Charge density wave transition in Na2Ti2Sb2O probed by 23Na NMR

Herein we investigated the electronic properties of layered transition-metal oxides Na2Ti2Sb2O by23Na nuclear magnetic resonance(NMR)measurement.The resistivity,susceptibility and specific heat measurements show a phase transition at approximately 114 K(TA).No splitting or broadening in the central line of23Na NMR spectra is observed below and above the transition temperature indicating no internal field being detected.The spin-lattice relaxation rate divided by T(1/T1T)shows a sharp drop at about 110 K which suggests a gap opening behavior.Below the phase transition temperature zone,1/T1T shows Fermi liquid behavior but with much smaller value indicating the loss of large part of electronic density of states(DOS)because of the gap.No signature of the enhancement of spin fluctuations or magnetic order is found with the decreasing temperature.These results suggest a commensurate charge-density-wave(CDW)phase transition occurring.     

Sodium-23 MAS NMR study on nuclear quadrupole interaction in Na1−xAgxNO2

Ag-impurity effects on the first- and second-order quadrupole interaction (QI) at ²³Na site in an isomorphic mixed system, Na_1−xAg_xNO₂ (x=0, 0.0084, 0.026, 0.079, 0.094, 0.16), have been investigated by employing ²³Na (I=3/2) magic angle spinning nuclear magnetic resonance (MAS NMR) technique. The central transition (CT) and satellite transition (ST) are simultaneously observed with this system. From the spectral analysis, the quadrupole parameter and its distribution width are obtained as a function of Ag concentration. From the intensity loss of CT MAS centerband and of the envelope function of ST MAS sidebands due to impurities, the range of their influence on the second- and first-order QI is estimated. The estimated ranges contain the second and first neighbouring Na sites from the resonating ²³Na nucleus for the first- and second-order QI, respectively.     

23Na nuclear magnetic resonance investigation of the lattice distortion in α-NaxV2O5

The full temperature dependence of the electric field gradient tensor at the Na sites has been determined by nuclear magnetic resonance (NMR) in the temperature range 8–330 K in α-Na_{x 2}O₅ (x = 0.996). Above the spin-Peierls transition (T _c = 34.7 K), only a single Na site is observed in agreement with the Pmmn space group proposed to describe this compound as the first example of a 1/4-filled ladder system. Below T_c, eight distinct quadrupolar²³Na sites are observed according to the distortion wave vector k_c = (1/2, 1/2, 1/4) previously reported. In addition, the opening of a spin gap is evidenced by a rapid drop of the magnetic hyperfine shift²³K at T_c. The results are discussed in the context of a charge-order-driven spin-Peierls transition.     

Complex magnetic differentiation of cobalts in Na x CoO2 with 22 K Néel temperature

Single crystals of sodium cobaltates Na _x CoO₂ with x ≈ 0.8 were grown by the floating zone technique. Using electrochemical Na de-intercalation method we reduced the sodium content in the as-grown crystals down to pure phase with 22 K Néel temperature and x ≈ 0.77. The ⁵⁹Co NMR study in the paramagnetic state of the T _N = 22 K phase permitted us to evidence that at least 6 Co sites are differentiated. They could be separated by their magnetic behavior into three types: a single site with cobalt close to non-magnetic Co³⁺, two sites with the most magnetic cobalts in the system, and the remaining three sites displaying an intermediate behavior. This unusual magnetic differentiation calls for more detailed NMR experiments on our well characterized samples.     

