I present an ab initio study of the phase stability and dielectric constant (κ 0) of Hf 1− x Ge x O 2 alloy as a function of Ge concentration in the range 0≤ x≤ 0.25. I determine the critical concentration x c at which the monoclinic structure (the stable phase at low x and at low temperatures) has a phase transition into the high-κ 0 fluorite structure. I found that, at x c, the fluorite phase presents a dielectric constant that is more than 50% higher than the orientationally averaged dielectric constant of undoped bulk monoclinic phase. I studied the effect of temperature on this phase transition. I identified the microscopic mechanisms which influence the dielectric properties of the fluorite phase finding that the decrease of the Born effective charge of Ge with respect to that of Hf is locally compensated by a variation of the Born effective charge of the O atoms which are neighbors of the Ge. I predict that the bulk modulus of this …