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 Cd_7- Mn 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.