Composition-controlled metal-insulator transitions and minimum metallic conductivity in the oxide systems LaNi1−xMxO3 (M = Cr,Mn, Fe,or Co)

The composition-controlled metal-insulator transition in the perovskite systems LaNi_1?xM_xO₃ (M = Cr, Mn, Fe, and Co) has been investigated by transport measurements over the temperature range 12–300 K. These systems, which have critical electron densities (n_c) in the range (1–2) × 10²⁰ electrons cm^?3, exhibit sharp metal-insulator transitions at the base temperature. The corresponding minimum metallic conductivity (σ_min), separating the localized and itinerant electronic regimes, is of the order of 10² ohm^?1 cm^?1. Particular attention is paid to the idea of σ_min scaling with n_c, and our present results are compared with earlier studies of the metal-insulator transition in low (e.g., Ge:Sb) and high (e.g., metal-ammonia, supercritical Hg) electron-density systems. A link is established between the transport and magnetic properties of the title systems at the metal-insulator transition.     

Microstructure and rheological properties of polybutadienes

The viscosities, rubbery deformations, densities, and their dependence on temperature have been measured for several series of polybutadienes with molecular weights ranging from 5,000 to 400,000 and differing in proportions of cis and trans structures (cis content from 40 to 95%). On the basis of the viscosity measurements the critical molecular weight M_c has been determined, corresponding to a sharp change in the nature of the viscosity versus molecular weight dependence. Rubbery deformations are displayed pronouncedly in specimens with M > M_c and are closely related to the appearance of non-Newtonian flow. The value of M_c depends on the relative content of cis and trans forms. When M > M_c, the initial viscosity is a parameter sensitive to the microstructure of polybutadienes, so that with at a single molecular weight, depending on the ratio of cis and trans units, the viscosity may vary over a more than tenfold range. The glass transition temperature and activation energy of viscous flow rise regularly with increasing trans content in the polymer chain, these parameters becoming independent of the molecular weight for specimens with M > M_c within a series of polybutadienes of equal microtacticity. Thermomechanical investigations of polybutadienes also made it possible to define more accurately the boundaries of the crystallization region and the dependence of the melting point on the microtacticity. The results obtained are discussed on the basis of modern ideas of polymer structure.     

Comparative curing kinetics of 1,4-bis (4-diaminobenzene-1-oxygen) n-butane and 4,4′-bis-(diaminodiphenyl) methane with tetraglycidyl methylene dianiline systems

Aromatic amine curing agent with flexible unit in backbone, 1,4-bis (4-diaminobenzene-1-oxygen) n-butane (DDBE), was synthesized, and the structure was confirmed by FT-IR and ¹H NMR. The curing kinetics of tetraglycidyl methylene dianiline (TGDDM, or AG80) using DDBE and 4,4′-bis-(diaminodiphenyl) methane (DDM) as curing agents, respectively, were comparatively studied by non-isothermal DSC with a model-fitting Málek approach and a model-free advanced isoconversional method of Vyazovkin. The dynamic mechanical properties and thermal stabilities of the cured materials were investigated by DMTA and TG, respectively. The results showed that the activation energy of AG80/DDBE system was slightly higher than that of AG80/DDM system. ?esták-Berggren model can generally simulate well the reaction rates of these two systems. DMTA measurements showed that the storage modulus of cured AG80/DDBE is similar to that of cured AG80/DDM at the temperature below glass transition temperature (T _g) and lower than that of cured AG80/DDM at the temperature above glass transition temperature, while T _g of cured AG80/DDBE is lower than that of cured AG80/DDM. TG showed that the thermal stabilities of these two cured systems are similar.     

Synthesis of molecular magnetics based on cobalt trisoxalate. Structural and magnetic properties of NBun 4[MnIICoIII(C2O4)3]

Bimetallic oxalate-bridged complexes Q[M^IICo(C₂O₄)₃] (Q=Me₄N⁺, Buⁿ ₄P⁺; M^II=Mn, Co, Ni, Cu, Zn) were synthesized. Single crystals of [NBuⁿ ₄][Mn^IICo^III(C₂O₄)₃] were studied by XRD. Unit cell parametersa=b=9.242(3) Å,c=54.524(13) Å; space groupR3c. Magnetic measurements indicate the absence of a magnetic phase transition up to the temperature of liquid helium. The XRD data confirm the presence of Co^III ions with a low-spin configuration in the crystal.     

