Effet Jahn-Teller cooperatif et affinite tétraedrique des ions Mn2+ et Zn2+ dans le système Mn3O4Zn2SnO4

Comparison between synthesis in air and in vacuum of the solid solution tMn₃O₄ + (1 ? t)Zn₂SnO₄, and cristallographic study of the nonoxidized compounds allowed us to establish the distribution and the electronic configuration of cations in tetrahedral (A) and octahedral (B) sites. The competitive aspect of Zn²⁺ and Mn²⁺ ions to occupy tetrahedral site is discussed. In air, the non-oxidizable character of Mn²⁺ on an A-site is clearly borne out, whereas the B-site displaced manganese oxidizes to Mn³⁺. In vacuum, the critical concentration of Mn³⁺ ion at the octahedral site, involving a cooperative Jahn-Teller effect, is about 50%.An important fact has also been put forward: the microscopic distortion of the oxygen octahedra, which the ratio of long and short anion-cation distances expresses, is equal to the unit-cell macroscopic deformation that the ratio $\text{c}\text{a}\text{√2}$ represents.     

Study of Pr1−xMn1+xO3 perovskites

The structural and magnetic properties of the Pr_1?xMn_1+xO₃ perovskites were studied. The increase of x (i.e., $\text{Pr}\text{Mn}\text{< 1}$) leads to the decrease of the orthorhombic deformation and of the Néel temperature and, simultaneously, to an increase of the ferromagnetic contribution. The latter effect is explained from the suggested distribution of the cations (Pr³⁺_1?xMn²⁺_x)_A(Mn³⁺_1?xMn⁴⁺_x)O^2?₃ by the double exchange of Mn³⁺Mn⁴⁺ pairs at the B—sublattice.     

Synthèse et caractérisation d'une série de titanates pérowskites isotypes de [CaCu3](Mn4)O12

A series of titanates which have perovskite-like arrangements and are isostructural with [CaCu₃](Mn₄)O₁₂ have been synthesized. The total charge of the A sites can be modified (1) by substituting the Ca²⁺ cations with monovalent ones and the tetravalent manganese cations of the B sites by a mixture of (Ti⁴⁺ + M⁵⁺) in which M = Ta, Nb, Sb, or (2) by substituting the Ca²⁺ cations by a combination of cations plus vacancies. In this case, if the total charge of the A sites is 2, one obtains compounds such as $\text{[}\text{Th}{}^{\mathrm{4+}}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{□}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{Cu}{}_3\text{](Ti}{}_4\text{)O}{}_{12}$ and $\text{[T}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{□}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{Cu}{}_3\text{](Ti}{}_4\text{O}{}_{12}\text{(T =}\text{rare earth}\text{)}$; on the contrary, if the charge is less than 2, then one has to compensate it by changing that of the B sites. This leads to compounds such as [□Cu₃](Ti₂M₂)O₁₂ (M = Ta, Nb, Sb).     

Synthesis,composition, and magnetic properties of the ferrimagnetic NdCu3−xMn4+xO12 perovskite-like phases

A magnetic oxide with composition close of NdCu₃Mn₄O₁₂ with a perovskite-related cubic structure ($\text{a ? 7.30}$ Å, space group Im3, Z = 2) has been synthesized by using either the high-pressure or the hydrothermal technique. The composition is strongly dependent on the synthesis conditions. A partial reduction of Mn⁴⁺ in the octahedral sites, resulting in a partial substitution of Cu²⁺ by Mn³⁺ in the Jahn-Teller sites, leads to the actual formula Nd(Cu²⁺_3?xMn³⁺_x)(Mn⁴⁺_3?xMn³⁺_1+x)O₁₂. For the compound synthesized at 650°C/2 kbar, the value of the substitution parameter x, as determined by neutron diffraction, is 0.32. For samples synthesized at higher temperatures, larger values of x are obtained. The compound is ferrimagnetic with Néel temperature of 390 K and a spontaneous magnetization of 93 emu/g at 4 K (52 emu/g at room temperature). For larger x values, magnetizations up to 118 emu/g at 4 K are obtained.     

