CRYSTAL STRUCTURE OF ALPHA-CA2P2O7

The high-temperature form of calcium pyrophosphate, α-Ca2P2O7, crystallizes in the monoclinic space group P21/n with lattice parameters a = 12.66 (1) Å, b = 8.542 (8) Å, c = 5.315 (5) Å, and β = 90.3 (1)°. There are four molecules per unit cell yielding a calculated density of 2.95 g/cm3 compared to a measured value of 2.947 g/cm3. This structure is closely related to that of α-Ca2P2O7, although the latter has orthorhombic symmetry. Both cations show coordination numbers of eight with average cation-oxygen atom bond lengths of 2.54 and 2.51 A for the individual cation sites. The two ends of the anion deviate by 13° from the eclipsed configuration with a P-O-P angle of 130°. The P-O(-P) bond lengths average 1.60 Å while the terminal ones average to 1.51 Å. The individual P-O bond lengths range from 1.49 to 1.62 Å and have estimated standard deviations of the order of 0.01 Å. The structure was refined by a full-matrix least-squares procedure using 1416 reflections whose intensities were estimated visually. The final value for the conventional R value is 0.098. The diffraction pattern shows a marked orthorhombic character suggesting a high-temperature phase with Pbnm symmetry. © 1968, American Chemical Society. All rights reserved.