4. X-RAY LABORATORY

4.1. PREFACE

Probably the most important single event in 1996 concerns the experimental facilities at the X-ray laboratory. Based on the Faculty of Science supported materials science research program the Seifert diffractometer was supplemented with high and low temperature sample environment unit. These new facilities will be in potential use next year providing excellent possibilities in the studies of the properties of condensed matter.

The use of the synchrotron sources has been continued effectively. Two-year projects "Properties of high temperature superconductors" together with Warwick University (U.K.) and Cavendish Laboratory and "Magnetic scattering at high X-ray energies" together with Warwick University were completed successfully at the European Synchrotron Radiation Facility (ESRF). The latter experiment where extremely high X-ray photon energies up to 1 MeV were used, was selected to be one of the "ESRF Highlights" in 1996. High resolution inelastic scattering experiments were made both at ESRF and National Synchrotron Light Source (NSLS, USA) to study especially Fermi surface and electron-electron correlation effects. A theory project to support these experiments has been started with the Helsinki University of Technology.

The beam time of the four-circle diffractometer has been fully booked. There have been several visits from the Russian side (Academy of Sciences of Russia) related to the high temperature superconductor and nonlinear optical material projects. Also exchange of scientists has taken place between Charles University (Czech Republic) and X-ray laboratory.

Fruitful co-operation with Helsinki University of Technology has produced interesting results in tailored polymer systems. Versatile techniques including small- and wide angle scattering experiments at X-ray laboratory combined with time resolved studies at synchrotron sites have given new information about the order-disorder transition in polymer-surfactant systems. Several joint projects in materials science with the Department of Chemistry have also continued.

The role of external funding has been increasing. In 1996 about 1.1 MFIM was obtained, the main contributions came from the Academy of Finland, TEKES and ESRF. This funding was used to employ both M.Sc. and postgraduate students, for traveling expenses and updating scientific instruments. Altogether 9 full or part time researchers, paid by the external funding were working in the laboratory. These included two Graduate School (Materials Science Graduate School) students.

Seppo Manninen

Laboratory Chairman

4.2. SOLID STATE SPECTROSCOPY

ELECTRONIC PROPERTIES OF HIGH TEMPERATURE SUPERCONDUCTORS

Keijo Hämäläinen, Seppo Manninen, M.J. Cooper*, M.A.G. Dixon*, P. Lawson* and D.A. Cardwell**

The project, started in 1995, was continued at ESRF. Single crystal samples of YBa2Cu3O7 and Bi1.6Pb0.4Sr2Cu3O10-x, prepared and characterized at the Interdisciplinary Research Centre in Superconductivity, University of Cambridge, were studied both below and above the transition temperature using the high resolution spectrometer at the ESRF beam line ID15B. The anisotropy of the electron momentum distribution and its temperature dependence is analysed and will be compared with the predictions of band structure calculations.
*Department of Physics, Univ. Warwick, U.K.
**IIRC in Superconductivity, Cambridge, U.K.

MAGNETIC X-RAY SCATTERING AT RELATIVISTIC PHOTON ENERGIES

Seppo Manninen, Keijo Hämäläinen, M.J. Cooper*, P. Lawson*, D.N. Timms**, J.E. McCarthy***, P. Suortti*** and  T. Tschentscher***

Circularly polarized high energy photons have been used to study the magnetic contribution to the inelastic scattering cross section. Using the new superconducting wiggler at the ESRF beam line ID15A photon energies up to 1 MeV have been used to determine the magnetic contribution in Co and Fe samples [1,2]. Deviations from the scattering cross section, valid at lower energies were found and the results should be interpreted using full relativistic calculations.

