4. X-RAY LABORATORY

 

4.1. PREFACE


In 1998 four new research projects, funded by the Academy of Finland, were started at the X-ray laboratory. These include two projects related to X-ray spectroscopy, one involves studies on polymers, the other forming part of a joint national project concentrating on the properties of wood. Taking into account existing projects our research is now increasingly reliant on external funding, which amounted to about 2 MFIM in 1998. This meant that about 70% of our researchers were, including two Ph.D. students in the national materials physics graduate school, funded externally.

Visits to synchrotron radiation facilities have been a crucial part of the experimental research. In addition to two members of our laboratory, Veijo Honkimäki and Pekka Suortti, who spent the whole year at the European Synchrotron Radiation Facility (ESRF), France, 13 scientific visits were made to ESRF, National Synchrotron Light Source (NSLS), USA, and Hamburg Synchrotron Radiation Laboratory (HASYLAB) in Germany. The major research programs were related to (i) electron correlation and Fermiology (ESRF), (ii) momentum transfer dependence of the inelastic scattering cross section (ESRF), (iii) electronic properties of high temperature superconductors (ESRF), and (iv) resonant inelastic X-ray scattering (NSLS).

Applications of wide and small angle scattering form one of the most promising areas of the research in the X-ray laboratory. Co-operation with the Helsinki University of Technology (HUT), Finnish Forest Research Institute METLA, and laboratories in the Department of Chemistry, University of Helsinki have produced new important results in the broad area of weakly ordered materials.

The Finnish-Russian teamwork in the use of the four-circle diffractometer has continued. Non-linear optical materials and langasites, used as substrates for surface acoustic wave filters, have been studied in 1998. Applications related to some shape memory and smart materials were carried out with HUT and the National Academy of Science of Ukraine.

Co-operation with the Department of Astronomy continued with the characterization of the SIXA X-ray detector array, part of the Russian X-ray telescope to be launched into inner space. The development of a commercial fluorescence analyzer has been continued in collaboration with Quantronics Oy.

In December we lost a long-term member of our laboratory when Riitta Kurki-Suonio passed away after a long illness. We all want to express our sympathy to Kaarle and the daughters.

March 9, 1999

Seppo Manninen
Laboratory Chairman
 
 

4.2. SOLID STATE SPECTROSCOPY


LINEAR DICHROISM IN DOUBLE PHOTOIONISATION AND NONRESONANT X-RAY SCATTERING
Sami Heinäsmäki

We studied theoretically the polarization dependence of angular patterns of electrons in double photoionization by randomly oriented systems for both nonresonant and resonant electron emission. The result is applicable in studies of linear and circular dichroism which gives information on the electron-electron correlation. We also demonstrate the possibility to extract orientational information in nonresonant X-ray scattering using linear dichroism. We assume photon beams in mixed polarization states and express the differential cross sections in terms of Stokes parameters.
 

HIGH RESOLUTION COMPTON SCATTERING STUDY ON Be
Simo Huotari, Keijo Hämäläinen, Seppo Manninen, W. Caliebe* and T. Buslaps**

The high resolution synchrotron-based Compton scattering studies on simple metals have been continued at ESRF beamlines ID15 and ID16. Our goal is to find a reason for the observed Compton defects, eg. profile asymmetry, peak shift, and broadening of the Fermi break unexplained by the accustomed theory of Compton scattering. The study was extended to include momentum transfer dependence of the failure of theory, especially the impulse approximation (IA). When comparing our ultra-high resolution data (D pz = 0.02 atomic units) to the theoretical profiles, we see clear broadening of the Fermi surface, as found in the earlier experiments on Be, Li and Na. The effect is evidently not due to IA failure, because it sustains also with high momentum transfer values. We suspect that the effect is most likely due to the many-electron spectral shape function arising from the Green's function theory of many-body problems. In the usual Compton scattering theory the spectral function is approximated by a delta function, but calculations show that it has a finite width and a double-peak structure. Theoretical approach to include this effect to the scattering cross section is under calculation.

