High-Pressure Magnetic-Susceptibility Studies of Spin-State Transaction

High-pressure magnetic susceptibility measurements have been carried out on Fe(dipy)2(NCS)2 and Fe(phen)2(NCS)2 in the pressure range 1–10 kbar and tempeature range 80–300 K in order to investigate the factors responsible for the spin-state transitions. The transitions change from first order to second or higher order upon application of pressure. The temperature variation of the susceptibility at different pressures has been analysed quantitatively within the framework of available models. It is shown that the relative magnitudes of the ΔG0 of high-spin and low-spin conversion and the ferromagnetic interaction between high-spin complexes determines the nature of the transition.

Correspondence between neutron depolarization and higher order magnetic susceptibility to investigate ferromagnetic clusters in phase separated systems

It is a tough task to distinguish a short-range ferromagnetically correlated cluster-glass phase from a canonical spin-glass-like phase in many magnetic oxide systems using conventional magnetometry measurements. As a case study, we investigate the magnetic ground state of La0.85Sr0.15CoO3, which is often debated based on phase separation issues. We report the results of two samples of La0.85Sr0.15CoO3 (S-1 and S-2) prepared under different conditions. Neutron depolarization, higher harmonic ac susceptibility and magnetic relaxation studies were carried out along with conventional magnetometry measurements to differentiate subtle changes at the microscopic level. There is no evidence of ferromagnetic correlation in the sample S-2 attributed to a spin-glass phase, and this is compounded by the lack of existence of a second order component of higher harmonic ac susceptibility and neutron depolarization. A magnetic relaxation experiment at different temperatures complements the spin glass characteristic in S-2. All these signal a sharp variance when we consider the cluster-glass-like phase (phase separated) in S-1, especially when prepared from an improper chemical synthesis process. This shows that the nonlinear ac susceptibility is a viable tool to detect ferromagnetic clusters such as those the neutron depolarization study can reveal.

A Magnetic Susceptibility Study of Spin-state Transitions in Rare-earth TrioxocobaItates( III)

Rare-earth trioxocobaltates(lll), Ln[CoO,], with Ln = Pr, Nd, Tb, Dy. and Yb exhibit low-spin to high-spin transitions of cobalt characterised by a maximum in the Ax-l against temperature plots where Ax is the cobalt contribution to the magnetic susceptibility. The susceptibility behaviour is distinct from that of La[CoO,] which shows a plateau in the x-I-T curve accompanied by a structural transition. The temperature at which the AX- I-T curve shows a maximum increases with the decrease in the size of the rare-earth ion. The susceptibility behavior of solid solutions of La,,Nd,CoO, has been investigated to see how the behaviour characteristic of Nd[CoO,] changes to that of La[CoO,].

Magnetic susceptibility studies of lead oxyhalide glasses containing transition metal oxides

Magnetic susceptibility studies of lead oxyhalide glasses containing high concentrations of transition metal oxides such as MnO and Fe2O3 have been performed. While they exhibit predominantly antiferromagnetic interactions, the low temperature (<100K) region is dominated by paramagnetic contributions. The behaviour in these glasses is found to be similar to that of covalent oxide glasses and is different from that of purely ionic sulphate glasses.

High-pressure magnetic susceptibility of the intermediate valence system EuPd2Si2

The pressure dependence (0-7 kbar) of the magnetic susceptibility is reported for the intermediate valence system EuPd2Si2 in the temperature interval 77-300K. It is found that the thermally induced valence transition becomes more gradual on application of pressure The characteristic fluctuation temperature Tf, also seems to be pressure dependent.

The effect of temperature on the magnetic susceptibility of vanadium pentoxide systems

The magnetic susceptibilities of certain vanadium pentoxide systems supported by kieselgur have been determined in the temperature interval 30° to 400° C. The plot of reciprocal susceptibility against temperature for all the systems studied indicates sudden deflections at temperatures which are about 150° lower than those of optimum catalytic activity. It has been suggested that these points may mark the temperatures of commencement of structural changes which may be responsible for the activity of these catalysts.

Evolution of magnetic order in the system LaNi1-xMnxO3 (x < 0.10)

Magnetic susceptibility measurements on dilute solid-solutions LaNi1-xMnxO3 (x < 0.1) have been carried out. With increasing x the magnetic susceptibility behaviour changes from Pauli paramagnetic to Curie-Weiss type. The temperature coefficient of resistance (TCR) changes sign around x = 0.03 but the system seems to be metallic in terms of showing a finite extrapolated conductivity at 0 K even when x = 0.10. The x = 0.10 system shows indications of spin-glass like behaviour.

