Apical Mn Research Articles

Structure, magnetism and electrical transport in La1-x-yPryCaxMnO3 (y = 0.30, 0.35, 0.40; x = 0.40): Consequences of the interplay between intrinsic ionic radius and extrinsic particle size

The structural, microstructural, magnetic and electrical transport properties have been correlatively studied to decipher the complex consequences of variation of intrinsic parameter ,the average La-site cationic radii and extrinsic particle size on the magneto-electrical phase coexistence in La1-x-yPryCaxMnO3 (y = 0.30, 0.35, 0.40; x = 0.40). Polycrystalline powders prepared by sol-gel route were sintered at 600 °C, 900 °C, 1100 °C, and 1300 °C to facilitate particle size variation. Rietveld refinement of the powder X-ray diffraction data confirms that all samples have an orthorhombic structure with Pnma space group. The lattice constants decrease with decreasing ionic radii, while at a fixed ionic radius, the enhanced particle size increases them. The average apical Mn−O−Mn bond angles and the average Mn−O bond lengths decrease with rising ionic radii but increase with particle size at a fixed composition. Magnetization measurements show sequential paramagnetic (PM)-antiferromagnetic (AFM)-ferromagnetic (FM) transitions on lowering the temperature is lowered. The AFM-FM transition exhibits (i) pronounced hysteresis between field-cooled cooling (FCC) and field-cooled warming (FCW) (ii) strong divergence between zero-field cooling (ZFC) and FCW curves. The low-temperature magnetic state transforms from spin glass-like to a cluster glass-like as the ionic radii or the particle size decreases. These behaviors are caused by the mesoscale modifications in the magnetic phase profile where the AFM and FM phases due to the interplay between the average La-site cationic radii and the particle size. In general, the FM component increases with an increase in the particle size, but it decreases at lower ionic radii. The presence of a large thermal hysteresis in the insulator-metal transition (IMT) coupled with the one in FCC-FCW magnetization at particle sizes ~200 nm–160 nm, and ~460-400 nm demonstrates that this particle size regime is the most conducive to the phase separation. IMT disappears at smaller ionic radii and larger particle sizes due to the enhancement of AFM and charge-ordering. The PM and AFM regimes in all samples reveal variable range hopping conduction with particle size and ionic radii dependent activation. The observed phenomena have been explained in terms of the interplay between the ionic radii and particle size induced changes in the average apical Mn−O−Mn bond angles and the average Mn−O bond length.

Trinuclear Manganese Complexes of Unsymmetrical Polypodal Diamino N3O3 Ligands with an Unusual [Mn3(μ-OR)4]5+ Triangular Core: Synthesis, Characterization, and Catalase Activity

Two new tri-Mn(III) complexes of general formula [Mn3L2(μ-OH)(OAc)]ClO4 (H3L = 1-[N-(2-pyridylmethyl),N-(2-hydroxybenzyl)amino]-3-[N'-(2-hydroxybenzyl),N'-(4-X-benzyl)amino]propan-2-ol; 1ClO4, X = Me; 2ClO4, X = H) have been prepared and characterized. X-ray diffraction analysis of 1ClO4 reveals that the complex cation possesses a Mn3(μ-alkoxo)2(μ-hydroxo)(μ-phenoxo)(4+) core, with the three Mn atoms bound to two fully deprotonated N3O3 chelating L(3-), one exogenous acetato ligand, and one hydroxo bridge, the structure of which is retained upon dissolution in acetonitrile or methanol. The three Mn atoms occupy the vertices of a nearly isosceles triangle (Mn1···Mn3 = 3.6374(12) Å, Mn2···Mn3 3.5583(13) Å, and Mn1···Mn2 3.2400(12) Å), with one substitution-labile site on the apical Mn ion occupied by terminally bound monodentate acetate. Temperature-dependent magnetic susceptibility studies indicate the presence of predominant antiferromagnetic intramolecular interactions between Mn(III) ions in 1ClO4. Complexes 1ClO4 and 2ClO4 decompose H2O2 at comparable rates upon initial binding of peroxide through acetate substitution, with retention of core structure during catalysis. Kinetic and spectroscopic studies suggest that these complexes employ the [Mn-(μ-oxo/aquo)-Mn](4+) moiety to activate peroxide, with the additional (μ-alkoxo)(μ-phenoxo)Mn(μ-alkoxo) metallobridge carrying out a structural function.

