Effect of Mn²⁺ substitution on the structure, properties and HER activity of cadmium phosphochlorides

Cadmium phosphochlorides, Cd4P2Cl3 and Cd7P4Cl6, possess cadmium atoms differently bonded to chlorine and phosphorus ligands. A combined experimental and theoretical study has been carried out to examine the effect of manganese substitution in place of cadmium in these compounds. Experimentally it is found that manganese prefers the Cd7P4Cl6 phase over Cd4P2Cl3. First-principles calculations reveal, stabilization of Cd7P4Cl6 upon Mn-substitution with a significant reduction in the formation energy when Mn2+ is substituted at Cd-sites coordinated octahedrally by Cl-ligands. Substitution of Mn2+ at two different Cd-sites in these compounds not only alters their formation energy differently but also causes a notable change in the electronic structures. In contrast to n-type conductivity in pristine Cd7P4Cl6, Mn2+ substituted Cd7-y Mn y P4Cl6 analogues exhibit p-type conductivity with a remarkable enhancement in the photochemical HER activity and stability of the system. Photochemical properties of pristine and substituted compounds are explained by studying the nature of charge carriers and their dynamics.