* Department of Physics, Univ. Warwick, U.K.
** Division of Physics, Univ. Portsmouth, U.K.
*** ESRF, Grenoble, France

1.P. Lawson, D.N. Timms, M.A.G. Dixon, M.J. Cooper, J.E. McCarthy, K. Hämäläinen, S. Manninen and T. Tschentscher, J. X-Ray Sci. Techn. 6 (1996) 299
2. J.E. McCarthy, M.J. Cooper, P. Lawson, D.N. Timms, S. Manninen, K. Hämäläinen and P.Suortti, J. Synch. Rad., in press

HIGH RESOLUTION COMPTON SCATTERING STUDY OF GaN SINGLE CRYSTALS

Keijo Hämäläinen, Jarkko Laukkanen, Seppo Manninen, A. Andrejczuk*, L. Dobrzynski* and G. Loupias**

GaN is one potential candidate for a blue laser construction. The detailed electronic and defect properties of GaN are, however, relatively poorely known. This is due to the difficulties in growing good quality single crystals. Together with the Nicolaus Copernicus University in Torun, Poland, which is at the moment the only place in the world to produce reasonable sized good quality GaN single crystals, we have started a pro ject to study the electronic structure of GaN. Simultaneously the defect structure and mechanism will be studied using positrons at Helsinki University of Technology and calculations on the band structure are performed at the Nihon University, Japan.

* Dept. of Physics, Univ. Warsaw, Poland
**Univ. Pierre et Marie Curie (Paris VI), France

STUDY OF ELECTRON CORRELATION EFFECTS USING HIGH RESOLUTION COMPTON SCATTERING

Seppo Manninen, C.-C. Kao*, W. Caliebe*, J.B. Hastings*

We have continued the ultra high resolution Compton scattering spectroscopy using syn\- chrotron radiation both at NSLS and ESRF. The measured directional Compton profiles from beryllium [1] and lithium show that there is a pronounced smearing of the Fermi b reak whose origin is unknown. In order to exclude the anisotropy effects we have measured Compton scattering from sodium which should behave as free electron gas. This opens up possibilities to measure and study the electron-electron correlation directly. The experi\-ments have been accomplished at ESRF beamline ID16 and the data analysis is in process.

* NSLS, Brookhaven National Lab., USA

1. K. Hämäläinen, S. Manninen, C.-C. Kao, W. Caliebe, J. B. Hastings, A. Bansil, S. Kaprzyk and P. M. Platzman, Phys. Rev. B 54 (1996) 5453

THE NEAR-THRESHOLD EVOLUTION OF THE CuKalpha3,4 SATELLITES

M. Fritsch*, Keijo Hämäläinen, C.C. Kao**, O. Gang* and M. Deutsch*

Our previous measurements [1] of the near-edge-photoexcited CuKalpha3,4 satellites showed that (a) the onsets of the alpha3 + alpha'3 and the alpha4 + alpha' lines are separated by 10-15 eV, and (b) the spectral shape saturates ~50 eV above threshold, above which only the intensity increases but no shape changes occur. Those studies did not resolve accurately the threshold energies, nor the spectral shape changes in the immediate vicinity of the threshold, issues that were addressed during the new experiments. The shape is observed to change considerably with increasing excitation energy due to the different thresholds and cross-section energy dependences of the individual lines underlying this highly overlapping spectrum. Quantitative analysis, now in progress, will determine these dependences and providing the first such data for the virtually unexplored adiabatic re gime of the X-ray satellite emission process.

*Physics Department, Bar-Ilan Univ., Israel
** NSLS, Brookhaven National Lab., USA

1. M. Deutsch, O. Gang, K. Hämäläinen and  C.-C. Kao, Phys. Rev. Lett. 76 (1996) 2424

FLUORESCENCE ANALYSIS OF NOBLE METALS IN MINE DRILL TEST HOLE ROCK SAMPLES

Keijo Hämäläinen, Jarkko Laukkanen, Veijo Honkimäki*, P. Jalas** and Seppo Manninen

Studies on fluorescence analysis of noble metals in samples with varying shape, composition and origin have been continued and expanded. Recent advances include analysis of rock samples from mine drill test holes. We have also constructed a hardware system to control the measurement, sample position and pre-analysis. The hardware is operated with new control software running under Linux operating system. The construction has turned out to be successful and greatly expands our flexibility in research. Development of the quantitative analysis software package based on the fundamental parameter approach is tightly bound to the construction of the hardware [1]. Our aim is to continue the work and develop a portable system for automatic noble metal analysis.

