New structural insights into Fe ₂ P ₂ O ₇ – unravelling an unresolved dispute and three reversible phase transitions

The debate in the literature whether the triclinic room-temperature crystal structure of iron(II) pyrophosphate (Fe₂P₂O₇) is centrosymmetric or not has been clearly resolved on the basis of new single-crystal X-ray intensity measurements. This study additionally revealed that Fe₂P₂O₇ undergoes three reversible phase transitions between -140 and 190°C, with the modifications denoted with decreasing temperature as β, α₃, α₂ and α₁. The room-temperature form, α₂-Fe₂P₂O₇, indeed crystallizes in a centrosymmetric but incommensurately modulated structure, a fact that has not been recognized for more than 40 years. For better comparison with the C-centred monoclinic thortveitite-type aristotype (space group type C2/m), the structure of the hettotype α₂-Fe₂P₂O₇ is described in the superspace group C1(αβγ)0 with a = 6.6393 (6), b = 8.4748 (6), c = 4.4839 (3) Å, α = 90.036 (5), β = 103.962 (7), γ = 92.929 (6)° and a modulation wavevector q = 0.4489 (3)a* + 0.2517 (3)b* + 0.3646 (3)c*. The α₂ modification undergoes two phase transitions towards periodic structures. On heating, a triclinic structure described in C1 with very similar lattice parameters is realized above 85°C for the corresponding α₃ modification. It can be considered as the non-modulated basic structure of the α₂ modification. At about 185°C, α₃-Fe₂P₂O₇ transforms to the thortveitite-type β modification, which remains stable up to at least 1000°C. On cooling the α₂ modification, a triclinic structure of the low-temperature α₁ modification forms below -140°C, which can be considered as a twofold superstructure of the α₃ modification with q = ½a* + ½b* + ½c*. The result of these phase transitions from the thortveitite-type β-modification via the triclinic α₃ phase and the incommensurately modulated triclinic α₂ modification to α₁-Fe₂P₂O₇ is the complete ordering of the pyrophosphate anion in the low-temperature phase with a P-O-P bridging angle of 151.91 (8)°. This ordering is accompanied by the lowering of the coordination number of one half of the Fe²⁺ ions from 6 to 5.