Crystal structure, vibrational spectra and thermal decomposition of a new tetrazinc(II) dipyrophosphate decahydrate, Zn4(P2O7)2 < eqid1 > ?10H2O

Abstract

A new zinc(II) pyrophosphate, Zn4(P2O7)2 ⋅10H2O, has been synthesized and characterized by single-crystal X-ray diffraction [orthorhombic space group Pnma, with unit cell parameters of a = 9.1508(2) A, b = 25.5271(5) A, c = 8.3574(2) A, Z = 4]. All the pyrophosphate anions show nonlinear P–O–P bonds with an average angle of 126.5∘. 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 Zn4(P2O7)2 10H2O is gradually heated, it decomposes and γ-Zn2P2O7 is formed at 481∘C. On further heating, γ-Zn2P2O7 is transformed into β-Zn2P2O7 at 750∘C. The conversion between the γ and β-Zn2P2O7 forms is irreversible and, on cooling β-Zn2P2O7 to room temperature, it reverts back to α-Zn2P2O7. The crystal structure of the new zinc(II) pyrophosphate material is compared with the known structures of the related anhydrous products α-, β-, and γ-Zn2P2O7.