Characterization of New 5M and 7M Polytypes of Niobia-Doped Ca2Ta2O7

Two new calcium tantalate polytypes have been prepared in the system Ca2Ta2O7–Ca2Nb2O7. Their structures have been determined and refined using single-crystal X-ray diffraction data and powder neutron diffraction data, and their dielectric properties have been measured in the 100 kHz to 10 GHz frequency range. The 7M polytype of pure Ca2Ta2O7 has monoclinic symmetry with a=12.726(1), b=7.380(1), c=42.538(1) ?, beta=95.77(1)°, Z=28, space group C2. The 7M Ca2Ta1.9Nb0.1O7 is isostructural, with a=12.7170(6), b=7.3728(3), c=42.464(2) ?, beta=95.755(5)°. The 5M Ca2 Ta1.8Nb0.2O7 is monoclinic with a=12.7528(4), b=7.3492(2), c=30.2442(8) ?, beta=94.245(2)°, Z=20, space group C2. The two polytypes have in common a closest-packing of (001) metal atom layers that alternately have the composition Ca3Ta and Ta3Ca. The polytypes have mixed hexagonal-cubic stacking sequences of the metal atom layers: 7M=hccccchccccccch ... , 5M=hccchccccch ... . The cubic-stacked blocks have weberite-type structures in both polytypes, in which hexagonal tungsten bronze type layers of corner-connected octahedra are interconnected via corner linking to isolated TaO6 octahedra in the Ca3Ta layers to give a [Ta2O7]4- 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.