Crystal structures and temperature-induced phase transitions of [formula omitted], and of its transformation to [formula omitted

Abstract

We present a new effective method for synthesizing Sr 2 MWO 6 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 Sr 2Mn 2+ WO 6 to be P 2 1 / 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 ) Å 3 . We show that the compound presents the following temperature induced phase-transition sequence: P 2 1 / n → I 4 / m → Fm 3 ¯ m . After a thermal treatment, 24 h at 870 K in air, the Sr 2 Mn 2 + WO 6 compound transforms irreversibly to Sr 2 Mn 3 + WO 6 + 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 Sr 2 Mn 3 + WO 6 + δ to be I 4 / m : Z = 2; a = 5.6353 ( 1 ) Å ; c = 8.0149 ( 1 ) Å and V = 254.53 ( 1 ) Å 3 . We show that the compound presents the following temperature induced phase-transition sequence: I 4 / m ( → I 4 / mmm ) → Fm 3 ¯ 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 Mn 3+ cation.