Expanding the Ruddlesden–Popper Manganite Family: The n = 3 La3.2Ba0.8Mn3O10 Member

La(3.2)Ba(0.8)Mn(3)O(10), a representative of the rare n = 3 members of the Ruddlesden-Popper manganites A(n+1)Mn(n)O(3n+1), was synthesized in an evacuated sealed silica tube. Its crystal structure was refined from a combination of powder X-ray diffraction (PXD) and precession electron diffraction (PED) data, with the rotations of the MnO(6) octahedra described within the symmetry-adapted mode approach (space group Cccm, a = 29.068(1) Å, b = 5.5504(5) Å, c = 5.5412(5) Å; PXD R(F) = 0.053, R(P) = 0.026; PED R(F) = 0.248). The perovskite block in La(3.2)Ba(0.8)Mn(3)O(10) features an octahedral tilting distortion with out-of-phase rotations of the MnO(6) octahedra according to the (Φ,Φ,0)(Φ,Φ,0) mode, observed for the first time in the n = 3 Ruddlesden-Popper structures. The MnO(6) octahedra demonstrate a noticeable deformation with the elongation of two apical Mn-O bonds due to the Jahn-Teller effect in the Mn(3+) cations. The relationships between the octahedral tilting distortion, the ionic radii of the cations at the A- and B-positions, and the mismatch between the perovskite and rock-salt blocks of the Ruddlesden-Popper structure are discussed. At low temperatures, La(3.2)Ba(0.8)Mn(3)O(10) reveals a sizable remnant magnetization of about 1.3 μ(B)/Mn at 2 K, and shows signatures of spin freezing below 150 K.

Structural modifications of disordered YMn1−xInxO3 solid solutions evidenced by infrared and Raman spectroscopies

Polycrystalline YMn1−xInxO3 compounds were obtained by solid state reaction and found to present a blue color for high substitution rate. X-ray diffraction analyses reveal the formation of solid solutions and the increase of cell parameters following Vegard's law. Far infrared reflectivity and Raman scattering spectra show the presence of random disorder and structural modifications without phase transition. A detailed analysis of some phonon modes allows to explain the structural changes upon doping. It was found that monocaping YO7 antiprisms are almost unaffected by substitution. However, results reveal the existence of two different MO5 (M = In, Mn) trigonal bipyramids. Their apical Mn–O distances are shorter than In–O bonds and both distances increase with the substitution rate, which is consistent with the evolution of the cell parameters.

Multireversible Redox Processes in Pentanuclear Bis(Triple-Helical) Manganese Complexes Featuring an Oxo-Centered triangular {MnII2MnIII(μ3-O)}5+or {MnIIMnIII2(μ3-O)}6+Core Wrapped by Two {MnII2(bpp)3}−

