The high lattice enthalpy of lithium fluoride is due to the small sizes of the Li+ and F− ions, causing the electrostatic interactions between them to be strong: a similar effect occurs for magnesium fluoride, consistent with the diagonal relationship between lithium and magnesium.
ðə hˈaɪ lˈæɾɪs ɛnθˈælpi ʌv lˈɪθiəm flˈʊɹɹaɪd ɪz dˈuː tə ðə smˈɔːl sˈaɪzᵻz ʌvðə lˈaɪ plˈʌs ænd ˈɛf mˈaɪnəs ˈaɪɑːnz, kˈɔːzɪŋ ðɪ ɪlˌɛktɹoʊstˈæɾɪk ˌɪntɚɹˈækʃənz bᵻtwˌiːn ðˌɛm təbi stɹˈɔŋ: ɐ sˈɪmɪlɚɹ ɪfˈɛkt əkˈɜːz fɔːɹ mæɡnˈiːziəm flˈʊɹɹaɪd, kənsˈɪstənt wɪððə daɪˈæɡənəl ɹᵻlˈeɪʃənʃˌɪp bᵻtwˌiːn lˈɪθiəm ænd mæɡnˈiːziəm.