The kerr effect in the vicinity of the transition from the isotropic to smectic a phase

The equilibrium electrooptical effect in the isotropic phase of seven liquid crystalline substances (4-n-decyloxy-4′-cyanobiphenyl, carbosilane dendrimer of the fourth generation with cyanobiphenyl terminal fragments, and the fourth, fifth, sixth, seventh, and tenth members of the homologous series of 4-n-acylphenylene 4′-n-alkoxybenzoates) was studied. The smectic A phases were found to exhibit weaker divergence of the Kerr constant in the vicinity of the T _c isotropic melt-liquid crystal phase transition temperature compared with the nematic phases. The difference between T _c and the temperature T* of the virtual second-order phase transition varied from 3.2 to 19.0 K for the smectic A phases, which substantially exceeded the value (T _c-T*) ≤ 1 K for the nematic phases. A theory of the electrooptical properties of isotropic melts in the vicinity of the T _c temperature of the phase transition from the isotropic to smectic A phase is developed. An equation relating the T _c-T* difference to the phenomenological coefficients of the expansion of the Helmholtz energy of an isotropic melt into a series in powers of the coordination and orientational order parameters is obtained.     

Thermosetting epoxidized polybutadiene/low-molecular weight polyphenylene oxide compatible system with quite low-dielectric constant and loss prepared through co-curing reaction

Through the redistribution reaction of PPO, the hydroxyl-terminated low molecular weight PPO (PPO-OH) was prepared. Furthermore, after the reacting of PPO-OH with methacrylic anhydride, the double-bond-terminated low-molecular weight PPO (R-PPO) were synthesized, its M_n was 2631 g/mol. The R-PPO were co-cured with epoxidized polybutadiene (JP-100) to prepare JP-100/R-PPO cured systems. The JP-100/R-PPO cured systems showed single-phase morphology. The co-curing reaction with R-PPO could effectively improve the thermal performance of JP-100, the glass transition temperature (T_g) of cured JP-100/R-PPO systems raised greatly. For the cured JP-100/R-PPO (100/30) system, its T_g was 192°C. The dielectric constant and dissipation factor of cured JP-100/R-PPO systems were both decreased and showed good frequency stability. The dielectric constant and dissipation factor at 1 MHz of cured JP-100/R-PPO (100/30) system were 2.61 and 0.0038, respectively, were obviously lower than those of cured JP-100 system (which were 3.10 and 0.0099, respectively). The JP-100/R-PPO systems exhibited an excellent thermal stability. The 5% weight loss temperature (T_5%) of cured JP-100/R-PPO system was around 340°C.     

Exciton transitions in quasi-one-dimensional crystals. 9,10-dichloroanthracene

Polarized reflection spectra of the first singlet transition of the α-crystalline form of 9,10-dichloroanthracene are reported. Crystal faces (001), (011) and (010) were examined in spectral range 450 to 350 nm at two temperatures, 5 K and 300 K. Two systems of transitions were observed. The first system is assigned to neutral excitons. Spectral similarities with unsubstituted anthracene and arguments based on the one-dimensional stacking of molecules are used to construct a model of the exciten band structures. The M-polarized ππ* molecular transition gives rise to a four branch band with two allowed transitions. The 0-0_b (A_g → A_u) transition lies 50–100 cm^?1 above the bottom of the exciton band and the 0-0_c′ (A_g → B_u) transition lies at the top of the band. In the reflection spectrum the Davydov splitting c′b for transverse excitons is 210 cm^?1. The exciton band of the 00 molecular transition is not isolated but overlaps the two-particle manifold of the 0–1 vibronic transition. As a result of the 0–1_c′ transition is unexpectedly strong in the spectra of the (010) face. The second system is polarized along the stack-axis a and starts 2500 cm^?1 above the first system. It is tentatively assigned as |a(A_g → B_u) charge transfer exciton transition in agreement with earlier observations.     

The structural chemistry of Bi14MO24 (M=Cr, Mo, W) phases: bismuth oxides containing discrete MO4 tetrahedra

The phases Bi₁₄MO₂₄ (M=Cr, Mo, W) have been studied using differential scanning calorimetry, variable temperature X-ray powder diffraction and neutron powder diffraction. All three compounds were found to undergo a phase change, on cooling, from the previously reported tetragonal symmetry (I4/m) to monoclinic symmetry (C2/m). Transition temperatures were determined to be ∼306 K (M=W) and ∼295 K (M=Mo), whereas a gradual transition between 275 and 200 K was observed for M=Cr. The high and low temperature structures are very similar, as indicated by the relationship between the monoclinic and tetragonal unit cell parameters: a_m=√2a_t, b_m=c_t, c_m=a_t, β∼135°. High-resolution neutron powder diffraction data, collected at 400 and 4 K, were used to establish the nature of the transition, which was found to involve a reduction in the statistical possibilities for orientation of the MO₄ tetrahedra. However, in both tetragonal and monoclinic variants, a degree of orientational disorder of the tetrahedra occurs to give partially occupied sites in the average unit cell.     