The chemical oxidation and electronic spectra of the complexes trans-[M(CNR)2(Ph2PCH2CH2PPh2)2] (M = Mo or W)

Oxidation of the complexes $\text{trans}$-[M(CNR)₂(dppe)₂] (A) (M = Mo or W; R = Me, Bu^t or CH₃C₆H₄-$\text{4}$; dppe = Ph₂PCH₂CH₂PPh₂) with diiodine or silver (I) salts gives the paramagnetic cations $\text{trans}$-[M(CNR)₂(dppe)₂]⁺, (M = Mo, R = CH₃C₆H₄-$\text{4}$; M = W, R = Bu^t) and $\text{trans}$-[M(CNR)₂(dppe)₂]²⁺ (M = Mo, R = Me or CH₃C₆H₄-$\text{4}$; M = W, R = Me or Bu^t). Mixtures of products are generally produced when dichlorine or dibromine are the oxidising agents, however pure salts, the seven-coordinate complex cations [MX(CNC₆H₄CH₃-$\text{4}$)₂(dppe)₂]⁺ (B, X = Cl or Br) have been isolated. A simple molecular orbital scheme is proposed for complexes (A) and used to discuss their electronic spectra and their oxidation.     

Synthesis and crystal structure of (ThCu3)(Mn3+2Mn4+2)O12, a new ferrimagnetic perovskite-like compound

Single crystals of [ThCu₃](Mn³⁺₂Mn⁴⁺₂]O₁₂, a ferrimagnetic perovskite-like compound, have been synthesized by hydrothermal conditions at 600°C and 2 kbar. They have been found to be cubic, of space group Im3, with a = 7.359 Å, and isostructural with [NaMn₃](Mn³⁺₂Mn⁴⁺₂)O₁₂. The crystal structure has been refined by single-crystal X-ray diffraction data. The Th⁴⁺ cations are surrounded by slightly distorted icosahedra; the ThO distance is 2.556 Å. The Cu²⁺ cations are also surrounded by 12 oxygens, which are arranged as three mutually perpendicular rectangles of different size, the smallest and the largest of which are almost squares. The three sets of CuO distances are 1.973, 2.800, and 3.238 Å. The octahedral MnO distance is 1.950 Å. A test based on neutron diffraction powder data indicated that the square sites are occupied by only the Cu²⁺ cations.     

Ln(GaM2+)O4 and Ln(AlMn2+)O4 compounds having a layer structure [Ln = Lu,Yb, Tm,Er, Ho,and Y,and M = Mg,Mn, Co,Cu, and Zn]

A series of new compounds Ln(GaM²⁺)O₄ and Ln(AlMn²⁺)O₄ having a layer structure were successfully prepared [Ln = Lu, Yb, Tm, Er, Ho, and Y, and M = Mg, Mn, Co, Cu, and Zn]. The synthesis conditions and the unit cell parameters for 23 compounds have been determined. These compounds are isostructural with YbFe₂O₄ (space group $\text{R}\text{3}\text{m, a = 3.455(1)}$ Å, and c = 25.109(2) Å).     

Cation ordering in Ca2La8(SiO4)6O2

The distribution of La³⁺ and Ca²⁺ over the cation sites in Ca₂La₈(SiO₄)₆O₂ was determined by single-crystal X-ray diffraction. Ca₂La₈(SiO₄)₆O₂ has the apatite structure, and all available evidence indicates that the space group is $\text{P6}{}_3\text{m}$, thus precluding a completely ordered structure. The 6h lattice sites are occupied by La³⁺. In contrast, the 4f sites are occupied equally by La³⁺ and Ca²⁺ ions. Consideration of the properties of the La³⁺ and Ca²⁺ ions suggests that this distribution is thermodynamically favored for this composition. A simple Ising model suggests ordered columns. These would not be precluded by space group $\text{P6}{}_3\text{m}$, if the correlation between adjacent columns were random.     

Fluorescence quantum yields and cascade-free lifetimes of state selected CO+2, COS+, CS+2 and N2O+ determined by photoelectron—photon coincidence spectroscopy

The details and principles of an apparatus built for measurements of fluorescence quantum yields and cascade-free lifetimes of open-shell cations are reported. These rely on the detection of coincidences between energy selected photoelectrons and undispersed photons. The results of such measurements for CO⁺₂, COS⁺, CS⁺₂ and N₂O⁺ in selected vibrational levels of their excited states are presented. Non-unity fluorescence quantum yields are found for some vibronic levels of CO⁺₂($\text{B}$), COS⁺ ($\text{A}$), N₂OP⁺($\text{A}$) and a non-exponential decay is observed for CS⁺₂($\text{A}$). The data yield the following values for the radiative lifetimes: CO⁺₂($\text{A}$) 124 ± 6 ns, CO⁺₂($\text{B}$) 140 ± 7 ns, COS⁺($\text{A}$) 550 ± 50 ns and N₂O⁺($\text{A}$) 240 ± 12 ns.     

