Magnetic properties and negative colossal magnetoresistance of the rare earth Zintl phase EuIn2As2

Large, high quality single crystals of a new Zintl phase, EuIn(2)As(2), have been synthesized from a reactive indium flux. EuIn(2)As(2) is isostructural to the recently reported phase EuIn(2)P(2), and it is only the second reported member of the group of compounds with formula AM(2)X(2) (A = alkali, alkaline earth, or rare earth cation; M = transition or post-transition metal; and X = Group 14 or 15 element) that crystallizes in the hexagonal space group P6(3)/mmc (a = 4.2067(3) A, c = 17.889(2) A and Z = 2). The structure type contains layers of A(2+) cations separated by [M(2)X(2)](2-) layers along the crystallographic c-axis. Crystals of the title compound were mounted for magnetic measurements, with the crystallographic c-axis oriented either parallel or perpendicular to the direction of the applied field. The collective magnetization versus temperature and field data indicate two magnetic exchange interactions near 16 K, one involving Eu(2+)...Eu(2+) intralayer coupling and the other involving Eu(2+)...Eu(2+) coupling between layers. EuIn(2)As(2) is metallic and magnetoresistive, as is the isostructural phosphide, and both compounds have coincident resistivity and magnetic ordering transitions, consistent with the observation of colossal magnetoresistance. Negative colossal magnetoresistance (MR = {[rho(H) - rho(0)]/rho(H)} x 100%) of up to -143% (at T = 17.5 K, H = 5 T) is observed for EuIn(2)As(2), approximately half of that observed for the more resistive phosphide, which has a higher magnetic ordering temperature and local moment coupling strength.     

Crystal structure and a giant magnetoresistance effect in the new Zintl compound Eu3Ga2P4

Single-crystalline samples of a new Zintl compound, Eu(3)Ga(2)P(4), have been synthesized by a Ga-flux method. Eu(3)Ga(2)P(4) is found to crystallize in a monoclinic unit cell, space group C2/c, isostructural to Ca(3)Al(2)As(4). The structure is composed of a pair of edge-shared GaP(4) tetrahedra, which link by corner-sharing to form Ga(2)P(4) two-dimensional layers, separated by Eu(2+) ions. Magnetic susceptibility showed a Curie-Weiss behavior with an effective magnetic moment consistent with the value for Eu(2+) magnetic ions. Below 15 K, ferromagnetic ordering was observed and the saturation magnetic moment was 6.6 μ(B). Electrical resistivity measurements on a single crystal showed semiconducting behavior. Resistivity in the temperature range between 280 and 300 K was fit by an activation model with an energy gap of 0.552(2) eV. The temperature dependence of the resistivity is better described by the variable-range-hopping model for a three-dimensional conductivity, suggesting that Eu-P bonds are involved in the conductivity. A large magnetoresistance, up to -30%, is observed with a magnetic field H = 2 T at T = 100 K, suggesting strong coupling of carriers with the Eu(2+) magnetic moment.     

A novel antiferromagnetic organic superconductor kappa-(BETS)(2)FeBr(4) [where BETS = bis(ethylenedithio)tetraselenafulvalene].

The electrical and magnetic properties of kappa-(BETS)(2)FeBr(4) salt [where BETS = bis(ethylenedithio)tetraselenafulvalene] showed that this system is the first antiferromagnetic organic metal at ambient pressure (T(N) = 2.5 K). The characteristic field dependence of the magnetization at 2.0 K indicates a clear metamagnetic behavior. The small resistivity drop observed at T(N) clearly shows the existence of the interaction between pi metal electrons and localized magnetic moments of Fe(3+) ions. In addition, this system underwent a superconducting transition at 1.1 K. That is, kappa-(BETS)(2)FeBr(4) is the first antiferromagnetic organic metal exhibiting a superconducting transition below Néel temperature. The magnetic field dependence of the superconducting critical temperature indicated that the superconductivity in this system is strongly anisotropic also in the conduction plane because of the existence of the metamagnetically induced internal field based on the antiferromagnetic ordering of the Fe(3+) 3d spins in contrast to the cases of the other conventional organic superconductors. Furthermore, the specific heat measurement exhibited a lambda-type large peak of zero-field specific heat corresponding to the three-dimensional antiferromagnetic ordering of high-spin Fe(3+) ions. The lack of distinct anomaly in the C(p) vs T curve at T(c) suggests the coexistence of the superconductivity and the antiferromagnetic order below T(c).     

