Thin film metal–ferroelectric–metal capacitors with an equal mixture of hafnium oxide and zirconium oxide as the ferroelectric material are fabricated using iridium oxide as the electrode material. The influence of the oxygen concentration in the electrodes during crystallization anneal on the ferroelectric properties is characterized by electrical, chemical, and structural methods. Forming gas, O₂, and N₂ annealing atmospheres significantly change the ferroelectric performance. The use of oxygen-deficient electrodes improves the stabilization of the ferroelectric orthorhombic phase and reduces the wake-up effect. It is found that oxygen-rich electrodes supply oxygen during anneal and reduce the amount of oxygen vacancies, but the nonferroelectric monoclinic phase is stabilized with a negative impact on the ferroelectric properties.