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

Cadmium phosphochlorides, Cd 4 P 2 Cl 3 and Cd 7 P 4 Cl 6 , 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 Cd 7 P 4 Cl 6 phase over Cd 4 P 2 Cl 3 . First-principles calculations reveal, stabilization of Cd 7 P 4 Cl 6 upon Mn-substitution with a significant reduction in the formation energy when Mn 2+ is substituted at Cd-sites coordinated octahedrally by Cl-ligands. Substitution of Mn 2+ 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 Cd 7 P 4 Cl 6 , Mn 2+ substituted Cd 7− y Mn y P 4 Cl 6 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. Mn 2+ prefers the Cd-sites having larger number of tightly bounded Cl-ligands. Pure Cd 7 P 4 Cl 6 exhibits n-type conductivity whereas Cd 5.8 Mn 1.2 P 4 Cl 6 exhibits p-type conductivity. The HER activity of Cd 7− y Mn y P 4 Cl 6 is superior to that of pristine Cd 7 P 4 Cl 6 .