Structure and crystal chemistry as a function of composition across the wide range nonstoichiometric (1-epsilon)ZrO2 center dot epsilon SmO1.5, 0.38 < epsilon < 0.55, oxide pyrochlore system

The extent of the pyrochlore solid solution field (at 1500 degrees C) in th e (1-epsilon)ZrO2.epsilon SmO1.5 system has been determined as running from 38.5 to 55 mol% SmO1.5. Subtle changes in crystal structure have been inve stigated as a function of composition across this solid solution field usin g a wide-angle convergent beam electron diffraction technique. Systematic c hanges in the one unknown oxygen atom fractional coordinate x are found as a function of composition and investigated from a crystal chemical point of view using the bond length - bond valence approach. The parameter x is fou nd to be similar to 0.352 for epsilon = 0.40 and 0.348 for epsilon = 0.45 a nd 0.55. Both values of x are significantly larger than the value (x = 0.34 2) found for the stoichiometric 50 mol% SmO1.5 specimen by the same techniq ue. The increased x value found for nonstoichiometric compositions on eithe r side of the ideal pyrochlore composition (epsilon = 0.5) is consistent wi th a systematic shift toward the average value for this parameter in the so -called "defect fluorite" structure (x = 0.375), the phase field for which appears on either side of the relevant pyrochlore solid solution field. (C) 1999 Academic Press.