Characterization of New 5M and 7M Polytypes of Niobia-Doped Ca2Ta2O7
I E. Grey, Robert S. Roth, W G. Mumme, J Planes, Leonid A. Bendersky, C Li, J Chenavas
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 frequency range 100 kHx - 10 GHz. The 7M polytype of pure Ca2Ta2O7 has monoclinic symmetry with a = 12.712 (1), b = 7.372(1), c = 42.485(1) , b = 95.76(1) , Z= 28, space group C2. 7M Ca2Ta1.9Nb0.1O7 is isostructural, with a = 12.7170 (6), b = 7.3728(3), c = 42.64(2) , b = 95.755(5) . 5M Ca2Ta1.8Nb0.2O7 is monoclinic with a = 12.7528 (4), b = 7.349(2), c = 30.2442(8) , b = 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.
, Roth, R.
, Mumme, W.
, Planes, J.
, Bendersky, L.
, Li, C.
and Chenavas, J.
Characterization of New 5M and 7M Polytypes of Niobia-Doped Ca<sub>2</sub>Ta<sub>2</sub>O<sub>7</sub>, Journal of Solid State Chemistry
(Accessed December 1, 2021)