High-resolution powder X-ray diffraction measurements have been used to determine the low-temperature crystal structures of the hydrofluorocarbons 1,1,1,2-tetrafluoroethane (HFC-134a) and 1,2-difluoroethane (HFC-152a). Below their melting points both compounds form body-centred-cubic orientationally-disordered phases with a = 5.7606(2) Angstrom for HFC-134a and a = 5.5433(1) Angstrom for HFC-152a, at 120 K. These transform to ordered structures on cooling, which were solved by autoindexing, intensity extraction and direct methods with the EXPO program, and refined via the Rietveld method. HFC-134a is monoclinic, space group P2(1)/c, a = 8.7569(4) Angstrom, b = 9.3523(1) Angstrom, c = 9.0784(4) Angstrom, beta = 106.479(5)degrees, at 80 K, Z = 8. HFC-152a is monoclinic, space group Pc, a 8.6417(2) Angstrom, b = 7.6748(1) Angstrom, c = 7.9123(2) Angstrom, beta 113.367(1)degrees, at 90 K, Z = 6. The packing of the molecules in the low-temperature phases is influenced by the formation of weak C-H...F interactions.
