Crystal structures and temperature-induced phase transitions of Sr2Mn2+W6+O6Sr2Mn2+W6+O6, and of its transformation to Sr2Mn3+W6+O6+δSr2Mn3+W6+O6+δ

We present a new effective method for synthesizing Sr2MWO6Sr2MWO6 double perovskite oxides: the co-precipitation route at 1220 K in nitrogen environment. Using conventional X-ray diffraction methods, we have confirmed the room-temperature space group of Sr2Mn2+WO6WO6 to be P21/nP21/n: Z = 2; a=5.6764(1)Å;b=5.6752(1)Å;c=8.0144(1)Å;β=90.12(1)° and V=258.18(1)Å3V=258.18(1)Å3. We show that the compound presents the following temperature induced phase-transition sequence: P21/n→I4/m→Fm3¯m. After a thermal treatment, 24 h at 870 K in air, the Sr2Mn2+WO6Sr2Mn2+WO6 compound transforms irreversibly to Sr2Mn3+WO6+δSr2Mn3+WO6+δ. This transformation has been confirmed by EPR and XANES measurements. Using conventional X-ray diffraction methods, we have shown the room-temperature space group of Sr2Mn3+WO6+δSr2Mn3+WO6+δ to be I4/mI4/m: Z = 2; a=5.6353(1)Å;c=8.0149(1)Å and V=254.53(1)Å3. We show that the compound presents the following temperature induced phase-transition sequence: I4/m(→I4/mmm)→Fm3¯m. The tetragonal-to-tetragonal phase transition is suggested to be present, it is not observed as it is very weak; it is attributed to the presence of the Jahn-Teller active Mn3+ cation.