Dielectric, X-Ray, and Optical Study of Ferroelectric<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cd</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Nb</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>7</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>and Related Compounds

Single crystals of ${\mathrm{Cd}}_{2}$${\mathrm{Nb}}_{2}$${\mathrm{O}}_{7}$ have been grown, and dielectric, x-ray, and optical studies have been carried out on these above and below the ferroelectric Curie point. The x-ray analysis confirms the pyrochlore structure, and gives the oxygen parameter $x=0.305$ at room temperature. The symmetry below the Curie point, -85\ifmmode^\circ\else\textdegree\fi{}C, is uncertain, due to the very slight lattice distortion: $\frac{c}{a}$ is about 1.0005 if the ferroelectric phase is tetragonal. The spontaneous polarization along the cubic (111) axis is 6 microcoulombs/${\mathrm{cm}}^{2}$ at -180\ifmmode^\circ\else\textdegree\fi{}C.Partial replacement of Cd by Pb or Ca, or of Nb by Ta, decreases the Curie temperature of ${\mathrm{Cd}}_{2}$${\mathrm{Nb}}_{2}$${\mathrm{O}}_{7}$. Dielectric measurements revealed no anomaly in several other ${A}_{2}{B}_{2}{\mathrm{O}}_{7}$ compounds. Lattice parameters are reported for ${\mathrm{Pb}}_{2}$${\mathrm{Ta}}_{2}$${\mathrm{O}}_{7}$, ${\mathrm{Y}}_{2}$${\mathrm{Ti}}_{2}$${\mathrm{O}}_{7}$, and ${\mathrm{Cd}}_{2}$${\mathrm{Nb}}_{2}$${\mathrm{O}}_{6}$.

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