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The structure of sodium-Y zeolite, Na56Si1360384Al56, containing chemisorbed deuterated benzene has been studied for two different benzene coverages at 4 K and room temperature by powderneutron diffraction. The bare dehydrated zeolite has also been examined. The space group is Fd3m with cell parameters, at room temperature, of a= 24.85 (3) A, 24.83 (3) A, and 24.81 (3) A for the bare zeolite, low-coverage, and high-coverage samples, respectively. At 4 K a= 24.85 (3) A at low coverage and a= 24.81 (3) A for thehigh-coverage sample. The sodium ions are distributed over three sites whose positions at room temperature for the bare zeolite are Na (l) at 32e (x, x, x), x= 0.2345 (2); Na (2) at 32e, x= 0.0507 (4), and Na (3) at 16c (0, 0, 0), with fractional occupancies of 100 (2)%, 58 (2)%, and 44 (2)%, respectively. At 4 K, the benzene molecules are localized on two distinct sites within the cavities of the zeolite …