (super)exchange interaction (rather than the Jahn-Teller interaction) in the stabilization of striped superstructure in the Mn-site-doped La1/3 Ca2/3 MnO3 system