Synthesis and magnetic structure of the layered manganese oxide selenide Sr2MnO2Ag1.5Se2

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

Crystal and magnetic structures of the oxide sulfides CaCoSO and BaCoSO

CaCoSO, synthesized from CaO, Co, and S at 900 °C, is isostructural with CaZnSO and CaFeSO. The structure is non-centrosymmetric by virtue of the arrangement of the vertex-sharing CoS3O tetrahedra which are linked by their sulfide vertices to form layers. The crystal structure adopts space group P63mc (No. 186), and the lattice parameters are a = 3.7524(9) Å and c = 11.138(3) Å at room temperature with two formula units in the unit cell. The compound is highly insulating, and powder neutron diffraction measurements reveal long-range antiferromagnetic order with a propagation vector k = (1/3, 1/3, 1/2). The magnetic scattering from a powder sample can be modeled starting from a 120° arrangement of Co(2+) spin vectors in the triangular planes and then applying a canting out of the planes which can be modeled in the magnetic space group Ccc (space group 9.40 in the Belov, Neronova, and Smirnova (BNS) scheme) with Co(2+) moments of 2.72(5) μB. The antiferromagnetic structure of the recently reported compound BaCoSO, which has a very different crystal structure from CaCoSO, is also described, and this magnetic structure and the magnitude of the ordered moment (2.75(2) μB) are found by experiment to be similar to those predicted computationally

Magnetic ordering in the layered oxyselenides Sr2CoO2Ag2Se2 and Ba2CoO2Ag2Se2

The antiferromagnetic structures of Sr2CoO2Ag2Se2 and Ba2CoO2Ag2Se2 are solved using powder neutron diffraction. Both compounds adopt the same magnetic structure, based on a √2a × √2a × c expansion of the nuclear cell with magnetic space group PC42/n (86.72 in the Belov-Neronova-Smirnova notation). This structure is adopted as a result of nearest-neighbour antiferromagnetic interactions within the CoO2 planes. The refined long-range-ordered magnetic moments of Sr2CoO2Ag2Se2 and Ba2CoO2Ag2Se2 are 3.7(1) and 3.97(3) μB per Co ion respectively. The refined moments are significantly greater than the value predicted from just considering the spin (3 μB); this is attributed to a significant orbital contribution to the magnetic moment in an analogous manner to that previously observed for Sr2CoO2Cu2S2 and the values conform to a relationship between the shape of the distended CoO4Ch2 (Ch = S, Se) octahedron and the size of the ordered moment established for a series of related compounds

Muon spin rotation study of the layered oxyselenide Sr2CoO2Ag2Se2

The results of a muon spin rotation experiment on the layered oxyselenide Sr2CoO2Ag2Se2 are presented. The compound contains square-planar CoO2 layers and is found to exhibit a phase transition at 160.4(1) K to an antiferromagnetic configuration of the Co2+ ions. Density functional theory calculations were performed in order to determine the stopping site of the muon within the unit cell. The calculated magnetic dipole field at the muon stopping site was shown to be consistent with the proposed magnetic structure

Synthesis, structure, and compositional tuning of the layered oxide tellurides Sr2MnO2Cu2–xTe2 and Sr2CoO2Cu2Te2

The synthesis and structure of two new transition metal oxide tellurides, Sr2MnO2Cu1.82(2)Te2 and Sr2CoO2Cu2Te2, are reported. Sr2CoO2Cu2Te2 with the purely divalent Co2+ ion in the oxide layers has magnetic ordering based on antiferromagnetic interactions between nearest neighbors and appears to be inert to attempted topotactic oxidation by partial removal of the Cu ions. In contrast, the Mn analogue with the more oxidizable transition metal ion has a 9(1)% Cu deficiency in the telluride layer when synthesized at high temperatures, corresponding to a Mn oxidation state of +2.18(2), and neutron powder diffraction revealed the presence of a sole highly asymmetric Warren-type magnetic peak, characteristic of magnetic ordering that is highly two-dimensional and not fully developed over a long range. Topotactic oxidation by the chemical deintercalation of further copper using a solution of I2 in acetonitrile offers control over the Mn oxidation state and, hence, the magnetic ordering: oxidation yielded Sr2MnO2Cu1.58(2)Te2 (Mn oxidation state of +2.42(2)) in which ferromagnetic interactions between Mn ions result from Mn2+/3+ mixed valence, resulting in a long-range-ordered A-type antiferromagnet with ferromagnetic MnO2 layers coupled antiferromagnetically

Synthesis, structure, and compositional tuning of the layered oxide tellurides Sr2MnO2Cu2–xTe2 and Sr2CoO2Cu2Te2

The synthesis and structure of two new transition metal oxide tellurides, Sr2MnO2Cu1.82(2)Te2 and Sr2CoO2Cu2Te2, are reported. Sr2CoO2Cu2Te2 with the purely divalent Co2+ ion in the oxide layers has magnetic ordering based on antiferromagnetic interactions between nearest neighbors and appears to be inert to attempted topotactic oxidation by partial removal of the Cu ions. In contrast, the Mn analogue with the more oxidizable transition metal ion has a 9(1)% Cu deficiency in the telluride layer when synthesized at high temperatures, corresponding to a Mn oxidation state of +2.18(2), and neutron powder diffraction revealed the presence of a sole highly asymmetric Warren-type magnetic peak, characteristic of magnetic ordering that is highly two-dimensional and not fully developed over a long range. Topotactic oxidation by the chemical deintercalation of further copper using a solution of I2 in acetonitrile offers control over the Mn oxidation state and, hence, the magnetic ordering: oxidation yielded Sr2MnO2Cu1.58(2)Te2 (Mn oxidation state of +2.42(2)) in which ferromagnetic interactions between Mn ions result from Mn2+/3+ mixed valence, resulting in a long-range-ordered A-type antiferromagnet with ferromagnetic MnO2 layers coupled antiferromagnetically

Unexpected behaviour in derivatives of Barluenga's reagent, Hal(Coll)2X (Coll = 2,4,6-trimethyl pyridine, collidine; Hal = I, Br; X = PF6, ClO4 & BF4)

For the collidine analogues of Barluenga's Reagent (IPy2BF4) reported, a flat cation is necessary for the generation of a modulated phase, in keeping with the “Ratchet Model” theory [Kim et al., Crystal Growth & Design, 2014, 14, 6294]. Attempts to study “diffuse modulation” in Br(Coll)2ClO4 have shown that these non-Bragg features disappear very rapidly on exposure to synchrotron radiation, an effect thought to be caused by the radiation damage disrupting the lattice vibrations that cause the modulation