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