Pulsed laser deposited KNbO3_3 thin films for applications in high frequency range

Potassium niobate thin films were grown by pulsed laser deposition on various substrates. Influence of deposition conditions on film characteristics was studied. Structural investigation evidenced that single phase polycrystalline randomly oriented films were grown on sintered alumina whereas epitaxial films were grown on (100)SrTiO$_3$ and (100)MgO substrates. The microstructure was highly controlled by the structural characteristics. Interdigited capacitors built from KNbO3 films on two different substrates (alumina and MgO) showed the strong influence of the structural characteristics on the dielectric behavior. The variation of the equivalent capacitance measured on the interdigital capacitor on MgO was 6.4% at 2.5 GHz while it was 1.5% on alumina, in both cases for a moderate applied field of $\sim$15 kV cm$^{-1}$. The results show the potentiality of these ferroelectric materials for use in frequency agile microwave electronics

Substrate-controlled allotropic phases and growth orientation of TiO2 epitaxial thin films

International audienceTiO2 thin films were grown by pulsed laser deposition on a wide variety of oxide single-crystal substrates and characterized in detail by four-circle X-ray diffraction. Films grown at 873 K on (100)-oriented SrTiO3 and LaAlO3 were (001)-oriented anatase, while on (100) MgO they were (100)-oriented. On (110) SrTiO3 and MgO, (102) anatase was observed. On M-plane and R-plane sapphire, (001)- and (101)-oriented rutile films were obtained, respectively. On C-plane sapphire, the coexistence of (001) anatase, (112) anatase and (100) rutile was found; increasing the deposition temperature tended to increase the rutile proportion. Similarly, films grown at 973 K on (100) and (110) MgO showed the emergence, besides anatase, of (110) rutile. All these films were epitaxically grown, as shown by ' scans and/or pole figures, and the various observed orientations were explained on the basis of misfit considerations and interface arrangement

Study of non-stoichiometric BaSrTiFeO3 oxide dedicated to semiconductor gas sensors

International audienceDeveloping instrumentation systems compatible with the European RoHS directive (restriction of hazardous substances) to monitor our environment is of great interest for our society. Our research therefore aims at developing innovating integrated systems of detection dedicated to the characterization of various environmental exposures. These systems, which integrate new gas sensors containing lead-free oxides, are dedicated to the detection of flammable and toxic gases. We have firstly chosen to study semiconductor gas sensors implemented with lead-free oxides in view to develop RoHS devices. Therefore thick films deposited by spin-coating and screen-printing have been chosen for their robustness, ease to realize and ease to finally obtain cost-effective sensors. As crystalline defects and ionic vacancies are of great interest for gas detection, we have decided to study a non-stoichiometric composition of the BaSrTiFeO3 sensible oxide. Nonstoichiometric BaSrTiFeO3 lead-free oxide thick films were deposited by screen-printing on polycrystalline AFO3 substrates covered by a layer of Ag-Pd acting as bottom electrode. The physical characterizations have revealed a crystalline structure mainly composed of BaTiO3 pseudo-cubic phase and Ba4Ti12O27 monoclinic phase for the powder, and a porous microstructure for the thick films. When compared to a BSTF thick film with a stoichiometric composition, a notable increase in the BSTF dielectric constant value was observed when taking into account of a similar microstructure and grain size. The loss tangent mean value varies more softly for the non-stoichiometric BaSrTiFeO3 films than for the perovskite BSTF film as tanδ decreases from 0.45 to 0.04 when the frequency increases from 100 Hz to 1 MHz. © Published under licence by IOP Publishing Ltd

Solid state phase equilibria in the Fe-Ga-As system

Solid state phase equilibria in the ternary Fe-Ga-As diagram were determined at 600 °C using powder X-ray diffraction as experimental techniques. Very limited solid solubilities were measured in the binary constituent Fe-Ga and Fe-As compounds. In the Fe-rich part of the diagram, a ternary phase was evidenced which corresponds in fact to a solid solution into which Ga and As substitute one another on the same sublattice. This phase, which can be expressed by the general formula Fe3Ga2−xAsx (0.20 ≤ x ≤ 1.125), crystallizes in hexagonal symmetry; it is structurally derived from the NiAs type-structure (B81) and can be considered, owing to its c/a ratio (1.23 ≤ c/a ≤ 1.28) close to $\sqrt{3}/\sqrt{2}$, as hexagonal-pseudocubic. The original feature of this experimental diagram is the occurrence of a tie-line between this ternary phase and the semiconductor GaAs which differs significantly from the theoretical diagram, estimated from simplified calculations proposed by Schmid-Fetzer. In particular, it is the first time that such a ternary phase Mx(Ga, As)y (M = transition metal) is found to be in thermodynamic equilibrium with GaAs. This fact suggests that the solid state interdiffusions which occur during the annealing of a Fe/GaAs heterostructure could lead to a ferromagnetic, epitaxial and stable Fe3GaAs/GaAs contact

The crystal structures of two ternary Mx(Ga, As)y phases (M ≡ Rh, Pd) with Rh5Ge3- and Cr12P7-type derivative structures

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Solid state phase equilibria in the Er-Ga-As system

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Solid state phase equilibria in the Fe–Ga–Sb ternary system at 600 °C

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Solid state phase equilibria in the Fe–Ga–Sb ternary system at 600 °C

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Crystal Structures of Zr 2 Mn 12 P 7 and (Mo,Mn) 12 P 7

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