A new pentanuclear bis(triple-helical) manganese complex has been isolated and characterized by X-ray diffraction in two oxidation states: [{Mn(II)(μ-bpp)(3)}(2)Mn(II)(2)Mn(III)(μ-O)](3+) (1(3+)) and [{Mn(II)(μ-bpp)(3)}(2)Mn(II)Mn(III)(2)(μ-O)](4+) (1(4+)). The structure consists of a central {Mn(3)(μ(3)-O)} core of Mn(II)(2)Mn(III) (1(3+)) or Mn(II)Mn(III)(2) ions (1(4+)) which is connected to two apical Mn(II) ions through six bpp(-) ligands. Both cations have a triple-stranded helicate configuration, and a pair of enantiomers is present in each crystal. The redox properties of 1(3+) have been investigated in CH(3)CN. A series of five distinct and reversible one-electron waves is observed in the -1.0 and +1.50 V potential range, assigned to the Mn(II)(4)Mn(III)/Mn(II)(5), Mn(II)(3)Mn(III)(2)/Mn(II)(4)Mn(III), Mn(II)(2)Mn(III)(3)/Mn(II)(3)Mn(III)(2), Mn(II)Mn(III)(4)/Mn(II)(2)Mn(III)(3), and Mn(III)(5)/Mn(II)Mn(III)(4) redox couples. The two first oxidation processes leading to Mn(II)(3)Mn(III)(2) (1(4+)) and Mn(II)(2)Mn(III)(3) (1(5+)) are related to the oxidation of the Mn(II) ions of the central core and the two higher oxidation waves, close in potential, are thus assigned to the oxidation of the two apical Mn(II) ions. The 1(4+) and 1(5+) oxidized species and the reduced Mn(4)(II) (1(2+)) species are quantitatively generated by bulk electrolyses demonstrating the high stability of the pentanuclear structure in four oxidation states (1(2+) to 1(5+)). The spectroscopic characteristics (X-band electron paramagnetic resonance, EPR, and UV-visible) of these species are also described as well as the magnetic properties of 1(3+) and 1(4+) in solid state. The powder X- and Q-band EPR signature of 1(3+) corresponds to an S = 5/2 spin state characterized by a small zero-field splitting parameter (|D| = 0.071 cm(-1)) attributed to the two apical Mn(II) ions. At 40 K, the magnetic behavior is consistent for 1(3+) with two apical S = 5/2 {Mn(II)(bpp)(3)}(-) and one S = 2 noninteracting spins (11.75 cm(3) K mol(-1)), and for 1(4+) with three S = 5/2 noninteracting spins (13.125 cm(3) K mol(-1)) suggesting that the {Mn(II)(2)Mn(III)(μ(3)-O)}(5+) and {Mn(II)Mn(III)(2)(μ(3)-O)}(6+) cores behave at low temperature like S = 2 and S = 5/2 spin centers, respectively. The thermal behavior below 40 K highlights the presence of intracomplex magnetic interactions between the two apical spins and the central core, which is antiferromagnetic for 1(3+) leading to an S(T) = 3 and ferromagnetic for 1(4+) giving thus an S(T) = 15/2 ground state.

Inelastic Neutron Scattering on an Mn10 Supertetrahedron: Assessment of Exchange Coupling Constants, Ferromagnetic Spin Waves and an Analogy to the Hückel Method

The synthesis, crystal structure and magnetic characterisation by magnetisation and inelastic neutron scattering (INS) of a mixed-valent Mn(10) supertetrahedral aggregate [Mn(III)(6)Mn(II)(4)(μ(4)-O)(4)(μ(3)-N(3))(3)(μ(3)-Br)(Hmpt)(6)(Br)]Br(0.7)(N(3))(0.3)·2MeOH·3MeCN (1) (H(3)mpt=3-methylpentan-1,3,5-triol) is reported. The magnetic core of the molecule can be described as an octahedron of six S=2 Mn(III) ions with four faces, each capped by a S=5/2 Mn(II) ion such as to form the supertetrahedron. Unlike most related complexes, the molecular symmetry is slightly reduced from approximately T(d) to C(3). The magnetic data reveal a total spin of S=22 in the ground state due to ferromagnetic exchange couplings within the molecule. The combined INS and magnetic data permits the accurate determination of the exchange coupling constants. Two types are found. The couplings between the Mn(III) ions in the inner octahedron are characterised by J(a)=18.4(3) K, whereas the couplings between the apical Mn(II) ions to the neighbouring Mn(III) ions are given by J(b)=7.3(2) K. The significantly larger coupling strength J(a) as compared to J(b), and the near-T(d) symmetry have profound consequences on the energy spectrum, which are discussed and carefully analysed. In particular, the observed INS spectra can consistently be reproduced by a simplified model in which the inner octahedron is replaced by one large spin of length S(0)=12. This model provides intuitive insight into the structure of the magnetic spectrum. Additionally, the magnetic excitations at low temperature are analysed within the frame of ferromagnetic linear spin-wave theory, which permits an analytical calculation of the energy levels. For ferromagnetic clusters, a close analogy to the Hückel method of electronic structure calculation can be drawn, which allows one to grasp the results of the spin-wave theory or the magnetic excitation spectrum, respectively, in a chemical language.

