Marina G. Rozova

Inducing superconductivity and structural transformations by fluorination of reduced YBCO

Abstract Bulk superconductivity with Tc up to 94 K has been induced by fluorination of non-superconducting YBa2Cu3O6.11 using XeF2 as a fluorination agent. Strong changes on X-ray patterns were found after fluorination of reduced YBCO. High resolution electron microscopy of superconducting samples showed the presence of a new phase with c ≈ 13 A which exists as noticeable areas included within a matrix of the 123 structure or which occurs as isolated defects with a limited extension. All fluorinated compounds exhibited a strong disorder along the c-direction resulting in a ‘loss’ of c-parameter on X-ray patterns. The present results demonstrate that fluorine indeed enters the YBCO structure with a significant structural rearrangement for the high level of fluorination. The structure of the fully fluorinated YBa2Cu3O6F2 phase, possibly responsible for superconductivity, has been deduced from high resolution electron microscopy.

Synthesis and crystal structure of the Sr2Al1.07Mn0.93O5 brownmillerite

A new brownmillerite-type compound Sr2Al1.07Mn0.93O5 was synthesized. The crystal structure was determined using electron diffraction and high resolution transmission electron microscopy and refined from X-ray powder diffraction data (space group Imma, a = 5.4358(1) A, b = 15.6230(4) A, c = 5.6075(1) A, RI = 0.036, RP = 0.023). The structure is characterized by a disordered distribution of the tetrahedral chains in L and R configuration and a partial occupation of the octahedral position by the Mn3+ and Al3+ cations. The relationships between the crystal structures of Sr2Al1.07Mn0.93O5 and its A2B′MnO5 analogues (A = Ca, Sr, B′ = Al, Ga) and the structural reasons for the different types of tetrahedral chain ordering in brownmillerites are discussed. The temperature dependences of the magnetic susceptibility and specific heat reveal that the compound is antiferromagnetically ordered below TN = 105 K.

Slicing the Perovskite Structure with Crystallographic Shear Planes: The AnBnO3n-2 Homologous Series

A new A(n)B(n)O(3n-2) homologous series of anion-deficient perovskites has been evidenced by preparation of the members with n = 5 (Pb(2.9)Ba(2.1)Fe(4)TiO(13)) and n = 6 (Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16)) in a single phase form. The crystal structures of these compounds were determined using a combination of transmission electron microscopy and X-ray and neutron powder diffraction (S.G. Ammm, a = 5.74313(7), b = 3.98402(4), c = 26.8378(4) Å, R(I) = 0.035, R(P) = 0.042 for Pb(2.9)Ba(2.1)Fe(4)TiO(13) and S.G. Imma, a = 5.7199(1), b = 3.97066(7), c = 32.5245(8) Å, R(I) = 0.032, R(P) = 0.037 for Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16)). The crystal structures of the A(n)B(n)O(3n-2) homologues are formed by slicing the perovskite structure with (101)(p) crystallographic shear (CS) planes. The shear planes remove a layer of oxygen atoms and displace the perovskite blocks with respect to each other by the 1/2[110](p) vector. The CS planes introduce edge-sharing connections of the transition metal-oxygen polyhedra at the interface between the perovskite blocks. This results in intrinsically frustrated magnetic couplings between the perovskite blocks due to a competition of the exchange interactions between the edge- and the corner-sharing metal-oxygen polyhedra. Despite the magnetic frustration, neutron powder diffraction and Mössbauer spectroscopy reveal that Pb(2.9)Ba(2.1)Fe(4)TiO(13) and Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16) are antiferromagnetically ordered below T(N) = 407 and 343 K, respectively. The Pb(2.9)Ba(2.1)Fe(4)TiO(13) and Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16) compounds are in a paraelectric state in the 5-300 K temperature range.

