Jean Michel Dance

Characterization of six-coordinated iron (V) in an oxide lattice

Abstract On the basis of a discussion of the correlations between the stability of the d 3 configuration and the symmetry and size of the cation site, six-coordinated iron (V) has been stabilized in the La 2 LiFeO 6 perovskite-type oxide and characterized by magnetic susceptibility, Mossbauer resonance and EPR measurements.

Sur la stabilisation dans un reseau oxygene du nickel (+III) a spin fort en octaedrique

Abstract The authors show that increasing the size of the octahedral site in K 2 NiF 4 − and perovskite-type oxides favours the high spin configuration of Ni (+III). This electronic configuration appears for the first time in BaLaNiO 4 and Sr 2 NbIn 0 . 9 Ni 0 . 1 O 6 thanks to the size increase brought about by the presence in the lattice either of baryum or indium. A low spin-high spin transition is observed at rising temperature by an EPR investigation and discussed in terms of covalency and symmetry.

Comparative study based on bond competition considerations of the electronic configuration of nickel(III) in the K2NiF4-type oxides Sr0.50La1.50Mg0.50Ni0.50O4 and Sr1.50La0.50Ti0.50Ni0.50O4

Abstract Two new K2NiF4-type oxides Sr0.50La1.50Ni0.50Mg0.50O4 and Sr1.50La0.50Ti0.50Ni0.50O4 have been prepared. X-ray diffraction, magnetic measurements, and EPR investigations characterize a low-spin configuration in the octahedral sites of low spin Ni(III) (t62ge1g). The nature of the distortion has been determined by comparing theg∖ andg⊥ values at room temperature. While a strong elongation of the NiO6 octahedra has been observed for La2Li0.50Ni0.50O4, quasi-regular octahedra can be expected for Sr0.50La1.50Mg0.50Ni0.50O4 (g∖ ⋍ g⊥) and compressed ones seem to occur in Sr1.50La0.50Ti0.50Ni0.50O4 (g∖ > g⊥). An explanation of this structural evolution is proposed based on bond competition considerations.

Etude de la structure magnetique et de la transition de typr “spin-flop” de Rb2FeF5

Abstract The nature of the magnetic interactions in the chain compound Rb2FeF5 has been investigated using neutron diffraction and magnetic measurements under high applied fields. Rb2FeF5 orders antiferromagnetically at TN = 8.0 ± 0.5 K; the magnetic structure is of the AZ + GX mode and the moment of the Fe3+ ion extrapoled to 0K is 3.5 ± 0.2 μB, this low value being due to zero-point spin reduction. Within a chain the Fe3+ ions are antiferromagnetically coupled with an exchange constant of J/k = −8.8 K. A spin-flop behavior has been observed and interpreted on the basis of the molecular field theory. The critical field was found to be HC = 65 kOe at 1.7 K.

Un exemple de chaînes antiferromagnetiques alternees de spin : CuWO4

Abstract The ordering of Cu and W atoms in the distorted wolframite structure of CuWO 4 gives rise to zigzag chains of edge sharing CuO 6 octahedra linked by similar chains of WO 6 octahedra. The existence of two kinds of connections between the CuO 6 octahedra leads to alternating antiferromagnetic interactions along the chains. The magnetic behavior is explained using a Heisenberg type model of Duffy and Barr with the largest exchange integral : J = − 67 K and an alternation parameter : a = 0.8. k B EPR measurements have shown that threedimensional ordering occurs at T N ⋍ 24 K .

Influence of competing bonds along the c-axis on the local distortion of the (CoO6) octahedron in K2NiF4-type oxides A′0.50La1.50Mg0.50Co0.50O4 (A′ = Ca, Sr and Ba)

Abstract The local distortion of the (CoO 6 ) octahedron in the K 2 NiF 4 -type oxides with the general formula A 0.50 La 1.50 Mg 0.50 Co 0.50 O 4 (A = Ca, Sr, Ba) was investigated by E.P.R. and Mossbauer resonance spectroscopies. The increase of the ionicity of the average [(A 0.25 La 0.75 )-0] bond from A = A to Ba induces an enhancement of the covalency of the competing (Co-O) bond along the c-axis, leading to a decrease of the elongation for such a (CoO 6 ) octahedron in the lattice.

