Crystal Structure Relation between Tetragonal and Orthorhombic CsAlD₄: DFT and Time-of-Flight Neutron Powder Diffraction Studies

The crystal structures of orthorhombic and tetragonal CsAlD₄ were refined from time-of-flight neutron powder diffraction data starting from atomic positions predicted from DFT calculations. The earlier proposed crystal structure of orthorhombic CsAlH₄ is confirmed. For tetragonal CsAlH₄, DFT calculations predicted a crystal structure in I4₁/amd as potential minimum structure, while from neutron diffraction studies of CsAlD₄ best refinement is obtained for a disordered structure in the space group I4₁/a, with a = 5.67231(9) Å, c = 14.2823(5) Å. While the caesium atoms are located on the Wyckoff position 4b and aluminium at Wyckoff position 4a, there are two distinct deuterium positions at the Wyckoff position 16f with occupancies of 50 % each. From this structure, the previously reported phase transition between the orthorhombic and tetragonal polymorphs could be explained.