Magnetic susceptibility studies on ternary oxides of copper(II)

The magnetic susceptibilities of a large number of ternary oxides of copper having structural features common to the presently identified phases of high-temperature superconductors have been studied in the temperature range 14-300 K. The systems studied are Ln2CuOP( Ln = La, Pr, Nd, etc.), Sr2CuO2CI2,B i2Cu0,, Ca2Cu03,S r2Cu03,S rCu02, MgCu203,B a2Cu3O4CI2Y, 2Cu205,Y2BaCu0,, BaCu02, Li2Cu02, etc. Cu2+ ions take different coordinations, like isolated square planar, square pyramidal or distorted-tetrahedral and octahedral, in these compounds. These compounds also exhibit different varieties of possible magnetic superexchange interactions like 180' or 90' Cu-0-Cu or Cu-0-0-Cu types as well as direct Cu-Cu interactions. Compounds in which there are extended 180' Cu-0-Cu interactions show a low, nearly temperature-independent susceptibility (100 X lod emu/mol). The estimated value of J for the Cu-0-Cu interaction is between 800 and 1500 K in these compounds. Isolated Cu2+ ions in which there are no 180' or close to 180" Cu-0-Cu interactions show Curie-Weiss susceptibility behavior. Compounds with only Cu-0-0-Cu interaction show evidence for the onset of antiferromagnetic coupling between 30 and 50 K. The superexchange rules are useful for explaining the qualitative features of the results. The possibility of disproportionation of Cu2+ ion when there are short Cu-Cu distances as in Bi2Cu04 is discussed. The extended geometry of the copper-oxygen framework seems to be more important than the local geometry around the Cu2+ ion in determining the magnetic properties.

Crystal structures, spectral and magnetic properties of (µ-hydroxo)(µ-acetato)dicopper(II) complexes containing chelating amines

The reaction of [Cu2(O2CMe)4(H2O)2] with N, N, N′, N′-tetramethylethane- 1,2-diamine (tmen) in ethanol yielded the dicopper(II) complex [Cu2(OH)(O2CMe)(tmen)2][ClO4]21. A similar reaction with N, N- dimethylethane- 1,2-diamine (dmen) afforded a crystalline product 2 in which two dicopper(II) complexes, [Cu2(OH)(O2CMe)(dmen)2][ClO4]22a and [Cu2(OH)(O2CMe)(H2O)2(dmen)2][ClO4]22b, are cocrystallized in a 1 : 1 molar ratio along with 2NaClO4. The crystal structures of 1 and 2 have been determined. The complexes have an asymmetrically dibridged [Cu2(µ-OH)(µ-O2CMe)]2+ core. The co-ordination geometry of the metal is square planar (CuO2N2). The copper atoms in 2b have a square-pyramidal CuO3N2 co-ordination sphere. The Cu Cu distances and Cu–O–Cu angles in 1, 2a and 2b are 3.339(2), 3.368(3), 3.395(7)Å, 120.1(2), 116.4(1) and 123.6(2)°, respectively. Complex 1 exhibits an axial ESR spectrum in a methanol glass giving g∥= 2.26 (A∥= 164 × 10–4 cm–1) and g⊥= 2.04. The ESR spectra obtained from the bulk material of the dmen product are indicative of the presence of two dimers, viz. complex 2a(g∥= 2.25, A∥= 165 × 10–4 cm–1; g⊥= 2.03) and 2b(g∥= 2.19, A∥= 184 × 10–4 cm–1; g⊥= 2.0). Variable-temperature magnetic susceptibility measurements on these complexes show an intramolecular antiferromagnetic coupling in the dimeric core. The fitting parameters are J=–27.8 cm–1, g= 2.1 for complex 1 and J=–10.1 cm–1, g= 2.0 for 2. The magnetostructural properties of the complexes are discussed. There is a linear correlation of the –2J values with the Cu Cu distances among dibridged complexes having square-planar copper(II) centres.