Palladium compounds with +III oxidation state

Abstract The trivalent oxidation state of palladium has been stabilized and unequivocally characterized in several fluoride series. NaPdF 4 , whose structure can be derived from the KBrF 4 type, is obtained under high pressure and is quenched back to ambient conditions. Elpasolite compounds of A 2 BPdF 6 formulation (A, B = alkali metal) are synthesized via solid state reactions. Between 200 and 500 K every A 2 BPdF 6 compound undergoes a tetragonal cubic phase transition. The ESR spectra are characteristic of a doublet ground state. The t 6 2g e 1 g configuration of Pd(+III) is stabilized versus the t 5 2g e 2 g high-spin alternative by an important Jahn-Teller splitting of the 2 E g state. In most cases ESR spectra are consistent with tetragonally elongated PdF 6 octahedra; an orthorhombic component occurs for Cs 2 KPdF 6 and Cs 2 RbPdF 6 . ESR spectra with tetragonal (a-K 2 NaPdF 6 ) and orthorhombic (b-Cs 2 KPdF 6 ) symmetries.

Ferromagnetic ordering in the MMnF6 series (M = Ni, Cu, Zn, Pd)

Abstract Neutron diffraction experiments performed on MMnF 6 compounds (M = Ni, Cu, Zn, Pd) have shown that both NiMnF 6 and ZnMnF 6 crystallize in the rhombohedral R 3 space group with a cationic ordering, whereas PdMnF 6 shows cationic disordering (R 3 c, a = 5.46 A, α = 54.45°). CuMnF 6 has a monoclinic distortion derived from the MnF 3 structural type ( a = 8.57 A, b = 4.85 A, c = 13.46 A, β = 92.12°). The magnetic properties can be correlated with the structure: the R 3 ordered compounds show a ferromagnetic behavior ( T C (NiMnF 6 ) = 39 K; T C (ZnMnF 6 ) = 9.5 K; T C (CdMnF 6 ) = 8 K), while the other two have an antiferromagnetic ordering.

High-pressure synthesis of cubic perovskite CaLaMgMnO5.5+x

Oxygen-deficient perovskite CaLaMgMnIV1−xMnVIxO5.5+x (indicated by P) has been prepared at 930°C for 10 mn using high oxygen pressure (60 kbar). X-ray diffraction study indicates that P has a simple cubic unit cell (a = 3.923A). The average oxidation state of Mn in P was determined by redox titration before (4.13) and after (4.03) thermal treatments at 300, 900, and 1100°C and was confirmed by thermogravimetry analysis. Two paramagnetic configurations are observed from the EPR spectrum: One isotropic signal characterized by g = 1.994, ΔH = 950 G, and an anisotropic signal with g⊥ = 1.978, g‖ = 1.938. They correspond respectively to octahedrally coordinated Mn(IV) and to tetrahedrally coordinated Mn(VI). According to magnetic susceptibility measurements, the constant spin-only moment observed between 77 and 650 K (μeff = 3.8 μB) is consistent with a Mn(IV)—4A2g ground term—(μeff = 3.87 μB). Such behavior can be explained by the low Mn(VI) content in P.

A new perovskite-type oxide (BaLa)(MgMn)O6−x(x≅0.21) prepared under high oxygen pressure and its physico-chemical characterization

Abstract A new cubic perovskite compound BaLaMgMnO 6− x has been prepared under high temperature (900°C) and high oxygen pressure (60–70 kbar) using in situ thermal decomposition of KClO 3 in a belt apparatus. X-Ray diffraction (XRD), redox titration, thermogravimetric analysis (TGA), and X-ray photoelectron (XPE) spectroscopy investigation have shown that x is close to 0.21 under these synthetic conditions. Mn(V) in octahedral coordination could be postulated.