C2O(X3Σ−): absolute reaction rates measured by laser induced fluoresence

Absolute rate constants are reported for reactions of C₂O($\text{X}\text{?}$³Σ^?) under pseudo-first-order decay conditions. C₂O is generated by laser photodissociation of C₃O₂ at 266 nm, and detected by dye-laser induced fluorescence on the $\text{A}\text{?}$³Π_i-$\text{X}\text{?}$³Σ^? transition. Rate constants of (433 ± 12), (3.30 ± 0.12) and (1.12 ± 0.05) × 10^?13 cm³ molecule^?1 s^?1 are reported for reactions with NO, O₂ and isobutene. The NO value is approximate due to an apparent dark reaction between NO and C₃O₂. Upper limits of 1 × 10^?14 cm³ molecule^?1 s^?1 are reported for reactions with H₂, CO₂, C₃H₂ and C₂H₄. The C₂O + C₃O₂ reaction does not follow pseudo-first-order decay kinetics. Two explanations are proposed to explain this observation. Results are compared with previous relative rate measurements and are discussed in terms of their relevance to combustion chemistry.     

Zur Kinetik der Spinellbildung von β-Ga2O3 mit zweiwertigen Oxiden II. Tracerdiffusion von 63Ni2+ und 67Ga3+ im Nickel-Gallium-Spinell

The self-diffusion coefficients of ⁶³Ni²⁺ and ⁶⁷Ga³⁺ have been measured in nickel-gallium spinels of the general formula $\text{Ni}{}_{\mathrm{1?y}}\text{Ga}{}_{\mathrm{<mtext>2+</mtext><mtext>2y</mtext><mtext>3</mtext>}}\text{O}{}_4$ between 1653 and 1838 K using the restactivity method. Thin radioactive oxide layers are produced on inactive sample surfaces by sedimentation from aqueous solution. The sample composition corresponds to the solubility limit of NiO, respectively, β-Ga₂O₃ in the spinel lattice.It has been concluded that diffusion of Ni²⁺-and Ga³⁺-ions occurs via vacant cation sites.The values obtained for the two cations, which are in the same order of magnitude, are used to calculate the rational reaction constants for NiGa₂O₄ formation after Schmalzried.There is a satisfying agreement between these reaction constants and those values determined by thickness measurement of spinel reaction layers; the agreement between values of activation energy is very good.It is suggested that NiGa₂O₄ formation proceeds after the Wagner mechanism of counterdiffusion of cations.     

Mecanismes d'isomerisation de l'ion C6H5COC3H7+• en phase gazeuse

Isomerisation reactions of 1-phenyl 1-butanone X^+? ($\text{1}$) and five other C₁₀H₁₂O^+? ions are demonstrated to proceed via a single intennediate ($\text{a}$); the H₂O elimination occurs from the tetralol structure ($\text{3}$).     

Optical and ESR investigations of lanthanum aluminates LaMg1−xMnxO19 single crystals with magnetoplumbite-like structure

New lanthanum aluminates LaMAl₁₁O₁₉ (M²⁺ = Ni, Co, Mn, Mg_1?xMn_x, 0 ≤ x ≤ 1), with magnetoplumbite-like structure have been obtained as single crystals. This paper is particularly devoted to the Mn²⁺ and $\text{Mg}{}^{\mathrm{2+}}\text{Mn}{}^{\mathrm{2+}}$ mixed compounds, which exhibit promising luminescent properties. Several characteristics of the crystals are given. The absorption spectra of the materials, as grown, are assigned to Mn²⁺ ions in tetrahedral sites. After annealing in air new absorptions attributed to octahedral Mn³⁺ ions, appear. The ESR spectra of Mn²⁺ in all these crystals exhibit axial symmetry. For x ≤ 0.25 they arise from isolated Mn²⁺ ions in slightly distorted tetrahedral sites and reveal a strong disorder effect. For x ≥0.5 the spectra consist of a single line, attributed to clusters of magnetically interacting Mn²⁺ ions.     

