Determination of complex structures by combined neutron and synchrotron X-ray powder diffraction

Abstract

THE feasibility of determining crystal structures from powder diffraction data has improved substantially during the past decade. Early work using laboratory X-ray data^1,2 has been followed by studies that take advantage of the higher resolution provided by synchrotron X-ray³ and neutron⁴ diffraction instrumentation. Other advances have been made in the computational aspects of the problem^5,6. Nevertheless, there has remained a disparity between the complexity of structures that can be solved, ab initio, from powder data, and those that can in principle be refined by the Rietveld profile method⁷. For example, refinements with up to 34 atoms⁸ and 132 positional parameters⁹ have been reported, but the most complex unknown structure to be solved from powder data contains only 17 atoms in the asymmetric unit cell¹⁰. Here we describe the solution and refinement of Ga₂(HPO₃)₃.4H₂O, a novel framework structure with 29 atoms in the asymmetric unit cell and 117 structural parameters, by the combined use of synchrotron X-ray and neutron powder diffraction. Exploiting the complementary nature of these techniques further extends the power of powder diffraction for structure determination.