New calcium tantalate polytypes with weberite-related structures have been prepared by doping Ca 2 Ta 2 O 7 with Sm 2 Ti 2 O 7 . Trigonal 6-layer (6T) and monoclinic 5-layer (5M) polytypes containing ∼6 and 10 mol%, respectively of the Sm 2 Ti 2 O 7 component were prepared and the structures were determined and refined using X-ray data on flux-grown single crystals. The 6T phase, Ca 1.89 Ta 1.86 Sm 0.16 Ti 0.10 O 7 , has a=7.353(1), c=36.264(1) Å, Z=12, space group P3 1 , and the 5M phase, Ca 1.80 Ta 1.80 Sm 0.24 Ti 0.17 O 7 , has a=12.763(1), b=7.310(1), c=30.190(1) Å, β=94.09°(1), Z=20, space group C2. The 6T structure is related to the 3T weberite structure, having a cubic close packed metal atom array as in fluorite and a stacking of (001) metal atom layers (≡{111} f ) that alternately have the composition Ta 3 Ca and Ca 3 Ta. Hexagonal tungsten bronze type layers of corner-connected octahedra are interconnected by isolated TaO 6 octahedra in the Ca 3 Ta layers, giving an octahedral framework of composition [Ta 2 O 7 ] 4− in which the eight-coordinated calcium atoms are distributed. The octahedra in the Ca 3 Ta layers have only four anions involved in corner sharing. The different weberite polytypes are distinguished by the geometry of the two terminal anions, cis- or trans. In the 6T structure there is an alternation of cis- and trans- octahedra in successive Ca 3 Ta layers. The 5M polytype differs from the weberite polytypes in having regular twinning of the metal atom lattice on (001), giving a stacking sequence hccchccccc. . . . The c-stacked blocks alternatively have the weberite and pyrochlore structures, with ordering of Sm into the Ca sites in the pyrochlore blocks.