Phase diagram and some physical properties of V2O3+x1 (0 ? x ? 0.080)

The magnetic and electric properties of V₂O_3+x were investigated by measurements of magnetic susceptibility, electrical resistivity, magnetotorque, Mössbauer of doped ⁵⁷Fe, and NMR of ⁵¹V, and the results were compared with those of the (V_1?xTi_x)₂O₃ system or highly pressured V₂O₃. The results obtained are as follows: (1) The metallic state shows an antiferromagnetic ordering at T_N (x). The value of T_N for metallic V₂O₃, obtained by interpolation to x = 0, shows the coincidence between V₂O_3+x and the (V_1?xTi_x)₂O₃ system. (2) Magnetic susceptibility of V₂O_3+x is expressed as χ_M(V₂O_3+x) = (1?x)χ_M(V³⁺) + xχ_M(V⁴⁺). χ_M(V⁴⁺) obeys the Curie-Weiss law $\text{(χ}{}_{\mathrm{<mtext>M</mtext>}}\text{(}\text{V}{}^{\mathrm{4+}}\text{) =}\text{0.77}\text{T}\text{+ 17)}$. (3) In the insulating phase, the electrical resistivity ? is expressed as a common equation: $\text{? = 10}{}^{\mathrm{?1.8}}\text{exp}\text{(}\text{E}\text{kT}\text{)}$. This implies that the substitution of Ti or nonstoichiometry (V⁺⁴ + metal vacancies) has little influence on the carrier mobility (or bandwidth). (4) There is a critical length in the c-axis (? 14.01 Å) where the metal-insulator transition takes place. This suggests that the length of the c-axis plays an important role in the metal-insulator transition of V₂O₃-related compounds.     

A pentagonal tunnel structure with copper in square planar coordination: The oxides KCuNb3O9 and KCuTa3O9

The structure of two new oxides KCuTa₃O₉ and KCuNb₃O₉ has been solved from X-ray powder data and by electron microscopy. Both compounds are orthorhombic, space group Pnc2 with $\text{a ? 8.8}$ Å, $\text{b ? 10.1}$ Å, and $\text{c ? 7.6}$ Å. Their host lattice is built up from corner-sharing MO₆ octahedra (M = Nb, Ta) forming pentagonal tunnels where the K⁺ ions are located. The copper ions are located in distorted perovskite CaCu₃Mn₄O₁₂-type cages and exhibit a square planar coordination. The relationships between these oxides and the TTB, HTB, ITB, and Ba_0.15WO₃ structures are discussed.     

The crystal structure of Na7Mg4.5(P2O7)4

The crystal structure of Na₇Mg_4.5(P₂O₇)₄ has been solved by direct methods from the three-dimensional X-ray data. The space group is $\text{P}\text{1}$. The crystal structure consists of Mg²⁺, Na⁺, and P₂O^4?₇ ions. One magnesium atom at symmetry center (0,0,0) and two sodium atoms at ±(?0.0421, ?0.0596, 0.2230) display occupation factors 0.5 each. A short interatomic distance between these Na⁺ and Mg²⁺ ions (1.80 ± 0.01 Å) excludes the occupation of both sites in the same unit cell. The crystal structure of Na₇Mg_4.5(P₂O₇)₄ consists of unit cells containing Na₈Mg₄(P₂O₇)₄ or Na₆Mg₅(P₂O₇)₄ with a statistical occurrence 1:1.Each Mg²⁺ ion is octahedrally coordinated by six O^2? ions at distances 1.979 – 2.270 Å. The coordination polyhedra around the Na⁺ ions are ill-defined. The bond angles POP in the P₂O^4?₇ groups are 126.6 and 133.6° (±0.3°). The final reliability factor R is 7.1%.     

The synthesis and crystal structure of CaCu3Mn4O12: A new ferromagnetic-perovskite-like compound

Single crystals of CaCu₃Mn₄O₁₂, a new ferromagnetic perovskite-like compound (T_c ? 160°C), have been synthesized at 50 kbar and 1000°C. By X-ray analysis it was found to be cubic (a = 7.241Å), space groupIm3 with two molecules per unit cell. The two Ca²⁺ and six Cu²⁺ cations occupy theA sites of the ideal perovskite structure, while the eight Mn⁴⁺ cations occupy theB sites. In theIm3 space group the sites occupied by the calcium and copper cations have different point symmetry and therefore the 12-oxygen polyhedra have different distortions. The Ca cations are surrounded by slightly distorted icosahedra, the CaO distance is 2.562Å. The twelve oxygens around the copper cations are arranged as three mutually perpendicular rectangles of different size, the smallest and the largest of which are almost squares. The three sets of Cu0 distances are 1.942, 2.707, and 3.181Å, respectively. The octahedral Mn0 distance is 1.915Å. This arrangement is similar to that found in NaMn₃Mn₄O₁₂. A comparison between the two structures and a discussion of their thermal data are given.     