* HASYLAB, DESY, Hamburg, Germany
** European Synchrotron Radiation Facility, Grenoble, France
 

CHARGE TRANSFER EXCITATION STUDIES UTILIZING RESONANT INELASTIC X-RAY SCATTERING
Keijo Hämäläinen, J.P. Hill*, Simo Huotari, C.-C. Kao**, L.E. Berman**, A. Kotani+, T. Ide+, J.L. Peng++ and R.L. Greene++

The physics of strongly correlated electron systems is currently attracting enormous attention because it underlies some of the most challenging and important questions in solid state physics, including especially high-TC superconductivity. Of basic interest in these systems is their electronic structure, and in particular the electronic excitation spectrum, which determines the electron dynamics. However, theoretical calculations of such properties are complicated by the strong electron correlations which mean that neither conventional band theories, nor local cluster calculations are adequate. Recently, work has begun to extend cluster calculations to larger numbers of atoms in order to correctly account for solid state (non-local) effects. While such approaches seem promising, there is a clear need for further experimental tests of these models. We have accomplished experimental studies on the polarization and momentum dependence of the 6 eV charge transfer excitation in Nd2CuO4, using resonant inelastic X-ray scattering utilizing synchrotron radiation. The incident polarization is shown to select the intermediate states participating in the resonant process and resonances associated with 1s -> 4ps and 1s -> 4pp transitions are resolved. No enhancement is observed for the 1s3d10L well-screened intermediate states in either case. Numerical calculations show that this is the result of non-local effects. The intensity and position of the excitation was found to be independent of the momentum transfer within 3.5 - 7.9 Å-1.

* Department of Physics, Brookhaven National Laboratory, USA
** National Synchrotron Light Source, Brookhaven National Laboratory, USA
+ Institute for Solid State Physics, University of Tokyo, Japan
++ Center for Superconductivity Research, Department of Physics and Astronomy, University of Maryland, USA
 

FLUORESCENCE ANALYSIS OF NOBLE METALS
Keijo Hämäläinen, Jarkko Laukkanen, Seppo Manninen, P. Jalas*, S. Hemminki*, J. Vierumäki* and Veijo Honkimäki**

The research and development project on the quantitative fluorescence analysis of noble metals, based on the fundamental parameter approach, has been continued to a more advanced level. Together with our industrial partner, we started the development of a professional, semi-automatic fluorescence analyzer with a certificate-level accuracy.

* Quantronics Oy, Helsinki, Finland
** European Synchrotron Radiation Facility, Grenoble, France
 

CHARACTERIZATION OF THE SIXA X-RAY DETECTOR ARRAY
Jarkko Laukkanen, Miguel A. Gosálvez, Keijo Hämäläinen, L. Alha*, J. Huovelin* and O. Vilhu*

Extensive studies on the SIXA X-ray detector were conducted both for the single pixel prototype unit, and for the 19-pixel detector array (the final qualification model). The results of these studies will be further used to develop a standard set of characterization and calibration methods for the final flying model before the scheduled launch. The setup consisted of a W-anode X-ray tube (operated at 40 kV maximum), a single crystal Si [111] monochromator and motorized X,Z tables with full computer control with the \tt spec software package. Both the dedicated SIXA ground station system and the \tt spec controlled MCA were used in the studies.

* Department of Astronomy, University of Helsinki, Finland
 

FERMI SURFACE AND ELECTRON CORRELATION EFFECTS IN SIMPLE METALS UTILIZING HIGH RESOLUTION COMPTON SCATTERING
Seppo Manninen, Keijo Hämäläinen, W. Caliebe*, C.-C. Kao**, T. Buslaps+, M. Krisch+ and A. Bansil++

High resolution Compton scattering spectrometers at NSLS (X21) and at ESRF (ID15 and ID16) have been used to study single crystals of Li, Be and Na. The momentum resolution, as good as 0.02 a.u. of momentum makes it possible to study the structure of the Fermi surface and electron-electron correlation effects which are both crucially effected by the resolution. The results clearly show that calculations made within the local density approximation do not explain the observed data. Deviations close to the Fermi surface are a sign of electron correlation effects. An effort to produce the momentum density based on the directional experimental Compton profiles is in progress. This would lead to an experimental value of the renormalization parameter describing the size of the correlation effect.

