P. Paufler

Voltage-controlled epitaxial strain in La0.7Sr0.3MnO3∕Pb(Mg1∕3Nb2∕3)O3-PbTiO3(001) films

Epitaxially grown La0.7Sr0.3MnO3 thin films show resistance modulations induced by the inverse piezoeffect of the employed Pb(Mg1∕3Nb2∕3)O3-PbTiO3(001) (PMN-PT) substrates. The in-plane strain state of the films can continuously be tuned by application of a piezovoltage to PMN-PT. The lattice deformation of a PMN-PT(001) substrate was quantified by x-ray measurements under an electric field. Variation of in-plane lattice parameters by ∼0.06% reversibly changes the resistance of the manganite films by up to 9% at 300 K and shifts the magnetic Curie temperature. Films of different thicknesses from 50 to 290 nm, offering different as-grown strain states, have been studied.

Thermal stability of Mo/Si multilayers with boron carbide interlayers

Abstract Mo/Si multilayer systems with boron carbide (B 4 C) diffusion barrier layers were deposited on sapphire and silicon substrates by DC magnetron sputtering. Samples were subsequently annealed in vacuum at temperatures between 100 and 800 °C for duration of between 20 min and 30 h. Thermally stimulated solid state reactions have been characterized by X-ray analysis methods. Mo/Si multilayers without barrier layers are stable up to 100 °C. Interdiffusion was observed to start by 150 °C. It was found that B 4 C diffusion barrier layers with thicknesses between 0.3 and 1.0 nm, depending on the stack sequence, give rise to an increase of the thermal stability up to 400 °C. The impact of thermal treatments, at various temperatures and annealing times, on thickness and composition of the interdiffusion layers was investigated by X-ray reflectometry, wide angle X-ray scattering, cross-sectional high resolution transmission electron microscopy and fluorescence extended X-ray absorption fine structure measurements in combination with excitation of X-ray standing waves. The last method was used to investigate the short-range order of Mo/Si multilayers depth-resolved.

On the temperature dependence of the hardness of quasicrystals

Abstract Results of mechanical micro- and nanotesting of single-quasicrystalline phases in the Al–Co–Ni-, Al–Pd–Mn- and RE–Mg–Zn-systems (RE=Y, Ho or Dy) are reported. Particular emphasis was devoted to the icosahedral phase in the Y–Mg–Zn-system, since — owing to its low melting point — high homologous temperatures Th can be attained almost without heating. At room temperature indentation of quasicrystals with icosahedral structure results in extended formation of type-III cracks, i.e. shear cracks with different height levels of the crack banks. This indentation-induced surface fragmentation is considerably less pronounced with decagonal phases, indicating that the quasicrystalline structure has a strong influence on brittleness. In contrast to the fracture behaviour the Meyer hardness value HM does not seem to follow a sequence HM (three-dimensional quasicrystal)》HM (two-dimensional quasicrystal)》HM (crystalline matter) but is mainly determined by the sequence of the solid→liquid transformation (or decomposition) temperature Tm, i.e. HM(Al73Ni14Co13)>HM(Al70Pd21Mn9)>HM(Y10Mg30Zn60). The hardness seems to be primarily governed by the complexity of the structure and by type and strength of bonding. At room temperature, all quasicrystals exhibit a strong indentation size effect with the hardness increasing with decreasing load. This effect becomes inverted at higher temperatures. The hardness variation with temperature depends on load: negligible variation for very small loads (nanotesting), very strong variation for moderate loads, and less drastic variation for higher loads. This observation is interpreted in the frame of two models: indentation-induced fragmentation, and dislocation involved deformation.

Structure refinement of the superconducting phase YNi2B2C as a function of temperature in the range 25–300 K

Abstract X-ray powder and single crystal diffraction has been used for structure refinement of various YNi 2 B 2 C samples in the temperature range 25–300 and 180–300 K, respectively. Linear thermal expansion is strikingly anisotropic. Coefficients vary between α a =4.1×10 −7 K −1 (25 K)–1.2×10 −5 K −1 (300 K) and α c =−6.1×10 −7 K −1 (25 K)–4.5×10 −7 K −1 (300 K) while the axial ratio c / a of the unit cell decreases with increasing temperature T . Moreover, a small increase of structure parameter z B with T was found. Anisotropic mean square atomic displacements remain almost constant for C and B but increase considerably for both Ni and Y atoms. The influence of the starting composition on the net content and on structure was studied for a series of samples. Lattice parameters as derived from Rietveld refinement deviated significantly among different samples. Relations between structure geometry, superconducting transition temperature, the residual resistance ratios and composition of the samples were discussed.

X-ray absorption fine structure study of short-range order of iron in Fe/Al multilayers

Results of quantitative analysis of fluorescence x-ray absorption fine structure (XAFS) experiments at the Fe–K absorption edge are presented for a 6*(4.3 nm Fe/10.4 nm Al) multilayer prepared by pulsed laser deposition. Fluorescence XAFS experiments have been combined with excitation of x-ray standing waves. This combination in one experiment allowed for a depth-controlled excitation of Fe fluorescence and hence for a depth-resolved analysis of short-range order. Depth-resolved analysis showed that instead of sharp Fe/Al or Al/Fe interfaces extended interlayer regions exist. The structure retained that of bulk α-Fe. In the upper half of the Fe layer 37 at. % Al as nearest and next nearest neighbors of Fe were found, whereas in the lower half 80 at. % Al atoms occur. Thus the Fe/Al interface (deposition of Fe on Al) should be characterized by an intermixing zone significantly larger in comparison to that of the Al/Fe interface (deposition of Al on Fe). By conventional XAFS measurement carried out at a fix...

