Magnetic and structural properties of high quality magnetocaloric MnFe 4 Si 3 single crystals are investigated macroscopically and on an atomic scale. Refinements of combined neutron and x-ray single crystal diffraction data introduce a new structural model in space group P 6 characterized by partial ordering of Mn and Fe into layers perpendicular to c on one of the transition metal sites. A second transition metal site is exclusively occupied by iron. MnFe 4 Si 3 has a phase transition to a ferromagnetically ordered phase at approximately 300 K and displays a strong anisotropy of the 1 magnetization and the magnetocaloric effect with the easy axis of magnetization in the a, b-plane. This is confirmed by a refinement of the magnetic structure in the magnetic spacegroup P m ′ which shows that the spins on the sites with mixed occupancy of Mn and Fe are aligned in the a, b-plane. A significant magnetic moment for site exclusively occupied by iron could not be refined. The thermal evolution of the lattice parameters exhibit an anisotropic behavior and clearly reflects the onset of magnetic ordering. Comparison of the ordered moment and the effective paramagnetic moment hints towards itinerant magnetism in the system.

Na 2 RuO 4 , prepared from Na 2 O 2 and RuO 2 via high oxygen pressure synthesis, crystallises monoclinic in space group P2 1 / c (a ϭ 10.721(6), b ϭ 7.033(4), c ϭ 10.871(6) Å , β ϭ 119.10(4)°, Z ϭ 8, 2503 unique reflections, R 1 ϭ 0.049). Structure determination from single crystal data shows that the compound consists of infinite chains of RuO 5 trigonal bipyramids connected through their axial vertices. The Na cations connect the pseudohexagonally packed 1 ϱ [RuO 3 O 2/2 ] chains and are coordinated by six or seven oxygen atoms, respectively. The compound exhibits an one-dimensional spin system with µ ϭ 2.80 µ B and Θ ϭ Ϫ222 K and a threedimensional antiferromagnetic ordering below 50 K. Na 2 RuO 3 was obtained from Na 2 RuO 4 at 850°C under a flow of argon. The struc-

The pressure-induced crystal structure of lead sulfide (PbS) above 2.2 GPa has been studied with single-crystal x-ray diffraction in a diamond anvil cell at room temperature. It has been found to be twinned and of the TlI type (Cmcm, Z=4), in which the Pb atoms are surrounded by seven S atoms in a capped trigonal prism coordination. The twin laws in relation to the parent B1 (NaCl) type structure (Fm ̄3m, Z=4) at atmospheric pressure have been discussed.

The crystal structure of the bismuth silicon oxide Bi 12 SiO 20 was determined by single-crystal x-ray diffraction at ambient conditions and at high pressure. Single-crystal intensity data between 0.0001 and 16.8(3) GPa were collected in house with Mo Kα radiation and with synchrotron radiation (λ = 0.45Å) at HASYLAB (D3), while lattice parameters were measured up to 23.0(3) GPa. The large cavities which exist in the crystal structure and host the lone electron pairs of the Bi 3+ ions are considerably compressed at high pressure. The crystal structure, however, remains stable and the lone electron pair is stereochemically active up to at least 16.8 GPa. A larger compression in the direction of the lone electron pairs by shear deformation was not observed. Raman spectra of Bi 12 SiO 20 were measured on powder samples during pressure decrease from 39.1(1) GPa down to ambient pressure and on single crystals during pressure increase up to 12.50(3) GPa. Density functional perturbation theory was used to compute Raman frequencies and intensities at ambient pressure and to investigate pressure-induced changes up to 50 GPa.

LiLuF 4 scheelite (I 4 1 /a, Z = 4) has been investigated at high pressures using synchrotron angle-dispersive x-ray powder diffraction in a diamond anvil cell at room temperature. At 10.7 GPa, it reversibly undergoes a tricritical phase transition to the fergusonite structure (C12/c1, Z = 4), a distorted modification of the scheelite type. No other phase transition occurs in this material up to 19.5 GPa, the highest pressure in this study. Such a high-pressure behaviour is compared with the pressure-induced transformations in LiYF 4 and LiGdF 4 , adding on to our knowledge of the structural systematics in LiMF 4 compounds.