Structural and magnetic inhomogeneity, phase transitions, magnetoresonance and magnetoresistive properties of La0.6 − x Pr x Sr0.3Mn1.1O3 (x = 0–0.6)

Ceramic samples of manganite perovskites La_{0.6 ? x} Pr _x Sr_0.3Mn_1.1O₃ (x = 0?0.6) have been studied using the X-ray diffraction, resistive, magnetic (χ_ac, ⁵⁵Mn NMR), microscopic, and magnetoresistive methods. It has been found that an increase in the praseodymium concentration x leads to a transition from the rhombohedral R $\bar 3$ c (x = 0–0.3) to orthorhombic Pbnm (x = 0.4–0.6) perovskite structure. It has been shown that the real perovskite structure contains anion and cation vacancies, whose concentrations increase with an increase in the praseodymium concentration x. A decrease in the metal-insulator phase transition temperature T _mi and the ferromagnetic-paramagnetic phase transition temperature T _c with increasing x correlates with an increase in the concentration of vacancies weakening the high-frequency electronic exchange Mn³⁺ ? Mn⁴⁺. For compositions with x = 0 and 0.1, when the lattice contains not only vacancies but also nanostructured clusters with Mn²⁺ in the A-positions, there is an anomalous hysteresis. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra of the compounds has revealed a high-frequency electronic exchange of the ions Mn³⁺ Mn⁴⁺ in the B-positions and a local heterogeneity of their surrounding by other ions (La²⁺, Pr³⁺, Sr²⁺) and vacancies. The phase diagram has demonstrated that there is a strong correlation between the composition, imperfection of the perovskite structure, phase transition temperatures T _mi and T _c , and magnetoresistive properties.     

Structural and magnetic inhomogeneities,phase transitions, 55Mn nuclear magnetic resonance,and magnetoresistive properties of La0.6 − x Nd x Sr0.3Mn1.1O3-δ ceramics

The structure, lattice imperfection, and properties of ceramic samples La_{0.6 ? x} Nd _x Sr_0.3Mn_1.1O_3-δ (x = 0–0.4) have been investigated using the X-ray diffraction, resistive, magnetic (χ_ac, ⁵⁵Mn NMR), magnetoresistive and microscopic methods. It has been shown that there is a satisfactory agreement between the concentration decrease in the lattice parameters a of the rhombohedral (x = 0, 0.1, 0.2) and cubic (x = 0.3, 0.4) perovskite structures and the average ionic radii $\bar R$ for the lattice containing anion vacancies, cation vacancies, and nanostructured clusters with Mn²⁺ ions in A-positions. With an increase in the neodymium concentration x, the vacancy-type imperfection increases, the cluster-type imperfection decreases, the temperatures of metal-semiconductor phase transition T _ms and ferromagnetic-paramagnetic phase transition T _C decrease, and the content of the ferromagnetic phase decreases. The anomalous hysteresis is associated with the appearance of unidirectional exchange anisotropy induced in a clustered perovskite structure consisting of a ferromagnetic matrix and a planar antiferromagnetic cluster coherently coupled with it. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra has revealed a high-frequency electronic double exchange (Mn³⁺-O^2?-Mn⁴⁺) ? (Mn⁴⁺-O^2?-Mn³⁺) and an inhomogeneity of the magnetic and charge states of manganese due to the heterogeneous environment of the manganese ions by other ions and defects. The observed changes in the resonant frequency and width of the resonance curve are caused by changes in the ratio Mn³⁺/Mn⁴⁺ and magnetic inhomogeneity. An increase in the neodymium concentration x leads to a decrease in the ferromagnetic phase content determined from the dependences 4πNχ_ac(T) and the ⁵⁵Mn NMR curves. The phase diagram characterizes an interrelation between the composition, the imperfection of the structure, and the transport, magnetic, and magnetoresistive properties of lanthanum neodymium manganite perovskites. It has been found that there is a correlation between the imperfection, magnetic inhomogeneity, coercive force, and magnetoresistance effect exhibited by the perovskite structure.     

