Screw- Type Motion and Its Impact on Cooperativity in BaNa2Fe[VO4]2.

BaNa2Fe[VO4]2 contains a Jahn-Teller active ion (FeII, 3d6, high-spin) in an octahedral coordination. On the basis of a combination of temperature-dependent X-ray diffraction and Mössbauer and Raman spectroscopies, we demonstrate the coupling of lattice dynamics with the electronic ground state of FeII. We identify three lattice modes combined to an effective canted screw- type motion that drives the structural transition around room temperature from the high-temperature ( P3̅) via intermediate phases to the low-temperature phase ( C2/ c). The dynamics of the electronic ground state of Fe(II) are evident from Mössbauer data with signatures of a motion-narrowed doublet above 320 K, a gradual evolution of the 5Eg electronic state below 293 K, and finally the signature of the thermodynamically preferred orbitally nondegenerate ground state (5Ag) of Fe(II) below 100 K. The continuous nature of the transition is associated with the temperature-dependent phonon parameters derived from Raman spectroscopy, which point out the presence of strong electron-phonon coupling in this compound. We present a microscopic mechanism and evaluate the collective component leading to the structural phase transition.

BaMn9[VO4]6(OH)2: a unique canted antiferromagnet with a chiral "paddle-wheel" structural feature.

BaMn(9)[VO(4)](6)(OH)(2) was synthesized by hydrothermal methods. We evaluated the crystal structure based on the two possible space groups P2(1)3 and Pa3̅ [a = 12.8417(2) Å] using single-crystal and powder X-ray diffraction techniques. The structure contains three-dimensionally linked Mn(9) units of a chiral "paddle-wheel" type. Experimental IR and Raman spectra were analyzed in terms of fundamental vanadate and hydroxide vibrational modes. The magnetic properties were investigated, and the specific heat in applied fields was studied. The dominant magnetic interactions (Mn(2+), S = 5/2) are of antiferromagnetic origin, as indicated by a Curie-Weiss fit above 175 K with Θ ≈ -200 K. Canting of the spins on the geometrically frustrated triangle segment of the structural feature is considered to account for the ferrimagnetic type of long-range order at T(C) ≈ 18 K. We propose a model for the spin structure in the ordered regime. Dielectric constants were measured and indicate a magnetodielectric effect at T(C), which is assigned to spin-lattice coupling.

Crystal structure and magnetic properties of the S = 1/2 quantum spin system Cu7(TeO3)6F2 with mixed dimensionality.

The new oxofluoride Cu7(TeO3)6F2 has been synthesized by hydrothermal synthesis. It crystallizes in the triclinic system, space group P1. The crystal structure constitutes a Cu-O framework with channels extending along [001] where the F(-) ions and the stereochemically active lone-pairs on Te(4+) are located. From magnetic susceptibility, specific heat, and Raman scattering measurements we find evidence that the magnetic degrees of freedom of the Cu-O-Cu segments in Cu7(TeO3)6F2 lead to a mixed dimensionality with single Cu S = (1)/2 moments weakly coupled to spin-chain fragments. Due to the weaker coupling of the single moments, strong fluctuations exist at elevated temperatures, and long-range magnetic ordering evolves at comparably low temperatures (TN = 15 K).

Mild hydrothermal crystal growth, structure, and magnetic properties of ternary U(IV) containing fluorides: LiUF5, KU2F9, K7U6F31, RbUF5, RbU2F9, and RbU3F13.

