This Letter presents neutron diffraction data from a novel, biologically relevant, lyotropic membrane system which is highly alignable (#1.0 ± mosaic) in a magnetic field and gives rise to a number of welldefined Bragg reflections. The system, composed of two different phosphorylcholine lipids, undergoes a rare nematic ! smectic phase transition upon doping the system with paramagnetic ions (e.g., 2.7 wt % Tm 31 ). In addition, the isotropic phase occurs at a lower temperature than the smectic phase, in contrast to other lyotropic systems and in contrast to the phase behavior predicted by the McMillan model [Phys. Rev. A 4, 1238(1971] of smectic ordering. [S0031-9007(96)

Ten compounds belonging to the series of oxygen-deficient perovskite oxides Ca(2)Fe(2-x)Mn(x)O(5) and CaSrFe(2-x)Mn(x)O(5+y), where x = 1/2, 2/3, and 1 and y ≈ 0-0.5, were synthesized and investigated with respect to the ordering of oxygen vacancies on both local and long-range length scales and the effect on crystal structure and magnetic properties. For the set with y ≈ 0 the oxygen vacancies always order in the long-range sense to form the brownmillerite structure containing alternating layers of octahedrally and tetrahedrally coordinated cations. However, there is a change in symmetry from Pnma to Icmm upon substitution of Sr for one Ca for all x, indicating local T(d) chain (vacancy) disorder. In the special case of CaSrFeMnO(5) the neutron diffraction peaks broaden, indicating only short-range structural order on a length scale of ~160 Å. This reveals a systematic progression from Ca(2)FeMnO(5) (Pnma, well-ordered tetrahedral chains) to CaSrFeMnO(5) (Icmm, disordered tetrahedral chains, overall short-range order) to Sr(2)FeMnO(5) (Pm3m, destruction of tetrahedral chains in a long-range sense). Systematic changes occur in the magnetic properties as well. While long-range antiferromagnetic order is preserved, the magnetic transition temperature, T(c), decreases for the same x when Sr substitutes for one Ca. A review of the changes in T(c) for the series Ca(2)Fe(2-x)M(x)O(5), taking into account the tetrahedral/octahedral site preferences for the various M(3+) ions, leads to a partial understanding of the origin of magnetic order in these materials in terms of a layered antiferromagnetic model. While in all cases the preferred magnetic moment direction is (010) at low temperatures, there is a cross over for x = 0.5 to (100) with increasing temperature for both the Ca(2)Fe(2-x)Mn(x)O(5) and the CaSrFe(2-x)Mn(x)O(5) series. For the y > 0 phases, while a brownmillerite ordering of oxygen vacancies is preserved for the Ca(2) phases, a disordered Pm3m cubic perovskite structure is always found when Sr is substituted for one Ca. Long-range magnetic order is also lost, giving way to spin glass or cluster-glass-like behavior below ~50 K. For the x = 0.5 phase, neutron pair distribution function (NPDF) studies show a local structure related to brownmillerite ordering of oxygen vacancies. Neutron diffraction data at 3.8 K show a broad magnetic feature, incommensurate with any multiple of the chemical lattice, and with a correlation length (magnetic domain) of 6.7(4) Å.

