Structural analysis of CuGeO3: Relation between nuclear structure and magnetic interaction

Spin-Peierls Distortion of TiPO4 Causing a Transition from a Magnetic to a Nonmagnetic Insulating State and Its Effect on the Thermoelectric Properties: Density Functional Theory Analysis

Density functional theory calculations were carried out to probe the nature of the electronic structure change in TiPO₄ before and after its spin-Peierls distortion at 74.5 K, which is characterized by the dimerization in the chains of Ti³⁺ (d¹) ions present in TiPO₄. These calculations suggest strongly that the electronic state of TiPO₄ is magnetic insulating before the distortion, but becomes nonmagnetic insulating after the distortion. Consistent with this suggestion, the phonon dispersion relations calculated for TiPO₄ show that the undistorted TiPO₄ is stable, while the distorted TiPO₄ is not, if each Ti³⁺ ion has a spin moment, and that the opposite is true if each Ti³⁺ ion has no spin moment. These observations suggest that the driving force for the spin-Peierls distortion is the formation of direct metal-metal bonds leading to the dimerized chains of Ti³⁺ ions. The abrupt change in the electronic structures from a magnetic insulating state to a nonmagnetic insulating state explains why the spin-Peierls distortion of TiPO₄ exhibits a first-order character. Although the two electronic states of TiPO₄ before and after the distortion have a band gap, the substantial spin-Peierls distortion is found to enhance the thermoelectric properties of TiPO₄.

Charge and Bonding in CuGeO3 Nanorods

We combine infrared and Raman spectroscopies to investigate finite length scale effects in CuGeO₃ nanorods. The infrared-active phonons display remarkably strong size dependence whereas the Raman-active features are, by comparison, nearly rigid. A splitting analysis of the Davydov pairs reveals complex changes in chemical bonding with rod length and temperature. Near the spin-Peierls transition, stronger intralayer bonding in the smallest rods indicates a more rigid lattice which helps to suppress the spin-Peierls transition. Taken together, these findings advance the understanding of size effects and collective phase transitions in low-dimensional oxides.

Dramatic enhancement of spin–spin coupling and quenching of magnetic dimensionality in compressed silver difluoride

The high-pressure HP2 form of AgF₂ features Ag₂F₇³⁻ units that are theoretically predicted to host extremely strong antiferromagnetic interactions, surpassing those seen in copper(ii) oxides.

Lattice and magnetic dynamics of a quasi-one-dimensional chain antiferromagnet PbFeBO4

A group of recently synthesized orthorhombic Pnma crystals PbMBO₄ (M  =  Cr, Mn, Fe) demonstrates a number of unusual structural and magnetic properties. We report on polarized Raman scattering study of the lattice and magnetic dynamics in single crystals of an antiferromagnet PbFeBO₄ below and above [Formula: see text] K. Polarization properties of the observed magnetic excitations below T _N as well as intense quasi-elastic scattering support the quasi-one-dimensional character of the magnetic structure of PbFeBO₄. Frequency overlapping of magnetic excitations and low-frequency phonons in the range of 90-200 cm^-1 leads to pronounced asymmetric anomalies thus confirming intrinsic coupling of magnetic and lattice subsystems. This conclusion is also supported by observation of anomalous temperature behaviour of higher frequency phonons in the vicinity of T _N. Experimental investigations are supported by relevant magnetic symmetry analysis which allows us to explain previously observed anomalous results.

Magnetic ordering of defects in a molecular spin-Peierls system

With interest in charge transfer compounds growing steadily, it is important to understand all aspects of the underlying physics of these systems, including the properties of the defects and interfaces that are universally present in actual experimental systems. For the study of these defects and their interactions a spin-Peierls (SP) system provides a useful testing ground. This work presents an investigation within the SP phase of potassium TCNQF₄ where anomalous features are observed in both the magnetic susceptibility and ESR spectra for temperatures between 60 K and 100 K. Muon spin spectroscopy measurements confirm the presence of these anomalous magnetic features, with low temperature zero-field data exhibiting the damped oscillatory form that is a characteristic signature of static magnetic order. This ordering is most likely due to the interaction between structurally correlated magnetic defects in the system. The critical behaviour of the temperature dependent muon spin rotation frequency indicates that a 2D Ising model is applicable to the magnetic ordering of these defects. We show that these observations can be explained by a simple model in which the magnetic defects are located at stacking faults, which provide them with a 2D structural framework to constrain their interactions.

Spin gap in the zigzag spin-1/2 chain cuprate Sr(0.9)Ca(0.1)CuO(2)

We report a comparative study of (63)Cu nuclear magnetic resonance spin lattice relaxation rates T(1)(-1) on undoped SrCuO(2) and Ca-doped Sr(0.9)Ca(0.1)CuO(2) spin chain compounds. A temperature independent T(1)(-1) is observed for SrCuO(2) as expected for an S=1/2 Heisenberg chain. Surprisingly, we observe an exponential decrease of T(1)(-1) for T<90 K in the Ca-doped sample evidencing the opening of a spin gap. The data analysis within the J(1)-J(2) Heisenberg model employing density-matrix renormalization group calculations suggests an impurity driven small alternation of the J(2)-exchange coupling as a possible cause of the spin gap.

Structural aspects of spin-chain and spin-ladder oxides

An overview is presented of the various oxidic compounds, that exhibit low-dimensional magnetic properties. These include spin-ladder and spin-chain compounds as well as compounds with a low-dimensional electronic band responsible for the magnetic order. The basic structures at room temperature are presented, and their dimensionality is discussed. The structural variations with temperature and magnetic field are analysed. Particular attention is given to the superstructures as they occur upon phase transitions involving the development of magnetic order and charge order. The consequences of these superstructures for the interpretation of the physical properties are reviewed.