One- and two-step spin-crossover behavior of [Fe(II)(isoxazole)(6)](2+) and the structure and magnetic properties of triangular [Fe(III)(3)O(OAc)(6)(isoxazole)(3)][ClO(4)

The structure and spin-crossover magnetic behavior of [Fe(II)1(6)][BF(4)](2) (1 = isoxazole) and [Fe(II)1(6)][ClO(4)](2) have been studied. [Fe(II)1(6)][BF(4)](2) undergoes two reversible spin-crossover transitions at 91 and 192 K, and is the first two-step spin transition to undergo a simultaneous crystallographic phase transition, but does not exhibit thermal hysteresis. The single-crystal structure determinations at 260 [space group P3, a = 17.4387(4) A, c = 7.6847(2) A] and at 130 K [space group P1, a = 17.0901(2) A, b = 16.7481(2) A, c = 7.5413(1) A, alpha = 90.5309(6) degrees, beta = 91.5231(6) degrees, gamma = 117.8195(8) degrees ] reveal two different iron sites, Fe1 and Fe2, in a 1:2 ratio. The room-temperature magnetic moment of 5.0 mu(B) is consistent with high-spin Fe(II). A plateau in mu(T) having a moment of 3.3 mu(B) centered at 130 K suggests a mixed spin system of some high-spin and some low-spin Fe(II) molecules. On the basis of the Fe-N bond distances at the two temperatures, and the molar fraction of high-spin molecules at the transition plateau, Fe1 and Fe2 can be assigned to the 91 and 192 K transitions, respectively. [Fe(II)1(6)][ClO(4)](2) [space group P3, a = 17.5829(3) A, c = 7.8043(2) A, beta = 109.820 (3) degrees, T = 295 K] also possesses Fe1:Fe2 in a 1:2 ratio, and magnetic measurements show a single spin transition at 213 K, indicating that both Fe1 and Fe2 undergo a simultaneous spin transition. [Fe(II)1(6)][ClO(4)](2) slowly decomposes in solutions containing acetic anhydride to form [Fe(III)(3)O(OAc)(6)1(3)][ClO(4)] [space group I2, a = 10.1547(7) A, b = 16.5497(11) A, c = 10.3205(9) A, beta = 109.820 (3) degrees, T = 200 K]. The isosceles Fe(3) unit contains two Fe.Fe distances of 3.2844(1) A and a third Fe.Fe distance of 3.2857(1) A. The magnetic data can be fit to a trinuclear model with H = -2J(S(1)xS(2) + S(2)xS(3)) - 2J(13)(S(1)xS(3)), where J = -27.1 and J(13) = -32.5 cm(-1).     

Cu(HCO2)2(pym) (pym = pyrimidine): low-dimensional magnetic behavior and long-range ordering in a quantum-spin lattice

We synthesized and structurally and magnetically characterized the novel 3D coordination polymer Cu(HCO2)2(pym) (pym = pyrimidine). The compound crystallizes in the monoclinic space group C2/c with a = 14.4639(8) A, b = 7.7209(4) A, c = 8.5172(5) A, beta = 126.076(2) degrees, and V= 768.76(7) A3. In the structure buckled layers of Cu(HCO2)2 are interconnected by pym ligands to afford 1D Cu-pym-Cu chains. Bulk magnetic susceptibility measurements show a broad maximum at 25 K that is indicative of short-range magnetic ordering. Between 12 and 300 K a least-squares fit of the chi(T) data to a mean-field-corrected antiferromagnetic chain model yielded excellent agreement for g = 2.224(3), J/kB = -26.9(2) K, and zJ'/kB = -1.1(3) K. Below approximately 3 K a transition to long-range magnetic ordering is observed, as suggested by a sharp and sudden decrease in chi(T). This result is corroborated by muon spin relaxation measurements that show oscillations in the muon asymmetry below T(N) = 2.802(1) K and rapidly fluctuating moments above T(N).     