Local and average structures and magnetic properties of of Sr2FeMnO5+y, y = 0.0, 0.5. comparisons with Ca2FeMnO5 and the effect of the A-site cation Ramezanipour, Farshid; Greedan, John E.; Siewenie, Joan; Proffen, Th.; Ryan, Dominic H.; Grosvenor, Andrew P.; Donaberger, Ronald L.Contact us / Contactez nous: nparc.cisti@nrc-cnrc.gc.ca. Oxygen-deficient perovskites can have a variety of structures depending on different factors including the degree of anion deficiency and the composition. In this article, the focus is on oxygen-deficient perovskites with the formula A 2 BB 0 O 5+x . The oxygen vacancies can order to form the brownmillerite structure in which the B cations are octahedrally coordinated, forming corner-sharing layers, and the B 0 cations are tetrahedrally coordinated, forming corner-sharing chains. (Figure 1). This longrange vacancy ordering results in a supercell that generally has dimensions with respect to the cubic perovskite cell constant, a p , of a br ≈ 2 1/2 a p , b br ≈ 4a p , and c br ≈ 2 1/2 a p . A number of different space group symmetries are observed, depending on subtle differences in the ordering of the tetrahedral chains within the unit cell. The chains can show either a right-handed (R) or lefthanded (L) orientation and the correlation between the intraand interlayer chain orientations and the resulting space groups are summarized in ABSTRACT: Sr 2 FeMnO 5+y was synthesized under two different conditions, in air and in argon, both of which resulted in a cubic, Pm3m, structure with no long-range ordering of oxygen vacancies. The unit cell constants were found to be a 0 = 3.89328(1) Å for argon (y = 0.0) and a 0 = 3.83075(3) Å for air (y = 0.5). In contrast, Ca 2 FeMnO 5 retains long-range brownmillerite oxygen vacancy ordering for either air or argon synthesis. Remarkably, Sr 2 FeMnO 5.0 oxidizes spontaneously in air at room temperature. A neutron pair distribution function (NPDF) study of Sr 2 FeMnO 5.0 (Ar) showed evidence for local, brownmillerite-like ordering of oxygen vacancies for short distances up to 5 Å. M€ ossbauer spectroscopy results indicate more than one Fe site for Sr 2 FeMnO 5+y (Ar and air), consistent with the noncubic local structure found by NPDF analysis. The isomer shifts and quadrupole splittings in both air-and argon-synthesized materials are consistent with the 3+ oxidation state for Fe in sites with coordination number four or five. This is confirmed by an L-edge XANES study. Mn is almost entirely in the 3+ state for Sr 2 FeMnO 5.0 (Ar), whereas Mn 4+ is predominantly present for Sr 2 FeMnO 5.5 (air). Magnetic susceptibility data show zero-fieldcooled/field-cooled (ZFC/FC) divergences near 50 K for the Ar sample and 25 K for the air sample, whereas Ca 2 FeMnO 5 is longrange G-type antiferromagnetically ordered at 407(2) K. Hyperfine magnetic splitting, observed in temperature-dependent M€ ossbauer measurements, indicates short-range magnetic correlations that persist up to 150 K for Sr 2 FeMnO 5.0 (Ar) and 100 K for Sr 2 FeMnO 5.5 (air), well above the ZF...

The local and average crystal structures and magnetic properties of the oxygen-deficient perovskite Sr(2)Fe(1.5)Cr(0.5)O(5+y) were studied using powder X-ray and neutron diffraction, neutron-pair distribution function analysis, and electron energy-loss spectroscopy. This material crystallizes in the cubic Pm3m space group, with a = 3.94491(14) Å. The oxygen vacancies are distributed randomly throughout the perovskite-type structure, and the average coordination number of the Fe(Cr) sites is 5. Refinement of the neutron diffraction data indicates y ∼ 0.05. This is in discordance with an earlier report on a material with the same nominal composition and cell constant. Electron energy-loss Cr L(2,3)-edge spectroscopy shows that Cr(3+) is present, which is also contrary to previous speculation. Neutron-pair distribution function studies show that a brownmillerite-like model involving ordered vacancies and alternating octahedral and tetrahedral coordination at the metal sites, gives a better description of the local structure out to ∼5 Å. A remarkable phenomenon determined by neutron diffraction in Sr(2)Fe(1.5)Cr(0.5)O(5) is the occurrence of a long-range G-type antiferromagnetic ordering with T(c) ≈ 565 K because cubic oxygen-deficient perovskites with B-site disorder usually do not undergo transitions to magnetically ordered states. The observation of long-range antiferromagnetic order and the T(c) value are in accordance with previous Mössbauer spectroscopic studies.

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