* HASYLAB at DESY, Hamburg, Germany
** National Synchrotron Light Source, Brookhaven National Laboratory, USA
+ European Synchrotron Radiation Facility, Grenoble, France
++ Northeastern University, Boston, USA
 

THEORETICAL AND EXPERIMENTAL STUDY OF X-RAY INELASTIC SCATTERING FROM SIMPLE METALS
Aleksi Soininen, Keijo Hämäläinen, Seppo Manninen, A. Kaprolat*, W. Schülke** and C. Sterneman**

Inelastic scattering experiments at high momentum transfer, referred as Compton scattering, are usually interpreted within so-called impulse approximation. It assumes that the electron binding energies are negligible compared with the energy transferred to the ejected electron. We have studied the validity of this approximation starting from an electron gas and calculated the double differential inelastic scattering cross section in terms of the spectral functions of a many-electron system. The spectral functions were calculated using the GW-approximation. The calculated cross sections for simple metals had a clear asymmetry, in agreement with recent high resolution experiments. These calculations are important both in the planning of new experiments and also in the interpretation of the observed data.

* European Synchrotron Radiation Facility, Grenoble, France
** University of Dortmund, Germany
 

CORRELATION EFFECTS IN MOMENTUM SPACE: A QUANTUM MONTE CARLO STUDY FOR He AND Be ATOMS
Aleksi Soininen and A. Harju*

Recently variational Monte Carlo (VMC) has been used to study the correlation effects to momentum density and Compton profiles of some solids (i.e. Li, Si). On the other hand, there has not been comparisons of the VMC momentum densities in the cases where the nearly exact configuration interaction (CI) densities are available. In this project we have calculated the radial momentum density of different types (VMC) wave functions for few atoms. By comparing these results to those obtained from CI calculations we hope to get insight in to how well the Monte Carlo based optimization of the wave functions is able to reproduce the momentum density of these systems.

* Laboratory of Physics, Helsinki University of Technology, Finland
 
 

4.3. CRYSTALLINE MATERIALS


THE STRUCTURE OF NONLINEAR OPTICAL MATERIALS
Merja Blomberg, N. Sorokina*, I.A. Verin* and V.I. Simonov*

Low-temperature studies on the structure of compounds of the KTiOPO4-family have been continued. KTiOPO4 is a well-known nonlinear optical material which is extensively used for frequency doubling of laser light. Because the efficiency of the KTiOPO4-related compounds is essentially higher than that of other crystalline materials used for this aim, synthesis of new compounds of this family and studies of their crystal structure and physical properties is of great importance. Single-crystal X-ray diffraction measurements on the structure of (Cs,Rb)TiOAsO4 at 20 K were performed. Results of the structural study of TlTiOPO4 single crystals at 11 K were published [12].

* Institute of Crystallography, Academy of Sciences of Russia, Moscow, Russia
 

STRUCTURE OF LANGASITES
Merja Blomberg, V.N. Molchanov*, I.A. Verin* and V.I. Simonov*

Langasite (La3Ga5SiO14, LGS) is one of the most promising substrate materials for surface acoustic wave filters in the next-generation mobile telecommunications equipment. Langasite offers temperature stability comparable to that of quartz crystals while providing a much better piezoelectric performance. An interesting scientific problem related to crystals of the LGS-family is the origin of dielectric and elastic anomalies in their material constants. Low-temperature data from dielectric measurements indicate a possible phase transition near 40 K. Studies of the changes in the crystal structure of langasite at temperatures below the room temperature were started.

* Institute of Crystallography, Academy of Sciences of Russia, Moscow, Russia
 

TWIN BOUNDARY MOTION STUDIES ON Ni2MnGa
Merja Blomberg, N.I.Glavatska* and K. Ullakko**

The collaboration with the Helsinki University of Technology  in the area of magnetic shape memory materials has been continued. The aim is to study the mechanisms enabling large magnetically induced strains in the martensitic phase of Ni2MnGa. The proposed explanation involves twin boundary motion, which would favor reversible size increase for those domains correctly oriented with respect to the magnetic field. Following the effect of the magnetic field on the intensity and shape of a number of reflections in the x-ray diffraction patterns could provide a proof for this explanation. Preliminary measurements at room temperature and at 240 K with and without a magnetic field were performed.

* Institute for Metal Physics, National Academy of Science of Ukraine, Kiev, Ukraine
** Helsinki University of Technology, Finland
 

SMART MATERIALS
Veli Eteläniemi, Sakari Vahvaselkä, Y. Ezer*, A. Sozinov**, V. K. Lindroos* and K. Ullakko*

Smart materials as shape memory, magnetic recording, magnetostrictive and magneto-optic materials, are new technology materials that are interesting due to their ability to react to the changes in their environment. Their properties may change due to, for instance, pressure, temperature, luminosity, electric and magnetic field. Our purpose is to develop materials that are sensitive to the magnetic field. A property that is not widely applied so far.