An indication of magnetic-field-induced superconductivity in a bifunctional layered organic conductor,kappa-(BETS)(2)FeBr(4)

Hybrid systems consisting of the conducting layers of organic donor molecules and the magnetic layers of inorganic anions have been focused on as possible bifunctional materials, whose conducting properties can be tuned by controlling the magnetic state of the anion layers on an application of magnetic field. Here we report the magnetoresistance of the antiferromagnetic organic superconductor, kappa-(BETS)2FeBr4 [BETS = bis(ethylenedithio)tetraselenafulvalene], consisting of the two-dimensional superconducting layers of the BETS semications and the insulating layers of the FeBr4- anions. Due to the metamagnetic nature of the Fe3+ spin system, characteristic resistivity decrease was observed just below the antiferromagnetic superconductor-to-ferromagnetic metal transition at 1.6 T. Furthermore, an indication of the onsets of the magnetic-field-induced superconductivity was discovered around 12.5 T.     

Synthesis,X-ray powder structure,and magnetic properties of layered NiII methylphosphonate, [Ni(CH3PO3)(H2O)], and NiII octadecylphosphonate, [Ni(CH3-(CH2)17-PO3)(H2O)

[Ni(CH(3)PO(3))(H(2)O)] (1) and [Ni(CH(3)-(CH(2))(17)-PO(3))(H(2)O)] (2) were synthesised by reaction of NiCl(2).6 H(2)O and the relevant phosphonic acid in water in presence of urea. The compounds were characterised by elemental and thermogravimetric analyses, UV-visible and IR spectroscopy, and their magnetic properties were studied by using a SQUID magnetometer. The crystal structure of 1 was determined "ab initio" from X-ray powder diffraction data and refined by the Rietveld method. The crystals of 1 are orthorhombic, space group Pmn2(1), with a=5.587(1), b=8.698(1), c=4.731(1) A. The compound has a hybrid, layered structure made up of alternating inorganic and organic layers along the b direction of the unit-cell. The inorganic layers consist of Ni(II) ions octahedrally coordinated by five phosphonate oxygen atoms and one oxygen atom from the water molecule. These layers are separated by bilayers of methyl groups and van der Waals contacts are established between them. A preliminary structure characterisation of compound 2 suggests the crystallisation in the orthorhombic system with the following unit-cell parameters: a=5.478(7), b=42.31(4), c=4.725(3) A. The oxidation state of the Ni ion in both compounds is +2, and the electronic configuration is d(8) (S=1), as determined from static magnetic susceptibility measurements above 50 K. Compound 1 obeys the Curie-Weiss law at temperatures above 50 K; the Curie (C) and Weiss (theta) constants were found to be 1.15 cm(3) K mol(-1) and -32 K, respectively. The negative value of theta indicates an antiferromagnetic exchange coupling between near-neighbouring Ni(II) ions. No sign of 3D antiferromagnetic long-range order is observed down to T=5 K, the lowest measured temperature. Compound 2 is paramagnetic above T=50 K, and the values of C and theta were found to be 1.25 cm(3) K mol(-1) and -24 K, respectively. Below 50 K the magnetic behavior of 2 is different from that of 1. Zero-field cooled (zfc) and field-cooled (fc) magnetisation plots do not overlap below T=21 K. The irreversible magnetisation, DeltaM(fc-zfc), obtained as a difference from fc and zfc plots starts to increase at T=20 K, on lowering the temperature, and it becomes steady at T=5 K. The presence of spontaneous magnetisation below T=20 K indicates a transition to a weak-ferromagnetic state for compound 2.     

Effect of included guest molecules on the normal state conductivity and superconductivity of beta''-(ET)(4)[(H(3)O)Ga(C(2)O(4))(3)].G (G = pyridine,nitrobenzene)

Normal state conductivity and superconductivity together with bulk magnetic susceptibility and magnetization measurements have been measured for two molecular charge-transfer salts: beta' '-(ET)4[(H3O)Ga(C2O4)3]G (ET = bis(ethylenedithio)tetrathiafulvalene, G = pyridine for compound I and nitrobenzene for compound II). With the exception of the included guest molecules (G) the crystal structures are almost identical. Both show minima in their electrical transport at 130 K for I and at 160 K for II, but at lower temperatures their behaviors differ markedly. The resistance of I reaches a maximum at 50 K with a further small peak at 2 K and possible superconductivity only below 2 K, whereas that of II increases continuously down to 7.5 K, where an abrupt transition to a superconducting state occurs.     

