Synthesis and crystal structure of two new phosphates with Alluaudite type structure: (M, Mn)₃Fe(PO₄)₃ (M= Ca, Cd)

Abstract

Two new phosphates with alluaudite type structure (M, Mn)3Fe(PO4)3 (M= Ca, Cd), namely Ca1.54Mn1.46Fe(PO4)3 and Cd0.66Mn2.34Fe(PO4)3, has been synthesized by a solid state reaction and characterized by single-crystal X-ray diffraction. The two compounds crystallize in the monoclinic system with C2/c space group. Their open framework results from (Fe1/Mn1)2O10 units of edge-sharing (Fe/Mn)O6 octahedra, which alternate with M(1)O6 octahedra (M(1)=Ca1/Mn2 or Cd1/Mn2) that form infinite chains running along [10-1] direction. These chains are linked together through the common corners of PO4 tetrahedra giving rise to two types of tunnels occupied by bivalent cations Ca2+ or Cd2+ and Mn2+.