The crystalline structure is of cubic CsCl type at room temperature, but it lowers to the orthorhombic thallium iodide type upon cooling, the transition temperature being likely affected by the impurities.
ðə kɹˈɪstɐlˌaɪn stɹˈʌktʃɚɹ ɪz ʌv kjˈuːbɪk sˌiːˈɛs sˌiːˈɛl tˈaɪp æt ɹˈuːm tˈɛmpɹɪtʃɚ, bˌʌt ɪt lˈoʊɚz tə ðɪ ˌɔːɹθoːɹhˈɑːmbɪk θˈæliəm ˈaɪədˌaɪd tˈaɪp əpˌɑːn kˈuːlɪŋ, ðə tɹænsˈɪʃən tˈɛmpɹɪtʃɚ bˌiːɪŋ lˈaɪkli ɐfˈɛktᵻd baɪ ðɪ ɪmpjˈʊɹɹᵻɾiz.