*  ESRF, F-38043 Grenoble Cedex 9, France
** Jalas Engineering Oy

1. V. Honkimäki, K. Hämäläinen and S. Manninen, X-Ray Spectrom. 25 (1996) 215

PARAMETRIZATION OF RESONANT PHOTOIONIZATION CROSS SECTIONS

Sami Heinäsmäki, T. Åberg* and G. Kutluk**

The Fano parametrization of photoionization cross sections in the region of a resonance has been studied using a novel approach based on time-independent multichannel scattering theory. The total cross section is expressed in a factorized form using three parameters, the Fano parameter, the relative energy and the correlation factor. In the parametrization the possibility of both radiative and nonradiative decay is taken into account. We show that the Fano parameter depends on both the radiative and nonradiative widths which may be strongly energy dependent. The parametrization is applied to atomic spectra of rare earth elements, where the parametrized cross sections are shown to model extremely well the 4d -> 4f giant resonances.

* Laboratory of Physics, Helsinki University of Technology, 02150 Espoo, Finland
** Hiroshima synchrotron radiation center, 1-3-1 Kagamiyama, Higashi-Hiroshimashi 739, Japan

CIRCULAR DICHROISM IN RADIATIVE AND NONRADIATIVE INELASTIC XUV SCATTERING

Sami Heinäsmäki and T.Åberg*

Differences in cross sections for left and right circularly polarized light are analyzed for radiative and nonradiative inelastic scattering by randomly oriented systems. It is shown that special experimental arrangements result in finite dichroism. In the nonradiative case we have shown using time-independent resonant scattering theory that dichroism also exists for a J=1/2 intermediate virtual hole in contrast to results based on the assumption that the formation of the hole occurs separately from the decay. In the radiative scattering the dichroism is related to the polarization state of the detected radi ation [1]. The results of [1] were generalized by showing that dichroism exists beyond the dipole approximation and is proportional to the generalized oscillator strength. We will use dichroism for tests of commonly used treatments of inelastic X-ray scattering such as the impulse approximation.

*Laboratory of Physics, Helsinki University of Technology, 02150 Espoo, Finland

1. T. Åberg and S. Heinäsmäki, submitted to Appl. Phys. A

MANY-BODY EFFECTS IN COMPTON SCATTERING

Aleksi Soininen, B. Barbiellini*, Keijo Hämäläinen, Seppo Manninen

Inelastic X-ray scattering, better known as Compton scattering, is widely used tool in study of electronic structure of solids. Theories which are currently employed in interpreting the experimental results rely mainly on static, initial state dependent picture of the scattering process. Recent experimental and theoretical studies have shown that there is need to incorporate some final state effects into this picture. Our project is to study theoretically a nd numerically the effects of many-body interactions on the final state. At the moment we are numerically studying the electron-electron interactions in free electron gas. This should give us qualitative, and to a certain degree a quantitative, understand ing how these interactions change the valence electron part of the Compton profile.

*Laboratory of Physics, Helsinki University of Technology, 02150 Espoo, Finland

4.3. STUDIES ON CRYSTALLINE MATERIALS

STRUCTURE OF HIGH-Tc SUPERCONDUCTORS

Merja Blomberg, Matti Merisalo, M.A. Gosálvez, V.N. Molchanov*, I.P. Makarova* and V.I. Simonov*

The main project in the collaboration between our X-ray physics laboratory and the Institute of Crystallography in Moscow has been to study the structure of high-T superconductors. The aim is to perform systematic high-precision X-ray diffraction studies to determine the crystallographic fine structure of the samples both in the normal state and in the superconducting state. The results will be used to determine factors controlling the physical properties of these materials. The experimental work, collection of three-dimensional X-ray diffraction data at temperatures 10...300 K, has been carried out using the four-circle diffractometer and the attached low-temperature device. This work was continued by low-temperature measurements on YBa2Cu4O8, which were completed during the year. Analysis of the structure, especially the thermal vibrations of atoms near the superconducting transition point, is in progress. The results for the low-temperature structure of Nd2CuO4 were published [1].

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

1. I.P. Makarova, V.I. Simonov, M.K. Blomberg and M.J. Merisalo, Acta Cryst. B 52 (1996) 93-9

STRUCTURE OF NONLINEAR OPTICAL MATERIALS

Merja Blomberg, Matti Merisalo, M.A. Gosálvez, N. Sorokina*, I. Verin* and V.I. Simonov*

Low-temperature measurements on the structure of KTiOPO and TlTiOPO 4 have been carried out using the four-circle diffractometer of the X-ray physics laboratory. 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.

