My actual position is shared between Hahn-Meitner-Institut and the Institut für Kristallographie at Tübingen. As HMI currently operates the most powerful neutron source in Germany, my work is concentrated on crystal chemistry using this fascinating sonde.

The materials involved in my work are manganites of the perovskite type. Different series of solid solutions have been prepared in Tübingen and are under study at the moment:

• Ho_1‚xCa_xMnO₃,
• Y_1‚xCa_xMnO₃,
• Sc_1‚xCa_xMnO₃.

In the regime of relatively low Ho-content, two phase transitions are observed, a crystallographic and a magnetic one. The crystal symmetry changes from Pnma (No. 62) to P2₁/n (No. 14), the critical temperature being a function of the Ho-content (primarily). The magnetic ordering is ferromagnetic below a certain temperature, the so-called Curie-temperature, T_C.

We try to separate both transitions by observing the lattice parameters at different temperatures. Monoclinic and magnetic phase transitions have different influence on lattice parameters, unit cell faces and volume. Additionally, strain moduli can be calculated from the lattice parameters. The temperature dependance af all these parameters is modeled by phenomenological means (e.g. using polynomial fits).

Neutron diffraction data are analyzed in order to elucidate the magnetic structure of these materials. This is possible, because a neutron "feels" the moment of magnetic species in a sample. The effect of this mechanism on the diffraction patterns is well understood and therefore the magnetic structure can be calculated once the crystallographic structure is known. With SIMREF 2.5, a Rietveld software developed by H. Ritter, diffraction patterns from different sources can be used in order to refine one single structural model.