Crystal structure of α- and β-Na2U2O7: From Rietveld refinement using powder neutron diffraction data

• Na2U2O7 has been prepared by means of a solid state reaction.
• The structure has been determined using neutron diffraction.
• At 293 K, the compound adopts a monoclinic structure with space group P21/a.
• The structure at 773 K is closely related, and is described in the space group C2/m.

The crystal structures of α- and β-Na2U2O7 have been determined from neutron powder diffraction. At 293 K, the compound α-Na2U2O7 has a monoclinic unit cell, space group P21/a, with a=12.7617(14) Å, b=7.8384(10) Å, c=6.8962(9) Å, β=111.285(9)°, and Z=4. At 773 K, β-Na2U2O7 is also monoclinic, space group C2/m, with a=12.933(1) Å, b=7.887(1) Å, c=6.9086(8) Å, β=110.816(10)°, and Z=4. The structures can be described by layers U2O72− built from corner linked deformed UO6 octahedra with pentagonal UO7 bipyramids in between linked with common edges to each other and to the octahedra. The Na atoms occupy the interlayer space. The Na2U2O7 layers are similar as in K2U2O7, but with a different stacking sequence of the layers. The layers in β-Na2U2O7 are more symmetric. The relationship with the compounds A2U2O7 (A=K, Rb, and Cs) is discussed.

The U2O7 layer of α-Na2U2O7 (left) at 293 K and of β-Na2U2O7 (right) at 773 K determined by neutron diffraction. Some Na atoms between the layers are visible.Figure optionsDownload as PowerPoint slide