Crystal structure, vibrational spectra and thermal decomposition of a new tetrazinc(II) dipyrophosphate decahydrate, Zn4(P2O7)2•10H2O

A new zinc(II) pyrophosphate, Zn-4(P2O7)(2).10H(2)O, has been synthesized and characterized by single-crystal X-ray diffraction [orthorhombic space group Pnma, with unit cell parameters of a = 9.1508(2) Angstrom, b = 25.5271(5) Angstrom, c = 8.3574(2) Angstrom, Z = 4]. All the pyrophosphate anions show nonlinear P-O-P bonds with an average angle of 126.5degrees. The framework of this new pyrophosphate is made up of packed layers of ZnO6 octahedra connected by double-tetrahedra P2O7 groups and a layer of Zn(H2O)(6) units. The [P2O7](4-) anions adopt a bent, near-staggered conformation. The absence of coincidences for the majority of the IR and Raman bands is in accord with the centrosymmetric structure of the material. The vibrational spectra have been interpreted in part on the basis of factor group effects. The structural changes occurring during heating have been investigated by TG-DSC, powder X-ray diffraction, and IR and Raman spectroscopy. When Zn-4(P2O7)(2) 10H(2)O is gradually heated, it decomposes and gamma-Zn2P2O7 is formed at 481degreesC. On further heating, gamma-Zn2P2O7 is transformed into beta-Zn2P2O7 at 750degreesC. The conversion between the gamma- and beta-Zn2P2O7 forms is irreversible and, on cooling beta- Zn2P2O7 to room temperature, it reverts back to alpha-Zn2P2O7. The crystal structure of the new zinc(II) pyrophosphate material is compared with the known structures of the related anhydrous products alpha-, beta-, and gamma-Zn2P2O7.