Abstract
The synthesis of a high-purity sample of the layered oxide selenide Sr2MnO2Ag1.5Se2 is reported. At ambient temperature it crystallises in the space group I4/mmm with two formula units in the unit cell and lattice parameters a=4.08771(1)Å, c=19.13087(8)Å. The compound displays mixed-valent manganese in a formal oxidation state close to +2.5 and powder neutron diffraction measurements reveal that below the Néel temperature of 63(1)K this results in an antiferromagnetic structure which may be described as A-type, modelled in the magnetic space group PI4/mnc (128.410 in the Belov, Neronova and Smirnova (BNS) scheme) in which localised Mn moments of 3.99(2) μB are arranged in ferromagnetic layers which are coupled antiferromagnetically. In contrast to the isostructural compound Sr2MnO2Cu1.5S2, Sr2MnO2Ag1.5Se2 does not display long range ordering of coinage metal ions and vacancies, nor may significant amounts of the coinage metal readily be deintercalated using soft chemical methods.
Highlights
Oxide chalcogenides of composition A2MO2X2Ch2 (A=Sr, Ba; M=1st row transition metal or Zn, X=Cu, Ag and Ch=S, Se) were first discovered by Zhu and Hor in 1997 [1,2]
We conclude that a variation in Ag occupancy of about 1% is accessible using the purification with I2; we cannot rule out that a similar variation in Ag occupancies is accessible via a purely solid state synthesis route for this compound
Magnetic susceptibility measurements in the Curie-Weiss regime produce an effective moment of 5.45(1) μB per Mn ion, consistent with the mixed valence (Mn oxidation state of +2.5), and a positive Weiss temperature suggesting that the strongest interactions between Mn moments are ferromagnetic
Summary
Oxide chalcogenides of composition A2MO2X2Ch2 (A=Sr, Ba; M=1st row transition metal or Zn, X=Cu, Ag and Ch=S, Se) were first discovered by Zhu and Hor in 1997 [1,2]. We have previously shown [5] that the compound originally reported as Sr2MnO2Cu2S2 [2] is highly copper deficient in the chalcogenide layers and the composition accessible in a high temperature synthesis is Sr2MnO2Cu1.5S2 In this compound and in the selenide analogue Sr2MnO2Cu1.5Se2 [5,6] the Cu deficiency (25% of the Cu sites are vacant) means that the average oxidation state of Mn is approximately +2.5. In this work we verify the difficulties in achieving high purity material using purely solid state synthesis techniques, but we report that an I2 purification step results in the isolation of Sr2MnO2Ag1.5Se2, with ~99 wt% purity This has enabled us to compare and contrast the crystal structure, magnetic ordering and chemical properties with those of the Cu-containing chalcogenide analogues using synchrotron powder X-ray diffraction (PXRD) and powder neutron diffraction (PND)
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