A metal-insulator transition in R2O2Bi with an unusual Bi2- square net (R = rare earth or Y)

A series of tetragonal ThCr(2)Si(2)-type compounds, R(2)O(2)Bi (R = rare earth or Y), are synthesized in which an unusual Bi(2-) anion forms a square net layer that is sandwiched between (R(2)O(2))(2+) fluorite layers. Two-dimensional (2D) electronic bands around the Fermi energy are predominantly composed of 6p(x)6p(y) orbitals in the Bi(2-) square net, which contains a positive hole per Bi(2-) ion. The decrease in the size of the square net caused by reducing the size of the R ion enhances the electrical conductivity because of the hole, resulting in a "chemical pressure"-induced metal-insulator transition.     

Intriguing interconnections among phase transition, magnetic moment, and valence disproportionation in 2H-perovskite related oxides

In this paper we report the crystal growth, structure determination, and magnetic properties of the 2H-perovskite related oxides, Sr(5)Co(4)O(12) and Sr(6)Co(5)O(15), as well as the charge disproportionation and associated phase transition of Sr(5)Co(4)O(12). Sr(5)Co(4)O(12) and Sr(6)Co(5)O(15) are the (m = 2, n = 3) and (m = 1, n = 1) members of the A(3m+3n)A'(n)B(3m+n)O(9m+6n) family, respectively. Sr(6)Co(5)O(15) crystallizes in the space group R32 with lattice parameters of a = 9.5020(10) ? and c = 12.379(8) ?. The structure solution shows that Sr(6)Co(5)O(15) is isostructural with Sr(6)Rh(5)O(15). Magnetic measurements do not indicate any long-range magnetic order, although the Weiss temperature of -248 K indicates the presence of dominant antiferromagnetic interactions. Sr(5)Co(4)O(12) crystallizes in the space group P-3c1 with lattice parameters of a = 9.4705(10) ? and c = 20.063(5) ? at room temperature. The single crystal structure solution revealed that the cobalt ions in the trigonal prismatic sites of Sr(5)Co(4)O(12) undergo a structural transition at ~170 K, where the cobalt atoms are in the center of the trigonal prisms below this temperature and move partially toward the faces above this temperature. This structure transition is accompanied by a change in the magnetic moment of the oxide and can be related to a valence disproportionation of the cobalt ions and a concomitant Jahn-Teller distortion. In addition, specific heat, Seebeck coefficient, electric conductivity, and magnetic measurements as well as bond valence sum calculations were carried out for Sr(5)Co(4)O(12). Sr(5)Co(4)O(12) exhibits strong magnetic anisotropy but no long-range magnetic order.     

An analysis of the rare earth contribution to the magnetic anisotropy in RCo5 and R2Co17 compounds

An attempt has been made to treat the rare earth contribution to the magnetocrystalline anisotropy in RCo₅ and R₂Co₁₇ compounds with a single ion model using a Hamiltonian of the form:

$\text{H=}\text{B}{}_2{}^0\text{O}{}_2{}^0\text{+gμ}{}_{\mathrm{B}}\text{J·H}{}_{\mathrm{<mtext>ex</mtext>}}$

H_ex is regarded as arising mainly from the cobalt sublattice. Eigenvalues and eigenfunctions for the above Hamiltonian were obtained when the exchange field is perpendicular to the c-axis and compared with those when it is parallel to the c-axis. For values of H_ex estimated from experiment it is found that the sign of B₂⁰ determines the direction preference of the rare earth sublattice magnetization. Comparison of theory with experiment shows that the correct sign of B₂⁰ can be predicted on the point charge model considering only the effect of rare earth nearest neighbors. The calculations also predict that the quantity |K_1R(0) + K_2R(0)| is nearly equal to the crystal field overall splitting (CFOAS) determined in the absence of exchange and is independent of the magnitude of the exchange field, provided that the exchange field is sufficiently large. The temperature dependence of |K_1R + K_2R| has also been calculated and found to agree semiquantitatively with available experimental results.     