Single crystals of several ternary alkali uranium fluorides, LiUF5, KU2F9, K7U6F31, RbUF5, RbU2F9, and RbU3F13, have been obtained in a mild hydrothermal process using UO2(CH3CO2)2(H2O)2 as the uranium source. Their crystal structures were determined by single crystal X-ray diffraction. The uranium in the starting reagent was successfully reduced from U(6+) to U(4+) in a dilute hydrofluoric acid environment, aided by the presence of a copper salt. All materials exhibit highly complex crystal structures that range from two-dimensional to three-dimensional. The U(4+) cations are found in high (UF8 and UF9) coordination environments. The magnetic susceptibility measurements yielded effective magnetic moments of 3.01-3.83 µB for the U(4+) cations. The temperature dependent magnetic susceptibility measurements confirmed that the U(4+) cation exhibits a nonmagnetic singlet ground state at low temperatures. No long-range magnetic order was observed for any of the above compositions down to 2 K. Optical and thermal behaviors of the fluorides were also investigated.

AAg2(M'(1/3)M(2/3))[VO4]2: synthesis, magnetic properties, and lattice dynamics of honeycomb-type lattices.

The magnetic honeycomb lattice series of compounds, AAg2(M'1/3M2/3)[VO4]2 with A = Ba(2+), Sr(2+), M' = Mg(2+), Zn(2+), and M = Mn(2+), Co(2+), and Ni(2+), have been synthesized and their physical properties are reported. This series of compounds contains the M' and M cations in a 1:2 ratio on a single crystallographic site. In an ordered arrangement, this could generate a magnetic honeycomb-type lattice. Presented X-ray diffraction data, spectroscopic measurements of lattice dynamics, along with ab initio calculations, magnetic, and specific heat data for these compounds clearly point toward the formation of magnetic honeycomb-type lattices.

Application of a mild hydrothermal approach containing an in situ reduction step to the growth of single crystals of the quaternary U(IV)-containing fluorides Na4MU6F30 (M = Mn2+, Co2+, Ni2+, Cu2+, and Zn2+) crystal growth, structures, and magnetic properties.

A family of rare U(IV)-containing quaternary fluorides, Na4MU6F30 (M = Mn(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+)), was synthesized in single crystal form via a mild hydrothermal technique utilizing an in situ U(VI) to U(IV) reduction step. The modified hydrothermal route is described, and the conditions to obtain single crystals in high yield are detailed. The crystal structures were determined by single crystal X-ray diffraction. The isostructural fluorides crystallize in a new structure type in the trigonal space group P3̅c1. They exhibit a complex three-dimensional crystal structure consisting of corner- and edge-shared UF9 and MF6 polyhedra. The main building block, a U6F30(6-) group, is arranged to create two distinct hexagonal channels, inside which MF6 octahedra and Na(+) cations are located. The copper-containing member of the series, Na4CuU6F30, is unusual in that the Cu(2+) cation exhibits a rare symmetrical coordination environment consisting of six identical Cu-F bond distances, indicating the lack of the expected Jahn-Teller distortion. Magnetic susceptibility measurements of Na4ZnU6F30 yielded an effective magnetic moment of 3.42 µB for the U(4+) (f(2)) cation in the structure. Measurements of the other members containing magnetic transition-metal cations in addition to U(4+), Na4MU6F30 (M = Mn(2+), Co(2+), Ni(2+), and Cu(2+)) yielded total effective magnetic moments of 10.2, 9.84, 8.87, and 8.52 µB for the Mn-, Co-, Ni-, and Cu-containing materials, respectively. No evidence for long-range magnetic ordering was found down to 2 K. Measurements of the magnetization as a function of applied magnetic field at 2 K for Na4MnU6F30 confirmed that the U(4+) magnetic cation exhibits a nonmagnetic singlet ground state at low temperature. Thermal stability measurements and UV-vis diffuse reflectance spectroscopy are also reported.

Synthesis, crystal structures, magnetic, and thermal properties of divalent metal formate-formamide layered compounds.

