  ZrCo alloy is promising to substitute uranium for handling hydrogen isotope
storage in thermonuclear reactor. The alloying substitution of Zr in ZrCo with
Hf or Ti can enhance the ability of anti-disproportionation. In this work,
Zr$_{0.75}$Hf$_{0.25}$Co and Zr$_{0.75}$Ti$_{0.25}$Co were considered in the
framework of density functional theory, aimed to investigate the properties of
the alloying substitution of Zr with Hf or Ti in ZrCo. Our results found that
the optimized lattice constants of the alloying substitutions,
Zr$_{0.75}$Hf$_{0.25}$Co and Zr$_{0.75}$Ti$_{0.25}$Co, are smaller than ZrCo.
The thermodynamic stability reduces in the order Zr$_{0.75}$Hf$_{0.25}$Co >
ZrCo > Zr$_{0.75}$Ti$_{0.25}$Co, as demonstrated by the enthalpy of formation.
The valence electrons are mainly localized at the ion core and the chemical
bonds are polarized in Zr$_{0.75}$Hf$_{0.25}$Co and Zr$_{0.75}$Ti$_{0.25}$Co
analogous to ZrCo.