Why magnetic field? One reason for that is that the changes take place rapidly. Piezo crystals under electric field are one example of this kind fast reactivity. X-ray diffraction method is one possibility to get information from the atomic positions under magnetic field. From the stress measurements we already knew that the size of the sample changes when magnetic field is applied. By diffraction studies we try to find out information about the physical reasons which will then help us to develop materials where, for instance, the shape memory effect is larger.

* Helsinki University of Technology, Finland
** Institute for Metal Physics, National Academy of Science of Ukraine, Kiev, Ukraine
 

SEEMANN-BOHLIN THIN FILM DIFFRACTOMETER
Szabolcs Galambosi and Merja Blomberg

The new Huber Guinier-diffractometer was installed and tested. Strictly monochromatic CuKa1 radiation is used in focusing geometry. By using a small angle of incidence to the surface of the specimen, large diffraction intensities can be obtained even with thin films. Stress measurements in thin films were demonstrated using a 800 Å thick gold layer vaporized on a silicon wafer. Due to the sensitivity of the equipment to slight misalignments, angular corrections to the diffraction data using internal standards were found to be necessary. Very low stresses (approx. 50 MPa) can be measured routinely. The diffractometer can also be used as a normal powder diffractometer in symmetric and asymmetric transmission and reflection geometries, still employing the Seemann-Bohlin focusing.
 
 

4.4. WEAKLY ORDERED MATERIALS


THE STRUCTURE OF WOOD CELLULOSE IN NORWAY SPRUCE
Seppo Andersson, Ritva Serimaa, Matti-Paavo Sarén, Simo Siiriä, P. Saranpää*, E. Pesonen* and Timo Paakkari

The structure of cellulose in the wood of Norway spruce (Picea abies L. Karst.) was studied by X-ray scattering and optical microscopy. The orientation of cellulose is an important property of wood cells (tracheids), affecting both the shrinkage and tensile strength of wood. Microfibril angle (MFA) is a quantity, which represents the orientation of cellulose in the cell wall along the stem axis. The techniques applied are wide angle X-ray scattering (WAXS), small angle X-ray scattering (SAXS), and optical methods (polarizing microscopy). WAXS experiments were made by using both the reflections (002) and (040) to obtain information on both the MFAs and the shape of the cross section of the cell. The dimensions of the elementary cellulose fibrils were also estimated by both X-ray scattering techniques. Microscope pictures of transverse sections of wood samples were analysed by an image processing program made for the purpose. The metrics of the wood cells are obtained by comparing the experimental data with mathematically produced parallel data.

* Finnish Forest Research Institute METLA, Finland
 

X-RAY SCATTERING STUDIES ON EDIBLE POTATO TUBERS
Teemu Ikonen, Kaija Jokela, Ritva Serimaa, T. Väänänen*, L. Pietilä** and E. Pehu+

The structure of starch granules of potato tubers is studied by wide and small angle x-ray scattering (WAXS, SAXS). One aim of the study is to reveal structural factors causing problems in the processing behaviour of Finnish potato tubers. Experiments are made by using fresh potato slices since it was observed that their structure is not equal to that of isolated starch.

Cultivated potatoes, wild species (Solanum acaule and Solanum brevidens), and hybrids between wild species and cultivated potatoes were studied. The size of the crystallites and crystallinity vary with the potato species and the humidity of the slice. The size of the crystallites in hybrids was found to be greater than that of the wild species. Also the amount of crystalline starch in the wild species was lower than in most hybrids.

* Department of Chemistry, University of Helsinki, Finland
** Boreal Plant Breeding, Jokioinen, Finland
+ Department of Plant Production, University of Helsinki, Finland
 

NANOSTRUCTURES OF COMPLEXES OF CATIONIC STARCH AND ANIONIC SURFACTANTS
Teemu Ikonen, Mika Torkkeli, Ritva Serimaa, J. Merta*, E. Kontturi* and P. Stenius*

Combining surfactants with oppositely charged polyelectrolytes is one way for producing ordered materials. Such materials have biological significance, since polynucleic acids and proteins are polyelectrolytes and lipids are surfactants. In this study complexes of cationic starch (CA) and anionic surfactants are synthesized for modification of mechanical properties of polymer materials. According to the SAXS experiments the structures formed in aqueous solutions of CA and alkanoates and sodium dodecyl sulfate depended from the length of the hydrocarbon chain of the surfactant. Cylindrical, lamellar, and spherical micelles were obtained, which formed well ordered larger scale crystalline structures.