The electrochemical Peltier effect observed with electrode reactions of Fe(II)/Fe(III) redox couples at a gold electrode

The electrochemical Peltier effect was studied at a gold electrode in solutions containing some Fe(II)/Fe(III) redox couples by measuring the local temperature change in the electrode/solution interphase under controlled-potential and controlled-current polarization. Relative values of the electrochemical Peltier coefficient for the cathodic process at equilibrium potential, which is denoted by (Π_c)_I=0, were determined by analyzing the observed temperature change as a function of current. The values of (Π_c)_I=0 were found to be positive for the Fe(H₂O)₆²⁺/Fe(H₂O)₆³⁺ systems in HClO₄ (1 M), HNO₃ (1 M), H₂SO₄ (0.5 M), and HCl (1 M), their magnitudes being very similar in the first three acid solutions, but smaller in the HCl solution. On the other hand, a negative value of (Π_c)_I=0 was obtained in the case of a Fe(CN)₆^4?/Fe(CN)₆^3? couple in a H₂SO₄ (0.5 M) solution. Such a difference in the Peltier coefficient is considered to be due to the difference in the ionic species of iron involved in the electrode reaction.     

Room temperature ferromagnetism in undoped and Fe doped ZnO nanorods: Microwave-assisted synthesis

One-dimensional (1D) undoped and Fe doped ZnO nanorods of average length ∼1 μm and diameter ∼50 nm have been obtained using a microwave-assisted synthesis. The magnetization (M) and coercivity (H_c) value obtained for undoped ZnO nanorods at room temperature is ∼5×10⁻³ emu/g and ∼150 Oe, respectively. The Fe doped ZnO samples show significant changes in M -H loop with increasing doping concentration. Both undoped and Fe doped ZnO nanorods exhibit a Curie transition temperature (T_c) above 390 K. Electron spin resonance and Mössbauer spectra indicate the presence of ferric ions. The origin of ferromagnetism in undoped ZnO nanorods is attributed to localized electron spin moments resulting from surface defects/vacancies, where as in Fe doped samples is explained by F center exchange mechanism.     

The crystal structure and phase transitions of LiBaPO4

Polymorphism in lithium barium phosphate (LiBaPO₄) was investigated by synchrotron X-ray diffraction (sXRD) and high temperature X-ray diffraction (HT-XRD). Two modifications were isolated using different cooling rates from the synthesis temperature to room temperature. The slowly-cooled sample exhibited a monoclinic structure with a Cc space group (denoted as M-LiBaPO₄) while the quenched sample belonged to a trigonal system with a P31c space group (denoted as T-LiBaPO₄). In both structures, LiO₄ and PO₄ tetrahedra are linked alternatively by sharing each corner in the lattice, forming tridymite-type six-membered rings. The voids in the anionic framework are filled by Ba atoms. The monoclinic distortion in M-LiBaPO₄ can be attributed to “polyhedral tilting” which shifts the axial bridging oxygens from the centroid of the PO₄ tetrahedron, breaking the three-fold symmetry in the trigonal structure. The HT-XRD data upon heating from 298 to 1373 K indicate successive structural transitions as follows; M-LiBaPO₄ (Cc) → T-LiBaPO₄ (P31c) → H-LiBaPO₄ (P6₃) → O-LiBaPO₄ (Pmcn). H-LiBaPO₄ and O-LiBaPO₄ denote the phases exhibiting the hexagonal and orthorhombic systems, respectively. The thermal evolution of the crystal structure of LiBaPO₄ is quite similar to that of LiKSO₄. The sequences of space group change in both compounds are nearly identical and only the transition temperatures differ.     

Thermal study of glass transitions in binary systems of simple hydrocarbons

Glass transition phenomena of four binary systems composed of simple hydrocarbons were studied by means of the differential thermal analysis (DTA). For all the systems, a definite glass transition was observed and a monotonous relation between the glass transition temperature (T _g) and composition (x) was obtained. The composition dependence ofT _g was analyzed in terms of the entropy theory based on the regular solution model. The theoretical prediction could not reproduce our results other than (1-butene)_x(1-pentene)_1?x system. This disagreement is considered to be due to deviations of the present systems from the regular solution, and the accompanying excess configurational entropy S_c ^E was estimated as a function of composition. Extraordinarily large values of S _c ^E ? were obtained for (propene)_x(propane)_1?x and (propene)inx(1-pentene)_1?x systems.     

