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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) Å for HFC-134a and  a  = 5.5433(1) Å 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. HCF-134a is monoclinic, space group  P 2 1 / c ,  a  = 8.7569(4) Å,  b  = 9.3523(1) Å,  c  = 9.0784(4) Å ,  β  = 106.479(5)°, at 80 K,  Z  = 8. HFC-152a is monoclinic, space group  Pc ,  a  = 8.6417(2) Å,  b  = 7.6748(1) Å,  c  = 7.9123(2) Å,  β = 113.367(1)º, 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.

Crystal structures of hydrofluorocarbons from powder X-ray diffraction data: HFC-134a and HFC-152a