Neutron Scattering - JUWEL - Forschungszentrum Jülich

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POWDER DIFFRACTOMETER SPODI 19 Typically, the monochromator crystals are installed at a vertical focusing unit of 200 – 300 mm, allowing optimization of the intensity distribution at the sample position with respect to the monochromator – sample distance or the sample height. On the other hand, the vertical beam divergence results in a smearing of the Debye-Scherrer rings along the detector height (this effect depends also on the sample height). At the high-resolution powder diffractometer SPODI, 15 Germanium wafer-stack crystals with a (551)-orientation are used. Different wavelengths between 1.0 and 2.6 Å can easily be selected by rotation of the monochromator unit (without changing the monochromator take-off angle 2�m), i.e. by selecting different (hkl) reflection planes. In general, large wavelengths are advantageous to investigate structures exhibiting large d-values. This is the case for large unit cells, but in particular for magnetic ordering. With decreasing wavelengths, larger Q-values can be achieved. Thus, with lower wavelengths, more reflections can be observed in the same scattering angle range. Low wavelengths are in particular advantageous for the analysis of thermal displacement parameters or static disorder phenomena. Detector array At constant-wavelength diffractometers the data are collected in an angle-dispersive manner at equidistant 2� points. Detector systems based on 3 He have been most commonly used due to their very high efficiency. Now, the world wide shortage of 3 He demands and promotes the development of alternatives, in particular scintillator based systems. Classical high-resolution powder diffractometers, such as D2B (ILL), SPODI (FRM II), BT1 (NIST), ECHIDNA (ANSTO) use multidetector/multicollimator systems. The data are collected by 3 He tubes while the beam divergence is limited by Soller collimators. Such systems enable high Q-resolution over a broad scattering angle regime, while the resolution does not depend on the sample diameter. On the other hand, a multidetector concept requires a data collection by stepwise positioning of the detector array to collect the full diffraction pattern. Therefore, kinetic measurements are not feaible due to the fact that the sample must not change during the collection of a pattern. The detector array of SPODI consists of 80 3 He tubes, which are position sensitive in the vertical direction. Thus, two-dimensional raw data are obtained, which allow to rapid check for sample crystallinity, alignment and possible preferred orientation effects. The conventional diffraction patterns (intensity vs. scattering angle 2�) are derived from the twodimensional raw data by integration along the Debye-Scherrer rings.
20 POWDER DIFFRACTOMETER SPODI 7. Experiment: Phase- and structure analysis of lead titanate at various temperatures samples Lead zirconate titanates PbZr1-xTixO3 („PZT“) exhibit piezo-, pyro- and ferroelectric properties. Piezoelectricity describes the generation of an electric polarisation as a consequence of a mechanical deformation – or the other way round the development of a macroscopic strain by an electric field. The crystallographic condition of piezoelectricity is the lack of an inversion center: as the balance points of negative and positive charge do not coincide the displacements of the ions in the electric field results in a polarization. Pyroelectrticity refers to a spontaneous polarization of a material as a function of temperature. Ferroelectrics are special pyroelectric materials, in which the polarization can be switched by an electric field, resulting in a ferroelectric hysteresis. The electromechanical properties of PbZr1-xTixO3 can be understood by their phase transformation behaviour. At high temperatures they exhibit the perovskite structure with simple cubic symmetry (space group Pm-3m). Because of its symmetry (inversion center) this phase is not piezoelectric but paraelectric. During cooling, titanium-rich samples undergo a phase transition to a tetragonal phase (space group P4mm). This phase transformation is accompanied by atomic displacements. In particular, the Ti 4+ /Zr 4+ are shifted in the opposite direction than O 2- ions, resulting in a dipole moment or a spontaneous polarisation. The material exhibits ferroelectric behaviour, with a polar axis in the direction of the pseudocubic c-axis, i.e. [001]c . Zirconium rich samples undergo a phase transition towards a rhombohedral phase (space group R3m) during cooling. In this case, the atomic displacements result in a polar axis in direction [111]c with respect to the parent pseudocubic lattice. Materials PbZr1-xTixO3 with compositions (Zr/Ti ratios) close to the so called morphotropic phase boundary between rhombohedral and tetragonal phase, show the highest piezoelectric response, i.e. the largest macroscopic strain as a function of the applied electric field. These compositions are therefore most interesting for technological applications. The piezoelectric properties can be modified further by adding doping elements to substitute Pb 2+ or Ti 4+ /Zr 4+ ions.
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SPHERES 13 4. Summarize the tempera
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________________________ DNS Neutro
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DNS 3 1 Introduction Polarized neut
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DNS 5 Monochromator horizontal- and
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DNS 7 3 Preparatory Exercises The p
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DNS 9 important and always necessar
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DNS 11 Contact �� Phone:
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________________________ KWS-3 Very
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KWS-3 3 1 Introduction Ultra small
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KWS-3 5 2. The standard Q-range of
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KWS-3 7 Table 2. Samples for practi
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KWS-3 9 References [1] Eskilden, M.
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________________________ RESEDA Res
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RESEDA 3 1 Introduction Neutrons ar
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RESEDA 5 various length values l ac
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RESEDA 7 In this expression, a norm
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RESEDA 9 spin flip from up to down
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RESEDA 11 The neutrons are counted
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