When the alkali metals react with the heavier elements in the carbon group (silicon, germanium, tin, and lead), ionic substances with cage-like structures are formed, such as the silicides M4Si4 (M = K, Rb, or Cs), which contains M+ and tetrahedral  ions.
wˌɛn ðɪ ˈælkəlˌaɪ mˈɛɾəlz ɹɪˈækt wɪððə hˈɛviɚɹ ˈɛlɪmənts ɪnðə kˈɑːɹbən ɡɹˈuːp sˈɪlɪkən, dʒɜːmˈeɪniəm, tˈɪn, ænd lˈiːd, aɪˈɑːnɪk sˈʌbstənsᵻz wɪð kˈeɪdʒlˈaɪk stɹˈʌktʃɚz ɑːɹ fˈɔːɹmd, sˈʌtʃ ɐz ðə sˈɪlɪsˌaɪdz ˈɛm fˈoːɹ sˈiː fˈoːɹ ˈɛm ˈiːkwəlz kˈeɪ, ˌɑːɹbˈiː, ɔːɹ sˌiːˈɛs, wˌɪtʃ kəntˈeɪnz ˈɛm plˈʌs ænd tˌɛtɹɐhˈiːdɹəl ˈaɪɑːnz.