X-ray single-crystal structure and magnetic properties of Fe[CH(3)PO(3))] x H(2)O: a layered weak ferromagnet

The crystal and molecular structure of the layered weak-ferromagnet Fe[CH(3)PO(3)] x H(2)O has been solved by X-ray single-crystal diffraction techniques. Crystal data for Fe[CH(3)PO(3)] x H(2)O are the following: orthorhombic space group Pna2(1); a =17.538(2), b = 4.814(1), c = 5.719(1) A. The structure is lamellar, and it consists of alternating organic and inorganic layers along the a direction of the unit cell. The inorganic layers are made of Fe(II) ions octahedrally coordinated by five phosphonate oxygen atoms and one from oxygen of the water molecule. Each phosphonate group coordinates four metal ions, through chelation and bridging, making in this way a cross-linked Fe-O network. The resultant layers are then separated by bilayers of the methyl groups, with van der Waals contacts between them. The compound is air stable, and it dehydrates under inert atmosphere at temperatures above 120 degrees C. The oxidation state of the metal ion is +2, and the electronic configuration is d(6)( )()high spin (S = 2), as determined from dc magnetic susceptibility measurements from 150 K to ambient temperature. Below 100 K, the magnetic moment of Fe[CH(3)PO(3)] x H(2)O rises rapidly to a maximum at T(max) approximately equal to 24 K, and then it decreases again. The onset of peak at T = 25 K is associated with the 3D antiferromagnetic long-range ordering, T(N). The observed critical temperature, T(N), is like all the other previously reported Fe(II) phosphonates, and it appears to be nearly independent of the interlayer spacing in this family of hybrid organic-inorganic layered compounds. Below T(N), the compound behaves as a "weak ferromagnet", and represents the third kind of magnetic materials with a spontaneous magnetization below a finite critical temperature, ferromagnets and ferrimagnets being the other two types.     

Controlled synthesis and magnetic properties of 2D and 3D iron azide networks infinity2[Fe(N3)2(4,4'-bpy)] and infinity3[Fe(N3)2(4,4'-bpy)

Controlled synthesis of transition metal complexes with mixed ligands has led to two new compounds with the same empirical formula [Fe(N3)2(4,4'-bpy)] (4,4'-bpy=4,4'- bipyridine). The compound 2D-[Fe(N3)2(4,4'-bpy)] (I) contains end-on (EO) bridging azido ligands. It crystallizes in the orthorhombic crystal system, space group Cmmm (No. 65): a=11.444(2) A, b=15.181(3) A, c=3.458(1) A, V=600.8(2) A(3), and Z=2. The compound 3D-[Fe(N3)2(4,4'-bpy)] (II) contains end-to-end (EE) azido bridges. It belongs to the tetragonal crystal system, space group P4(1)2(1)2 (No. 92): a=8.132(1) A, b=8.132(1) A, c=16.708(3) A, V=1104.9(5) A(3), and Z=4. Crystals of I and II have been grown by the diffusion method. Phase-pure samples of both compounds have been obtained by means of an optimal solution synthesis. Spontaneous long-range magnetic ordering was found in both I and II, with I being a metamagnet, and II being a ferromagnet. For I, in the low-field region, multiple transitions at TN1=20 K and TN2=5 K were observed, and these indicated the existence of Fe moment reorientation. Heat capacity measurements on II confirmed ferromagnetic transition at TC=20 K.     