Synthesis, crystal structure and magnetic properties of a polymeric copper(II) Schiff-base complex having binuclear units covalently linked by isonicotinate ligands

The polynuclear copper(II) complex [{Cu2L(O2CC5H4N)}. C2H5OH](x) (1), where H3L is a 1∶2 Schiff base derived from 1,3-diaminopropan-2-ol and salicylaldehyde, has been prepared and structurally characterized. The structure consists of a one-dimensional zigzag chain in which the binuclear [Cu2L](+) units are covalently linked by isonicotinate ligands to give a syndiotactic arrangement of the copper ions protruding outside the chain. In the basic unit, the copper(II) centres are bridged by an alkoxo and a carboxylato ligand, giving a Cu ... Cu distance of 3.492(3) Angstrom and a Cu-O-Cu angle of 130.9(2)degrees. While one copper centre has a square-planar geometry, the other copper is square-pyramidal with the pyridine nitrogen being the axial ligand. The visible electronic spectrum of 1 shows a broad d-d band at 615 nm. The complex shows a rhombic X-band EPR spectral pattern in the polycrystalline phase at 77 K. Magnetic susceptibility measurements in the temperature range 22 to 295 K demonstrate the antiferromagnetic behaviour of 1. A theoretical fit to the magnetic data is based on a model assuming 1 as an equimolar mixture of copper atoms belonging to an antiferromagnetically coupled one-dimensional Heisenberg chain with the other copper atoms outside the chain behaving like paramagnetic centres.

Cu-II-azide polynuclear complexes of Cu-4 building clusters with Schiff-base co-ligands: synthesis, structures, magnetic behavior and DFT studies

Three new copper-azido complexes Cu-4(N-3)(8)(L-1)(2)](n) (1), Cu-4(N-3)(6)(L-2)(2)(H2O)(2)] (2), and Cu-4(N-3)(6)(L-3)(2)](n) (3) L-1 is the imine resulting from the condensation of pyridine-2-carboxaldehyde with N-methylethylenediamine, HL2 and HL3 are the condensation products of 2-hydroxy-3-methoxybenzaldehyde with N,N-diethylethylenediamine and N-ethylethylenediamine respectively] have been synthesized by using 0.5 molar equivalents of the Schiff base ligands with Cu(NO3)(2)center dot 3H(2)O and an excess of NaN3. Single crystal X-ray structures show that the basic unit of these complexes contains very similar Cu-4(II) building blocks. While 1 and 3 have overall 1D structures, 2 forms discrete tetranuclear clusters due to blocking of two coordination sites on the tetranuclear cluster by water molecules. Magnetic susceptibility measurements over a wide range of temperatures exhibit the presence of both antiferromagnetic and ferromagnetic exchanges within the tetranuclear unit structures. Density functional theory calculations (using B3LYP functional and two different basis sets) have been performed on the complexes 1-3 to provide a qualitative theoretical interpretation of their overall magnetic behavior.

Solvent-templated supramolecular isomerism in 2D coordination polymer constructed by (Ni2CoII)-Co-II nodes and dicyanamido spacers: drastic change in magnetic behaviours

Two heterometallic coordination polymers (CPs) have been prepared using (NiL)-L-II](2)Co-II (where H2L = N,N'-bis(salicylidene)-1,3-propanediamine) as nodes and dicyanamido spacers by varying the solvent for synthesis. Structural characterizations revealed that methanol assisted the formation of a two-dimensional (4,4) connected rhombic grid network of (NiL)(2)Co(NCNCN)2](infinity) (1a) whereas relatively less polar acetonitrile afforded a different superstructure {(NiL)(2)Co(NCNCN)(2)]center dot CH3CN}(infinity) (1b) with a two-dimensional (4,4) connected square grid network. The presence of acetonitrile molecules in the structure of 1b seems to change the spatial orientation of the terminal metalloligands NiL] from pseudo-eclipsed in 1a to staggered-like in 1b around the central Co(II). These structural changes in the nodes together with the conformationally flexible dicyanamido spacers, which are cis coordinated to the Co(II) in both trinuclear units, led to the differences in the final 2D network. Variable-temperature magnetic susceptibility measurements revealed that this supramolecular isomerism led to a drastic transition from spin-frustrated antiferromagnetism for 1a to a dominant ferromagnetic behaviour for 1b. The geometrical differences in Ni2Co coordination clusters (CCs) which are scalene triangular in 1a but nearly linear in 1b, are held responsible for the changes of the magnetic properties. The DFT calculations of exchange interactions between metal centres provide a clear evidence of the role played by the fundamental geometrical factors on the nature and magnitude of the magnetic coupling in these pseudo-polymorphic CPs.