XRD and DSC study of the formation and the melting of a new zeolite like borosilicate CsBSi5O12 and (Cs,Rb)BSi5O12 solid solutions

Polycrystalline CsBSi5O12 was prepared from a stoichiometric mixture by solid-state reaction above 1000 °C. The solid solutions Cs1–xRbxBSi5O12 were obtained at 1000 °C during a long heat treatment of polycrystalline Cs1–xRbxBSi2O6 boropollucites (xRb = 0, 0.05, 0.2, 0.4). A new borosilicate compound and its solid solutions were studied using X-ray powder diffraction (XRD), annealing, differential scanning calorimetry (DSC), and thermogravimetry (TG). For Cs,Rb-boropollucites the new phase formation is accompanied by significant mass losses detected by DSC and TG. The following mechanism of phase transformations is assumed: (Cs,Rb)BSi2O6 → (Cs,Rb)BSi5O12 + (Cs,Rb)BO2↑. The zeolite phase forms as a result of the boropollucite decomposition over 1000 °C. Zeolite decomposes also on further heating and the SiO2 reflections are observed in the XRD pattern only. Thus above 1000 °C both boropollucite and zeolite phases are unstable presumably due to the ability of the alkali cations to leave the structure. Using XRD the unit cell parameters of CsBSi5O12 have been determined in the orthorhombic crystal system: a = 16.242(4) Å, b = 13.360(4) Å, c = 4.874(1) Å. The compound is isostructural with the zeolite compound CsAlSi5O12. In the crystal structure of Cs1–xRbxBSi5O12 solid solutions the changes of cell parameters are insignificant under the substitution of Cs by Rb atoms that indicates a very limited substitution range.

Crystal structure of K1-xCsxBSi2O6 (x = 0.12, 0.50) boroleucite solid solutions and thermal behaviour of KBSi2O6 and K0.5Cs0.5BSi2O6

Abstract The crystal structures of two K1-xCsxBSi2O6 solid solutions have been refined at room temperature by the Rietveld method: x = 0.12, a = 12.6858(4) Å, Rwp = 7.66%, RF = 5.56% and x = 0.50, a = 12.8480(2) Å, Rwp = 7.64%, RF = 3.10%. They are isostructural to cubic KBSi2O6 with the space group I4̅3d. The structure is built up from (Si,B)O4 tetrahedra linked in four-, six- and eightfold rings which are forming a three-dimensional borosilicate framework. The framework contains large cavities that are placed in continuous channels along the [111] directions. The Cs and K atoms occupy the positions in the channels statistically. Thermal behaviour of KBSi2O6 and K0.5Cs0.5BSi2O6 has been studied by high-temperature powder X-ray diffraction within the temperature range of 293-1073 K. A new tetragonal polymorph of KBSi2O6 has been found in situ under heating. The new polymorphic I4̅3d (cubic) – Ia3̅d (cubic) transition and the new Ia3̅d cubic polymorphic phase has been proposed for K1-xCsxBSi2O6 from our experimental and literature data on crystal structures and thermal expansion of leucites. The structural relaxation under cationic (K, Cs) substitutions and under heating has been investigated.

Structural and magnetic phase transformation in metastable Fe-Cr alloys induced by ion irradiation

Unusual metastable paramagnetic phases have been observed in Fe–Cr thin films (thickness about 40 nm) fabricated by pulsed laser deposition. In the present article, x-ray diffraction and Mossbauer spectroscopy have been applied to follow the structural and magnetic phase transformation in these alloys induced by ion irradiation with a projected range positioned in the center of the films. It has been found that the critical dose for the transformation to the more stable body-centered cubic (bcc) structure depends on the initial phase of the film and the ion mass. The initial body-centered tetragonal phase, which forms in the alloys with low Cr content (∼30 at. %), can be completely transformed to the bcc phase already by a dose of 5×1015 Cr/cm2, whereas the primitive orthorhombic phase of roughly equiatomic Fe–Cr alloys is about four times more resistant against ion bombardment. A five times higher Ne ion dose is required to induce the same transformation as by the Cr bombardment. The observed effects are...

Nanomechanical characterization of Al–Co–Ni decagonal quasicrystals

This paper addresses some issues related to the nanomechanical responses of decagonal quasicrystals using nanoindentation techniques. A Hysitron Triboscope with a Berkovich indenter was used to carry out the nanoindentation tests on single crystals of Al–Co–Ni decagonal phase. The anisotropy in terms of nanohardness at higher load was observed, whereas it was not possible to establish the same at lower loads. However, the Youngs modulus was found to be indistinguishable in all these cases. Discontinuity in load displacement, which is known as a ‘pop-in’ effect, was observed frequently at various load regime. It was found that this discontinuity was not due to cracking or a phase transformation effect, but to a plastic yielding phenomena and nanodeformation of the material. Significant differences between the nanohardness and microhardness were noticed and are discussed based on various experimental parameters and the consequent mechanical response of materials.

X-ray investigation of metastable crystalline phases in co-deposited Fe–Cr alloy nanometer films

Abstract Fe100−xCrx (x=24.5–61.7, 100 at.%) alloy nanometer films prepared by crossed-beam pulsed laser deposition were examined by wide-angle X-ray scattering. On varying the Cr content, x, a change of phase content is observed starting from the known body-centered cubic phase at low Cr content. Indications for three new metastable Fe–Cr phases were found to form in body-centered tetragonal, face-centered orthorhombic and primitive orthorhombic lattice types, respectively.