Neutron and X-ray diffraction study of hydrogarnet Ca3Al2(O4H4)3

The crystal structure of Ca₃Al₂(O₄D₄)₃ was refined from time-of-flight neutron powder data collected at 300, 200, and 100 K. The D atom is located 0.906(1) Å (300 K) from oxygens surrounding the vacant 4 tetrahedral site. With the complete replacement of Si⁴⁺ by 4^H+ in the grossular structure, the tetrahedral d-O distance increases from 1.645 to 1.950 Å. Structural adjustments in response to tetrahedral expansion include a decrease in the length of the shared octahedral edge relative to the unshared edge and a corresponding increase in the Ca(l)-O(4) distance in the dodecahedron. At low temperatures, the D tetrahedron rotates ∼1° about the 4 axis relative to the rigid oxygen tetrahedron and becomes slightly more regular. The O-D distances at 300 and 100 K are within one standard deviation. Single-crystal X-ray intensity data were also obtained at 300 K from the same sample used in the neutron-diffraction experiments. The short O-H distance (0.65 Å) determined from the X-ray data is characteristic of mineral structures with non-H-bonded OH groups and OH groups with weak O-H…O interactions.