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


STRUCTURE OF URANIUM INTERMETALLICS

Merja Blomberg, Matti Merisalo, M.A. Gosálvez,  R. Kuzel*, L. Havela** and V. Sechovsky**

The study of the temperature dependence of the structure of uranium intermetallics UNiGa, URhAl, URuAl and URuGa has been continued. The aim is to determine the type of lattice distortions at the magnetic ordering temperatures and the temperature dependence of the effective atomic radius of uranium, and to attain a better knowledge of the lattice thermodynamics. The results obtained will be interpreted in context of high-pressure magnetic measurements.

* Department of Semiconductor Physics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic

** Department of Metal Physics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic

X-RAY RESONANCE EXCHANGE SCATTERING OF FERROMAGNETIC MATERIALS

Ritva Serimaa, F. Bartolome1,2, J.M. Tonnerre1, D. Raoux1, J. Chaboy2, L.M. Garca2, J. Bartolome2, H. Maruyama3, A. Marcelli4, F. Rietord5 and R. Simon5

X-ray resonance exchange scattering (XRES) is new method of investigating magnetic materials using synchrotron radiation. A computational method to extract the energy dependent dipolar and quadrupolar te rms of the magnetic scattering factor from experimental XRES data of ferromagnetic materials is developed. The analysis method was tested with simulated and experimental data of ferromagnetic Nd2Fe14B single crystal measured at LII and LIII absorption edges of Nd.

1 Laboratoire de Cristallographie, CNRS, 38042 Grenoble Cedex 09, France
2 Insitituto de Ciencia de Materiales de Aragon, CSIC-U. de Zaragoza, 50009 Zaragoza, Spain
3 Dept. of Physics, Okayama Universtiy, 3-1-1 Tauahima-Naka, Okayama 700, Japan
4 INFN Laboratori Nazionali di Frascati, 00044 Frascati, Italy
5 ESRF, B.P. 220, F-38043 Grenoble Cedex, France

4.4. STUDIES ON WEAKLY ORDERED MATERIALS

SELF-ORGANIZATION IN POLYMER-SURFACTANT SYSTEMS

Mika Torkkeli, Ritva Serimaa, Sakari Vahvaselkä, M. Toivola, O. Ikkala*, J. Ruokolainen*, J. Tanner*, M. Saariaho*, G. ten Brinke** and M. Luyten**

Co-operation with the polymer physics group of Materials Physics Laboratory at Helsinki University of Technology is continued. Novel micro-phase separated structures in polymer-amphiphile systems were observed, reminescent to block copolymer or liquid crystalline type structures. Experiments were made mainly with poly(4-vinyl pyridine), P4VP, which allows many types of interactions. For instance, with pentadecyl phenol P4VP forms microphase separated material via hydrogen bonding. The phase behaviour of polymer-amphiphile systems were studied both with SAXS and WAXS with X-ray laboratory equipment and with synchrotron radiation a t HASYLAB and Daresbury. The presence of an order-disorder transition from a self-assembled lamellar structure to a homogeneous state similar to comb-polymers was observed for the first time.[1] The conditions under which this transition occurs are now su rveyd and are of theoretical and experimental interest.

* Dept. of Technical Physics, Helsinki University of Technology
** Dept. of Polymer Science and Materials Science Center, Univ. of Groningen

1. J. Ruokolainen, G. ten Brinke, O. Ikkala, M. Torkkeli, R. Serimaa, Macromolecules 29 (1996) 3409
 

THE PHASE BEHAVIOUR OF THE GRAFTED AND SULFONATED POLY(VINYLIDENE FLUORIDE) MEMBRANES

Milja Karjalainen, Ritva Serimaa, Sakari Vahvaselkä, Mika Torkkeli, Veli Eteläniemi, Eero Rauhala, M. Paronen*, S. Hietala*, and F. Sundholm*

X-ray scattering studies of grafted and sulfonated poly(vinylidene fluoride) (PVDF) membranes with high proton-conductivity [1] were continued in co-operation with the Department of Chemistry. In earlier studies the ioni c conductivity of the membranes were found out to increase with the content of sulfonic acid groups and with decreasing crystallinity [1]. In 1996 studies on the temperature dependence of the structure of the grafted and sulfonated membranes were started . Understanding of the rich phase behaviour of the starting material, PVDF, and the grafted and sulfonated membrane are important from the point of view of applications.

