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E-Verhandlungen 2003
Programm und Abstracts der Sitzung SYEC 2

Neutron scattering under extreme P/T conditions

•Stefan Klotz
Physique des Milieux Condensés, Université P&M Curie, B77, 4 Place Jussieu, 75252 Paris, France

This talk will first give a brief overview of new experimental possibilities for high pressure neutron scattering up to 25 GPa and in the temperature range 80-2000 K. In the second part I will focus on very recent results on the structure and the transformations of amorphous ices at low temperatures, obtained by neutron diffraction at the ISIS facility. The structural pressure dependence of high density amorphous ice (HDA) to 2.2 GPa will be presented [1], and the relation of HDA to the recently discovered 'very high density amorphous' form (VHDA) will be discussed. The first in-situ diffraction data of the pressure induced transformation of LDA (low density amorphous ice) to HDA will be shown [2]; the impact of these results on the ongoing water debate will be outlined.

[1] S. Klotz et al., Phys. Rev. Lett. 89, 285502 (2002).

[2] S. Klotz et al., Science, under review.

New advanced high pressure nitrides and their properties

•Andreas Zerr, Gerhard Miehe, Marcus Schwarz, Roland Schmechel, Robert Kolb und Ralf Riedel
TU-Darmstadt, FB Material- und Geowissenschaften, Petersenstr. 23, 64287 Darmstadt, Germany

The laser-heated diamond anvil cell is a unique tool which allows for the synthesis of nitrides from elements at pressures up to 40 GPa and temperatures to 3000 K. Using this technique we discovered c-Si₃N₄, the most prominent member of a new family of the group-14 element nitrides with cubic spinel structure. It was found that c-Si₃N₄ exhibits a unique combination of advanced properties. It has high elastic moduli (B₀ = 290 GPa, G₀ = 148 GPa) and very high hardness of about 35 GPa. The fundamental band gap of c-Si₃N₄ was found to be about 3.3 eV. The outstanding property of c-Si₃N₄ is its thermal (meta)stability in air to at least 1400⁰ C. Recently we also discovered another novel family of high pressure nitrides Me₃N₄ (Me = Zr, Hf) with Th₃P₄-structure where the Me-cations are eight-fold coordinated by N-anions. To our knowledge, this is the first observation of binary nitrides with such a high coordination number. Both compounds exhibit a high bulk modulus around 250 GPa indicating high hardness. Cubic Zr₃N₄ and Hf₃N₄ may be the first members of a larger group of transition metal and/or lanthanide nitrides with ferromagnetic or superconducting behaviour.

AC-calorimetry at very high pressure and low temperature

•Heribert Wilhelm¹ und Didier Jaccard^2
1Max Planck Institut für Chemische Physik fester Stoffe, Nöthnitzer-Str. 40, D-01187 Dresden
²DPMC, University of Geneva, Quai E.-Ansermet 24, CH-1211 Geneva 4

The pressure-temperature (p,T) phase diagram of strongly correlated electron systems at low temperature is commonly explored by electrical resistivity measurements for pressures up to 25 GPa, using an anvil-type of high pressure cell. It is, however, desirable to measure thermodynamic quantities, notably the specific heat in these extreme conditions. The accessible pressure range for specific heat experiments was limited to 2-3 GPa since adiabatic techniques demand large sample masses and thus, a large cell volume. For higher pressures a much smaller sample volume is required. Among the non-adiabatic methods, the ac-calorimetry is a suitable technique to be adopted to the conditions in an anvil-type of high pressure cell. A combined measurement of electrical resistivity r(T) and ac-calorimetry of the magnetically ordered CePd_2.02Ge_1.98 in the temperature range 0.3 K < T < 10 K for pressures up to 22 GPa will be reported. The pressure dependence of the Néel temperature could be deduced from the anomaly observed in the ac-signal and in r(T). The inverse of the ac-signal 1/V_ac µ C/T recorded at the lowest temperature is strongly pressure dependent and shows a pronounced peak in the vicinity of the magnetic/non-magnetic borderline.

The pulsed high magnetic field facility in Dresden

•T. Herrmannsdörfer¹, H. Krug¹, F. Pobell¹, S. Zherlitsyn¹, H. Eschrig², L. Schultz² und K.-H. Müller^2
1Forschungszentrum Rossendorf (FZR), P. O. Box 51 01 19, D-01314 Dresden
²Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden (IFW), P. O. Box 27 00 16, D-01171 Dresden

We are building a user facility for experiments in pulsed high magnetic fields at the FZR. Based on the experience obtained since 1999 from a pilot pulsed field lab with a 1 MJ / 10 kV capacitor bank and magnets for the field range up to 60 T at the IFW, the new large scale facility will be built until 2006. It will comprise a new laboratory building, a 50 MJ / 24kV capacitor bank, and high performance experimental equipment including various pulsed magnets for the field range up to 100 T / 10 ms. In order to offer a wide spectrum of experimental possibilities, the pulsed field magnets are planned with various bore diameters (up to 50 mm) and various pulse times (10 ms to 1 s). As a unique opportunity, infrared spectroscopy in the wavelength range between 5 and 150 mm will be possible at high magnetic fields by connecting the pulsed field lab to the free-electron-lasers of the superconducting electron linear accelerator ELBE of the FZR. The german Wissenschaftsrat has recently recommended the Dresden High Field Project without any hesitation. The project is a joint effort of the FZR, the IFW, the Max-Planck-Institut für chemische Physik fester Stoffe, the Max-Planck-Institut für Physik komplexer Systeme, and the Institut für Angewandte Physik of the TU Dresden.

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