If for element A, the formation enthalpy and formation entropy per vacancy created are 0.5 eV and 3k_B, respectively, then the equilibrium vacancy concentration (in mole fraction) at 500 K is _______ × 10^-4. (Round off to two decimal places)
Given: Boltzmann constant, k_B = 8.62 × 10^-5 eV⋅atom^-1⋅K^-1