Magnetocrystalline and structural phase transitions of compounds MOCl (M = Ti, V, Cr, Fe) in dependence on pressure and temperature.

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Compounds MOCl (M = Ti, V, Cr, Fe) are isostructural layered compounds, in which the M3+ ions possess a magnetic moment that is unique for each element. Compounds MOCl with M = V, Cr or Fe develop antiferromagnetic order with strong magnetoelastic coupling and incommensurate and commensurate superstructures at low temperatures. TiOCl supports strong magnetic exchange interactions along chains of Ti3+ ions only. The behavior of TiOCl is essentially different from the behavior of the other MOCl because of these quasi-1-dimensional (1D) magnetic chains: TiOCl undergoes a transition to a spin-Peierls state at low temperatures. On increasing pressure, structural phase transitions occur which are related to the optimization of the packing of chlorine atoms in the VanderWaals gap between the layers MOCl. This project proposes to determine the pressure dependence of the magnetocrystalline phase transitions of MOCl (M = V, Cr, Fe) by X-ray diffraction experiments in dependence on temperature and pressure. Next to the pressure dependence of the transition temperature, diffraction experiments will provide the pressure dependence of the structural distortions of the magnetically ordered phases. Magnetostructural transitions of the high-pressure phases will be investigated by low-temperature diffraction experiments at pressures higher than 15 GPa. The pressure dependence of the modulated crystal structure of CrOCl offers the unique possibility to determine magnetostructural order in dependence on this modulation. Previous work has revealed, that at room temperature the spin-Peierls and structural transitions of TiOCl both occur at a pressure of ~15 GPa. Temperature- and pressure dependent X-ray diffraction will be used to determine the pressure dependence of the spin-Peierls transition. The influence of the structural transition on the magnetostructural order will be investigated at higher pressures. The experimental results will contribute to the understanding of the interaction between magnetism and crystal structure. An important aspect is, that the magnetic moment and occupied orbitals can be varied within a single structure type through the choice of M = Ti, V, Cr, Fe, while pressure allows a continuous variation of the crystal structure, including a continuously increasing modulation.