Reversible Valence Transition Eu3+ → Eu2+ → Eu3+ in $${\text{Eu}}_{{{\text{1}}-x}}^{{{\text{2 + }}}}{\text{Eu}}_{x}^{{3 + }}M{{{\text{O}}}_{{{\text{3}} + x/{\text{2}}}}}$$ (M = Ti, Zr, Hf): An Analysis of XAFS and XRD Data