A.A. Bosak

Epitaxial phase stabilisation phenomena in rare earth manganites

Abstract Using injection, powder-flash and band-flash MOCVD techniques, epitaxial thin films of Nd, Ho, Y, Tm, Dy and Lu manganites were prepared. The formation of high-pressure phase NdMn 7 O 12 as a thin surface layer on Pnma Nd 1− x MnO 3+δ ( x ∼0.15) in an Nd-deficient system and HoMnO 3 , YMnO 3 , TmMnO 3 and LuMnO 3 as perovskite phases (known in the bulk only as high-pressure phases) on a LaAlO 3 substrate were observed. The remarkable reduction in the pressure needed for formation of the phases in epitaxial films in comparison with bulk samples is a manifestation of the epitaxial stabilisation discussed in the paper. The details of the crystal and domain structure of the films as observed by X-ray diffraction (XRD), selected-area electron diffraction (SAED) and high-resolution electron microscopy (HREM) techniques are described.

METAL-INSULATOR TRANSITION INDUCED BY 16O-18O OXYGEN ISOTOPE EXCHANGE IN COLOSSAL NEGATIVE MAGNETORESISTANCE MANGANITES

The effect of 16O→18O isotope exchange on the electric resistivity was studied for (La1−yPry)0.7Ca0.3MnO3 ceramic samples. It was found that at y=0.75, the substitution of 16O by 18O results in the reversible transition from a ferromagnetic metal (FM) to charge ordered (CO) insulator at zero magnetic field. The applied magnetic field (H⩾2 T) transformed the sample with 18O again to the metallic state and caused the increase in the FM transition temperature TC of the 16O sample. As a result, the isotope shift of TC at H=2 T was as high as 63 K. Such unique sensitivity of the system to oxygen isotope exchange, giving rise even to the metal–insulator transition, is discussed in terms of the isotope dependence of the effective electron bandwidth which shifts the balance between the CO and FM phases.

Synthesis, crystal structure and properties of manganese(II) hexafluoroacetylacetonates Mn(hfa)2(H2O)2 and KMn(hfa)3

Abstract Two manganese(II) hexafluoroacetylacetonate complexes, dihydrate, the Mn(hfa)2(H2O)2 (1), and the bimetallic KMn(hfa)3 (2) have been synthesized and characterized by X-ray single crystal structure analysis. They have a molecular structure and a polymeric structure, respectively. The high stability of 2 allowed sublimation without decomposition. It was then used as a precursor for MOCVD of fluoroperovskite, KMnF3.

Epitaxial stabilization of hexagonal RMnO3(R = Eu–Dy) manganites

Epitaxial thin films of RMnO3 (R = Sm, Eu, Gd, Dy) have been prepared using liquid injection MOCVD. The formation of a high-temperature hexagonal modification for DyMnO3 and bulk hexagonal EuMnO3 and GdMnO3, which does not exist for the bulk compounds, has been observed on ZrO2(Y2O3) (111) substrates at 900 °C due to epitaxial stabilization.

Hexagonal rare earth (R=Eu-Dy) manganites: XRD study of epitaxially stabilized films

Abstract Epitaxial thin films of RMnO 3 (R=Sm, Eu, Gd, Dy) were prepared using liquid injection MOCVD and studied by XRD. The formation of high-temperature hexagonal polymorphic structure of DyMnO 3 and non-existing-as-bulk hexagonal EuMnO 3 , GdMnO 3 and SmMnO 3 (traces) was observed on ZrO 2 (Y 2 O 3 ) (111) substrates at 900 °C due to epitaxial stabilization. For the thickness exceeding the critical one the oriented stable perovskite form grew on the hexagonal phase. Parallel deposition on perovskite or non-coherent substrates gave only the stable perovskite phase, epitaxially grown in the former case or oriented in the latter.

MOCVD of perovskites with metallic conductivity

Abstract Single source MOCVD techniques were used to prepare perovskite films with metallic conductivity (CaRuO 3 , LaNiO 3 , La 0.5 Sr 0.5 CoO 3 and (La,Pr) 0.7 (Sr,Ca) 0.3 MnO 3 ). Structural and electric properties of the epitaxial layers on the coherent substrates are close to that of the films grown by PLD and magnetron sputtering. Peculiarities of the growth occur on the worse matched substrates, such as a mixture of two orientations, each aligned in the plane of the interface (LaNiO 3 /MgO) and variant structures in the films on YSZ. XRD of the films points to a pseudocubic lattice for all R 1 − x A x MO 3 films in spite of the distortions in the bulk material.

Crystal structure of copper(II) pivaloyltrifluoracetonate adducts with o-phenanthroline and 2,2′-dipyridyl

Abstract The crystal structure of Cu(pta)2·Phen has been determined. The phenanthroline ligand occupies a position in the equatorial plane of the tetragonally distorted octahedron with equal CuN distances and equivalent β-diketonate-anions. Cu(pta)2·2,2′-Dipy is isostructural to Cu(pta)2·Phen. The volatility of adducts decreases in the sequence Cu(thd)2·Phen, Cu(pta)2·Phen and Cu(hfa)2·Phen changing inversely to that of the β-diketonates. The variation of packing mode caused by the difference of the molecular structure is considered to be responsible for the effect. The changes of the electronic spectra of copper(II) β-diketonates caused by adduct formation are discussed.

Electronic structure and phase separation in LaxMnO3 films (0.83≤x≤1.10): Optical and magnetooptical data

The variations in the electronic structure and the evolution of phase separation as a result of creation of vacancies or excess of lanthanum in lanthanum manganites are studied on the basis of analysis of optical and magnetic properties of LaxMnO3 epitaxial films (0.83≤x≤1.10) in the fundamental absorption range. The Kerr effect, the temperature dependences of resistivity, optical density, and magnetoabsorption of light indicate the charge and magnetic phase separation in the films. The fine structure observed in the spectrum is attributed to spectral overlapping of electron transitions with charge transfer and geometrical resonances reflecting an inhomogeneous nanoscopic structure of the films, which strongly depends on stoichiometry and stresses emerging during film deposition. It is shown that, in contrast to bulk polycrystals, the gradient of stresses over the film thickness significantly affects the phase separation in the films.

SYNTHESIS AND CRYSTAL STRUCTURE OF COPPER(II) 2,2,6,6-TETRAMETHYLHEPTAN-3,5-DIONATE ADDUCT WITH O-PHENANTHROLINE

Abstract The X-ray crystal structure of Cu(thd)2·Phen, which is the first adduct of Cu(thd)2 reported to be stable in the vapor phase, was determined. Acis-octahedral arrangement of chelate ligands around copper is of low symmetry due to Jahn-Teller distortion and the rigidity of the ligands. Peculiarities of the Cu(thd)2·Phen synthesis are discussed.

Phase separation and electronic structure in LaxMnO3 (0.83⩽x⩽1.10) films

Abstract The investigation of optical and magnetooptical properties of La x MnO 3 (0.83⩽ x ⩽1.10) films revealed the changes of spectra and of electronic structure caused by the deviation from the stoichiometry and by strain in the films. Transversal Kerr effect data, temperature dependencies of magnetotransmission and magnetoresistance give the evidence for existence of charge and magnetic phase separation. The experimental results are discussed within the theoretical model of effective medium.