Crystal structure of copper polysilicate, Cu[SiO3]

For 40 years the copper polysilicate Cu[SiO3], isotypic with the germanate prototype Cu[GeO3], was considered non-existent. Recently however we discovered this compound as a metastable decomposition product of the silicate mineral dioptase, Cu6[Si6O18] · 6 H2O. Its orthorhombic unit-cell was refined from Guinier data giving a = 4.6357(6) Å, b = 8.7735(11) Å and c = 2.8334(4) Å. Using powder diffraction data, the crystal structure was determined in the space group Pbmm with Z = 2 formula units, dx = 4.024 g· cm-3. The reliability factors after least-squares refinement are RF = 0.035 and wR 2 = 0.047. The crystal structure consists of einer single chains of silicate tetrahedra running along [001]. These chains are connected by chains of edge-sharing Cu04+2 ‘octahedra’ with two more distant oxygen neighbours, a result of the Jahn-Teller effect. The interatomic distances were found to be (2×) 1.582(7) Å and (2×) 1.640(7) Å for Si–O; (4×) 1.941(4) Å and (2×) 2.926(7) Å for Cu–O. Using this data, the cationic bond valence sums were calculated in accordance with the valencies and found to be 3.96 for Si4+, respectively 2.00 for Cu2+. The valence angles of about 119.5° obtained between silicon atoms and bridging oxygen atoms (ideally 109.47° for the tetrahedral O–Si–O angles and about 139° for the Si–O br –Si one) are evidently the topological limits for condensed silicate anions. Being a quasi-onedimensional antiferromagnetic material, the prototype Cu[GeO3] belongs to a few inorganic compounds that exhibit a Spin-Peierls transition. It is expected that Cu[SiO3] will also show magnetic properties of considerable interest.

Isotypism and low-dimensional physical properties of CuAs2/3H2/3O3 (theoparacelsite), the spin-Peierls phase CuGeO3 and CuSiO3

Theoparacelsite is a recently discovered orthorhombic copper arsenate hydroxide mineral with the ideal chemical formula Cu3As2O7(OH)2, but because of disorder only two smaller formula units CuAs2/3H2/3O3 are established in the orthorhombic unit cell with lattice parameters a = 4.6644(5) Å, b = 8.3212(8) Å and c = 2.9377(3) Å, and space group Pbmm. We show that this rare mineral is iso-typic to the unique inorganic spin-Peierls phase CuGeO3 (TSP = 14.3 K) and to the recently discovered copper polysilicate, CuSiO3, which is also a quasi-one-dimensional S = 1/2 anti-ferromagnetic chain compound (TN = 7.9 K). We discuss in detail the previously reported crystallographic data of the phases at room temperature in view of its ability to attain similar low-dimensional physical properties and conclude that the arsenate with its S = 1/2 spin chains of edge-sharing CuO4+2 octahedra may be a further inorganic compound that could show the spin-Peierls effect. Although the Cu-O(2)-Cu angle, which is considered to be important for the magnetic interaction and ground state properties, has been found for the arsenate (98.8°) almost identical to that of the germanate (99.1°), the modified inter-chain coupling of the arsenate as a defect variant of the CuGeO3 structure type may suppress the spin-Peierls effect and favour other states of magnetic order (chiral spin chains, AF order). With respect to the rarity of the mineral some remarks on synthesis routes are made.

Frustrated cuprate route from antiferromagnetic to ferromagnetic spin-1/2 Heisenberg chains: Li2ZrCuO4 as a missing link near the quantum critical point

From thermodynamics, local spin density approximation+Hubbard U studies and exact diagonalizations of a five-band Hubbard model on CuO2 stripes we find that Li2ZrCuO4 (Li2CuZrO4 in traditional notation) is close to a ferromagnetic critical point. Analyzing its susceptibility chi(T) and specific heat cp(T,H) within a Heisenberg model, we show that the ratio of the 2nd to the 1st neighbor exchange integrals alpha=-J2/J1 approximately 0.3 is close to the critical value alphac=1/4. Comparing with related chain cuprates we explain the rather strong field dependence of cp, the monotonic downshift of the peak of chi(T), and its increase for alpha-->alphac+0.

Heterogeneous catalyst design using stochastic optimization algorithms

We describe the application of two stochastic optimization algorithms to heterogeneous catalyst design. In particular, we discuss the optimal design of a two-component catalyst for the diffusion limited A + B --> 0 and A + B2 --> 0 reactions in which each of the reactants are adsorbed specifically on one of the two distinct catalytic sites. The geometric arrangement of the catalytic sites that maximizes the catalyst activity is determined by the use of a genetic algorithm and a simulated annealing algorithm. In the case of the A + B --> 0 reaction, it is found that the catalyst surface with the optimal active site distribution, that of a checkerboard, is approximately 25% more active than a random site distribution. A similar increase in catalytic activity is obtained for the A + B2 --> 0 reaction. While both the genetic and simulated annealing algorithms obtain identical optimal solutions for a given reaction, the simulated annealing algorithm is shown to be more efficient.

Neutron scattering study of the field-induced soliton lattice in CuGeO3

CuGeO3 undergoes a transition from a spin-Peierls phase to an incommensurate phase at a critical field of H(c) approximately 12.5 T. In the high-field phase a lattice of solitons forms, with both structural and magnetic components, and these have been studied using neutron scattering techniques. Our results provide direct evidence for a long-ranged magnetic soliton structure which has both staggered and uniform magnetizations with amplitudes that are broadly in accord with theoretical estimates. The magnetic soliton width gamma(m) and the field dependence of the incommensurability deltak(SP) are found to agree well with theoretical predictions.