Hyperfine Interactions 126 (2000) 313–317 313 Valence averaging and phase transitions in europium (Eu 2 VO 4 ) and iron (RSr 2 Fe 3 O 9 ) compounds I. Nowik a , R.H. Herber a , I. Felner a and V.P.S. Awana b a Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel b Instituto de Fisica, Unicamp, 13083-970 Campinas, SP, Brazil M¨ ossbauer studies of 151 Eu in Eu2VO4 reveal a very sharp valence averaging phase tran- sition at 450 K. Two equally intense absorption lines, differing in isomer shift by 12.7 mm/s, corresponding to Eu 2+ and Eu 3+ , 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) 57 Fe studies reveal averaging valence phase transitions coinciding with mag- netic order phase transitions at 190 K and 140 K, respectively. Two magnetic sextets, corresponding to 2Fe 3+ + Fe 5+ , collapse to two singlets corresponding to 2Fe 4+ + Fe 3+ . 1. Introduction Eu and Fe ions in solids may appear in more than one stable valence state. In most of these cases, the ions of different valence reside in crystallographically inequivalent sites. In a few cases, all of the Eu or Fe ions reside in equivalent sites, yet may have two (for Fe even more) different valence states, stable at low temperatures. When the temperature is raised, a critical temperature (T v ) is reached, above which only one average valence is present. Here we report this phenomenon, studied by X-ray diffraction, resistivity, magnetometry and 151 Eu and 57 Fe M¨ ossbauer spectroscopy, in Eu 2 VO 4 (T v = 450 K) [1], PrSr 2 Fe 3 O 9 (T v = 190 K) [2], and SmSr 2 Fe 3 O 9 (T v = 140 K). 2. Experimental details The sample of Eu 2 VO 4 was prepared by well known procedures [1,3]. The as-prepared compound contained ∼16% Eu 2 O 3 , as determined by magnetization and M¨ ossbauer studies. The crystal structure is tetragonal (I4/mmm). The preparation of RSr 2 Fe 3 O 9-δ (R = Pr, Sm) samples was described in [2]. The crystal structure is hexagonal (R3c) [4]. The oxygen deficiency δ was determined by analysis of the M¨ ossbauer spectra and by comparing these to those of LaSr 2 Fe 3 O 9-δ [5]; its value is less than 0.1. The M¨ ossbauer spectra were obtained by using a standard constant acceleration drive, a 151 Sm 2 O 3 source (200 mCi) for the Eu studies and a 57 Co:Rh  J.C. Baltzer AG, Science Publishers