<Superscript>23</Superscript>Na NMR in binary lithium-sodium cobaltite

Li_0.48Na_0.35CoO₂ lithium-sodium cobaltite was studied by means of wide-line ²³Na and ⁷Li NMR. A series of quantum-chemical calculations allowed to us determine the optimum positions of Li and Na atoms, to construct a map of the densities of electronic states near the Fermi level, and to estimate the electric field gradient on the Na nuclei. The results from these calculations are compared with experimental data from NMR and X-ray photoelectron spectroscopy     

Structural and magnetic inhomogeneities, phase transitions, 55Mn NMR, and magnetoresistive properties of La0.6Sr0.2Mn1.2 − x Nb x O3 ceramics

Ceramic samples of lanthanum strontium manganite perovskites La_0.6Sr_0.2Mn_{1.2 ? x} Nb _x O₃ (x = 0–0.3) annealed at temperatures of 1260 and 1500°C have been investigated using the X-ray diffraction, electron microscopic, resistive, magnetoresistive, and magnetic (χ_ac, ⁵⁵Mn NMR) methods. It has been found that there is a correlation between the increasing unit cell parameter a of the rhombohedral R $\bar 3$ c structure and the average ionic radius with increasing niobium concentration x and annealing temperature for the case where the lattice contains anion vacancies, cation vacancies, and nanostructured clusters. The observed increase in the electrical resistivity and decrease in the temperatures of metal-semiconductor phase transition T _ms and ferromagnetic-paramagnetic phase transition T _C with an increase in the niobium concentration x and the annealing temperature have been explained by the decrease in the content of the ferromagnetic phase, as well as by changes in the ratio Mn³⁺/Mn⁴⁺, the oxygen nonstoichiometry, and the concentration of defects weakening the high-frequency electronic exchange of the ions Mn³⁺ ? Mn⁴⁺. The presence of nanostructured clusters in the lattice has been confirmed by an anomalous hysteresis associated with the unidirectional exchange anisotropy of the interaction between the ferromagnetic matrix and antiferromagnetic clusters with Mn²⁺ and Nb³⁺ in distorted A-positions. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra and their computer decomposition have revealed a high-frequency electronic exchange and an inhomogeneity of the magnetic and valence states of manganese due to the nonuniform distribution of all ions and defects. Two types of magnetoresistive effects have been found: one effect, which is observed near the phase transition temperatures T _C and T _ms, is caused by scattering at intracrystalline nanostructured heterogeneities of the imperfect perovskite structure, and the other effect, which is observed in the low-temperature range, is induced by tunneling through intercrystalline mesostructured boundaries. The phase diagram has demonstrated that there is a strong correlation between the composition, structure, resistive and magnetic properties of rare-earth manganites.     

Knight shift in superconducting oxides BaPb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript>x</Subscript>O<Subscript>3</Subscript> (<Emphasis Type="Italic">x</Emphasis> < 0.35)

The Knight shift ²⁰⁷K_s for the ²⁰⁷Pb nuclei in the metal phase of the oxides BaPb_1?xBi_xO₃ (x < 0.35) has been analyzed as a function of the concentration. The shift, which is proportional to the density of states near the Fermi energy: ²⁰⁷K_s ～ N(E_F), reaches a maximum for an oxide with the maximum superconducting transition temperature T_c(x ≈ 0.25) = 12 K. A significant increase in the width of the shift distribution with the Bi concentration testifies to the formation of a nonuniform state of the electronic system in the conduction band of superconducting oxides, which is accompanied by an increase in short-wavelength contributions to the spin susceptibility. To detect the ²⁰⁷Pb NMR spectra in superconducting oxides with x > 0.2, the ¹⁷O-²⁰⁷Pb spin-echo double-resonance method is used, which provides successful detection of the ²⁰⁷Pb NMR signal with an anomalously high rate of spin-spin relaxation T ₂ ^?1 > 500 ms^?1. Thus, fundamental restrictions arising in investigations of rapidly relaxing ²⁰⁷Pb nuclei, which are “unobservable” in superconducting oxides BaPb_1?xBi_xO₃ when they are studied by traditional single-resonance methods of pulse NMR spectroscopy, have been overcome.     