Novel transition metal oxalatophosphates with a two-dimensional honeycomb structure: (H3TREN)[M2(HPO4)(C2O4)2.5] x 3H2O (M = Mn(II) and Fe(II), TREN = tris(2-aminoethyl)amine)

Two new layered transition metal oxalatophosphates, (H(3)TREN)[M(2)(HPO(4))(C(2)O(4))(2.5)].3H(2)O (M = Mn(II) and Fe(II)), have been synthesized by hydrothermal methods in the presence of a structure-directing organic amine, tris(2-aminoethyl)amine, and characterized by single-crystal X-ray diffraction and magnetic susceptibility. They are the first metal oxalatophosphates which adopt a two-dimensional honeycomb structure with the organic cations and water molecules intercalated in between. Within a layer, there are 12-membered pores made from 6 Mn, 1 phosphate, and 5 oxalate units. Measurements of field dependence of magnetization and variable-temperature susceptibilities under different fields were performed on a polycrystalline sample of the manganese compound. The results indicate a phase transition from a paramagnetic to an antiferromagnetic coupled state at about 12 K. Crystal data for the manganese compound follow: triclinic, space group Ponemacr; (No. 2), a = 8.8385(6) A, b = 9.0586(6) A, c = 16.020(1) A, alpha = 77.616(1) degrees, beta = 83.359(1) degrees, gamma = 68.251(1) degrees, and Z = 2. Crystal data for the iron compound are the same as those for the manganese compound except a = 8.7776(9) A, b = 8.9257(9) A, c = 15.884(2) A, alpha = 78.630(2) degrees, beta = 84.018(2) degrees, and gamma = 67.372(2) degrees.     

Synthesis, physical properties, multitemperature crystal structure, and 20 K synchrotron X-ray charge density of a magnetic metal organic framework structure, Mn3(C8O4H4)3(C5H11ON)2

A new magnetic metal organic framework material has been synthesized, Mn3(C8O4H4)3(C5H11ON)2, 1. Magnetic susceptibility measurements from 2 to 400 K reveal anti-ferromagnetic ordering at approximately 4 K and a total magnetic moment of 6.0 micro(B). The magnetic phase transition is confirmed by heat capacity data (2-300 K). The crystal structure is studied by conventional single-crystal X-ray diffraction data at 300, 275, 250, 225, 200, 175, 150, 125, and 100 K, and synchrotron data at 20 K. There is a phase transition between 100 and 20 K due to ordering of the diethylformamide molecules. The X-ray charge density is determined based on multipole modeling of a second 20 K single-crystal synchrotron radiation data set. The electron distributions around the two unique Mn centers are different, and both have substantial anisotropy. Orbital population analysis reveals large electron donation (1.7 e) to each Mn atom and the maximum possible number of unpaired electrons is 3.2 for both Mn sites. Thus, there is a considerable orbital component to the magnetic moment. Bader topological analysis shows an absence of Mn-Mn bonding, and the magnetic ordering is via super-exchange through the oxygen bridges. Formal electron counting suggests Mn2+ sites, but this is not supported by the Bader atomic charges, Mn1 = +0.11 e, Mn2 = +0.17 e. The topological measures show the dominant metal-ligand interactions to be electrostatic, and a simple exponential correlation is derived between Mn-O bond lengths and the values of nabla2rho at the bond critical points.     

A novel undecametallic iron(III) cluster with an S = (11)/(2) spin ground state

The reaction of [NEt(4)](2)[Fe(2)OCl(6)] with sodium benzoate, 4,6-dimethyl-2-hydroxypyrimidine (dmhp), and 1,1,1-tris(hydroxymethyl)ethane (H(3)thme) gives the undecametallic compound [NEt(4)][Fe(11)O(4)(O(2)CPh)(10)(thme)(4)(dmhp)(2)Cl(4)]. X-ray crystallography, EPR spectroscopy, bulk magnetic susceptibility studies, and low-temperature single-crystal magnetic measurements were used to characterize the compound. Magnetic measurements indicate an S = (11)/(2) ground state with the parameters g = 2.03 and D = -0.46 cm(-)(1). Single-crystal magnetic studies show hysteresis of molecular origin at T < 1.2 K with fast quantum mechanical tunneling at zero field.     

