Structure and Crystal Chemistry as a Function of Composition across the Wide Range Nonstoichiometric (1-epsilon)ZrO2·epsilonSmO1.5, 0.38

The extent of the pyrochlore solid solution field (at 1500°C) in the (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 investigated as a function of composition across this solid solution field using a wide-angle convergent beam electron diffraction technique. Systematic changes 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 found to be ~0.352 for epsilon=0.40 and 0.348 for epsilon=0.45 and 0.55. Both values of x are significantly larger than the value (x=0.342) found for the stoichiometric 50 mol% SmO1.5 specimen by the same technique. The increased x value found for nonstoichiometric compositions on either side of the ideal pyrochlore composition (epsilon=0.5) is consistent with 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.