Determination of the Pcmn/Ibm2 phase boundary at high temperatures in the system Ca2Fe2O5-Ca2Al2O5

Abstract

The crystals of Ca2(AlxFe1-x)2O5 with 0 ≤ x ≤ 0.4 were examined using powder XRD at temperatures between 25° and 1000°C. The Pcmn to Ibm2 phase transition was readily detected by the disappearance of the 131 and 151 reflections (h + k + l odd). Thus, the phase relationship has been determined as combined functions of the temperature and x-value. At 25°C, the space group changed from Pcmn (0 ≤ x ≤ 0.23) to Ibm2 (0.24 ≤ x ≤ 0.40) across x = 0.235. With increased x-value, the cell dimensions of both phases steadily decreased, showing the nearly complete continuity at that phase boundary. For the crystal with x = 0 (Ca2Fe2O5), the cell dimensions steadily increased during heating to 685°C, at which temperature the phase transition occurred. There was a slight discontinuous decrease in cell dimensions just after the transition. Subsequent heating to 1000°C of the Ibm2 phase led to a steady thermal expansion along the b-axis and c-axis, while a slight contraction occurred along the a-axis up to 800°C.