N. Nguyen

Intercroissance de feuillets “perovskites lacunaires” et de feuillets type chlorure de sodium: Les oxydes La2−xA1+xCu2O6−x2 (A = Ca, Sr)

New compounds La2−xA1+xCu2O6−x2 have been isolated with 0≤x≤0, 14 for A=Sr and x = 0.10 for A=Ca. These compounds crystallize in a tetragonal cell with a ⋍ 3.90 A and c ⋍ 20 A; the possible space group is I4/mmm. The structure of these compounds is derived from that of Sr3Ti2O7: it can be described as an intergrowth of ”oxygen defect double perovskite“ layers and of SrO — type layers. The coordination of the metallic ions is discussed.

MAGNETORESISTANCE IN THE OXYGEN DEFICIENT LNBACO2O5.4 (LN=EU, GD) PHASES

New “112” phases, LnBaCo2O5.4, with an ordered oxygen deficient perovskite structure, derived from the YBaFeCuO5-type were studied for Ln=Eu, Gd. The appearance of giant negative magnetoresistance in this structural type is demonstrated. Resistance ratio R0/R7u2002T reaches at least 10 at 10 K, i.e., is significantly larger than those observed in the other cobalt perovskites, such as La1−xSrxCoO3. These properties are linked to an original magnetic behavior of these materials that exhibit two types of transition—antiferromagnetic to ferromagnetic, and ferromagnetic to paramagnetic—as T increases. This magnetic behavior may be related to a possible I.S. and L.S. spin ordering of trivalent cobalt in pyramidal and octahedral coordinations, respectively.

Oxygen defect K2NiF4-type oxides: The compounds La2−xSrxCuO4−x2+δ

Oxygen defect K2NiF4-type oxides La2−xSrxCuO4−x2+δ have been synthesized for a wide composition range: 0 ≤ x ≤ 1.34. From the X-ray and electron diffraction study three domains have been characterized: orthorhombic compounds with La2CuO4 structure for 0 ≤ x < 0.10, tetragonal oxides similar to LaSrCuO4 for 0.10 ≤ x < 1 and several superstructures derived from the tetragonal cell (a ⋍ n.aLaSrCuO4 with n = 3, 4, 4.5, 5, 6) for 1 ≤ x ≤ 1.34. The compounds corresponding to 0 < x < 1 differ from the other oxides in that they are characterized by the presence of copper with two oxidation states: + 2 and + 3. A model structure for La0.8Sr1.2CuλO3.4, in which copper has only the + 2 oxidation state, and for which the actual cell is tegragonal—a = 18.804 A and c = 12.94 A—has been established. The particular structural evolution of these compounds is discussed in terms of a competition between the capability of Cu(II) to be oxidized to Cu(III) and the ordering of oxygen vacancies.

La2SrCu2O6: Neutron diffraction study

The oxide La2−xSr1+xCu2O6 is one of the rare layered copper oxides which do not exhibit superconducting properties. In order to understand this behavior a detailed neutron diffraction study of La2SrCu2O6 was undertaken. This study confirms the preliminary X-ray diffraction investigation. However it allows several original features concerning the oxygen atoms to be observed: existence of vacancies in the O(1) sites corresponding to the basal planes of the CuO5 pyramids and of extra oxygen on the O(3) site located between the copper layers. An analysis of the anisotropic thermal parameters shows also anomalies of the anisotropic factors s11 of O(2) and s33 of O(1). These features are interpreted in terms of defects which tend to create a three-dimensionnal framework. Morever the examination of the interatomic distances shows that the intercalation of oxygen between the copper planes is really favoured in this structure owing to the bigger size of the cation interleaved between the copper layers compared to the calcium or yttrium in other layered cuprates.

Sr2Mn2O5, an oxygen-defect perovskite with Mn(III) in square pyramidal coordination

Abstract An oxygen-defect perovskite Sr 2 Mn 2 O 5 was isolated by reduction of SrMnO 3−x perovskites in the presence of zirconium. Its structure, similar to that of Ca 2 Mn 2 O 5 , has been determined by X-ray powder diffraction and HREM. The orthorhombic cell has the parameters : a = 5.523(1) A , b = 10.761(5) A , c = 3.811(1) A . The possible space groups are Pbam and Pba2. The framework is built up from corner-sharing MnO 4 pyramids forming pseudo-hexagonal tunne l s running along 〈001〉 and perovskite tunnels running along 〈110〉 and 〈110〉. This oxide is antiferromagnetic with T N ⋍ 380 ± 10 K and θ P ⋍ 300 ± 10 K .

