Valence averaging and phase transitions in europium (Eu₂VO₄) and iron (RSr₂Fe₃O₉) compounds

Mössbauer studies of ¹⁵¹Eu in Eu₂VO₄ reveal a very sharp valence averaging phase transition at 450 K. Two equally intense absorption lines, differing in isomer shift by 12.7 mm/s, corresponding to Eu²⁺ and Eu³⁺, collapse to a single absorption line at the average isomer shift. The transition is confirmed by X-ray and resistivity measurements. In RSr₂Fe₃O₉ (R = Pr, Sm) ⁵⁷Fe studies reveal averaging valence phase transitions coinciding with magnetic order phase transitions at 190 K and 140 K, respectively. Two magnetic sextets, corresponding to 2Fe³⁺ + Fe⁵⁺, collapse to two singlets corresponding to 2Fe⁴⁺ + Fe³⁺.