Ammonium is expected to behave stably as a metal ( ions in a sea of delocalised electrons) at very high pressures (though less than the typical pressure where transitions from insulating to metallic behaviour occur around, 100 GPa), and could possibly occur inside the ice giants Uranus and Neptune, which may have significant impacts on their interior magnetic fields.
æmˈoʊniəm ɪz ɛkspˈɛktᵻd tə bᵻhˈeɪv stˈeɪbli æz ɐ mˈɛɾəl ˈaɪɑːnz ɪn ɐ sˈiː ʌv dᵻlˈɑːkəlˌaɪzd ᵻlˈɛktɹɑːnz æt vˈɛɹi hˈaɪ pɹˈɛʃɚz ðˌoʊ lˈɛs ðɐn ðə tˈɪpɪkəl pɹˈɛʃɚ wˌɛɹ tɹænsˈɪʃənz fɹʌm ˈɪnsəlˌeɪɾɪŋ tə mətˈælɪk bᵻhˈeɪvjɚɹ əkˈɜːɹ ɚɹˈaʊnd, wˈʌnhˈʌndɹɪd dʒˌiːpˌiːˈeɪ, ænd kʊd pˈɑːsᵻbli əkˈɜːɹ ɪnsˈaɪd ðɪ ˈaɪs dʒˈaɪənts jˈʊɹɹænəs ænd nˈɛptuːn, wˌɪtʃ mˌeɪhɐv sɪɡnˈɪfɪkənt ˈɪmpækts ˌɔn ðɛɹ ɪntˈɪɹiɚ mæɡnˈɛɾɪk fˈiːldz.