Valence instabilities, phase transitions, and abrupt lattice expansion at 5 K in the YbGaGe system

The powder synchrotron X-ray diffraction technique was used to study the thermal expansion behavior of the mixed valence layered compound, YbGa1.05Ge0.95 in the temperature range 3-1123 K. A surprising abrupt isosymmetric phase transition, accompanied by a dramatic volume increase (negative thermal expansion), was found at 5 K induced by a sudden Yb valence transition from +(2 + epsilon) toward +2. At high temperatures, the material undergoes a transformation to a highly disordered structure until it eventually collapses at 1123 K to a structure with isovalent Yb ions and flat Ga/Ge planes (AlB2 type).     

Pressure-induced structural, magnetic, and transport transitions in the two-legged ladder Sr3Fe2O5

The layered compound SrFeO(2) with an FeO(4) square-planar motif exhibits an unprecedented pressure-induced spin state transition (S = 2 to 1), together with an insulator-to-metal (I-M) and an antiferromagnetic-to-ferromagnetic (AFM-FM) transition. In this work, we have studied the pressure effect on the structural, magnetic, and transport properties of the structurally related two-legged spin ladder Sr(3)Fe(2)O(5). When pressure was applied, this material first exhibited a structural transition from Immm to Ammm at P(s) = 30 ± 2 GPa. This transition involves a phase shift of the ladder blocks from (1/2,1/2,1/2) to (0,1/2,1/2), by which a rock-salt type SrO block with a 7-fold coordination around Sr changes into a CsCl-type block with 8-fold coordination, allowing a significant reduction of volume. However, the S = 2 antiferromagnetic state stays the same. Next, a spin state transition from S = 2 to S = 1, along with an AFM-FM transition, was observed at P(c) = 34 ± 2 GPa, similar to that of SrFeO(2). A sign of an I-M transition was also observed at pressure around P(c). These results suggest a generality of the spin state transition in square planar coordinated S = 2 irons of n-legged ladder series Sr(n+1)Fe(n)O(2n+1) (n = 1, 2, 3, ...). It appears that the structural transition independently occurs without respect to other transitions. The necessary conditions for a structural transition of this type and possible candidate materials are discussed.     

High pressure synthesis of late rare earth RFeAs(O,F) superconductors; R = Tb and Dy

New TbFeAs(O,F) and DyFeAs(O,F) superconductors with critical temperatures T(c) = 46 and 45 K and very high critical fields, >/=100 T, have been prepared at 1100-1150 degrees C and 10-12 GPa, demonstrating that high pressure may be used to synthesise late rare earth derivatives of the recently reported RFeAs(O,F) (R = La-Nd, Sm, Gd) high temperature superconductors.     

The sodium D2-line in electric and magnetic fields. Part III: adiabatic hyperfine level transfer in perpendicularly crossed fields

Numerical calculations of the splitting of the hyperfine statesM _F of an atom influenced by crossed electric and magnetic fields were performed. These calculations show that, adiabatically changing the presence and the strength of the fields according to a well defined cycle, atoms populated in a given hyperfine levelF _i before the start of the field cycle may be found in another levelF _f after experiencing the field cycle.     

Kinetics of the Reactions between [(RS)2Fe(µ-S)2M(µ-S)2Fe(SR)2] n− (M = Mo, R = Ph, n = 2; M = V, R = Et, n = 3) and [Fe(SR)4]2−: Key Steps in the Assembly of Cuboidal Fe—S-based Clusters

