Professor Paolo G. Radaelli OSI

Abstract:

We have studied the crystal and magnetic structures of the magnetoelectric materials RMn2O5 (R = Tb, Ho, Dy) using neutron diffraction as a function of temperature. All three materials display incommensurate antiferromagnetic ordering below 40 K, becoming commensurate on further cooling. For R = Tb, Ho, a commensurate-incommensurate transition takes place at low temperatures. The commensurate magnetic structures have been solved and are discussed in terms of competing exchange interactions. The spin configuration within the ab plane is essentially the same for each system, and the radius of R determines the sign of the magnetic exchange between adjacent planes. The inherent magnetic frustration in these materials is lifted by a small lattice distortion, primarily involving shifts of the Mn3+ cations and giving rise to a canted antiferroelectric phase.

Abstract:

The crystal structure of deuterated natrolite, Na1.85Mg0.05Ca0.03[Al2.06Si 2.95O10]·nD2O, compressed in liquid D2O at 0.9 and 1.0 GPa has been determined from neutron powder diffraction data. At 0.9 GPa, the crystal structure is close to the original natrolite with the same space group Fdd2 and 1 % smaller unit cell volume. New water positions are found in addition to the original ones indicating the early stage of natrolite over-hydration. The unit cell volume of high-pressure phase stable at 1.0 GPa is expanded by 5.4 % with respect to initial natrolite. According to structural investigations, HP phase contains 3.5 water molecules pfu. Higher degree of hydration is accompanied by the drastic rearrangement of extra-framework subsystem, water molecules occupying four independent positions. Three of them belong to Na⁺ coordination sphere and together with three framework O-atoms form a distorted octahedron. Water molecule in the fourth position (occupancy = 0.5) has no contact to the cations. The evolution of natrolite structure with increasing pressure is discussed in terms of framework flexibility and hydrogen bonding rearrangement. © 2005 E. Schweizerbart'sche Verlagsbuchhandlung.

Abstract:

We have studied the magnetostructural phase diagram of multiferroic TbMn2O5 as a function of temperature and magnetic field by neutron diffraction. Dielectric and magnetic anomalies are found to be associated with steps in the magnetic propagation vector, including a rare example of a commensurate-incommensurate transition on cooling below 24 K, and in the structural parameters. The geometrically frustrated magnetic structure is stabilized by "canted antiferroelectric" displacements of the Mn3+ ions, an example of the magnetic Jahn-Teller effect. The Tb moments order ferromagnetically at low temperatures in an applied field, while the Mn magnetic structure is largely unchanged.