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A temperature-dependent, single crystal x-ray diffraction study of the giant magnetocaloric material, Gd5(Si2Ge2), across its Curie temperature (276 K) reveals that the simultaneous orthorhombic to monoclinic transition occurs by a shear mechanism in which the (Si, Ge)-(Si,Ge) dimers that are richer in Ge increase their distances by 0.859(3) A and lead to twinning. The structural transition changes the electronic structure, and provides an atomic-level model for the change in magnetic behavior with temperature in the Gd5(SixGe1-x)(4).

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Results are presented of single crystal structural, thermodynamic, and reflectivity measurements of the double-perovskite Ba2NaOsO6. These characterize the material as a 5d1 ferromagnetic Mott insulator with an ordered moment of ∼ 0.2 µB per formula unit and TC = 6.8(3) K. The magnetic entropy associated with this phase transition is close to Rln2, indicating that the quartet groundstate anticipated from consideration of the crystal structure is split, consistent with a scenario in which the ferromagnetism is associated with orbital ordering.PACS numbers: 75.50. Dd, 75.30.Cr, 71.70.Ej The interplay between spin, orbital and charge degrees of freedom in 3d transition metal oxides has proven to be a rich area of research in recent years. Despite the wide array of interesting physics found in these materials, much less is known about whether similar behavior can be found in related 4d and 5d systems, for which both the extent of the d-orbitals and larger spin-orbit coupling cause a different balance between the relevant energy scales. In this respect, oxides of osmium are of particular interest because the element can take formal valences from 4+ to 7+, corresponding to electron configurations 5d4 to 5d 1 . In this instance, we examine the simplest case of a 5d1 osmate for which the magnetic properties indicate that orbital ordering may indeed play a significant role.Simple oxides of osmium are typically Pauli paramagnets due to the large extent of the 5d orbitals. Examples include the binary oxide OsO 2 [1, 2] and the simple perovskites AOsO 3 (A= Sr, Ba) [3]. However, more complex oxides, including the double and triple perovskites La 2 NaOsO 6 [4], Ba 2 AOsO 6 (A = Li, Na) [5,6] and Ba 3 AOs 2 O 9 (A = Li, Na) [7], appear to exhibit local moment behavior. Presumably the large separation of Os ions in these more complex structures leads to a Mott insulating state, and indeed these and related materials are most often found to be antiferromagnetic. Of the above materials and their near relations containing no other magnetic ions, Ba 2 NaOsO 6 distinguishes itself as the only osmate with a substantial ferromagnetic moment (∼0.2 µ B ) in the ordered state [5].Weak ferromagnetism has been previously observed in other 5d transition metal oxides containing iridium. BaIrO 3 exhibits a saturated moment of 0.03 µ B , which has been attributed to small exchange splitting associated with charge density wave formation [8]. Sr 2 IrO 4 and Sr 3 Ir 2 O 7 exhibit similarly small saturated moments, attributed variously to either spin canting in an antiferromagnet due to the low crystal symmetry [9] or to a borderline metallic Stoner scenario [10,11]. The ferromagnetic moment in Ba 2 NaOsO 6 is substantially larger than in these materials. Furthermore, at room temperature the material has an undistorted double-perovskite structure, space group Fm3m (inset to Fig. 1) [5], in which OsO 6 octahedra are neither distorted nor rotated with respect to each other or the underlying lattice [12]. Such a high crystal symmetry, if preserve...