The reaction of [S₂Mo(μ-S)₂Fe(SPh)₂]²⁻ with [Fe(SPh)₄]²⁻ produces the dicuboidal cluster [{MoFe₃S₄(SPh)₃}₂(μ-(SPh)₃]³⁻ in a three stage process studied by ¹H-n.m.r. spectroscopy and stopped-flow spectrophotometry. The initial stage involves the formation of the linear trinuclear [(PhS)₂Fe(μ-S)₂Mo(μ-S)₂Fe(SPh)₂]²⁻ and the kinetics indicate an equilibrium reaction (k₁^Mo = 2.5±0.3x 10² dm³ mol⁻¹ s⁻¹, k₋₁^Mo = 0.8±0.1 s⁻¹). The second stage involves the reduction of [(PhS)₂Fe(μ-S)₂Mo(μ-S)₂Fe(SPh)₂]²⁻ by [Fe(SPh)₄]²⁻ to form [(PhS)₂Fe(μ-S)₂Mo(μ-S)₂Fe(SPh)₂]³⁻ which subsequently rearranges to form the voided cuboidal [MoFe₂S₄(SPh)₃]²⁻. The kinetics of the second stage exhibits a simple first order dependence on the concentrations of both [(PhS)₂Fe(μ-S)₂Mo(μ-S)₂Fe(SPh)₂]³⁻ and [Fe(SPh)₄]²⁻ (k₂^Mo = 25 ± 2 dm³ mol⁻¹ s⁻¹). The Kinetics of the third stage have not been studied but must involve incorporation of the final Fe and formation of the dicuboidal [{MoFe₃S₄(SPh)₃}₂(μ-SPh)₃]³⁻. The kinetics of the reaction between [(EtS)₂Fe(μ-S)₂V(μ-S)₂Fe(SEt)₂]³⁻ and [Fe(SEt)₄]²⁻ to form [{VFe₃S₄(SEt)₃}₂(μ-SEt)₃]³⁻ have also been studied. An important difference between this reaction and the formation of the analogous [{MoFe₃S₄(SPh)₃}₂(μ-SPh)₃]³⁻ is that the formation of [{MoFe₃S₄(SPh)₃}₂(μ-SPh)₃]³⁻ from [(PhS)₂Fe(μ-S)₂Mo(μ-S)₂Fe(SPh)₂]²⁻ involves a change in the redox state of the cluster, whilst the formation of [{VFe₃S₄(SEt)₃}₂(μ-SEt)₃]³⁻ from [(EtS)₂Fe(μ-S)₂V(μ-S)₂Fe(SEt)₂]³⁻ requires no change in redox state. The reaction between [(EtS)₂Fe(μ-S)₂V(μ-S)₂Fe(SEt)₂]³⁻ and [Fe(SEt)₄]²⁻ involves two stages. The kinetics of the faster phase is associated with a rate law analogous to that observed for the reaction between [(PhS)₂Fe(μ-S)₂Mo(μ-S)₂ Fe(SPh)₂]²⁻ with [Fe(SPh)₄]²⁻: a first order dependence on the concentrations of [(EtS)₂Fe(μ-S)₂V(μ-S)₂Fe(SEt)₂]³⁻ and [Fe(SEt)₄]²⁻ (k₂^V = 1.1±0.1 x 10³ dm³ mol⁻¹ s⁻¹. This observation indicates that whilst there is no change in redox state between [(EtS)₂Fe(μ-S)₂V(μ-S)₂Fe(SEt)₂]³⁻ and [{VFe₃S₄(SEt)₃}₂(μ-SEt)₃]³⁻, reduction is necessary to catalyse the conversion of the linear [(EtS)₂Fe(μ-S)₂V(μ-S)₂Fe(SEt)₂]³⁻ into the voided cuboidal [VFe₂S₄(SEt)₃]³⁻. The results of the studies reported in this paper together with those on other putative reactions involved in the assembly of cuboidal clusters, have been combined to present a scheme of the mechanism of cuboidal cluster assembly. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

A comparative study of the magnetic properties and phase separation behavior of the rare earth cobaltates, Ln0.5Sr0.5CoO3 (Ln=rare earth)

A comparative study of the magnetic properties of a few members of the Ln_0.5Sr_0.5CoO₃ family with different radii of the A-site cations, 〈r_A〉, in the range 1.19-1.40 Å has been carried out. The apparent T_c (where the magnetization undergoes an abrupt increase) decreases markedly with 〈r_A〉 as well as the size-disorder arising from the mismatch in the size of the A-site cations. The value of the magnetization at low temperatures decreases markedly with decrease in 〈r_A〉 or increase in size-disorder, suggesting that the relative proportion of the ferromagnetic (FM) species decreases relative to that of the paramagnetic (PM) species. Such a variation of the FM/PM ratio with composition and temperature is evidenced from the Mössbauer spectra of La_0.5Sr_0.5CoO₃ as well. The variation of the FM/PM ratio with 〈r_A〉 and size-disorder, as well as a local-probe study using ⁵⁹Co Nuclear magnetic resonance spectroscopy suggest that electronic phase separation is an inherent feature of the Ln_0.5Sr_0.5CoO₃ type cobaltates, with the nature of the different magnetic species in the phase-separated system varying with 〈r_A〉 and size disorder.     