23Na NMR in the relaxor ferroelectric Na1/2Bi1/2TiO3

The ²³Na NMR spectra of a Na_1/2Bi_1/2TiO₃ crystal are studied at frequencies of 79.4 and 15.7 MHz in the temperature range 150–720 K. It is revealed that, at all temperatures, the crystal contains regions with a nearly cubic matrix and polar clusters. The temperature dependence of the local distortion of the Na environment in the clusters is determined. The dynamics of the reorientation of the local cluster polarization in the tetragonal and trigonal NBT phases is analyzed.     

Magnetic properties of Y-type trigonal ferrites first-order magnetization processes in trigonal systems

Magnetic properties of the single crystal ferrite Ba₂(Zn_1-xCo_x)₂Fe₁₂O₂₂ with O ? x ? 1, which has a trigonal symmetry around the c-axis, were investigated. The presence of Co ion strongly affects the properties of the Y-type ferrite: easy cone of magnetization, spin reorientation phase transition and first-order magnetization processes (FOMP) have been observed in this system. The observed effects as well as the origin of the magnetocrystalline anisotropy are discussed on the basis of a phenomenological analysis. The anisotropy constants K₁, K₂ and K_t trigonal of the ferrite are given versus temperature and composition. The magnetic phase diagram of a trigonal system is also presented. The condition for the existence of FOMP are obtained in term of the anisotropy constants ratios.     

The role of kinetic effects in voltammetry studies of intercalation systems

The role of kinetic effects in voltammetry studies of intercalation systems is examined. These effects disguise features arising from the thermodynamic properties, such as from coexisting phases. It is shown that a quantitative analysis of the phase diagram of the intercalation system is nevertheless possible. The results are applied to Li_xVS₂.     

Domain configuration and dynamic processes in orthorhombic NaCN studied by 23Na NMR satellite lines and relaxation times

The orientational dependences of the ²³Na NMR spectra which were measured in a NaCN single crystal below T_c1 are related to the EFG tensor at the Na sites in different domains. The results are consistent with the assumption that 12 different domains may arise in the orthorhombic phase. Below T_c1 also the temperature dependences of the spin relaxation times T₁ and T_1ρ of the ²³Na nucleus were measured and related to dynamic processes. Particularly the activation energy and the frequency of the flip motions of the CN^--dipoles are determined.     

Low-temperature specific heat study of Ni85.5PxB18.5−x (0 ⩽ x ⩽ 18.5) metallic glasses

The low-temperature specific heat (LTSH) of the melt-quenched Ni_81.5P_x^- B_18.5?x amorphous alloy system, with 0 ? x ? 18.5, is presented. The decomposition of the LTSH into magnetic, lattice and electronic contributions shows that both Debye temperature θ_D and electronic specific heat coefficient decrease when the concentration of P increases.The electronic density of states N(E_F), deduced from γ for various Ni-metalloid alloys, is plotted as a function of the average electronic concentration xZ^M, where x is the metalloid concentration and Z^M is the chemical valence of M. Following Malozemoff et al.'s work, this plot is considered as a representation of the band structure and yields the change of the Fermi level with alloying.     

Proton-induced direct capture on 21Ne and 22Ne

The direct capture process in the reactions ²¹Ne(p, γ)²²Na and ²²Ne(p, γ)²³Na has been investigated at E_p = 0.3–1.6 MeV using neon gas enriched to 91 % in ²¹Ne and to 99 % in ²²Ne, respectively. The gas was recirculated in a differentially pumped gas target system of the extended-static and quasi-point supersonic jet type. For ²²Ne(p, γ)²³Na, the direct capture process has been observed to several final states in ²³Na up to E_x = 8.83 MeV excitation energy. The deduced spectroscopic factors C²S are in fair agreement with the corresponding values from stripping reactions. The capture transition into the ²³Na ground state exhibits broad structures, which resemble Ericson fluctuations. The data remove the previously reported discrepancies in C²S for the ²³Na ground state. The excitation functions for the ²¹Ne(p, γ)²²Na reaction are dominated by broad and intense resonances, which hampered the measurement of the direct capture process. The nuclear and astrophysical aspects of the results are discussed.     