Synthesis,X-ray powder structure,and magnetic properties of the new,weak ferromagnet iron(II) phenylphosphonate

A new molecule-based weak ferromagnet of formula Fe[C6H5PO3].H2O was synthesized. It was characterized by thermogravimetric analysis and UV-visible and infrared spectroscopy, and the magnetic properties were studied using a superconducting quantum interference device magnetometer. The crystal structure of the compound was determined "ab initio" from X-ray powder diffraction data and refined by the Rietveld method. The crystals of Fe[C6H5PO3].H2O are orthorhombic, space group Pmn2(1), with a = 5.668(8) A, b = 14.453(2) A, c = 4.893(7) A, and Z = 2. The title compound is isostructural with the previously reported lamellar M[C6H5PO3].H2O, M = Mn(II), Zn(II), and Cd(II). The inorganic layers are made of Fe(II) ions octahedrally coordinated by five phosphonate oxygen atoms and one from oxygen of the water molecule. These layers are then separated by bilayers of the phenyl groups, and van der Waals contacts are established between them. The refinement has shown that the phenyl rings are disordered in the lattice. The oxidation state of the metal ion is +2, and the electronic configuration is d6 (S = 2) high-spin, as determined from dc magnetic susceptibility measurements from 150 K to room temperature. Below 100 K, the magnetic moment of Fe[C6H5PO3].H2O rises rapidly to a maximum at TN = 21.5 K, and then it decreases again. The peak at TN is associated with the 3D antiferromagnetic long-range ordering. Below the critical temperature, the title compound behaves as a "weak" ferromagnet, which represents the third type of magnetic materials characterized by having a finite zero-field magnetization, ferromagnets and ferrimagnets being the other two types. The large coercive field (i.e., 6400 G) observed in the hysteresis loop at T = 10 K is rare in molecule-based materials; it can be ascribed to a pronounced spin-orbit coupling for the 5T2g ground state of the Fe(II) ion in the octahedral environment.     

Variable-temperature x-ray structural investigation of [Fe[HC(3,5-Me2pz)3]2](BF4)2 (pz= pyrazolyl ring): observation of a thermally induced spin state change from all high spin to an equal high spin-low spin mixture, concomitant with the onset of nonmerohedral twinning

The complex [Fe[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (pz = pyrazolyl ring) undergoes a phase transition that occurs concomitantly with a thermally induced spin conversion between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states. Above 204 K the compound is completely HS with the structure in the C2/c space group with Z = 4. A crystal structure determination of this phase was performed at 220 K yielding the cell constants a = 20.338(2) A, b = 10.332(1) A, c = 19.644(2) A, beta = 111.097(2) degrees, and V = 3851.5(6) A(3). There is one unique iron(II) site at this temperature. Below 206 K the compound converts to a 50:50 mixture of HS and LS. The radical change in the coordination sphere for half of the iron(II) sites, most notably a shortening of the Fe-N bond distances by ca. 0.2 A, that accompanies this magnetic transition causes a phase transition. The crystal system changes from C-centered monoclinic to primitive triclinic with Z = 2 with two half-molecules on independent inversion centers. A crystal structure determination was performed at 173 K in space group P1 with a = 10.287(2) A, b = 11.355(3) A, c = 18.949(4) A, alpha = 90.852(4) degrees, beta = 105.245(4) degrees, gamma = 116.304(4) degrees, and V = 1892.3(8) A(3). All specimens investigated below the phase transition temperature were determined to be nonmerohedral twins. Temperature cycling between these two forms does not appear to degrade crystal quality. Previous magnetic susceptibility measurements indicate a second, irreversible increase in the magnetic moment the first time the crystals are cooled below 85 K. A crystal structure determination at 220 K of a specimen precooled to 78 K was not significantly different from those not cooled below 220 K.     

Crystal structures and magnetic properties of complexes of M(II)Cl2 (M = Cu, Ni, and Co) coordinated with 4-(N-tert-butyloxyamino)-2-(methoxymethylenyl)pyridine: 2D magnetic anisotropy of the aminoxyl-Co(II) complex in the crystalline state