* Dept. of Chemistry, University of Helsinki

1. S. Holmberg, T. Lehtinen, J. Näsman, D. Ostrovskii, M. Paronen, R. Serimaa, F. Sundholm, G. Sundholm, L. Torell, and M. Torkkeli. J. Mater. Chem. 6(8) (1996) 1309-1317

DIRECT SULFONATION OF POLY(VINYL FLUORIDE)

Milja Karjalainen, Mika Torkkeli, Ritva Serimaa, Eero Rauhala, Sakari Vahvaselkä, Veli Eteläniemi, M. Paronen*, S. Hietala* and F. Sundholm*

Proton irradiation and consequent direct sulfonation of poly(vinyl fluoride) (PVF) films have been studied, with the purpose of producing new types of proton conductors. The irrad iations were performed with 2.5 MeV protons at the Accelerator Laboratory. The films were studied by X-ray scattering methods. Irradiation was observed to promote sulfonation, the effect increasing with ion dose and the linear energy transfer. Furthermor e, that crystallinity of PVF decreased in the sulfonation process. In addition, mass change, ion exchange capacity, water uptake and conductivity are being investigated.

*Dept. of Chemistry, University of Helsinki

STRUCTURAL STUDIES OF AMORPHOUS PLATINUM PYRIMIDINE GREENS

Ritva Serimaa, Veli Eteläniemi, Mika Torkkeli, Aleksi Soininen, Sakari Vahvaselkä,  T. Laitalainen*, J. Pitkänen* and R. Lehtilä*

The structures and properties of amorphous Pt-pyrimidine greens [1] were studied together with the Laboratory of Organic Chemistry (HU). The purpose of these studies is to develop synthesis for a stable and biologically active Pt-complex. Earlier anomalo us wide angle X-ray scattering (AWAXS) studies indicated that Pt-uridine green is a mixture of mono- and dinuclear Pt-complexes.[2] In 1996 Pt-nuclearity of amorphous Pt-uridine blue sulfate was studied by AWAXS at Pt K absorption edge (70 keV) at European Synchrotron Radiation Facility. Furthermore, a study on the thermochromic behaviour of Pt-greens is in progress. As a part of this study the structure and aggregation of Pt-complexes in aqueous solution were studied by EXAFS and SAXS as a function of temperature.

*Dept. of Chemistry, University of Helsinki

1. T. Laitalainen, R. Serimaa, S. Vahvaselkä and A. Reunanen. Inorg. Chim. Acta. 248 (1996) 121
2. R. Serimaa, V. Eteläniemi, T. Laitalainen, A. Bienenstock, S. Vahvaselkä, and T. Paakkari. Submitted for publication

LOW TEMPERATURE ATTACHMENT FOR THE THETA-THETA-DIFFRACTOMETER

Sakari Vahvaselkä

A low temperature attachment has been constructed for the vertical sample cuvette used together with the theta-theta-diffractometer. Nitrogen gas flow acts as coolant circulating through the cuvette in the vacuum chamber. Temperatures are controlled with a heat controller (Eurotherm 818) together with a thermocouple. For liquid hydrocarbons a temperature -130 °C is reached with the accuracy of 0.3 °C.

LOW TEMPERATURE STUDY OF LIQUID NORMAL ALCOHOLS

Sakari Vahvaselkä and Ritva Serimaa

The X-ray scattering intensities of liquid normal alcohols and pentane were measured near their melting points using the nitrogen cooling system together with the theta-theta-diffractometer. The scattering intensities and radial distribution functions of liquid normal alcohols were compared with those of pentane and with those measured in room temperature earlier. Differencies of results in the two temperatures were interpreted in terms of changes in conformations of molecules and in packing. The largest differences were observed in the intensity curves at small scattering angles which suggest continuation of this work with small-angle X-ray scattering studies.