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Abstract:

Transition-metal spinels (general formula AB 2X 4) have been, for many years, the subject of intense experimental and theoretical activity. Structurally, the most interesting feature of these systems is the fact that the B cation occupies the nodes of a pyrochlore lattice, which is known to be geometrically frustrated. Therefore, one can explore how the natural tendency of the transition metals to order in the charge, magnetic and orbital sectors is affected by geometrical frustration. Recently, orbital ordering has become a topical subject in a variety of both non-frustrated systems, such as manganites and other perovskites, and in the spinels. In this paper, I review the recent experimental activity on the subject of orbital ordering in transition-metal spinels and relate this to models of orbital ordering that are being developed by theoreticians. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

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±Õ·²Ô¶Ù2O, 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.