Dear Kaspars,

when you ground your crystal I suppose you change your sample a lot - then it's not a surprise that QY is changed as well.

Dear Kaspars,

i suppose, that QY in a single crystal might be smaller mainly due to these two reasons:

1) There occurs less reabsorbtion in the grounded or precipitated sample (smaller crystallite size). For that You should compare fluorescence/luminescence spectra of these samples;

2) There might be less deffects (non-emissive species) in a small crystallite than in a large single crystal, considering diffusion length of excitons in your system is typical. For that you should try to compare fluorescence/luminescence lifetimes of your samples;

Dear Kaspars,

This is an interesting point. It is clear that the emission intensity and therefore the QY depends on crystal size and morphology. This is the reason because it is very dangerous compare absolute intensities from different samples even if the same setup was used.

My suggestion is that all measurements are valid, but you should mention the conditions you performed them. When light interact with matter you have different processes depending on the excitation wavelength and the intrinsic properties of the material: absorption, reflection, refraction, scattering, emission, etc. The scattering will change a lot depending on the size of your samples. My point is that it is the main reason of the different values you obtained. It depends on size and excitation wavelength and, of course, on the refractive index of the media surrounding your sample powders. 

I hope the explanation is clear enough.

Chears,

Rafa