Synthesis, characterisation, and crystal structure of a novel U(V) brannerite UTi1.23Al0.77O6 and the UTi2-xAlxO6 system

The synthesis, characterisation and crystal structure of a novel U5+ (dominant) brannerite of composition U1.09(6)Ti1.29(3)Al0.71(3)O6 is reported, as determined from Rietveld analysis of high resolution powder neutron diffraction data. Examination of the UTi2-xAlxO6 system demonstrated the formation of brannerite structured compounds with varying Al3+ and U5+ content, from U0.93(6)Ti1.64(3)Al0.36(3)O6 to U0.89(6)Ti1.00(3)Al1.00(3)O6. Substitution of Al3+for Ti4+, with U5+ charge compensation, resulted in near-linear changes in the b and c unit cell parameters and the overall unit cell volume, as expected from ionic radii considerations. The presence of U5+ as the dominant oxidation state in near single phase brannerite compositions was evidenced by complementary laboratory U L3 edge and high energy resolution fluorescence detected (HERFD) U M4 edge X-ray Absorption Near Edge Spectroscopy. No brannerite phase was found for compositions with Al3+ / Ti4+ > 1, which would require U6+ contribution for charge compensation. These data expand the crystal chemistry of uranium brannerites to the stabilisation of dominant U5+ brannerites by substitution of trivalent cations, such as Al3+, on the Ti4+ site