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Phonon Spectrum in Alkali Metals at High Pressure

#2594


Inelastic Neutron Scattering Investigation of Alkali Metals at High Pressure

Tech Area / Field

  • PHY-SSP/Solid State Physics/Physics

Status
3 Approved without Funding

Registration date
22.08.2002

Leading Institute
Institute of Physical-Technical Problems, Russia, Moscow reg., Dubna

Supporting institutes

  • Institute for High Pressure Physics, Russia, Moscow reg., Troitsk\nVNIIEF, Russia, N. Novgorod reg., Sarov\nJoint Institute of Nuclear Research, Russia, Moscow reg., Dubna

Collaborators

  • Universita di Perugia / Dipartimento di Fisica, Italy, Perugia\nUniversity of Wisconsin-Madison / Department of Physics, USA, WI, Madison\nFraunhofer Institute Zerstörungsfreie Prüfverfahren / Branch Lab Dresden, Germany, Dresden\nArgonne National Laboratory (ANL) / Intense Pulsed Neutron Source Division, USA, IL, Argonne\nUniversity of Vienna / Institute for Experimental Physics, Austria, Vienna

Project summary

The project is aimed at studying of pressure-induced dependence of phonon spectrum in alkali metals in the pressure range from 0.8 to 2.5 GPa and temperature range 30-300 K comparing the experimental data with their electric resistivity. The phonon spectra in alkali metals are to be measured by incoherent inelastic neutron scattering technique.

Simultaneously with measuring the phonon spectra the structural dependence in these metals on both temperature and pressure by time-of-flight neutron diffraction technique will be determined. Greater pressures will enable to check the conclusions of the phonon spectrum theory and electric resistivity of alkali metals.

Measurement of electric resistivity of alkali metals after shock compression and unloading in the pressure range of 0.1-30 GPa, will, in particular, allow to investigate the feasibility of metal transition into molecular crystal, predicted in the paper of Neaton and Ashcroft [41].

The P-T dependences of thermodynamic functions (entropy, specific heat, thermal expansion coefficients) in Potassium and Rubidium [40] will be determined by technique of pulse-adiabatic pressure modulation up to 2.5 GPa.

All these data will enable to reconstruct the interaction potentials in alkali metals up to high pressures.

Theoretical approach.

The investigation of alkali metals Li, Na, K, Rb and Cs is of great interest for solid state physics. Their simple electronic structure permits to apply the theory of pseudopotentials for the description of their physical properties. Great compression ratio enables to study their [Р,Т] – phase diagram at relatively small pressures. Besides that, alkali metals under pressure show a number of unique properties: s-d – electron transition in Cesium and, possibly, in Rubidium, as well as a possible display of superconductivity in Cesium and Rubidium.

An important characteristic property of alkali metals is the fact that the pressure-induced dependence of electric resistivity in alkali metals is known to show the existence of a minimum at relatively small pressures of about 1 GPa.

At room temperature and atmospheric pressure, alkali metals crystallize in body-centered cubic (bcc) structure, that becomes, in general, with decreasing temperature or increasing pressure less stable than face-centered cubic close-packed (fcc) and hexagonal close-packed (hcp) structures. Actually, a second hcp-phase appears in Li at 78 K [8,9]. Na shows a second hcp-phase at a temperature of 36 K [10]. The possible mechanism of these transformations based on changed Gruneisen coefficients, are described in Ref. [8-10].

The project is aimed at gaining more detailed information about the changes of phonon spectra and the mechanism of electric resistivity in alkali metals under high pressure. To achieve this goal, the experiments on studying the lattice dynamics in alkali metals by inelastic neutron scattering are supposed to be carried out.

To this day, only several papers are available where some dispersion curves of alkali metals were studied. Mostly, they are related to bcc-hcp transition investigation in Lithium. Still less data exist on phonon state density in alkali metals at higher pressures. These data may be gained in inelastic incoherent neutron scattering experiments at the time-of-flight neutron spectrometers of KDSOG-M-type at the IBR-2 reactor [4]. The intensity at the KDSOG-M inverted geometry spectrometer where the experiment is to be carried out is by several orders of magnitude higher than intensity of a conventional inelastic scattering spectrometer for coherent neutron scattering. This is mainly due to a large solid angle of the detector system and simultaneous scanning of many Brillouin zones. This fact furnishes a hope to receive information on lattice dynamics in alkali metals at higher pressures.

The measurement of electric resistivity in alkali metals after shock compression and unloading in the pressure range 0.1-30 GPa is planned [42]. The results will be used for refinement of equations of state of alkali metals.

Experimental procedure.

Equipment and software to be used:

1. High flux pulsed reactor IBR-2, which is a high flux pulsed neutron source. The frequency of neutron bursts of the reactor is ~5 Hz, duration of a pulse is ~200 ms. Mean power of reactor is equal to 2 МW, pulse power is ~1,500 МW.


2. Neutron chopper, working simultaneously with pulsed reactor, designed to decrease the fast neutron background
3. High luminosity time-of-flight neutron spectrometer KDSOG-M [4], which was designed to analyze inelastic neutron scattering in a wide range of transmitted pulses and energies. The spectrometer resolution in the working mode using graphite crystal analyzer in the energy range of 0-20 meV is equal to 4% of the transmitted energy. The detector can work in the Beryllium filter mode, that approximately twice decreases the resolution but increases the luminosity by an order of magnitude. Simultaneously with inelastic incoherent neutron scattering measurements the spectrometer can be used for registration of diffraction pattern what is especially important for phase transformation investigations.
4. High resolution Fourier diffractometer HRFD [38] for precise investigation of structure (range of wave length – 0.8-6 Å, dhkl range – 0.5-6 Å, neutron flux on sample - 4×106 n/cm2/s, resolution (Dd/d)=1.5×10-3 for d=2 Å, sizes of standard sample - 6 mm in diameter, height - up to 60 mm.
5. Measuring modules for time-of-flight spectrometric measurements consisting of CAMAC- and VME-modules and PC/AT-type computers.
6. Software required for inelastic incoherent neutron scattering spectra and diffraction spectra calculations and g(w) calculations and refinement of atomic structure.

The experiments are to be carried out in high pressure cells made from Aluminum oxide ceramics (up to 2.5 GPa in the volume of 0.5 cm3) and in the cells of “toroid” type made from hard alloy of BK-6 type (up to 6 GPa in the volume of 0.06 cm3).

To improve the energy resolution of KDSOG-M spectrometer [4], it is planned to install new crystal-analysers made from pyrolytic graphite with mosaic of approx. 30ў.

Diffraction spectra measurements are planned to be carried out at high resolution Fourier diffractometer at the IBR-2 reactor (relative resolution is Dd/d=1.5×10-3 for d=2 Å) [38].

Experiments with alkali metals samples after shock compression and unloading in the pressure range 0.1-30 GPa are planned [42].


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