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, P3(2)12 (no. 153), Z = 3, a = 5.1699(2) angstrom, c = 18.9722(8) angstrom, 60 parameters, R-1 = 0.018, wR(2) = 0.051, 1265 unique reflections with F-o > 4 sigma(F-o)}. For the second polymorph a structure model was derived, using direct space methods followed by Rietveld refinement {Li4P2O7-pseudomono*: P1, a = 7.88468(5) angstrom, b = 10.30394(6) angstrom, c = 7.54581(4) angstrom, alpha = 89.8142(6)degrees, beta = 115.26(2)degrees, gamma = 90.2015(5)degrees, R-I = 7.62}. P-31-MAS-NMR measurements show delta(iso) = -2.9 ppm for Li4P2O7-pseudomono* and delta(iso) = -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 P-31-MAS-NMR measurements.