Electron energy-loss studies of NaxC60 compounds

The crystal and electronic structure of Na_xC₆₀ (4< x < 10) has been investigated using electron energy-loss spectroscopy in transmission. Electron scattering experiments confirm that the crystal symmetry remains face centered cubic over the entire intercalation range. The electronic structure shows a close similarity for the Na₆C₆₀ and Na₄C₆₀ compounds compared to the corresponding K and Rb systems. Upon intercalation to higher stochiometries up to Na₁₀C₆₀ the charge transfer from sodium to C₆₀ turns out to be incomplete, something that has never been observed before in alkali intercalated systems. Current theoretical calculations indicate that this can be understood by taking into account additional states related to the Na atoms in the octahedral site. In Na₁₀C₆₀ the partial filling of the t_1G level of C₆₀ leads to additional electronic transitions near the Fermi energy.     

Structure of the superdense LiC3 compound prepared by ball-milling

The intercalation of lithium into graphite by ball-milling leads to a powder whose X-ray pattern is close to that of LiC₆, but the density of the powder indicates the presence of an intercalation phase close to LiC₃. This compound is superdense since its volume concentration in lithium is twice that of LiC₆, the repeat distance along the c-axis being identical in both compounds. Moreover, contrarily to an other compound, LiC₂ synthesized under high pressure, which decomposes slightly (in several months) into LiC₆+Li, LiC₃ is stable.The ⁷Li NMR was very helpful to solve the different problems about this compound and the present work explains all the observations, how and why LiC₃ remains stable at ambient pressure.     

Transition from the Kondo regime to long-range magnetic order in the FexV1−x S system

A study is reported on the electrical and magnetic characteristics of the Fe_xV_1?x S solid-solution system with x≤0.5. A maximum in the temperature dependence of resistivity ρ(T) characteristic of the Kondo effect has been observed for small x(x<0.01). For x>0.1, long-range magnetic order sets in in the system with T _K ≈ 100 K. Near x=0.05, the Fe²⁺ impurity behavior crosses over to a magnetically ordered phase. The electronic properties of Fe_xV_1?x S are typical of those of strongly correlated electronic systems. Both the electrical and magnetic data imply that carrier delocalization is the strongest at x=0.4.     

A computational NICS and 13C NMR characterization of the substitution patterns of C70−2x(BN)x fullerenes (x=1–25)

A density functional study has been performed to investigate the electronic and magnetic properties of BN substituted fullerenes C_70?2x(BN)_x (x=1, 2, 3, 6, 9, 12, 15, 17, 19, 21, 23 and 25) based on the NMR parameters and NICS index. The calculated ¹³C chemical shielding (CS) tensors are found to be perturbed at the first and second neighbors of the doped atoms while the other distant carbon atoms not to be influenced significantly. ¹³C Chemical shifts (δ_iso) of the second neighbors of nitrogen and boron are significantly shifted to upfield and downfield (the second neighboring effects), respectively. Besides, chemical shifts are also affected by the curvature of the corresponding sites; for example, the perturbed sites at the caps yield smaller absolute values of chemical shifts than those located at the equator. Nucleus independent chemical shifts (NICS) at the cage centers of heterofullerenes (from ?25.29 to ?8.80) demonstrate that all the substituted species are aromatic, but less than C₇₀ (?27.29). The predicted NICS values may be useful for identification of the heterofullerenes through their endohedral ³He NMR chemical shifts.     

Charge-density waves in Nb1 + xS2 crystals

In-situ intercalation of surplus niobium during heating of 3R-Nb_{1 + x}S₂ (x ≥ 0.09) layered crystals in ultra-high vacuum up to 1500 K triggered an unusual inter-polytypic transition from a trigonally prismatic coordinated polytype into an octahedrally coordinated one. The new polytype contains charge-density waves with a commensurate-incommensurate phase transition, temperature-dependent on the intercalated niobium concentration. Charge-density waves in the Nb_{1 + x}S₂ compound, observed for the first time here, were studied by transmission electron diffraction and scanning tunnelling microscopy.