Its often seen for pyrochlore iridates that the lattice parameter calculated for single crystals is significantly smaller (up to 0.2 A difference) than that for the polycrystal.Also no significant stoichiometry difference exist between the two.
Actually this shouldn't be since the difference between these to objects is only the macroscopic size. The only reason could be that assumed parameters are clearly different, e.g. the focus which is quite fix for the single crystal but for a powder sample probably "deeper" than expected. Then the sample position (or the diffracting volume) is not described by the surface of the sample but by a layer which is schifted out of focus. This results in smaller Bragg angles which means bigger lattice parameters.
the other opportunity is that a determination of lattice parameters using single crystals is not that accuracte (maybe this has been changed over the last ten-twenty years using specific corrections). But even if this would be true it wouldn't explain the explained systematic deviation. Therefore I would assume the above mentioned problem of the materials density (absorption impact) could be a reason.
Yes. All the data sets correspond to ambient pressure-temperature. More-ever the pressure/low temperature studies also do not report this big a difference between lattice parameters (0.2 A) between the extreme points.
I have not work with pyrochlore iridates but worked with the defect pyrochlore H2Ta2O6xH2O (“defect” means that oxygen position (8b) is empty). What do you mean when say about difference in lattice parameter? Is it one material, i.e. powder after crush of single crystal? If so, it is fantastic result. If not, you can have “no significant stoichiometry difference” for cations, but, probably, it is due to the oxygen stoichiometry. Think about this possibility. It can depend on the different methods of preparation of powder and single crystals.
The powder is prepared via solid state synthesis however the single crystals are grown using flux method ( with KF being a popular choice) .But how exactly that will give rise to such drastic change in lattice parameters is not clear to me. Also in Pyrochlore iridates, minor changes in lattice parameter lead to very different physical properties (ex: Huge shift in Metal insulator transition temperature) But in spite of having different lattice parameters, the single crystal and polycrystalline sample for a given composition display similar physical properties.
Dear Prachi,
I repeat my suggestion. Minor changes in cations stoichiometry can lead to minor changes in lattice parameter which "leads to very different physical properties". But big changes in anion stoichiometry can lead to big changes in lattice parameter which does not lead to very different physical properties. Physical properties can mainly depend on the cations stoichiometry. Powder is prepared by sintering in oxygen, which means that oxygen content is 7 in formulae (A2Ir2O7) (material is completely oxidized). Single crystal is grown in flux, which means that oxygen content could be 6.5-6.8 (A2Ir2O6.5-6.8) (material is partly reduced). To check this idea you can take single crystal and anneal it in oxygen for several hours at ~1000 C. Then compare lattice parameter before and after annealing.
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