Electronic structure,magnetic properties,and mixed valence character of Ce2Ni3Si5 from first principles calculations

Cerium intermetallic compounds exhibit anomalous physical properties such as heavy fermion and Kondo behaviors. Here, an ab initio study of the electronic structure, magnetic properties, and mixed valence character of Ce₂Ni₃Si₅ using density functional theory (DFT) is presented. Two theoretical methods, including pure Perdew–Burke–Ernzerhof (PBE) and PBE + U , are used. In this study, Ce³⁺ and Ce⁴⁺ are considered as two different constituents in the unit cell. The formation energy calculations on the DFT level propose that Ce is in a stable mixed valence of 3.379 at 0 K. The calculated electronic structure shows that Ce₂Ni₃Si₅ is a metallic compound with a contribution at the Fermi level from Ce 4f and Ni 3d states. With the inclusion of the effective Hubbard parameter (U _eff), the five valence electrons of 5 Ce³⁺ ions are distributed only on Ce³⁺ 4f orbitals. Therefore, the occupied Ce³⁺ 4f band is located in the valence band (VB) while Ce⁴⁺ 4f orbitals are empty and Located at the Fermi level. The calculated magnetic moment in Ce₂Ni₃Si₅ is only due to cerium (Ce³⁺) in good agreement with the experimental results. The U _eff value of 5.4 eV provides a reasonable magnetic moment of 0.981 for the unpaired electron per Ce³⁺ ion. These results may serve as a guide for studying present mixed valence cerium‐based compounds. © 2017 Wiley Periodicals, Inc.     

Biferrocene-M(mnt)2 charge-transfer complexes (M = Ni, Co; mnt = maleonitriledithiolate). structure, valence states, and magnetic properties

Charge-transfer salts of branched-alkyl biferrocenes, (1',1' '-R2-1,1' '-biferrocene)[Ni(mnt)2] (1a, R = isopropyl; 2a, R = dineopentyl) and (1',1' '-R2-1,1' '-biferrocene)2[Co(mnt)2]2 (1b, R = isopropyl; 2b, R = dineopentyl), were prepared. Their valence states were investigated using X-ray crystallography and M?ssbauer spectroscopy. Complexes 1a and 1b show segregated-stack crystal structures that contain columns of acceptors, whereas structures of 2a and 2b, which contain bulky donors, are rather discrete. All of the complexes contain mixed-valent biferrocenium monocations. A two-step valence transition was found in complex 1a. The crystal contains two crystallographically independent cations: one undergoes valence localization below room temperature; the other undergoes valence localization below ca. 130 K. The former transition is derived from asymmetry of the crystal environment around the cation, whereas the latter one is caused by symmetry lowering coupled with a spin-Peierls transition (T(C) = 133.2 K) associated with the dimerization of the acceptors. This compound was found to exhibit a dielectric response based on valence tautomerization. Other complexes (1b, 2a, and 2b) show a valence-trapped state. In all complexes, charge localization was found to occur through local electrostatic interactions between the donor's cationic moiety and the acceptor's electronegative moieties.     

Spectral,thermal and magnetic studies of 2-chloro-5-nitrobenzoates of rare earth elements(iii)

The conditions of the formation of yttrium(III) and lanthanide(III) 2-chloro-5-nitrobenzoates were studied and their quantitative composition and solubilities in water at 298 K were determined. The compounds are dihydrates and their solubilities are of the order of 10^-3 mol dm^-3. The FTIR, FIR and X-ray spectra for the complexes were recorded. All complexes are crystalline compounds. Their thermal decomposition was studied only in the range 293-523 K since on heating above 523 K the complexes decompose explosively. Hydrated 2-chloro-5-nitrobenzoates of rare earth elements lose two crystallization water molecules in one step. The enthalpy of dehydration process was established and the magnetic moments of the complexes were determined in the range 77-300 K. The compounds obey the Curie-Weiss law. The 4f electrons do not participate in the formation of the Ln-O bonds. This revised version was published online in August 2006 with corrections to the Cover Date.     