Trinuclear Terpyridine Frustrated Spin System with a MnIV3O4 Core: Synthesis, Physical Characterization, and Quantum Chemical Modeling of Its Magnetic Properties

The trinuclear oxo bridged manganese cluster, [Mn(IV)(3)O(4)(terpy)(terpyO(2))(2)(H(2)O)](S(2)O(8))(2) (5) (terpy = 2,2':2'',6'-terpyridine and terpyO(2) = 2,2':2'',6'-terpyridine 1,1''-dioxide), was isolated in an acidic aqueous medium from the reaction of MnSO(4), terpy, and oxone as chemical oxidant. The terpyO(2) ligands were generated in situ during the synthesis by partial oxidation of terpy. The complex crystallizes in the monoclinic space group P21/n with a = 14.251(5) A, b = 15.245(5) A, c = 24.672(5) A, alpha = 90.000(5) degrees, beta = 92.045(5) degrees, gamma = 90.000(5) degrees, and Z = 4. The triangular {Mn(IV)(3)O(4)}(4+) core observed in this complex is built up of a basal Mn(mu-O)(2)Mn unit where each Mn ion is linked to an apical Mn ion via mono(mu-O) bridges. The facial coordination of the two tridentate terpyO(2) ligands to the Mn(mu-O)(2)Mn unit allows the formation of the triangular core. 5 is also the first structurally characterized Mn complex with polypyridinyl N-oxide ligands. The variable-temperature magnetic susceptibility data for this complex, in the range of 10-300 K, are consistent with an S = 1/2 ground state and were fit using the spin Hamiltonian H(eff) with S(1) = S(2) = S(3) = 3/2, J(a) = -37 (+/-0.5) and J(b) = -53 (+/-1) cm(-1), where J(a) and J(b) are exchange constants through the mono-mu-oxo and the di-mu-oxo bridges, respectively. The doublet ground spin state of 5 is confirmed by EPR spectroscopic measurements. Density functional theory (DFT) calculations based on the broken symmetry approach reproduce the magnetic properties of 5 very well (calculated values: J(a) = -39.4 and J(b) = -55.9 cm(-1)), thus confirming the capability of this quantum chemical method for predicting the magnetic behavior of clusters involving more than two metal ions. The nature of the ground spin state of the magnetic {Mn(IV)(3)O(4)}(4+) core and the role of ancillary ligands on the magnitude of J are also discussed.

Effect of uniaxial pressure on metal-insulator transition in (Sm1−yNdy)0.52Sr0.48MnO3 single crystals

We have investigated the effect of uniaxial pressure (P) on resistivity along the ab plane and c-axis in single crystals of (Sm1−yNdy)0.52Sr0.48MnO3 with y=0, 0.05, and 0.3. The application of pressure along the c-axis shifts the metal-insulator transition (MIT) to higher temperature, while MIT temperature decreases with P when it is applied perpendicular to the c-axis. This behavior is quite different from that observed in hydrostatic pressure and can be explained by considering the P dependence change in equatorial and apical Mn–O–Mn bond angles.

Control of the Barrier in Cyanide Based Single Molecule Magnets Mn(III)2Mn(II)3: Theoretical Analysis.