Oxydes ternaires de cuivre a valence mixte de type K2NiF4 deficitaires en oxygene:Evolution progressive d'un etat semi-conducteur vers un etat semi-metallique des oxydes La2−xSrxCuO4−x2+δ

Resume Magnetic and electron transport properties of the mixed valence copper oxides La 2−x Sr x CuO 4−x 2+δ (0 ⩽ x ⩽ 1.20) have been investigated in the temperature range 120–650 K. A progressive evolution of the conductivity from a semi-conductor to a semi-metallic behavior, as the amount of trivalent copper increases, has been observed. The positive sign of the Seebeck coefficient and the evolution of the conductivity and magnetic properties have been interpreted.

Intercroissances des structures de type perovskite et SrO deficitaires en oxygene : Les oxydes Ln2−xSr1+xCu2O6−x/2 (Ln = Sm, Eu, Gd)

Abstract New oxides Ln 2−x Sr 1+x O 6−x/2 (Ln = Sm, EU, GD), corresponding to oxygen deficient intergrowths of double perovskite and SrO layers have been isolated for 0.70 ≤ x ≤ 0.90. They are characterized by an orthorhombic cell, a ⋍ a p ⋍ 3.9 A , b ⋍ 3a p , and c ⋍ 20 A . A structural model has been obtained, showing that this structure, although closely related to that of La 2−x Sr 1+x Cu 2 O 6−x/2+ δ exhibits a different distribution of the oxygen vacancies, involving for copper several coordinations. The semi-conductive properties of these compounds, very different from the semi-metallic behaviour of La 2−x Sr 1+x Cu 2 O 6−x/2+ δ is explained by the distribution of the oxygen vacancies in the structure.

Colossal magnetoresistance properties of samarium based manganese perovskites

Abstract The study of the samarium manganites Sm 1−x (Sr,Ca) x MnO 3 has shown the magnetoresistance properties of this kind of compounds. It allowed us to evidence the effect of the two antagonist effects, interpolated cation size and Mn(IV) : Mn(III) ratio, on the transition temperature T max . By varying these two factors, we were able to get RR ratio of 10 5 at 75K under 5T for Sm 0.58 Sr 0.3 Ca 0.12 MnO 3 . From this study, we have evidenced that the Sm 0.5 Sr 0.5 MnO 3 compound exhibits CMR properties corresponding to a transition from a ferromagnetic metallic state to a paramagnetic semi-conducting state as T increases, contrary to that observed for Pr 0.5 Sr 0.5 MnO 3 and Nd 0.5 Sr 0.5 MnO 3 .

LiFeP2O7 : Structure and magnetic properties

In order to compare the structural and magnetic properties of the diphosphates AFeP2O7 (A=Li, Na, K), the structure of the LiFeP2O7 compound was redetermined and the magnetic measurements of the lithium, sodium and potassium phases were performed. It is shown that LiFeP2O7 does not exhibit any abnormal O-O distances contrary to the results obtained by previous authors. A new description of the structure is proposed. The three compounds exhibit an antiferromagnetic transition at TN ≈ 20 K confirming the results obtained by Hagenmuller et al (6). The paramagnetic temperature θp exhibits a regular evolution which can be explained on the basis of Fe-Fe distances and of the number of nearest Fe3+ ions surrounding one iron cation.

Complex magnetotransport in LaSr2Mn2O7

Abstract A detailed study of the magnetic and electrical transport properties of well characterized samples of the layered manganite LaSr 2 Mn 2 O 7 by a battery of magnetization studies reveal the complex interplay of ferromagnetic and antiferromagnetic interactions in this phase. The preponderantly ferromagnetic (in terms of sign) interactions that set in at around 490 K give way to antiferromagnetism at 230 K as evidenced by low-field studies. Interestingly, the antiferromagnetic transition is coupled with a sharp rise in the resistivity of the samples allowing us to speculate that in this phase, notionally comprising equal amounts of Mn III and Mn IV , the localized holes could be ordered on different lattice sites at low temperatures. The simultaneous presence of the two kinds of interactions in this layered phase give rise to complex electrical transport properties, including a re-entrant insulator at low temperatures with no magnetic signature in the transition. The last feature we ascribe to disorder-induced localization, important in these quasi-2D materials.