La 3 NSe 3 and La 3 PSe 3 : Two Isostoichiometric, but not Isostructural Lanthanum‐Selenide Derivatives

The two non ‐isostructural lanthanum pnictogenide selenides La 3 NSe 3 and La 3 PSe 3 are formed by oxidizing lanthanum metal with sodium azide (NaN 3 ) in the presence of selenium and an excess of iodine for eight days at 850 °C and by reacting the two binary compounds lanthanum sesquiselenide (La 2 Se 3 ) and lanthanum monophosphide (LaP) for four days at 900 °C in torch‐sealed glassy silica ampoules. La 3 NSe 3 crystallizes with the well‐known Sm 3 NS 3 ‐type structure in the orthorhombic space group Pnma with the lattice parameters a =1278.56(8) pm, b =424.18(3) pm, c =1358.73(9) pm, while La 3 PSe 3 adopts the cubic Th 3 P 4 ‐type structure in the space group I 4 ‾ 3 d with the lattice parameter a =901.42(6) pm, both for Z =4. It features one crystallographically distinct cationic La 3+ site and a mixed anionic site with both P 3− and Se 2− anions sharing the same crystallographic position in its crystal structure. In contrast, the crystal structure of La 3 NSe 3 is comprised of three different La 3+ cations with one N 3− and three distinct Se 2− anions. The key structural elements are N 3− centered (La 3+ ) 4 tetrahedra connected via vertices to form one‐dimensionally infinite chains∞ 1 { [ N ( La 1 , 2 ) 2 / 1 t( La 3 ) 2 / 2 v ] 6 + }( t =terminal, v =vertex‐sharing). The comparatively large La 3+ cation adopts coordination numbers of six (1×) and seven (2×) in La 3 NSe 3 , whereas in La 3 PSe 3 , the coordination number of the single cation site is eight .