Magnetic Susceptibilities of ThM2X2 Compounds (M = Cr,Mn, Fe,Co, Ni and Cu; X = Si and Ge)

Magnetic susceptibilities of a series of ternary intermetallic compounds of the general composition ThM₂X₂(M = Cr, Mn, Fe, Co, Ni and Cu; X = Si and Ge) were determined by means of FARADAY method in the temperature interval 100–570°K. The results confirm the previously proposed magnetic behaviour of these compounds, based on the observation of the basic crystallographic parameters as a function of the atomic number of the transition metal element. The compounds were found to be alternatively ferromagnetics and antiferromagnetics depending on the atomic number of the transition metal atom.     

La0.5R0.5Ba2Cu3O7,Pr0.5R0.5Ba2Cu3O7以及RBa2Cu3o7（R=稀土）的键共价性计

使用复杂晶体化学键理论计算了Ｌａ０．５Ｒ０．５Ｂａ２Ｃｕ３Ｏ７（Ｒ＝Ｐｒ,Ｎｄ,Ｓｍ,Ｅｕ,Ｇｄ,Ｄｙ,Ｙ,Ｈｏ,Ｅｒ,Ｔｍ,Ｙｂ,Ｌｕ）（Ｌａ－Ｒ１２３）,Ｐｒ０．５Ｒ０．５Ｂａ２Ｃｕ３Ｏ７（Ｒ＝Ｌａ,Ｎｄ,Ｓｍ,Ｅｕ,Ｇｄ,Ｄｙ,Ｈｏ,Ｙ,Ｅｒ,Ｔｍ,Ｙｂ,Ｌｕ）（Ｐｒ－Ｒ１２３）以及ＲＢａ２Ｃｕ３Ｏ７（Ｒ＝Ｌａ,Ｐｒ,Ｎｄ,Ｓｍ,Ｅｕ,Ｇｄ,Ｄｙ,Ｈｏ,Ｙ,Ｅｒ,Ｔｍ）（Ｒ１２３）中Ｃｕ－Ｏ键的键共价性,结果表明Ｐｒ－Ｒ１２３,Ｌａ－Ｒ１２３,以及Ｒ１２３都应具有超导性,而实验结果是Ｌａ０．５Ｐｒ０．５Ｂａ２Ｃｕ０７,Ｒ０．５,Ｐｒ０．５Ｂａ２Ｃｕ３Ｏ７（Ｒ＝Ｌａ,Ｎｄ,Ｓｍ,Ｅｕ,Ｇｄ）无超导性,产生这种矛盾的原因尚不明确,需要做进一步的研究。     

Five-, six- and eight-membered ring organosilicon chalcogenides of the types Z2(SiMe2)2E (Z = Me2Si, H2C; E = S, Se, Te), Z(SiMe2E)2MR2 (Z = Me2Si, H2C, O; E = S, Se, Te; M = Si, Ge, Sn, R = Me, Ph) and (SiMe2ZSiMe2E)2 (Z = Me2Si, H2C; E = S, Se)

Reactions of ClMe₂Si–Z–SiMe₂Cl (Z = SiMe₂ (1a), CH₂ (1c), O (1e)) with Li₂E (E = S, Se) yielded eight-membered ring compounds (SiMe₂ZSiMe₂E)₂ (3a–d) as well as acyclic oligomers (SiMe₂ZSiMe₂E)_x of different chain lengths. If 1:1 molar mixtures of 1a, 1c or 1e and a diorganodichlorosilane, -germane or -stannane (R₂MCl₂) are reacted with Li₂E (E = S, Se, Te), six-membered ring compounds Z(SiMe₂E)₂MR₂ (4a–7g) are formed exclusively. Five-membered rings Z₂(SiMe₂)₂E (Z = SiMe₂ (8a–c), CH₂ (9a–c); E = S, Se, Te) are obtained starting from the tetrasilane ClMe₂Si–(SiMe₂)₂–SiMe₂Cl (1b) or the disilylethane ClMe₂Si–(CH₂)₂–SiMe₂Cl (1d) by treatment with Li₂E. All products were characterized by multinuclear NMR spectroscopy (¹H, ¹³C, ²⁹Si, ¹¹⁹Sn, ⁷⁷Se, ¹²⁵Te, including coupling constants) and the effects of the different ring sizes towards NMR chemical shifts are discussed.     

