Characterization of new 5M and 7M polytypes of niobia-doped Ca_2Ta_2O_7

摘要

Two new calcium tantalate polytypes have been prepared in the system Ca_2Ta_2O_7-Ca_2Nb_2O_7. 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 Ca_2Ta_2O_7 has monoclinic symmetry with a = 12.726(1), b = 7.380(1), c = 42.538(1) A, #beta# = 95.77(1) deg, Z = 28, space group C2. The 7M Ca_2Ta_(1.9)Nb_(0.1)O_7 is isostructural, with a = 12.7170(6), b = 7.3728(3), c = 42.464(2) A, #beta# = 95.755(5) det. The 5M Ca_2Ta_(1.8)Nb_(0.2)O_7 is monoclinic with a = 12.7528(4), b = 7.3492(2), c = 30.2442(8) A, #beta# = 94.245(2) deg, Z = 20, space group C2. The two polytypes have in common a closest-packing of (001) metal atom layers that alternately have the composition Ca_3Ta and Ta_3Ca. The polytypes have mixed hexagonal-cubic stacking sequences of the metal atom layers: 7M = hccccchccccccch…, 5M = hccchccccch…. The cubic-stacked blocks have weberitetype structures in both polytypes, in which hexagonal tungsten bronze type layers of connected octahedra are interconnected via corner linking to isolated TaO_6 octahedra in the Ca_3Ta layers to give a [Ta_2O_7]~(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 approx 25 and with Q values of about 200.