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New Sr2MnGaO4.97complex oxide was synthesized by solid state reaction in sealed silica tubes at 950–1000°C. The Sr2MnGaO4.97crystal structure was refined from X-ray powder diffraction data. Sr2MnGaO4.97is based on the Ima2 brownmillerite-type structure with apically elongated MnO6octahedra due to a Jahn–Teller effect. Electron diffraction and high-resolution electron microscopy showed that local ordering of the left-and right-hand chains of GaO4tetrahedra in Sr2MnGaO4.97leads to a superstructure with a doubling of the bparameter of the orthorhombic unit cell. The formal oxidation state of Mn (VMn) can be varied by thermal treatments at elevated oxygen pressure (450°C, 20 bar of O2). The oxygen insertion induces a structure transformation in oxidized Sr2MnGaO5.47material with the formation of a tetragonal perovskite-like structure (a≃ap, c≃2cp) with oxygen vacancies located in the Ga layers. The oxidation is accompanied by a significant compression of the Mn–O apical distances and a suppression of the Jahn–Teller deformation. Both Sr2MnGaO4.97and Sr2MnGaO5.47can probably be treated as canted antiferromagnets with TN=150 and 80 K, respectively.

New layered oxides Ca2MnGaO5+δand Ca2MnGa1−xZnxO5+δ(x=0.1, 0.2) were synthesized by solid-state reaction at 1000–1300°C. The crystal structure of Ca2MnGaO5.045was studied using X-ray, neutron and electron diffraction, and high-resolution electron microscopy (HREM). The samples of Ca2MnGaO5.045consist of two closely intermixed brownmillerite-type phases with Pnmaor Ima2 space symmetry that possess very closely related structures that differ mainly by the orientation of the Ga–O chains. The formal Mn valence (VMn) in Ca2MnGaO5+δcan be varied either by heterovalent replacement of Ga3+for Zn2+or by oxygen insertion after treatment under 80 atm O2. In the latter case the compound with the composition Ca2MnGaO5.39forms built of domains of initial reduced material and fully oxidized Ca2MnGaO5.5phase. The Ca2MnGaO5.5structure was determined by HREM as belonging to the brownmillerite AnBn−1B′O3n−1(n=4) homologous series, in which three layers of BO6(B=Mn, Ga) octahedra form blocks separated by a single layer of B′O4(B′=Ga) tetrahedra. The increase of VMnfrom +3.09 to +3.78 is accompanied by a drastic compression of the structure along the b=4aperaxis due to a suppression of the Jahn-Teller deformation of the MnO6octahedra.

NbM2RCu2O8and TaM2RCu2O8(MBa, Sr; RPr, Nd, Sm, Eu and Gd) phases were synthesized. Electron diffraction study of TaSr2EuCu2O8showed the presence of a weak superstructure (a= 82a07), like that found for a number of similar phases. Rietveld refinement of this structure based on X-ray powder diffraction data was carried out in the space groupP4/mmm(R1= 0.038, Rp= 0.037) since no superstructure reflections were observed. Various types of cation replacement were performed in order to increase the copper valence. Monophasic Nb(Ta)1, Ti,Sr2EuCu2O8samples were synthesized atP(O2) = 1 bar andT= 1050°C up tox= 0.1 (Nb) and 0.3 (Ta), but no superconductivity was detected down to 12 K. High pressure syntheses were carried out to obtain Nb(Ta)Sr2Eu1, Ca, Cu2O8phases, but they were found to have a narrow homogeneity range that provides no opportunity of inducing superconductivity in such a way.

Compounds with the general formula HgBa2RCu2O6+δ(Hg-1212 phase, R = Eu, Gd, Ho, Er, Tm, Yb and Y) were synthesized by solid state reaction. The influence of synthesis conditions on the phase composition of these samples was investigated.