The H2 antagonist cimetidine forms many polymorphs, several of which have resisted structural analysis thus far. Using single-crystal X-ray measurements obtained from synchrotron radiation, the crystal structure of cimetidine form C has been solved. This layered structure crystallizes in space group
C2/
c with an unusually large lattice parameter,
a = 82.904 Å. The thickness of each layer
L is equal to
a′ =
a/6 = 13.82 Å, and
a = 6
a′ originates from a sixfold
LLLL′
L′
L′ sequence with
L and
L′ differing by 0.5
b. This packing is reminiscent of polytypic stacking in metals. A (3 + 1)-dimensional superspace model is derived and used to explain and predict many polytypic modifications. This model is characterized by (i) the (3 + 1)-dimensional symmetry group
X2/
c(α0γ)00, where
X = 0
0
; (ii) the lattice parameter
a′ and modulation vector
q = 1/
n(
a′*); (iii) the atomic positions of a single molecule of cimetidine form C; (iv) the primary variable, 1/
n. The model reproduces the previously solved structure, the 6M polytype, and generates the related polytypes
nM with lattice parameter
anM =
na′ for
n = 1, 2, 3, 4 and 5. A comparison of powder X-ray diffraction patterns available for cimetidine form C with those simulated for the
nM polytypes suggests that the powder samples published previously probably contain a mixture of various polytypes.