Structural and magnetic studies on cyano-bridged rectangular Fe2M2 (M = Cu, Ni) clusters

Using the tricyano precursor, (Bu4N)[(Tp)Fe(CN)3] (Tp = Tris(pyrazolyl) hydroborate) (1), four new tetranuclear clusters, [(Tp)Fe(CN)3Cu(Tp)]2.2H2O (2), [(Tp)Fe(CN)3Cu(bpca)]2.4H2O (3) (bpca = bis(2-pyridylcarbonyl)amidate anion), [(Tp)Fe(CN)3Ni(tren)]2(ClO4)2.2H2O (4) (tren = tris(2-amino)ethylamine), and [(Tp)Fe(CN)3Ni(bipy)2]2[(Tp)Fe(CN)3]2.6H2O (5) (bipy = 2,2'-bipyridine), have been synthesized and structurally characterized. The four clusters possess similar square structures, where FeIII and MII (M = CuII or NiII) ions alternate at the rectangle corners. There exist intermolecular - stacking interactions through pyrazolyl groups of Tp- ligands in complexes 2 and 4, which lead to 1D chain structures. Complex 5 shows a 3D network structure through the coexistence of - stacking effects and hydrogen-bonding interactions. Magnetic studies show intramolecular ferromagnetic interactions in all four clusters. The exchange parameters are +11.91 and +1.38 cm(-1) for clusters 2 and 3, respectively, while uniaxial molecular anisotropy can be detected in complex 3 due to the distorted core in its molecular structure. Complex 4 has a ground state of S = 3 and shows SMM behavior with an effective energy barrier of U = 18.9 cm(-1). Unusual spin-glass-like dynamic relaxations are observed for complex 5.     

Structural and magnetic properties of MCl2 (M = Fe, Mn, Co): acetonitrile solvates

MIICl2 (M = Mn, Fe, Co) as their acetonitrile solvates were isolated, and their structural, spectroscopic, and magnetic properties were studied. MCl2(NCMe)2 (M = Fe, Mn) form 1-D chains of octahedral MII ions with four bridging chlorides and two axial MeCN's. The presence of an axial distortion for MFe causes a significant magnetic anisotropy that increases significantly below 150 K; however, chiav [=(chi parallel + 2chi perpendicular)/3] almost coincides with the value obtained on a polycrystalline sample. MnCl2(NCMe)2 is a paramagnet with a weak antiferromagnetic coupling. Annealing FeCl2(NCMe)2 at 55 degrees C forms the monosolvate of FeCl2(NCMe) composition in which two chains collapse into a double chain with formation of Fe-Cl bonding such that half of the mu-Cl's becomes mu3-Cl's. This material orders magnetically below Tc = 4.3 K. For M = Co, paramagnetic tetrahedral [CoCl3(NCMe)]- anions are isolated.     

Infrared and Raman studies of phase transitions in metal–organic frameworks of [(CH3)2NH2][M(HCOO)3] with M=Zn,Fe

We report on temperature-dependent infrared (IR) and Raman studies of [(CH₃)₂NH₂][M(HCOO)₃] metal–organic frameworks (MOFs) with M=Zn, Fe. Based on Raman and IR data, an assignment of the observed modes to respective vibrations of atoms is proposed. Temperature-dependent studies revealed abrupt changes below 160 K that are attributed to the onset of first-order structural phase transition. The most pronounced changes are observed for the modes corresponding to the dimethylammonium cation, especially those involving motion of hydrogen atoms. This behavior proves that the phase transition has an order–disorder character and is associated with the ordering of protons. The abrupt splitting of some modes related to the formate ion indicates that this transition is also associated with significant distortion of the metal-formate framework.     

Generation and EPR Spectroscopy of the First Silenyl Radicals,R2C=Si.−R: Experiment and Theory

The first two persistent silenyl radicals (R₂C=Si^.?R), with a half‐life (t_1/2) of about 30 min, were generated and characterized by electron paramagnetic resonance (EPR) spectroscopy. The large hyperfine coupling constants (hfccs) (a(²⁹Si_α)=137.5–148.0 G) indicate that the unpaired electron has substantial s character. DFT calculations, which are in good agreement with the experimentally observed hfccs, predict a strongly bent structure (?C=Si?R=134.7–140.7°). In contrast, the analogous vinyl radical, R₂C=C^.?R (t_1/2≈3 h), exhibits a small hfcc (a(¹³C_α)=26.6 G) and has a nearly linear geometry (?C=C?R=168.7°).