Tetrapotassium pyrophosphates γ- and δ-K_4P_2O_7

摘要

The structures of γ- and δ-K_4P_2O_7 are solved by X-ray powder diffraction (conventional laboratory X-ray and synchrotron data, respectively), both in hexagonal symmetry (a_γ = 5.9645(3) A, c_γ= 14.4972(8) A, V_γ = 446.64(4) A~3 at 300 ℃, Z_γ= 2, space group P6_3/mmc; a_δ= 10.211 45(7) A, c_δ=42.6958(4) A, V_δ= 3855.59(7) A~3 at room temperature, Zδ= 18, space group P6_1) with cell-supercell relations a_δ ≈ a_γ3 and c_ δ≈ 3c_γ In the experimental conditions, the expected ply transition previously announced at 486 ℃ is not observed; the γ-form is stable at least up to the maximum temperature of our measurements (700 ℃). In the y-form, similar to the orthorhombic form of Na_4P_2O_7, idealized, the pyrophosphate group is in eclipsed conformation, the K~+ cations occupying three different coordinations. In the δδ-form, two of the three different [P_2O_7]~(4-) groups are staggered and one eclipsed, the K~+ cations occupying 12 independent sites.