Two types of binary mixtures were examined to optimize the siloxanyl fraction by filling the gap between the two Cub-phase-forming molecules with di- and tri-siloxanyl terminals. Adding siloxanyl to the disiloxanyl system largely inhibited crystallization, increasing the stability at room temperature of the meta-stable Ia3̄d phase obtained by cooling from the high-temperature phase. The effect was prominent for the mixtures containing both di- and tri-siloxanyl compounds. The most prominent result was obtained for the 50 : 50 mixture; the Ia3̄d phase was quite stable and survived at room temperature after more than 1 year, as if it were like a thermodynamically stable phase.