About us

Summary

The work description of the group of the laboratory of theory and computational instruments is the calculation of the electron structures, in particular focusing on the oxide materials on the border between the insulator and metal with strongly correlated electron systems.

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Laboratory of X-ray powder diffraction

X-ray powder diffraction is a basic analytical tool in material science for phase identification and characterization. It is a non-destructive method with easy sample preparation and mounting. Small quantity of sample (0.1 cm3 of powder volume) is sufficent for measurement in the standard sample holder, but basically any amount can be analyzed as well using special holder cut from silicon single crystal.

X-ray powder diffractometer Bruker D8 Advance(link is external) (CuKα radiation) was installed in 2001.

 

Bruker D8
Fig 1: X-ray powder diffractometer Bruker D8 Advance
Bruker D8
Fig 2: X-ray powder diffractometer with Karel Knížek.

The principal task is an identifications of crystalline phases in multiphase sample with the help of crystal structure database ICSD(link is external).

Molar ratio of phases with known crystal structure is calculated by whole pattern fitting using FullProf suite(link is external) based on the Rietveld method. The powder diffraction is the most accurate method for calculation of lattice parameters. The Rietveld method is further used for refining the crystal structurei.e. atom positions and occupancies, of the identified phases. The modulated structures are solved by JANA(link is external) program. The determination of the crystallite (grain) size of nano-materials (over the range approx. 3-300 nm) is based on the analysis of the peak broadening. Thompson-Cox-Hastings pseudo-Voigt profile is used to resolve instrumental, strain and size contributions. Instrumental resolution is determined by measuring strain-free tungsten powder with grain size 9.4 µm. Preferred orientation (texture) of solid samples (pellets) or thin films can be analysed by the rocking-curve method.

The temperature dependent structural characterization over the range 90-1200 K is realized using MRI TC-wide range(link is external) temperature chamber (Bruker XRD Non-ambient Stages(link is external)). The measurements can be performed under ambient atmosphere, vacuum or flowing gas. Two scans are typically measured for each temperature and the measured intensities are compared in order to check the thermal stability and reproducibility of the sample structure.

Kalvados software was developed in our laboratory for working with powder diffraction data and crystal structure files.

Contact person: Karel Knížek

Laboratory of computational instruments

Inter alia, one of the main goals is to use the newly developed methods for the calculation of crystal field of rare earth elements in various optical and magnetic materials. Calculations can be performed efficiently on large-scaled, centrally maintained and - in terms of the use of computing time - optimized computing instruments. An important part of said computational instruments is also required technical support with regard to both hardware and software. In this regard, we expect the use of two central computing clusters, which are members of the Institute of Physics hardware. This is a cluster LUNA (47 nodes with 16 CPUs, 95 GB RAM, 800 gigabytes scratch) and Kalpa (2 nodes 24 CPUs, 256 GB RAM, 450 gigabytes scratch). Clusters are managed by an organization METACenter, which provides an extensive and well-maintained software.

Contact person: Pavel Novák

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Description

Fig. 3.: Energy levels and g-factors for ion Nd3+ v NdMnO3