Work

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

Year: 2025

Type: article

Abstract: The debate in the literature whether the triclinic room-temperature crystal structure of iron(II) pyrophosphate (Fe 2 P 2 O 7 ) is centrosymmetric or not has been clearly resolved on the basis of new single-crystal X-ray intensity measurements. This study additionally revealed that Fe 2 P 2 O 7 undergoes three reversible phase transitions between −140 and 190°C, with the modifications denoted with decreasing temperature as β, α 3 , α 2 and α 1 . The room-temperature form, α 2 -Fe 2 P 2 O 7 , 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 C 2/ m ), the structure of the hettotype α 2 -Fe 2 P 2 O 7 is described in the superspace group C 1 (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 α 2 modification undergoes two phase transitions towards periodic structures. On heating, a triclinic structure described in C 1 with very similar lattice parameters is realized above 85°C for the corresponding α 3 modification. It can be considered as the non-modulated basic structure of the α 2 modification. At about 185°C, α 3 -Fe 2 P 2 O 7 transforms to the thortveitite-type β modification, which remains stable up to at least 1000°C. On cooling the α 2 modification, a triclinic structure of the low-temperature α 1 modification forms below −140°C, which can be considered as a twofold superstructure of the α 3 modification with q = ½ a * + ½ b * + ½ c *. The result of these phase transitions from the thortveitite-type β-modification via the triclinic α 3 phase and the incommensurately modulated triclinic α 2 modification to α 1 -Fe 2 P 2 O 7 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 Fe 2+ ions from 6 to 5.

Source: IUCrJ

DOI: 10.1107/s2052252525007547