Idle spin behavior of the shifted hexagonal tungsten bronze type compounds FeIIFeIII2F8(H2O)2 and MnFe2F8(H2O)2

Fe^IIFe^III₂F₈(H₂O)₂ and MnFe₂F₈(H₂O)₂, grown by hydrothermal synthesis ($\text{P ? 200}\text{MPa}\text{, T = 450}\text{or}\text{380°}\text{C}$), crystallize in the monoclinic system with cell dimensions (Å): a = 7.609(5), b = 7.514(6), c = 7.453(4), β = 118.21(3)°; and a = 7.589(6), b = 7.503(8), c = 7.449(5), β = 118.06(3)°, and space group $\text{C2}\text{m}\text{, Z = 2}$. The structure is related to that of $\text{WO}{}_3\text{·}\text{1}\text{3}\text{H}{}_2\text{O}$. It is described in terms of perovskite type layers of Fe³⁺ octahedra separated by Fe²⁺ or Mn²⁺ octahedra, or in terms of shifted hexagonal bronze type layers. Both compounds present a weak ferromagnetism below T_N (157 and 156 K, respectively). Mössbauer spectroscopy points to an “idle spin” behavior for Fe^IIFe^III₂F₈(H₂O)₂: only Fe³⁺ spins order at T_N, while the Fe²⁺ spins remain paramagnetic between 157 and 35 K. Below 35 K, the hyperfine magnetic field at the Fe²⁺ nuclei is very weak: H_hf = 47 kOe at T = 4.2 K. For MnFe₂F₈(H₂O)₂, Mn²⁺ spin disorder is expected at 4.2 K. This “idle spin” behavior is due to magnetic frustration.     

Polyèdre de coordination de l'etain(II) dans SnC2O4: Relations structurales entre SnC2O4, Na2C2O4 et Na2Sn(C2O4)2

The crystal structure of SnC₂O₄ has been determined by X-ray single-crystal techniques and refined to R = 0,018 for 1139 reflections. The cell is monoclinic, space group $\text{C2}\text{c}$ with Z = 4 formula units, the parameters being a = 10,375(3)Å. b = 5,504(2)Å, c = 8,234(3)Å, β = 125,11(2)°. The oxalato groups, located on symmetry centers, are chelated to two Sn atoms through one oxygen on each carbon atom, giving rise to an infinite string (SnC₂O₄)_n. The Sn(II) atom is one-side bonded to four oxygen atoms with two SnO bonds of 2,232(2) Å and two of 2,393(2) Å. The tin atom is in a distorted trigonal bipyramid SnO₄E, the lone pair E occupying one of the apices of the equatorial trigonal base of the polyhedron. Crystal structure comparison with disodium bisoxalatostannate(II), Na₂Sn(C₂O₄)₂, permits one to deduce SnC₂O₄ by crystallographic shear operation $\text{1}\text{8}\text{[34}\text{2}\text{](001)}$ of $\text{c}\text{2}$ periodicity. Na₂Sn(C₂O₄)₂ can be described as an intergrowth of SnC₂O₄ and Na₂C₂O₄ structures and consldered as the first member of a new series Na₂Sn_1+n(C₂O₄)_2+n with n integer ? 0.     

Crystal structures of some fluorite-related M7O12 compounds

Powder X-ray diffraction techniques have been used to determine the crystal structures of Ti₃Sc₄O₁₂, Nb_1.5Sc_5.5O₁₂, NbSc₆O₁₁F, NbSc₅HfO₁₂, Zr₃Er₄O₁₂, and Hf₃Sc₄O₁₂ and to reexamine the structures of Zr₃Sc₄O₁₂ and Zr₃Yb₄O₁₂. All the compounds have the same fluorite-related superstructure as reported previously for Pr₇O₁₂, UY₆O₁₂, Zr₃Sc₄O₁₂, and Zr₃Yb₄O₁₂. Most of the compounds exhibit cation ordering, a situation observed previously only in UY₆O₁₂. One-seventh of the cations occupy a set of special sites of symmetry $\text{3}$ and are octahedrally coordinated by oxygen. These cations are of the type that has the smallest available ionic radius. The remaining cations occupy a set of general sites randomly with respect to atom type and are sevenfold coordinated by oxygen. In Hf₃Sc₄O₁₂, only partial ordering of this nature occurs, while it is confirmed that no cation ordering occurs in Zr₃Sc₄O₁₂. Some aspects of the cation order-disorder situation are discussed.