* Laboratory of Forest Products Chemistry, Helsinki University of Technology, Finland
 

X-RAY SCATTERING STUDIES ON PROTON CONDUCTING MEMBRANES FOR FUEL CELLS
Kaija Jokela, Milja Karjalainen, Mika Torkkeli, Ritva Serimaa, Sakari Vahvaselkä, Veli Eteläniemi, M. Paronen* and F. Sundholm*

Structural investigations of proton conducting membranes have been continued by WAXS and SAXS methods. The aim of this study is to develop new, inexpensive membrane materials for fuel cell applications requiring high proton conductivity and good mechanical, thermal and chemical stability. The matrix material of the PVDF-based membranes has been changed during this year. The new matrix material is now poly(vinylidene fluoride-co-hexafluoropropylene), Kynar, which has lower inherent crystallinity than PVDF. Information about crystallinity, preferred orientation and macrostructure of the Kynar-based membranes was received by WAXS and SAXS measurements. WAXS studies have shown that Kynar-based membranes are more homogenous than PVDF-based. The preferred orientation of the Kynar-based membranes is also quite weak. According to SAXS experiments Kynar-based membranes contain PVDF lamellae.

* Department of Chemistry, University of Helsinki, Finland
 

STUDIES ON ION IMPLANTED SrS-BASED THIN FILMS
Milja Karjalainen, Ritva Serimaa, Reijo Lappalainen, Sanna Sevanto, W.-M. Li* and M. Leskelä*

Structural investigations of ion implanted SrS based thin films were continued. These films were studied for emitting blue light in electroluminescence displays. The structure of the films was studied by means of X-ray diffraction and the diffusion of the implanted ions was studied by means of nuclear resonance broadening and Rutherford backscattering spectrometry. We have observed that the luminescence properties of the films depend on the phosphor material, the implanted ion, amount of the ions, the annealing temperature, the crystallinity, the strain and the preferred orientation of the crystallites.

* Department of Chemistry, University of Helsinki, Finland
 

NANOSTRUCTURING OF RIGID ELECTROACTIVE POLYMERS DUE TO SELF-ORGANIZATION
M. Knaapila*, J. Ruokolainen*, O. Ikkala*, R. Mäkinen*, Mika Torkkeli, Kaija Jokela, Ritva Serimaa, T. Mäkelä**, A. P. Monkman+ and G. ten Brinke++

Hierarchical supramolecular nanostructures having multiple length scales have been demonstrated which allow their operation as electric switches [2]. In the first case, self-organization of flexible (coil-like) polymers have been used to create nano scale channels exhibiting ionic conductivity. Presently, similar concept is applied to conjugated electroactive polymers which are rigid and therefore typically have poor solubility. We have observed that high solubility of poly(p-pyridine) in selected amphiphiles is achieved via sulphonic acid doping. As a product, the system forms cylindrical co-crystallized nanostructures with a characteristic dimension of about 40 Å i.e. nano scale electronic conductors which might be controlled by forcing phase transitions on the nanostructures.

* Department of Engineering Physics and Mathematics, Helsinki University of Technology, Finland
** VTT Microelectronics, Espoo, Finland
+ Department of Physics, University of Durham, UK
++ Department of Polymer Science, and Materials Science Center, University of Groningen, The Netherlands
 

X-RAY SCATTERING AND DSC STUDIES ON ETHYLENE-$\alpha$COPYLMERS
A. Väänänen*, Mika Torkkeli, Kaija Jokela, P. Starck*, Ritva Serimaa, B. Löfgren** and J. Seppälä**

The structure of ethylene-1-olefin copolymers was studied by wide and small angle X-ray scattering (WAXS, SAXS) and differential scanning calorimetry (DSC). 1-butene, 1-hexene, 1-octene and 1-hexadecene were chosen as comonomers and both Ziegler-Natta and metallocene catalysts were used for polymerization. When the comonomer content is small, the crystallinity of the material is high and the material contains lamellar crystallites which are stacked with interlamellar amorphous regions. The crystallinity and the average size of the crystallites decrease with increasing content of co-monomer. When the comonomer content is large, the crystallites form poorly organized bundles.

* Polymer Science Center, Helsinki University of Technology, Finland
** Laboratory of Industrial Chemistry and Polymer Technology, Department of Chemical Technology, Helsinki University of Technology, Finland