Crystal structures and magnetic properties of the honeycomb-lattice antiferromagnet M2(pymca)3(ClO4), (M = Fe,Co, Ni)

We report on the syntheses, crystal structures, and magnetic properties of a series of transition metal coordination polymers M₂(pymca)₃(ClO₄), (pymca = pyrimidine-2-carboxylic acid, M = Fe (1), Co (2), and Ni (3)). These compounds are found to crystallize in a trigonal crystal system, space group P31m, with the lattice constants a = 9.727 Å and c = 5.996 Å for 1, a = 9.608 Å and c = 5.996 Å for 2, and a = 9.477 Å and c = 5.958 Å for 3 at room temperature. In these compounds, each pymca ligand connects to two M²⁺ ions, forming a honeycomb network in the ab plane. The temperature dependences of magnetic susceptibilities in these compounds show broad maxima, indicating antiferromagnetic interactions within two-dimensional honeycomb layers. We also observed an antiferromagnetic phase transition at low temperatures by magnetic susceptibility and heat capacity measurements. From the crystal structures and magnetic properties, we conclude that the compounds 1, 2, and 3 are good realizations of honeycomb-lattice antiferromagnets.     

Iron-57 Mössbauer effect study of the distribution of divalent and trivalent ions in potassium transition metal fluorides having the tetragonal bronze structure

Mössbauer studies are reported for compounds of the type KM^IIM^IIIF₆ (M = first-row transition metal ion, Zn, or Mg) which have the tetragonal bronze structure. The results of this investigation provide evidence that the trivalent ions are located on the 8(j) sites of the bronze structure and that the divalent ions are distributed over both the 8(j) and 2(c) sites.     

New defect vanadium dioxide phases

Two new monoclinic V₂O₄ phases were prepared at high pressure from the regular monoclinic (M₁) form of V₂O₄. The unit cell dimensions for the unmodified monoclinic (M₂) phase are: a = 9.083, b = 5.763, c = 4.532 Å, and β = 91.30°. The space group $\text{C 2}\text{m}$ is consistent with the crystallographic data. The new vanadium dioxide exhibited a structural transition and an abrupt, reversible change in resistivity (approx. 4 orders of magnitude) at 66°C similar to that observed in M₁-type V₂O₄. This new form of V₂O₄ is believed to be stabilized by chemical and structural defects. Controlled substitution of V⁵⁺ for V⁴⁺ in the structure led to yet another monoclinic (M₃) phase. This phase is closely related to the M₂ phase. The M₃ unit cell dimensions are: a = 4.506, b = 2.899, c = 4.617 Å, and β = 91.79°, having the space group $\text{P 2}\text{m}$. The substitution of V³⁺ yielded only monoclinic (M₁) derivatives. The modified products have varied semiconductor to metal transition temperatures which depend on the type and amount of substitution and defect structure.     

Crystal orientation and melting behavior of poly(ɛ-Caprolactone) under one-dimensionally “hard” confined microenvironment

Crystal orientation and melting behavior of poly(ε-caprolactone) in a diblock copolymer of poly(ε-caprolactone)-block-poly(2,5-bis[4-methoxyphenyl]oxycarbonyl)styrene) (PCL-b-PMPCS) was investigated. The degrees of polymerization of the PCL and PMPCS block are 200 and 98, respectively. With the PMPCS in a columnar liquid crystalline phase, the diblock is rod-coil one, which exhibits a lamellar phase morphology with the PCL layer thickness of 15.2 nm. Since the glass transition temperature of PMPCS block is much higher than the melting temperature of PCL, the crystallization of PCL is in a one-dimensionally "hard" confinement environment. Mainly on the basis of two-dimensional wide-angle X-ray diffraction experiments, we identified the orientation of PCL isothermally crystallized at various crystallization temperatures (Tcs). At high Tcs (Tc≥10℃), the c-axis of the PCL crystal is along the layer normal of the microphase-separated sturcture. Decreasing Tc can result in the tilting of PCL c-axis with respect to the layer normal. The lower the Tc is, the more the c-axis inclines. Meanwhile, the b-axis of PCL remains perpendicular to the layer normal. At a very low Tc of -78℃, the orientation of the PCL crystals is completely random. For the samples isothermally crystallized at Tc≤10℃, double melting behavior can be observed. While the low temperature endotherm reflects the melting of the crystals originally formed at the Tc applied, the high temperature one is associated with the crystals subjected to the process of recrystallization/reorganization upon heating due to the annealing effect.     

