Potassium forms a mixture of potassium peroxide and potassium superoxide, while rubidium and caesium form the superoxide exclusively.
pətˈæsiəm fˈɔːɹmz ɐ mˈɪkstʃɚɹ ʌv pətˈæsiəm pəɹˈɑːksaɪd ænd pətˈæsiəm sˌuːpɚɹˈɑːksaɪd, wˌaɪl ɹuːbˈɪdiəm ænd sˈiːsiəm fˈɔːɹm ðə sˌuːpɚɹˈɑːksaɪd ɛksklˈuːsɪvli.