Crystal structure and hydrogenation properties of Pd5As

The crystal structure of Pd5As was refined simultaneously from laboratory X-ray, synchrotron and neutron powder diffraction data (space group C2/m, a = 551.82(2) pm, b = 774.50(3) pm, c = 842.13(4), β = 99.037(2)°, Z = 4). This is in contrast to earlier work, which describes the structure in the non-centrosymmetric space group C2. Eight palladium atoms form a bicapped trigonal prism around arsenic. Two such polyhedra are connected by a common edge to double prisms (average distance As-Pd 252 pm). Palladium has got coordination numbers of 11 (average distance 276 pm) or 12 (average distance of 278 pm). The crystal structure of Pd5As represents a strongly distorted cubic closest packing as proven by crystallographic group-subgroup relationships. Pd5As does not take up hydrogen up to 5.0 MPa hydrogen pressure and temperatures up to 723 K. Quantum-mechanical calculations using DFT methods using an ab initio evolutionary algorithm confirm the crystal structure of Pd5As and reveal the stability of a hypothetical hydride Pd5AsH. The lack of reactivity might be due to kinetic hindrance because of an endothermic rearrangement of the metal matrix necessary for hydrogen incorporation. Above 5.8 GPa, however, this phase transition becomes thermodynamically favourable, making hydrogenation of Pd5As likely to occur.