Terence C. Gibb

The structural consequences of charge disproportionation in mixed-valence iron oxides. I. The crystal structure of Sr2LaFe3O8.94 at room temperature and 50 K

The crystal structure of the averaged-valence phase of the rhombohedrally distorted perovskite Sr{sub 2}LaFe{sub 3}O{sub 8.94} has been refined from neutron powder diffraction data collected at room temperature (spacegroup R{bar 3}c, a = 5.4712 {angstrom},{alpha} = 60.09{degree}). All iron sites are identical. The crystal and magnetic structures of the low-temperature antiferromagnetic phase showing a charge disproportionation have been similarly obtained at 50 K. The crystal structure has been refined in space group R{bar 3}c, a = 5.4580 {angstrom}, {alpha} = 60.19{degree}; the magnetic structure has been refined in P3m1 using a model derived from considerations of the exchange interactions between ideal localized electron configurations. The results are interpreted in terms of a charge density wave (CDW) and spin density wave (SDW) propagating along the (111) axis of the crystal and commensurate with the crystal lattice. The apparent absence of a periodic structural distortion (PSD) of the same period as the CDW is surprising, and it is suggested that local variations in the strain field brought about by the disordered arrangement of Sr and La may quench the PSD which would otherwise occur.

Spin-glass behavior in Sr2FeRuO6 and BaLaNiRuO6: A comparison with antiferromagnetic BaLaZnRuO6

The crystal structures of the double perovskites BaLaZnRuO6, BaLaNiRuO6, and Sr2FeRuO6 have been refined in space group I2c using neutron powder diffraction data collected at room temperature. Data collected at 5 K led to refinements in I2c for Sr2FeRuO6 and BaLaZnRuO6, and in a triclinic space group for BaLaNiRuO6. BaLaZnRuO6 shows A-type antiferromagnetism at 5 K whereas Sr2FeRuO6 and BaLaNiRuO6 show no evidence of long-range magnetic ordering. Mossbauer and magnetic susceptibility data indicating low temperature transitions (<50 K) in these latter two materials have been interpreted in terms of spin-glass behavior, attributable to the long-range structural disorder of the B-site cations in the perovskite structure.

The structural and magnetic properties of SrMnO3: A reinvestigation

Abstract Neutron diffraction and Mo¨ssbauer spectroscopy have revealed that the phase 4H-SrMnO 3 is paramagnetic at 290 K, contrary to earlier reports. Mo¨ssbauer spectroscopy on 57 Fe-doped samples has revealed antiferromagnetic ordering with T N = 278 ± 5K . The susceptibility maximum at ∼350 K is thought to represent only short-range coupling between Mn 4+ ions in the Mn 2 O 9 pairs of face-sharing octahedra in the structure.

Investigation of magnetic frustration in A2FeMO6(A Ca, Sr, Ba; M Nb, Ta, Sb) by magnetometry and Mössbauer spectroscopy

Perovskite-related compounds in the series A2FeMO6(A Ca, Sr, Ba; M Nb, Ta, Sb) have been prepared using the standard techniques of solid state chemistry and characterized by X-ray powder diffraction, magnetometry and Mossbauer spectroscopy. All but the 6H hexagonal compound Ba2FeSbO6 adopt cubic or pseudo-cubic crystal structures and show a magnetic susceptibility maximum at ca. 25 K. Field cooled and zero-field cooled susceptibility data show a hysteresis below this maximum. The Mossbauer spectra recorded at 4.2 K comprise a single magnetic hyperfine pattern typical of Fe3+, but with a high degree of line broadening. The spectra show a relaxational (nonBrillouin) collapse above 20 K. The data are described in terms of spin-glass behaviour and the factors controlling the magnetic properties are discussed.

Neutron diffraction study of the influence of structural disorderon the magnetic properties of Sr2FeMO6 (M=Ta,Sb)

The crystal structure of the perovskite Sr 2 FeTaO 6 has been refined by simultaneous analysis of X-ray and neutron powder diffraction data collected at 280 K; space group Pbnm, a=5.6204(3), b=5.6161(3), c=7.9266(3) A. The structure is of the GdFeO 3 type, with a disordered distribution of Fe and Ta over the six-coordinate cation sites. The structure of Sr 2 FeSbO 6 has been refined in a similar manner; space group P2 1 /n, a=5.6132(5), b=5.5973(5), c=7.9036(7) A, β=90.01(1)°. The two crystallographically distinct six-coordinate sites in Sr 2 FeSbO 6 are occupied in a partially ordered manner [0.795(6):0.205(6)] by Fe and Sb atoms. Neutron diffraction data collected from Sr 2 FeTaO 6 at 1.5 K show no evidence of long-range magnetic ordering and, in the light of previous susceptibility and Mossbauer measurements, it is concluded that Sr 2 FeTaO 6 is a spin glass below 23 K. Neutron diffraction data collected from Sr 2 FeSbO 6 at 1.5 K include magnetic Bragg peaks characteristic of a type I magnetic structure with an average ordered moment of 3.06(9) µ B per Fe atom on the Fe-dominated octahedral site, and no significant ordered moment on the second site. The magnetic Bragg scattering decreases to zero in the temperature interval 1.5≤T/K≤37(2). It is concluded that the partial cation ordering leads to the coexistence of a magnetically ordered spin system and a spin-glass system.