Three metal complexes, [M(II)Cl2(4NOPy-OMe)2] (M = Cu (1), Ni (2), and Co (3)), were prepared by mixing the corresponding metal chloride and 4-(N-tert-butyloxyamino)-2-(methoxymethylenyl)pyridine, 4NOPy-OMe, in 1:2 ratio. Complex 1 has two structures (complexes A and B) with similar coordination geometries, compressed octahedrons. In the crystal structure, complexes A and B locate alternately in short distances (C(radical)...C(beta) = 3.17 and 3.23 A) to form a 1-D chain structure. Complexes 2 and 3 are isomorphous and have a slightly distorted octahedral structure. In the crystal structure, both complexes have intermolecular short contacts (C(radical)...C(alpha) = 3.46 and 3.52 A for 2 and 3, respectively) to form the 2-D structures. The temperature dependence of the chi(mol)T values for the three complexes indicated that the magnetic interactions between the radicals and the metal ions within the complexes were ferromagnetic. By fitting a modified Fisher 1-D model to the data of the chi(mol)T vs T plot for 1, we estimated the intra- and intermolecular (intrachain) exchange coupling constants to be J1/kB = 60.2 and J2/kB = -7.02 K, respectively. On the other hand, complexes 2 and 3 showed steep increases of the chi(mol)T value below ca. 3 K, indicating that the long-range magnetic ordering is operating. The 1/chi(mol) vs T plot for 2 was analyzed by a Curie-Weiss model to give theta = 6.25 K and C = 2.02 cm3 K mol(-1) with g(Ni) = 2.25. Complex 3 was investigated in more detail using an orientated sample. Magnetic behavior strongly depends on the direction of the applied field, in which the c axis perpendicular to the ab plane is an easy axis for magnetization. Direct current (dc) and alternating current (ac) magnetic susceptibility measurements revealed that complex 3 had a magnetic phase transition of T(c) = 2.14 K and exhibited a glasslike magnetic behavior below T(c).     

A new inorganic-organic hybrid iron(III) arsenate: (C2H10N2)[Fe(HAsO4)2(H2AsO4)](H2O). Hydrothermal synthesis, crystal structure, and spectroscopic and magnetic properties

A new iron(III) arsenate templated by ethylenediamine, (C2H10N2) [Fe(HAsO4)2(H2AsO4)](H2O), has been prepared by hydrothermal synthesis. The unit-cell parameters are a = 8.705(3) A, b = 16.106(4) A, c = 4.763(1) A, beta = 90.63(3) degrees; monoclinic, P2(1) with Z = 2. The compound exhibits a chain structure along the c-axis with the ethylenediammonium cations as counterion. The chains show isolated FeO6 octahedra with two HAsO4 and one H2AsO4 tetrahedra per FeO6 octahedron. The ESR spectrum at 5.0 K is isotropic with a g-value of 2.0, which remains practically unchanged at room temperature. Magnetic measurements indicate the presence of antiferromagnetic interactions. A value of -0.835 K for the J-exchange parameter has been calculated by fitting the magnetic data to a model for antiferromagnetic chains of spin S = 5/2.     

Effect of Nonmagnetic Substitution on the Magnetic Properties and Charge-Transfer Phase Transition of an Iron Mixed-Valence Complex, (n-C(3)H(7))(4)N[Fe(II)Fe(III)(dto)(3)] (dto = C(2)O(2)S(2))

The iron mixed-valence complex (n-C(3)H(7))(4)N[Fe(II)Fe(III)(dto)(3)] exhibits a novel type of phase transition called charge-transfer phase transition (CTPT), where the thermally induced electron transfer between Fe(II) and Fe(III) occurs reversibly at ～120 K, in addition to the ferromagnetic phase transition at T(C) = 7 K. To investigate the mechanism of the CTPT, we have synthesized a series of magnetically diluted complexes (n-C(3)H(7))(4)N[Fe(II)(1-x)Zn(II)(x)Fe(III)(dto)(3)] (dto = C(2)O(2)S(2); x = 0-1), and carried out magnetic susceptibility and dielectric constant measurements and (57)Fe M?ssbauer spectroscopy. With increasing Zn(II) concentration (x), the CTPT is gradually suppressed and disappears at x ≈ 0.13. On the other hand, the ferromagnetic transition temperature (T(C)) is initially enhanced from 7 K to 12 K between x = 0.00 and 0.05, despite the nonmagnetic nature of Zn(II) ions, and then it decreases monotonically from 12 K to 3 K with increasing Zn(II) concentration. This anomalous dependence of T(C) on Zn(II) concentration is related to a change in the spin configuration of the ferromagnetic state caused by the partial suppression of the CTPT.     