Synthesis and investigation of novel Mn-based oxyfluoride Sr2Mn2O5–xF1+x

A new oxyfluoride Sr 2Mn2O5–xF1+x was synthesized by low temperature fluorination of Sr 2Mn2O5 with XeF2 and studied using X-ray powder diffraction, electron diffraction and high resolution electron microscopy. The oxyfluoride has a tetragonal perovskite-like structure with cell parameters a = 3.8069(8) A, c = 3.989(2) A (S.G. P4/mmm). No additional superstructure or lattice distortions were detected by electron microscopy. The Mn oxidation state was evaluated using bond valence sum calculations as close to VMn =+ 3.5. The tetragonal distortion of the perovskite structure arises from the Jahn–Teller deformation of the Mn(O,F) 6 octahedra.  2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Synthesis and crystal structure of the Sr_{2}Al_{1.07}Mn_{0.93}O_{5} brownmillerite

A new brownmillerite-type compound Sr2Al1.07Mn0.93O5 was synthesized. The crystal structure was determined using electron diffraction and high resolution transmission electron microscopy and refined from X-ray powder diffraction data (space group Imma, a = 5.4358(1) A, b = 15.6230(4) A, c = 5.6075(1) A, RI = 0.036, RP = 0.023). The structure is characterized by a disordered distribution of the tetrahedral chains in L and R configuration and a partial occupation of the octahedral position by the Mn3+ and Al3+ cations. The relationships between the crystal structures of Sr2Al1.07Mn0.93O5 and its A2B′MnO5 analogues (A = Ca, Sr, B′ = Al, Ga) and the structural reasons for the different types of tetrahedral chain ordering in brownmillerites are discussed. The temperature dependences of the magnetic susceptibility and specific heat reveal that the compound is antiferromagnetically ordered below TN = 105 K.

Effect of fluorination and high pressure on the structure and properties of the Hg-bearing superconducting Cu mixed oxides

Abstract The T c variation of HgBa 2 CuO 4+ δ (Hg-1201) and HgBa 2 CuO 4 F δ can be achieved by a change in the carrier concentration and by a compression of the structure under high pressure. Oxygenated and fluorinated series exhibit a cupola-shaped behavior for the T c vs. δ dependence, but the curves are shifted away from each other along the δ axis. NPD showed double amount of extra fluorine in comparison with extra oxygen for the oxygenated Hg-1201 phases with close T c s. An exchange of the extra oxygen by a double amount of fluorine causes a significant compression of the apical CuO bond distances, while the in-plane ones, as well as T c , do not vary. Fluorination of Hg-1223 resulted in a slight increase in T c in comparison with oxygenated material. The influence of the external pressure on the structure and T c of Hg-1201 strongly depends on the doping level. An increase in the extra oxygen content from underdoped to overdoped state results in the larger compression of the apical CuO and BaO Hg distances while the HgO 2 dumbbell as well as a distance between Ba and O from the (CuO 2 ) layers become practically pressure independent.

Structural transformation in fluorinated LaACuGaO5 (A=Ca, Sr) brownmillerites

Abstract The brownmillerite type phases LaSrCuGaO 5 and LaCaCuGaO 5 were fluorinated using XeF 2 and the resulting materials were investigated using X–ray diffraction, high resolution electron microscopy (HREM) and electron diffraction. For low content of XeF 2 , both materials undergo a decrease in orthorhombicity maintaining an a O ≈4 a P , b O ≈ a P 2 , c O ≈ a P 2 unit cell with space group Ima2 . On increasing the amount of XeF 2 , LaSrCuGaO 5 transforms to a tetragonal structure with cell parameters, a T ≈ a P , c T ≈2 a P (space group P4 / mmm ). The model for the tetragonal structure was proposed on the basis of HREM observations. The anion exchange reaction results in the replacement of one oxygen anion by two fluorines in the (GaO□) layers. This reaction is accompanied by a random occupation of the anion positions in the Ga-layers by oxygen, fluorine and anion vacancies, leading to a tetragonal symmetry. Different superstructures, due to local ordering of anions and anion vacancies, were observed by electron microscopy.

