Two new calcium tantalate polytypes have been prepared in the system Ca 2 Ta 2 O 7 +Ca 2 Nb 2 O 7 . Their structures have been determined and re5ned using single-crystal X-ray di4raction data and powder neutron di4raction data, and their dielectric properties have been measured in the 100 kHz to 10 GHz frequency range. The 7M polytype of pure Ca 2 Ta 2 O 7 has monoclinic symmetry with a โซุโฌ 12.726(1), b โซุโฌ 7.380(1), c โซุโฌ 42.538(1) A s , โซุโฌ 95.77(1)3, Z โซุโฌ 28, space group C2. The 7M Ca 2 Ta 1.9 Nb 0.1 O 7 is isostructural, with a โซุโฌ 12.7170( 6), b โซุโฌ 7.3728(3), c โซุโฌ 42.464(2) A s , โซุโฌ 95.755(5)3. The 5M Ca 2 Ta 1.8 Nb 0.2 O 7 is monoclinic with a โซุโฌ 12.7528(4), b โซุโฌ 7.3492(2), c โซุโฌ 30.2442(8) A s , โซุโฌ 94.245(2)3, Z โซุโฌ 20, space group C2. The two polytypes have in common a closest-packing of (001) metal atom layers that alternately have the composition Ca 3 Ta and Ta 3 Ca. The polytypes have mixed hexagonal-cubic stacking sequences of the metal atom layers: 7M โซุโฌ hccccchccccccch ... , 5M โซุโฌ hccchccccch ... . The cubic-stacked blocks have weberitetype structures in both polytypes, in which hexagonal tungsten bronze type layers of corner-connected octahedra are interconnected via corner linking to isolated TaO 6 octahedra in the Ca 3 Ta layers to give a [Ta 2 O 7 ] 4ุ framework within which the eight-coordinated calcium atoms are distributed. The polytypes display a broad resonance at about 8 GHz, with relative permittivities of &25 and with Q values of about 200.