  Lanthanum and yttrium compounds with composition RE2M2O7 (RE=Y, La; M = Ti,
Zr, Hf) have high density and high Z and can be doped with Ce onto the La and Y
sites. This makes these compounds good candidates for Ce activated scintillator
gamma-ray detectors particularly for the hafnate systems which have a very high
density. There is disagreement in the literature concerning La2Hf2O7:Ce as it
has been reported to show both bright as well as no Ce activated luminescence
by different experimental groups. We have performed first-principles electronic
structure calculations of these compounds doped with Ce using the
pseudopotential method based on the generalized gradient approximation in
density functional theory. The positions of the Ce 4f states relative to the
valence band maximum and the position of the Ce 5d states relative to the
conduction band minimum (CBM) of the host material are determined. We find,
unlike Ce activated La and Y compounds where the CBM is typically of La 5d or Y
4d character, that, in these systems the CBM is predominately of d character on
the Ti, Zr, Hf atoms. For all these compounds we also find that the Ce 5d state
lies above the CBM which would prevent any luminescence from the Ce site.
