While it was previously thought that the heavier alkali metals also formed octahedral hexaaqua ions, it has since been found that potassium and rubidium probably form the [K(H2O)8]+ and [Rb(H2O)8]+ ions, which have the square antiprismatic structure, and that caesium forms the 12-coordinate [Cs(H2O)12]+ ion.
wˌaɪl ɪt wʌz pɹˈiːviəsli θˈɔːt ðætðə hˈɛviɚɹ ˈælkəlˌaɪ mˈɛɾəlz ˈɔːlsoʊ fˈɔːɹmd ˌɑːktɐhˈiːdɹəl hˈɛksɑːkwə ˈaɪɑːnz, ɪt hɐz sˈɪns bˌɪn fˈaʊnd ðæt pətˈæsiəm ænd ɹuːbˈɪdiəm pɹˈɑːbəbli fˈɔːɹm ðə kˈeɪ ˈeɪtʃ tˈuː ˈoʊ ˈeɪt plˈʌs ænd ˌɑːɹbˈiː ˈeɪtʃ tˈuː ˈoʊ ˈeɪt plˈʌs ˈaɪɑːnz, wˌɪtʃ hæv ðə skwˈɛɹ ˌæntɪpɹɪsmˈæɾɪk stɹˈʌktʃɚ, ænd ðæt sˈiːsiəm fˈɔːɹmz ðə twˈɛlvkoʊˈɔːɹdᵻnət sˌiːˈɛs ˈeɪtʃ tˈuː ˈoʊ twˈɛlv plˈʌs ˈaɪɑːn.