Synthesis and characterization of a porous magnetic diamond framework, Co3(HCOO)6, and its N2 sorption characteristic

[Co3(HCOO)6](CH3OH)(H2O) (1), the isostructural analogue of the porous magnet of coordination framework [Mn3(HCOO)6](CH3OH)(H2O), and its desolvated form [Co3(HCOO)6] (2) were prepared and characterized by X-ray and neutron diffraction methods, IR, thermal analyses, and BET, and their magnetic properties were measured. The parent compound, 1, crystallizes in the monoclinic system, space group P21/c, a = 11.254(2) A, b = 9.832(1) A, c = 18.108(3) A, beta = 127.222(2) degrees , V = 1595.5(4) A3, Z = 4, R1 = 0.0329 at 180 K. It possesses a unit cell volume that is 9% smaller than [Mn3(HCOO)6](CH3OH)(H2O) due to the smaller radius of Co2+ ion. Compared with the parent compound 1, the desolvated compound 2 has slightly larger lattice with cell parameters of a = 11.2858(4) A, b = 9.8690(4) A, c = 18.1797(6) A, beta = 127.193(2) degrees , V = 1613.0(1) A3, R1 = 0.0356 at 180 K. The cell parameters of 2, obtained from neutron powder data at 2 K, are a = 11.309(2) A, b = 9.869(1) A, c = 18.201(3) A, beta = 127.244(8) degrees , V = 1617.3(5) A3. The pore volume reduces from 33% to 30% by replacing Mn by Co. The material exhibits a diamond framework based on Co-centered CoCo4 tetrahedral nodes, in which all metal ions have octahedral coordination geometry and all HCOO groups link the metal ions in syn-syn/anti modes. It displays thermal stability up to 270 degrees C. The compound easily loses guest molecules without loss of crystallinity, and it partly reabsorbs water from the atmosphere. Significant N2 sorption was observed for the desolvated framework suggesting that the material possesses permanent porosity. The magnetic properties show a tendency to a 3D long-range magnetic ordering, probably antiferromagnetic with a spin canting arrangement below 2 K.     

Molecular metamagnet [Ni(4ImNNH)(2)(NO(3))(2)] (4ImNNH = 4-imidazolyl nitronyl nitroxide) and the related compounds showing supramolecular H-bonding interactions

A new chelating radical ligand 4ImNNH (2-(4-imidazolyl)-4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide) was prepared, and complexation with divalent transition metal salts gave complexes, [M(4ImNNH)(2)X(2)], which showed intermolecular ferromagnetic interaction in high probability (7 out of 10 paramagnetic compounds investigated here). The nitrate complexes (X = NO(3); M = Mn (1), Co (2), Ni (3), Cu (4)) crystallize isomorphously in monoclinic space group P2(1)/a. The equatorial positions are occupied with two 4ImNNH chelates and the nitrate oxygen atoms are located at the axial positions. Magnetic measurements revealed that the intramolecular exchange couplings in 1, 2, and 4 were antiferromagnetic, while that in 3 was ferromagnetic with 2J/k(B) = +85 K, where the spin Hamiltonian is defined as H = -2J(S(1).S(2) + S(2).S(3)) based on the molecular structures determined as the linear radical-metal-radical triads. The intramolecular ferromagnetic interaction in 3 is interpreted in terms of orthogonality between the radical pi and metal dsigma orbitals. Compounds 1-3 exhibited intermolecular ferromagnetic interaction ascribable to a two-dimensional hydrogen bond network parallel to the crystallographic ab plane. Complex 3 became an antiferromagnet below 3.4 K and exhibited a metamagnetic transition on applying a magnetic field of 5.5 kOe at 1.8 K. The complexes prepared from metal halides, [M(4ImNNH)(2)X(2)] (X = Cl, Br; M = Mn, Co, Ni, Cu), showed intramolecular antiferromagnetic interactions, which are successfully analyzed based on the radical-metal-radical system. The crystal structures determined here on 1-4, [Mn(4ImNNH)(2)Cl(2)], and [Cu(4ImNNH)(2)Br(2)] always have intermolecular hydrogen bonds of H(imidazole).X(axial ligand)-M, where X = NO(3), Cl, Br. This interaction seems to play an important role in molecular packing and presumably also in magnetic coupling.