Structure and thermodynamic stability of UTa3O10, a U(v)-bearing compound.

Heating a mixture of uranyl(vi) nitrate and tantalum(v) oxide in the molar ratio of 2 : 3 to 1400 °C resulted in the formation of a new compound, UTa3O10. The honey colored to yellow brown crystals of UTa3O10 crystallize in an orthorhombic structure with the space group Fddd (no. 70), lattice parameters a = 7.3947(1), b = 12.7599(2), c = 15.8156(2) Å, and Z = 8. Vertex sharing [TaO6]7- octahedra of two crystallographically distinct Ta cations form a three dimensional tantalate framework. Within this framework, six membered rings of [TaO6]7- octahedra are formed within the (001) plane. The center of these rings is occupied by the uranyl cations [UO2]+, with an oxidation state of +5 for uranium. The pentavalence of U and Ta was confirmed by X-ray photoelectron spectroscopy and X-ray adsorption spectroscopy. The enthalpy of formation of UTa3O10 from Ta2O5, ß-U3O7, and U3O8 has been determined to be 13.1 ± 18.1 kJ mol-1 using high temperature oxide melt solution calorimetry with sodium molybdate as the solvent at 700 °C. The close to zero enthalpy of formation of UTa3O10 can be explained by closely balanced structural stabilizing and destabilizing factors, which may also apply to other UM3O10 compounds.

LuF[SeO3]: the structural chameleon of lanthanoid fluoride oxoselenates(IV).

LuF[SeO3] is a compound that can easily be obtained by a solid-state reaction of Lu2O3, LuF3, and SeO2 with CsBr as the fluxing agent. The outstanding property of LuF[SeO3] is the appearance of two phase transitions within a range of less than 200 K. With an increase in the coordination number for Lu(3+) from 7 to 8, the triclinic room-temperature modification changes at temperatures below -40 °C to the monoclinic low-temperature or high-pressure phase of LuF[SeO3]. At the same time, room-temperature modification retains the structure but gains higher symmetry at the second phase transition of around +110 °C. This second transition can even be observed under a microscope using polarized light to see twinning lamellae disappear and reappear during this reversible process.

Garnet-type Mn(3)Cr(2)(GeO(4))(3).

Single crystals of garnet-type trimanganese(II) dichrom-i-um(III) tris-[orthogermanate(IV)], Mn(II) (3)Cr(III) (2)(GeO(4))(3), were obtained by utilizing a chemical transport reaction. Corres-ponding to the mineral garnet with the general formula A(II) (3)B(III) (2)(SiO(4))(3), each of the four elements occupies only one crystallographically distinct position. Mn(2+) occupies the respective A position (Wyckoff site 24c, site symmetry 2.22), being surrounded by eight O atoms that form a distorted cube [d(Mn-O) = 2.291 (2) and 2.422 (2) Å, 4× each], while Cr(3+) on the B position (Wyckoff site 16a, site symmetry .-3.) is situated in a slightly distorted octa-hedron of six O(2-) anions [d(Cr-O) = 1.972 (2) Å, 6×]. In addition, the O atoms on general site 96h form isolated [GeO(4)](4-) tetra-hedra with Ge(4+) on site 24d [site symmetry -4..; d(Ge-O) = 1.744 (2) Å, 4×].

Rare-earth metal(III) oxide selenides M4O4Se[Se2] (M=La, Ce, Pr, Nd, Sm) with discrete diselenide units: crystal structures, magnetic frustration, and other properties.

The rare-earth metal(III) oxide selenides of the formula La4O4Se[Se2], Ce4O4Se[Se2], Pr4O4Se[Se2], Nd4O4Se[Se2], and Sm4O4Se[Se2] were synthesized from a mixture of the elements with selenium dioxide as the oxygen source at 750 degrees C. Single crystal X-ray diffraction was used to determine their crystal structures. The isostructural compounds M4O4Se[Se2] (M=La, Ce, Pr, Nd, Sm) crystallize in the orthorhombic space group Amm2 with cell dimensions a=857.94(7), b=409.44(4), c=1316.49(8) pm for M=La; a=851.37(6), b=404.82(3), c=1296.83(9) pm for M=Ce; a=849.92(6), b=402.78(3), c=1292.57(9) pm for M=Pr; a=845.68(4), b=398.83(2), c=1282.45(7) pm for M=Nd; and a=840.08(5), b=394.04(3), c=1263.83(6) pm for M=Sm (Z=2). In their crystal structures, Se2- anions as well as [Se-Se]2- dumbbells interconnect {[M4O4]4+} infinity 2 layers. These layers are composed of three crystallographically different, distorted [OM4]10+ tetrahedra, which are linked via four common edges. The compounds exhibit strong Raman active modes at around 215 cm(-1), which can be assigned to the Se-Se stretching vibration. Optical band gaps for La4O4Se[Se2], Ce4O4Se[Se2], Pr4O4Se[Se2], Nd4O4Se[Se2], and Sm4O4Se[Se2] were derived from diffuse reflectance spectra. The energy values at which absorption takes place are typical for semiconducting materials. For the compounds M4O4Se[Se2] (M=La, Pr, Nd, Sm) the fundamental band gaps, caused by transitions from the valence band to the conduction band (VB-CB), lie around 1.9 eV, while for M=Ce an absorption edge occurs at around 1.7 eV, which can be assigned to f-d transitions of Ce3+. Magnetic susceptibility measurements of Ce4O4Se[Se2] and Nd4O4Se[Se2] show Curie-Weiss behavior above 150 K with derived experimental magnetic moments of 2.5 micro B/Ce and 3.7 micro B/Nd and Weiss constants of theta p=-64.9 K and theta p=-27.8 K for the cerium and neodymium compounds, respectively. Down to 1.8 K no long-range magnetic ordering could be detected. Thus, the large negative values for theta p indicate the presence of strong magnetic frustration within the compounds, which is due to the geometric arrangement of the magnetic sublattice in form of [OM4]10+ tetrahedra.