The aim of this communication is to probe the possibility of increasing the barrier for reversal of magnetization in the family of new cyano-bridged pentanuclear Mn(III)2Mn(II)3 clusters in which single molecule magnet behavior has been recently discovered. In this context, we analyze the global magnetic anisotropy arising from the unquenched orbital angular momenta of ground terms (3)T1(t2(4)) of the two apical Mn(III) ions. The model takes into account the trigonal component of the crystal field, spin-orbit interaction in (3)T1(t2(4)), and an isotropic exchange interaction between Mn(III) and Mn(II) ions. The height of the barrier is shown to be sensitive to the change of the trigonal field stabilizing orbital doublet (3)E, which carries the first-order orbital magnetic contribution and enhances with an increase of the trigonal field. This result is expected to be useful for the more rational design of new cyano-bridged SMMs with high blocking temperatures.

Role of the Orbitally Degenerate Mn(III) Ions in the Single-Molecule Magnet Behavior of the Cyanide Cluster {[MnII(tmphen)2]3[MnIII(CN)6]2} (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline)

We report a new theoretical model that accounts for the unusual magnetic properties of the cyanide cluster ([MnII(tmphen)2]3[MnIII(CN)6]2) (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline). The model takes into account (1) the spin-orbit interaction, (2) the trigonal component of the crystal field acting on the ground-state cubic (3)T(1) terms of the apical Mn(III) ions, and (3) the isotropic contribution to the exchange interaction between Mn(III) and Mn(II) ions. The ground state of the cluster was shown to be the state with the total angular momentum projection |M(J)| = 15/2; the energies of the low-lying levels obtained from this treatment increase with decreasing |M(J)| values, a situation that leads to a barrier for the reversal of magnetization (U(eff) approximately 30 cm(-1)). The new model explains the recently discovered single-molecule magnet behavior of the ([MnII(tmphen)2]3[MnIII(CN)6]2)in contrast to the traditional approach that takes into account only the ground-state spin (S) and a negative zero-field splitting parameter (D(S) < 0).

Synthesis and crystal structure of the Sr 2MnGa(O,F) 6 oxyfluorides

The fluorine-containing derivatives of Sr 2MnGaO 5.5 were prepared by treatment with XeF 2 at temperatures ranging from 300°C to 600°C. The compounds crystallize in a tetragonal unit cell with a t≈ a p, c t≈2 a p ( a p—the parameter of the perovskite subcell). An increase in fluorine content is accompanied by a reduction of the Mn oxidation state due to a partial replacement of oxygen by fluorine. The crystal structure of Sr 2MnGaO 4.78F 1.22 was determined by electron diffraction and X-ray powder diffraction ( a=3.85559(2) Å, c=7.78289(6) Å, S.G. P4/ mmm, R I =0.012, R P=0.019). The structure consists of alternating (MnO 2), (SrO) and (GaO 0.78F 1.22) layers. The Ga atoms are situated in slightly elongated octahedra, the MnO 6 octahedra are characterized by two short apical Mn–O distances of 1.876(8) Å and four long equatorial ones of 1.9278(1) Å. This is interpreted as an “apically compressed” type of Jahn–Teller distortion, in contrast to the “apically elongated” one in the Sr 2MnGaO 5+ δ brownmillerites with different oxygen content. Possible structural reasons for the reversed Jahn–Teller effect are discussed.

Temperature study of MnAs local structure by EXAFS

Extended X-ray absorption fine structure measurements at the Mn K absorption edge on MnAs in the temperature range between 10 and 300 K are reported. In accordance with previous diffraction studies, a contraction of the unit cell has been found when lowering the temperature. The Debye–Waller factors of the Mn–Mn and Mn–As first sub-shells have been fitted with a correlated Debye model plus a temperature independent static disorder, obtaining a Debye temperature around 310 K. The fit provides information about the static contribution to the local disorder along the apical Mn–Mn and along the Mn–As interatomic distances. In particular the results on the Mn–As sub-shell show strict similarities with the manganese-oxide perovskites.

