Characterization of new 5M and 7M polytypes of niobia-doped Ca2Ta2O7

Two new calcium tantalate polytypes have been prepared in the system Ca2Ta2 O7- Ca2Nb2O7. Their structures have been determined and refined using singl e-crystal X-ray diffraction data and powder neutron diffraction data, and t heir dielectric properties have been measured in the 100 kHz to 10 GHz freq uency range. The 7M polytype of pure Ca2Ta2O7 has monoclinic symmetry with a = 12.726(1), b = 7.380(1), c = 42.538(1) Angstrom, beta = 95.77(1), Z = 2 8, space group C2. The 7M Ca2Ta1.9Nb0.1O7 is isostructural, with a = 12.717 0(6), b = 7.3728(3), c = 42.464(2) Angstrom, beta = 95.755(5)degrees. The 5 M Ca-2 Ta1.8Nb0.2O7 is monoclinic with a = 12.7528(4), b = 7.3492(2), c = 3 0.2442(8) Angstrom, beta = 94.245(2), Z = 20, space group C2. The two polyt ypes have in common a closest-packing of (001) metal atom layers that alter nately have the composition Ca3Ta and Ta3Ca. The polytypes have mixed hexag onal-cubic stacking sequences of the metal atom layers: 7M = hccccchccccccc h..., 5M = hccchccccch.... The cubic-stacked blocks have weberite-type stru ctures in both polytypes, in which hexagonal tungsten bronze type layers of corner-connected octahedra are interconnected via corner linking to isolat ed TaO6 octahedra in the Ca3Ta layers to give a [Ta2O7](4-) framework withi n which the eight-coordinated calcium atoms are distributed. The polytypes display a broad resonance at about 8 GHz, with relative permittivities of s imilar to 25 and with Q values of about 200. (C) 2001 Academic Press.