A series of layered divalent metal formate compounds, [M(HCOO)2(HCONH2)2] (M = Mn (1Mn), Ni (2Ni), Cu(3Cu), Zn(4Zn), Mg(5Mg)), have been prepared by solvothermal synthesis and their room temperature (RT) and low-temperature (LT) crystal structures, and thermal and magnetic properties determined. All the compounds contain octahedral metal ions connected by four anti-anti formato ligands to form (4,4) nets with the composition of M(HCOO)2. The oxygen atoms from two coordinating formamide ligands above and below the layer complete the MO6 distorted octahedral coordination. Order-disorder phase transformations involving the formamide ligands were observed in the 1Mn, 2Ni, and 4Zn compounds. Like transitions in related formate structures with perovskite like topology, the transitions correspond to the ordering of the amine groups of the terminating formamide ligands which are disordered at ambient temperature. The magnetic properties of the three magnetic members of the series 1Mn, 2Ni, and 3Cu were investigated using microcrystalline samples, over the temperature range of 2 K-300 K under different applied fields. All compounds belong to antiferromagnetic square lattices with S = 5/2, 1, and 1/2. Exchange constants for a nearest neighbor model are presented here. Specific heat measurements indicate magnetic long-range order at lower temperatures, S = 5/2 (antiferromagnetic) and S = 1 (ferrimagnetic).

Antimony tartrate transition-metal-oxo chiral clusters.

A chiral precursor K2Sb2(L-tartrate)2 was used for the assembly of three homochiral heterometallic antimony(III)-tartrate transition-metal-oxo clusters: Mn(H2O)6[Fe4Mn4Sb6(µ4-O)6(µ3-O)2(l-tartrate)6(H2O)8]·10.5H2O (1), [V4Mn5Sb6(µ4-O)6(µ3-O)2(L-tartrate)6(H2O)13]·9.5H2O (2), and (H3O)[Ni(H2O)6]2[NiCrSb12(µ3-O)8(µ4-O)3(l-tartrate)6]·6H2O (3). In 1 and 2, the antimony tartrate dimer precursor decomposes and recombines to form Sb3(µ3-O)(L-tartrate)3 chiral trimers, which act as scaffolds to construct negative-charged [Fe4Mn4Sb6(µ4-O)6(µ3-O)2(L-tartrate)6](2-) in 1 and neutral [V4Mn5Sb6(µ4-O)6(µ3-O)2(L-tartrate)6] in 2. The scaffold is flexible and accommodates different types of transition-metal-oxo clusters due to the different possible coordination modes of the L-tartrate ligand. In 3, a two-level chiral scaffold Sb3(µ3-O)(L-tartrate)3Sb3 is formed from the precursor. Two such scaffolds are linked by three bridging oxygen atoms to form a cavity occupied by one Cr(3+) ion and one Ni(2+) ion disordered over two positions. Cr(3+) and Ni(2+) ions are located in two face-shared MO6 octahedra at the center of a negatively charged [NiCrSb12(µ3-O)8(µ4-O)3(L-tartrate)6](3-) cluster.

Ba(1-x)Na(x)Ti2Sb2O (0.0 ≤ x ≤ 0.33): a layered titanium-based pnictide oxide superconductor.

A new layered Ti-based pnictide oxide superconductor, Ba(1-x)Na(x)Ti(2)Sb(2)O (0.0 ≤ x ≤ 0.33), is reported. X-ray studies revealed that it crystallizes in the tetragonal CeCr(2)Si(2)C structure. The undoped parent compound, BaTi(2)Sb(2)O [P4/mmm; a = 4.1196(1) Å; c = 8.0951(2) Å], exhibits a charge density wave (CDW)/spin density wave (SDW) transition at 54 K. Upon chemical doping with Na, the CDW/SDW transition is systematically suppressed, and superconductivity arises with the critical temperature (T(c)) increasing to 5.5 K. Bulk superconductivity was confirmed by resistivity, magnetic, and heat capacity measurements. Like the high-T(c) cuprates and the iron pnictides, the superconductivity in BaTi(2)Sb(2)O arises from an ordered state. Similarities and differences between BaTi(2)Sb(2)O and the cuprate and iron pnictide superconductors are discussed.