The crystal and magnetic structures of Sr2CoFeO5

Abstract The crystal and magnetic structures of the brownmillerite Sr 2 CoFeO 5 have been refined from neutron powder diffraction data collected at room temperature, space group Icmm , a = 5.6243(2), b = 15.6515(5), c = 5.5017(2) A. The transition metal ions are partially ordered over the 6- and 4-coordinate cation sites; in addition the 4-coordinate sites themselves have a positional disorder. The compound shows G-type antiferromagnetic order at room temperature with ordered magnetic moments of 3.2 and 2.9 μ B , aligned along z , at the 6- and 4-coordinate sites, respectively. The low magnitude of the ordered moments is discussed.

Ruthenium-99 Mössbauer studies of the magnetic properties of ternary and quaternary ruthenium(IV) oxides

Ruthenium-99 Mossbauer spectra have been obtained for the ternary oxides SrRuO3, CaRuO3, BaRuO3, and Y2Ru2O7 and for the quaternary oxides Sr(Ru1–xIrx)O3(x= 0·1 and 0·2) and Sr(Ru0·7Mn0·3)O3. The spectrum of SrRuO3 reveals the first example of a hyperfine magnetic field in a ruthenium compound; the internal field of 352 kG at 4·2 K is compatible with the ferromagnetic moment derived from neutron-diffraction data, and confirms the collective-electron magnetism model for this perovskite. A more precise value for the E2/M1 mixing ratio of 2·72 ± 0·17 has been obtained for the 99Ru Mossbauer γ transition. The oxide CaRuO3 shows only a single line and hence no magnetic ordering even at 4·2 K; it is not antiferromagnetic as previously reported but is Curie-Weiss paramagnetic. The oxide BaRuO3 also gives a singlet; there is no long-range antiferromagnetic ordering, but within each cluster of three face-sharing RuO6 units localised pairing of the 4d electrons takes place. The oxide Y2Ru2O7 is magnetically ordered at 4·2 K, with a hyperfine field of 126 kG. Substitution of ruthenium by iridium in the oxide SrRuO3 reduces the field at ruthenium by approximately 60 kG for each iridium nearest neighbour, whereas in the case of manganese substitution the reduction for each manganese neighbour is only 22 kG.

The crystal and magnetic structures of Sr2LaFe3O8

Abstract The crystal and magnetic structures of the anion-deficient perovskite Sr 2 LaFe 3 O 8 (space group Pmma , a = 5.5095(1), b = 11.8845(5), c = 5.6028(1) AA) have been refined from X-ray and neutron powder diffraction data collected at room temperature. The crystal structure consists of layers of octahedral (O) and tetrahedral (T) iron-oxygen polyhedra arranged in the stacking sequence … OOTOOT … perpendicular to theyˆaxis of the unit cell. The magnetic structure is that of a G-type antiferromagnet with ordered magnetic moments of 3.77(5) and 3.15(11) μ B at the octahedral and tetrahedral sites, respectively. The low moment at the tetrahedral site is consistent with the observed disorder and magnetic anisotropy.

A study of the new perovskite solid solution series SrFexRu1−xO3−y by ruthenium-99 and iron-57 Mössbauer spectroscopy

Abstract The magnetic and structural properties of the solid solution SrFexRu1−xO3−y (0 ⩽ x ⩽ 0.5) have been studied using 57Fe and 99Ru Mossbauer spectroscopy and other techniques. These phases, which are here reported for the first time, have a distorted perovskite structure. The iron substitutes exclusively as Fe3+ and thereby causes oxygen deficiency, but has little effect on the magnetic behaviour of the Ru4+ until x > 0.2, whereupon the metallic band system begins to revert to a localized electron structure. The properties of a sample with x = 0.3 are complex and intermediate in character. For x > 0.3 the oxygen deficiency is reduced by substantial oxidation to Ru5+ until at x = 0.5 the system corresponds to Sr2Fe3+Ru5+O6.

The crystal structure and magnetic properties of SrLaFeSnO6 and SrLaNiSbO6

The crystal and magnetic structures of the perovskite-related materials SrLaFeSnO6 and SrLaNiSbO6 have been refined using neutron diffraction data collected at room temperature and 1.7 K. SrLaFeSnO6 adopts the orthorhombic space group Pbnm with disordered SrLa and FeSn cations and is magnetically ordered as a G-type antiferromagnet at 1.7 K. SrLaNiSbO6 differs in that a partial order of the Ni and Sb cations results in the space group P21n and a Type I antiferromagnetic structure. The magnetic susceptibility shows a maximum at 39 and 26 K, respectively, but there is a significant difference between the field-cooled and zero-field-cooled susceptibility above these temperatures. This effect persists to at least 250 K in SrLaFeSnO6. An explanation is developed in terms of magnetic clusters which increase in size with decreasing temperature until a long-range ordered structure is finally achieved.