Two New, Metastable Polymorphs of Lithium Pyrophosphate Li4P2O7

By quenching a melt of Li4P2O7 two new, thermodynamically metastable polymorphs are obtained as biphasic mixtures. The crystal structure of one polymorph was determined from single‐crystal X‐ray diffraction data {Li4P2O7‐trig*: colorless, trigonal, P3212 (no. 153), Z = 3, a = 5.1699(2) Å, c = 18.9722(8) Å, 60 parameters, R1 = 0.018, wR2 = 0.051, 1265 unique reflections with Fo > 4σ(Fo)}. For the second polymorph a structure model was derived, using direct space methods followed by Rietveld refinement {Li4P2O7‐pseudomono*: P1, a = 7.88468(5) Å, b = 10.30394(6) Å, c = 7.54581(4) Å, α = 89.8142(6)°, β = 115.26(2)°, γ = 90.2015(5)°, RI = 7.62}. 31P‐MAS‐NMR measurements show δiso = –2.9 ppm for Li4P2O7‐pseudomono* and δiso = –6.6 ppm for the trigonal polymorph. Substitution of about 10 % lithium by copper(I) is possible in the trigonal as well as in the long known triclinic polymorph as is evidenced by the results of SXRD, XRPD, and 31P‐MAS‐NMR measurements. Two thermodynamically metastable polymorphs of Li4P2O7 have been synthesized by melt quenching as well as by solid state reaction. Both were crystallographically charac‐terized.