Synthesis, structures and properties of the new lithium cobalt(II) phosphate Li _4Co(PO _4) _2

摘要

α-Li _4Co(PO _4) _2 has been synthesized and crystallized by solid-state reactions. The new phosphate crystallizes in the monoclinic system (P2 _1/a, Z=4, a=8.117(3) ?, b=10.303(8) ?, c=8.118(8) ?, β=104.36(8) ?) and is isotypic to α-Li _4Zn(PO _4) _2. The structure of α-Li _4Co(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>4σ (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 _4Co(PO _4) _2:α-Li _4Co(PO _4) _2442°C β-Li _3PO _4LiCoPO _4 ?773°C β-Li _4Co(PO _4) _2quenchingto25°C α-Li _4Co(PO _4) _2 According to HT-XRPD at θ=850°C β-Li _4Co(PO _4) _2 (Pnma, Z=2, 10.3341(8) ?, b=6.5829(5) ?, c=5.0428(3) ?) is isostructural to γ-Li _3PO _4. The powder reflectance spectrum of α-Li _4Co(PO _4) _2 shows the typical absorption bands for the tetrahedral chromophore [Co ~(II)O _4].