Synthesis, structures and properties of the new lithium cobalt(II) phosphate Li4Co(PO4)2

Abstract α- Li 4 Co(PO 4 ) 2 has been synthesized and crystallized by solid-state reactions. The new phosphate crystallizes in the monoclinic system ( P 2 1 /a , Z =4, a =8.117(3) A, b =10.303(8) A, c =8.118(8) A, β =104.36(8) A) and is isotypic to α- Li 4 Zn(PO 4 ) 2 . The structure of α- Li 4 Co(PO 4 ) 2 has been determined from single-crystal X-ray diffraction data { R 1 =0.040, wR 2 =0.135, 2278 unique reflections with F o >4 σ ( F o )}. The crystal structure, which might be regarded as a superstructure of the wurtzite structure type, is build of layers of regular CoO 4 , PO 4 and Li1O 4 tetrahedra. Lithium atoms Li2, Li3 and Li4 are located between these layers. Thermal investigations by in-situ XRPD, DTA/TG and quenching experiments suggest decomposition followed by formation and phase transformation of Li 4 Co(PO 4 ) 2 : α- Li 4 Co(PO 4 ) 2 ⟹ 442 ° C β -Li 3 PO 4 +LiCoPO 4 ⇌ 773 ° C β- Li 4 Co(PO 4 ) 2 ⟹ q u e n c h i n g t o 25 ° C α- Li 4 Co(PO 4 ) 2 According to HT-XRPD at ϑ = 850 ° C β- Li 4 Co(PO 4 ) 2 ( Pnma , Z =2, 10.3341(8) A, b =6.5829(5) A, c =5.0428(3) A) is isostructural to γ- Li 3 PO 4 . The powder reflectance spectrum of α- Li 4 Co(PO 4 ) 2 shows the typical absorption bands for the tetrahedral chromophore [Co II O 4 ].