On the characterization of BiMO2NO3 (M=Pb, Ca, Sr, Ba) materials related with the Sillén X1 structure

The compounds BiMO₂NO₃, with M=Pb, Ca, Sr, and Ba, were obtained as single-phase products from solid-state reactions in an atmosphere of nitrous gases. The oxide nitrates with Pb and Ca crystallize in the tetragonal space group I4/mmm with two formula units per unit cell; the oxide nitrates with Sr and Ba crystallize in the orthorhombic space group Cmmm with four formula units per unit cell. Lattice parameters at room temperature are a=397.199(4), c=1482.57(2) pm for M=Pb; a=396.337(5), c=1412.83(3) pm for M=Ca; a=1448.76(3), b=567.62(1), c=582.40(1) pm for M=Sr and a=1536.50(8), b=571.67(3), c=597.55(3) pm for M=Ba. The structures, which were refined by powder X-ray diffraction, consist of alternating [BiMO₂]⁺ and [NO₃]⁻ layers stacked along the direction of the long axis. IR and thermogravimetric data are also given. The various M²⁺ cations in BiMO₂NO₃ are compatible with each other; therefore and because of their layer-type structure, these compounds are interesting precursors for oxide materials, e.g., the HTSC compounds (Bi,Pb)₂Sr₂Ca_n−1Cu_nO_x.     

The study of phase transition of KHF2 doped with sodium ions with Raman intensities of the lattice modes as the order parameter

The phase transition of doped K_1-xNa_xHF₂ is studied with the K⁺ translatory mode and the HF⁻₂ librational mode, which are Raman active in the low temperature phase while inactive in the high temperature phase, as the order parameter. Particular attention is paid to the elucidation of the critical exponent β which is defined as the parameter relating the lattice Raman intensity to the temperature near T_c through (T_c - T)^β. β values for various dopings are interpreted to understand the dynamics of the phase transition in the critical region. The analysis shows that a very small amount of doping results in a drastically large effect in the critical region. Further doping does not induce further a critical effect in a proportional way. This enables one to define the undoped and doped systems with a very small amount of dopant as two distinct states.     

Crystal structure of Na2MMgP2O8 (M: Ba, Sr, Ca) orthophosphates and their luminescence properties activated by Eu; analogous structural behaviors of glaserite-type phosphates and silicates

Rietveld refinements of X-ray powder diffraction data and vibrational spectroscopy have confirmed the crystal structure of Na₂MMgP₂O₈ (M: Ba, Sr, Ca) prepared by a standard solid state reaction. They have glaserite-type layered structure. Na₂MMgP₂O₈ has a trigonal P3? form for M=Ba, and monoclinic P2₁/c forms for M=Sr and Ca. The observed structural transition is analogous to the corresponding layered orthosilicate M₃MgSi₂O₈.Eu²⁺-doped Na₂MMgP₂O₈ exhibits an intense blue to violet emission under ultraviolet excitation, based on 5d-4f electron transition of Eu²⁺ ions. The emission character is very sensitive to the structural transition induced by M²⁺ and the subsequent site symmetry changes.     

Phase formation features in the systems M2MoO4-Fe2(MoO4)3 (M=Rb, Cs) and crystal structures of new double polymolybdates M3FeMo4O15

The systems M₂MoO₄-Fe₂(MoO₄)₃ (M=Rb, Cs) were shown to be non-quasibinary joins of the systems M₂O-Fe₂O₃-MoO₃. New compounds M₃FeMo₄O₁₅ were revealed along with the known MFe(MoO₄)₂ and M₅Fe(MoO₄)₄. The unit cell parameters of the new compounds are a=11.6192(2), b=13.6801(3), c=9.7773(2) Å, β=92.964(1)°, space group P2₁/c, Z=4 (M=Rb) and a=11.5500(9), b=9.9929(7), c=14.513(1) Å, β=90.676(2)°, space group P2₁/n, Z=4 (M=Cs). In the structures of M₃FeMo₄O₁₅ (M=Rb, Cs), a half of the FeO₆ octahedra share two opposite edges with two MoO₆ octahedra linked to other FeO₆ octahedra through the bridged MoO₄ tetrahedra by means of the common oxygen vertices to form the chains along the a axis. The difference between the structures is caused by diverse mutual arrangements of the adjacent polyhedral chains.