Structure and magnetotransport properties of the layered manganites Re1.2Sr1.8Mn2O7(RE = La, Pr, Nd)

We have examined the magnetotransport properties and the structure, by Rietveld refinement of powder X-ray data, of the phases RE1.2Sr1.8Mn2O7(RE = La, Pr, Nd). We find that on cooling, La2Sr1.8Mn2O7 undergoes a transition to a nearly perfect ferromagnet with 90% magnetization at 1.45 T, as reported by earlier workers, but the Pr and Nd phases show only a small magnetization that grows gradually as the temperature is decreased. There seems to be significant correlation between electrical transport and the Jahn-Teller elongation of the apical Mn–O bonds in these systems. The elongation of the apical Mn-O bonds forces the nine-coordinate rock-salt site to be occupied preferentially by the smaller rare-earth-metal cations. This preferential occupation is reliably obtained from the X-ray refinement. All three title phases show a magnetoresistance ratio of about 4 (corresponding to a magnetoresistance, [R(0)–R(H)]/R(0), of about 75%) at a field of 7 T and temperatures around 100 K.

Crystal Structure of Paramagnetic DyMn2O5 at 298°K

Paramagnetic DyMn2O5 crystallizes in the orthorhombic system with lattice constants a=7.2940±0.0008, b=8.5551±0.0008, and c=5.6875±0.0008 Å, in the space group Pbam(D2h9). There are four DyMn2O5 per unit cell. The crystal structure was solved by a combination of three-dimensional Patterson and Fourier series and refined by the method of least squares. The integrated intensities of 5094 reflections were measured by PEXRAD, with 976 symmetry-independent structure factors significantly above background. The final agreement factor between measured and calculated structure factors is 0.0401. There are two independent Mn atoms. One is pentacoordinated, with a slightly distorted tetragonal pyramidal configuration: two basal Mn–O distances are 1.926±0.006 Å, the other two are 1.911±0.005 Å, and the apical Mn–O bond, nearly normal to the basal plane, is 2.020±0.005 Å. The other Mn occupies a slightly compressed octahedron, with four planar Mn–O bonds of 1.940±0.004 Å and two, normal to this plane, of 1.873±0.004 Å. The octahedral Mn environment closely resembles that in β-MnO2. The Dy is surrounded by a somewhat distorted square-antiprismatic polyhedron of oxygen atoms, the Dy–O distance ranging from 2.341±0.005 to 2.469±0.005 Å, the average being 2.390 Å. The thermal vibrations are significantly anisotropic. The room-temperature paramagnetism could mask an antiferromagnetic ordering of the octahedral Mn atoms.

The crystal and molecular structure of cis-1,2-difluorovinylpentacarbonylmanganese

Crystals of cis-1,2-difluorovinylpentacarbonylmagnanese, CFHCF·Mn(CO)5, are monoclinic, a 6·32, b 13·94, c 10·89 Å, β 90·5°, space group P21/n. The structure was refined by least-squares methods, the final R being 0·17 for 696 reflexions. The structure contains discrete cis-CFHCF·Mn(CO)5 molecules, the manganese atom being co-ordinated octahedrally to five carbonyl groups and one difluorovinyl group. The plane of the CFHCF– group is perpendicular to the adjacent square-plane of carbonyl groups, and is staggered with respect to the Mn–C bonds. Deviations from octahedral regularity are discussed. The Mn–C distance to the CFHCF– group (1·95 Å) is significantly longer than the Mn–CO lengths, and the apical Mn–carbonyl bond (1·73 Å) is possibly a little shorter than the other four (1·77–1·83, mean 1·79 Å). The C–F bonds (1·48 Å) are longer than normal. The intermolecular distances correspond to van der Waals interactions, most of the shorter contacts involving fluorine atoms.