Are there any adhkar (remembrances of God) to be recited in the silent prayer when the Imam prolongs his recitation and there is a pause between the completion of the surah and the takbir (proclamation of God's greatness)?

It is recommended for the worshipper to recite something from the Qur’an or supplicate during a silent prayer, so that they may occupy themselves with the remembrance of Allah and not be susceptible to intrusive thoughts, nor remain silent. It is permissible to combine two or more surahs in a single rak'ah.