New Structural Topologies in a Series of 3d Metal Complexes with Isomeric Phenylenediacetates and 1,3,5-Tris(1-imidazolyl)benzene Ligand: Syntheses, Structures, and Magnetic and Luminescence Properties

In this article we present the syntheses, characterizations, magnetic and luminescence properties of five 3d-metal complexes, Co(tib)(1,2-phda)](n)center dot(H2O)(n) (1), Co-3(tib)(2)(1,3-phda)(3)(H2O)](n)center dot(H2O)(2n) (2), Co-5(tib)(3)(1,4-phda)(5)(H2O)(3)](n)center dot(H2O)(7n) (3), Zn-3(tib)(2)(1,3-phda)(3)](n)center dot(H2O)(4n) (4), and Mn(tib)(2)(H2O)(2)](n)center dot(1,4-phdaH)(2n)center dot(H2O)(4n) (5), obtained from the use of isomeric phenylenediacetates (phda) and the neutral 1,3,5-tris(1-imidazolyl)benzene (tib) ligand. Single crystal X-ray structures showed that 1 constitutes 3,5-connected 2-nodal nets with a double-layered two-dimensional (2D) structure, while 2 forms an interpenetrated 2D network (3,4-connected 3-nodal net). Complex 3 has a complicated three-dimensional structure with 10-nodal 3,4,5-connected nets. Complex 4, although it resembles 2 in stoichiometry and basic building structures, forms a very different overall 2D assembly. In complex 5 the dicarboxylic acid, upon losing only one of the acidic protons, does not take part in coordination; instead it forms a complicated hydrogen bonding network with water molecules. Magnetic susceptibility measurements over a wide range of temperatures revealed that the metal ions exchange very poorly through the tib ligand, but for the Co(II) complexes the effects of nonquenched orbital contributions are prominent. The 3d(10) metal complex 4 showed strong luminescence with lambda(max) = 415 nm (lambda(ex) = 360 nm).

Temperature dependent magnetic, dielectric and Raman studies of partially disordered La2NiMnO6

We report a detailed magnetic, dielectric and Raman studies on partially disordered and biphasic double perovskite La2NiMnO6. DC and AC magnetic susceptibility measurements show two magnetic anomalies at T-C1 similar to 270 K and T-C2 similar to 240 K, which may indicate the ferromagnetic ordering of the monoclinic and rhombohedral phases, respectively. A broad peak at a lower temperature (T-sg similar to 70 K) is also observed indicating a spin-glass transition due to partial anti-site disorder of Ni2+ and Mn4+ ions. Unlike the pure monoclinic phase, the biphasic compound exhibits a broad but a clear dielectric anomaly around 270 K which is a signature of magneto-dielectric effect. Temperature-dependent Raman studies between the temperature range 12-300 K in a wide spectral range from 220 cm(-1) to 1530 cm(-1) reveal a strong renormalization of the first as well as second-order Raman modes associated with the (Ni/Mn)O-6 octahedra near T-C1 implying a strong spin-phonon coupling. In addition, an anomaly is seen in the vicinity of spin-glass transition temperature in the temperature dependence of the frequency of the anti-symmetric stretching vibration of the octahedra. (C) 2014 Elsevier Ltd. All rights reserved.

Cu-II-Azide Polynuclear Complexes of Three Different Building Clusters with the Same Schiff-Base Ligand: Synthesis, Structures, Magnetic Behavior, and Density Functional Theory Studies

Three copper-azido complexes Cu-4(N-3)(8)(L-1)(2)(MeOH)(2)](n) (1), Cu-4(N-3)(8)(L-1)(2)] (2), and Cu-5(N-3)(10)(L-1)(2)](n) (3) L-1 is the imine resulting from the condensation of pyridine-2-carboxaldehyde with 2-(2-pyridyl)ethylamine] have been synthesized using lower molar equivalents of the Schiff base ligand with Cu(NO3)(2)center dot 3H(2)O and an excess of NaN3. Single crystal X-ray structures show that the basic unit of the complexes 1 and 2 contains Cu-4(II) building blocks; however, they have distinct basic and overall structures due to a small change in the bridging mode of the peripheral pair of copper atoms in the linear tetranudear structures. Interestingly, these changes are the result of changing the solvent system (MeOH/H2O to EtOH/H2O) used for the synthesis, without changing the proportions of the components (metal to ligand ratio 2:1). Using even lower proportions of the ligand, another unique complex was isolated with Cu-5(II) building units, forming a two-dimensional complex (3). Magnetic susceptibility measurements over a wide range of temperature exhibit the presence of both antiferromagnetic (very weak) and ferromagnetic exchanges within the tetranuclear unit structures. Density functional theory calculations (using B3LYP functional, and two different basis sets) have been performed on the complexes 1 and 2 to provide a qualitative theoretical interpretation of their overall magnetic behavior.