While the larger alkali metals (K, Rb, and Cs) initially form MC8, the smaller ones initially form MC6, and indeed they require reaction of the metals with graphite at high temperatures around 500 °C to form.
wˌaɪl ðə lˈɑːɹdʒɚɹ ˈælkəlˌaɪ mˈɛɾəlz kˈeɪ, ˌɑːɹbˈiː, ænd sˌiːˈɛs ɪnˈɪʃəli fˈɔːɹm ˌɛmsˈiː ˈeɪt, ðə smˈɔːlɚ wˈʌnz ɪnˈɪʃəli fˈɔːɹm ˌɛmsˈiː sˈɪks, ænd ˌɪndˈiːd ðeɪ ɹᵻkwˈaɪɚ ɹɪˈækʃən ʌvðə mˈɛɾəlz wɪð ɡɹˈæfaɪt æt hˈaɪ tˈɛmpɹɪtʃɚz ɚɹˈaʊnd fˈaɪvhˈʌndɹɪd dᵻɡɹˈiːz sˈiː tə fˈɔːɹm.