Synthesis, Crystal Structure, NMR Studies, and Thermal Stability of Mixed Iron-Indium Phosphates with Quasi-One-Dimensional Frameworks

The hydrothermal synthesis, single-crystal structure analysis, spectroscopic studies, and thermal stability of the compounds Ca2(In1-xFex)(PO4)(HPO4)2·H2O (0 ≤ x ≤ 1) are reported. The framework of these new phases is based on linear chains (//[101]) formed by (MO6) octahedra and (PO4)/(HPO4) tetrahedra sharing corners. The (HPO4) groups and water molecules link the chains through hydrogen bonding to form layers stacked perpendicular to the c axis. The calcium cations are located between the layers and are coordinated by nine oxide anions. Crystal data:  Ca2In(PO4)(HPO4)2·H2O, space group C2/c (No. 15), a = 7.573(1) Å, b = 15.838(1) Å, c = 9.3126(7) Å, β = 113.55(1)° Ca2(In0.5Fe0.5)(PO4)(HPO4)2·H2O, C2/c (No. 15), a = 7.548(2) Å, b = 15.670(3) Å, c = 9.241(2) Å, β = 113.62(3)° Ca2Fe(PO4)(HPO4)2·H2O, C2/c (No. 15), a = 7.503(2) Å, b = 15.477(2) Å, c = 9.142(1) Å, β = 113.60(2)°. The phases lose two water molecules between 350 and 600 °C to form the series Ca2(In1-xFex)(PO4)(P2O7) (0 ≤ x ≤ 1), which are isostructural with Ca2V(PO4)(P2O7). Solid state magic angle spinning (MAS) 31P NMR of Ca2In(PO4)(HPO4)2·H2O confirms two phosphorous moieties in roughly a 2:1 ratio. A CP-MAS buildup study yielded polarization transfer rates (TIS-1) of 2128 and 1597 s-1 for the HPO4 (-2.4 ppm) and PO4 (-0.9 ppm) sites, respectively. A 1H-31P WISE experiment indicates the presence of motional narrowing and hydrogen exchange between the water molecules and hydroxyl protons.