Structural and Magnetic Phase Transitions in the AnBnO3n-2 Anion-Deficient Perovskites Pb2Ba2BiFe5O13 and Pb1.5Ba2.5Bi2Fe6O16

Novel anion-deficient perovskite-based ferrites Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 were synthesized by solid-state reaction in air. Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 belong to the perovskite-based A(n)B(n)O(3n-2) homologous series with n = 5 and 6, respectively, with a unit cell related to the perovskite subcell a(p) as a(p)√2 × a(p) × na(p)√2. Their structures are derived from the perovskite one by slicing it with 1/2[110]p(101)p crystallographic shear (CS) planes. The CS operation results in (101)p-shaped perovskite blocks with a thickness of (n - 2) FeO6 octahedra connected to each other through double chains of edge-sharing FeO5 distorted tetragonal pyramids which can adopt two distinct mirror-related configurations. Ordering of chains with a different configuration provides an extra level of structure complexity. Above T ≈ 750 K for Pb2Ba2BiFe5O13 and T ≈ 400 K for Pb(1.5)Ba(2.5)Bi2Fe6O16 the chains have a disordered arrangement. On cooling, a second-order structural phase transition to the ordered state occurs in both compounds. Symmetry changes upon phase transition are analyzed using a combination of superspace crystallography and group theory approach. Correlations between the chain ordering pattern and octahedral tilting in the perovskite blocks are discussed. Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 undergo a transition into an antiferromagnetically (AFM) ordered state, which is characterized by a G-type AFM ordering of the Fe magnetic moments within the perovskite blocks. The AFM perovskite blocks are stacked along the CS planes producing alternating FM and AFM-aligned Fe-Fe pairs. In spite of the apparent frustration of the magnetic coupling between the perovskite blocks, all n = 4, 5, 6 A(n)Fe(n)O(3n-2) (A = Pb, Bi, Ba) feature robust antiferromagnetism with similar Néel temperatures of 623-632 K.

Structural features, oxygen and fluorine doping in Cu-based superconductors

The variation of structures and superconducting properties by changing extra oxygen or fluorine atoms concentration in Hg-based Cu mixed oxides and YBa2Cu3O6+δ was studied. The data obtained by NPD study of Hg-1201 can be considered as an evidence of the conventional oxygen doping mechanism with 2δ holes per (CuO2) layer. The extra oxygen atom was found to be located in the middle of the Hg mesh only. Different formal charges of oxygen and fluorine inserted into reduced 123 structure results in its distinct variations. The fluorine incorporation into strongly reduced YBa2Cu3O6+δ causes a significant structural rearrangement and the formation of a new compound with a composition close to YBa2Cu3O6F2 (tetragonal a=3.87A and c≈13 A), which structure was deduced from the combined results of X-ray diffraction, electron diffraction and high resolution electron microscopy. Fluorination treatment by XeF2 of nonsuperconducting 123 samples causes an appearance of bulk superconductivity with Tc up to 94K.

Synthesis and structure of new double oxides of mercury and rare earth elements: R2HgO4 (R =La, Nd, Sm, Eu, and Gd)

Abstract New double oxides with mercury and five rare earth elements, R2HgO4 (R =La, Nd, Sm, Eu, and Gd), were synthesized and characterized by X-ray powder diffraction, EDS analysis, electron microscopy, and thermal analysis. They all have the structure which was determined for one of the compounds, Nd2HgO4, from powder diffractometer data. Nd2HgO4 has a monoclinic unit cell, space group C2/m, Z = 4, and the cell parameters a = 13.8737(8)A, b = 3.7936(4)A, c = 10.240(2)A, and β = 121.200(9)°. The mercury atoms are linearly coordinated to two oxygen atoms, and the two independent neodymium atoms coordinate seven oxygen atoms, which form polyhedra of slightly different shapes and sizes.