| Popis: |
Here, we present a comprehensive study that encompasses changes within the crystal and magnetic structure in the brownmillerite-type phase Ca2Fe2O5 induced by the substitution of Fe3+ with Ga3+. 61 synthetic single-crystal samples of Ca2Fe2xGaxO5 0.00x1.328 have been investigated by single-crystal X-ray diffraction at 25 C. We find that pure Ca2Fe2O5 and samples up to x1.0 have space group Pnma, Z=4, whereas samples with x>1.0 show I2mb symmetry, Z=4. The Raman spectroscopic measurements exhibit that the change from Pnma to I2mb space group symmetry is reflected by a significant shift of two Raman modes below 150 cm1. These Raman modes are obviously linked to changes in the CaO bond lengths at the phase transition. 57Fe Mössbauer spectroscopy was used to characterize the cation distribution and magnetic structure as a function of composition and temperature. Thereby, the strong preference of Ga3+ for the tetrahedral site is verified, as an independent method besides XRD. At room-temperature, Ca2Fe2xGaxO5 solid solution compounds with 0x1.0 are antiferromagnetic ordered, as revealed by the appearance of magnetically split sextets in the Mössbauer spectra; samples with higher Ga3+ contents are paramagnetic. Over and above, the substitution of Fe3+ by Ga3+ results in the appearance of sharp, additional magnetic hyperfine split sextets, which can be attributed to cluster configurations within the individual tetrahedral chains. The temperature-dependent (20720 K) Mössbauer study reveals a transition from the magnetically ordered to the paramagnetic state at a temperature of about 710 K for the Ca2Fe2O5 end-member. (VLID)2956672 |
