Order-disorder phase transition in ZrV2D3.6

The deuterated C15-type Laves phase ZrV₂D_3.6 undergoes a structural phase transition near room temperature (T ≈ 325 K). In the cubic high-temperature phase the deuterium atoms are disordered over two types of tetrahedral interstices, the centres of which are 1.3 Å apart. In the tetragonal low-temperature phase the D atoms are ordered and occupy only the energetically more favourable interstices. The tetragonal structure is isotypic with the low-temperature phase of HfV₂D₄. The shortest D—D distance is 2.1 Å.     

Synthesis and transport properties of La2NiO4

La₂NiO₄ compounds were prepared by a modified sol–gel auto-combustion method, which is a low-temperature combustion synthesis procedure using microwave-assisted sol–gel as precursors. The high-temperature transport properties of the samples were investigated. The band structure, total density of states (DOS), and partial density of states (PDOS) of low-temperature orthorhombic (Bmab) phase and high-temperature tetragonal (I4/mmm) phase for La₂NiO₄ were calculated in order to study the transport properties of the as-obtained samples.     

Semiconductor-metal transition in novel Cd2Os2O7

Cd₂Os₂O₇ has been prepared for the first time and has the pyrochlore structure with a cubic cell edge of 10.17 Å at room temperature. Electrical, magnetic, and DSC measurements on single crystals of this compound show a sharp transition at 225 K which we interpret to be an electronic, second-order, metal-semiconductor transition. The low-temperature semiconducting phase is probably antiferromagnetic.     

Ground-state magnetism of chromium-substituted LiMn2O4 spinel

Comparative crystal structure and magnetic properties studies have been conducted on quaternary powder spinel samples LiMn_1.82Cr_0.18O₄ obtained by two different synthesis methods, glycine-nitrate (GN) and ultrasonic spray-pyrolysis (SP). Although both samples possess the same spinel structure of the cubic space group Fd3¯m, their low-temperature magnetic properties display significant differences. While the SP sample undergoes only spin-glass transition at the freezing temperature T_f=20 K, the GN sample possesses more complicated low-temperature magnetic behavior of the reentrant spin-glass type with the Néel temperature T_N=42 K and freezing temperature T_f=22 K. High-temperature magnetic susceptibility of both samples is of the Curie–Weiss type with the effective magnetic moments in agreement with the nominal compositions. This fact together with the results of the chemical analysis discards the existence of the diversity in chemical compositions as a possible cause for the observed differences in the low-temperature magnetism. On the other hand, the crystal structure analysis done by the Rietveld refinement of the X-ray powder diffraction data points to the strong influence of the cation distribution on the ground-state magnetism of these systems. An explanation of this influence is proposed within the framework of a collective Jahn–Teller effect.     

颗粒超导体Ba2YCu3O7-δ的低温电阻特征

本文中研究了单相Ba₂YCU₃O_7-δ样品的低温电阻与外磁场、温度以及测量电流之间的关系。结果表明,外磁场的大小、方向,以及测量电流的大小,显著地改变着样品的低温电阻行为。从样品的颗粒结构特征对结果进行了讨论。 关键词：     

Crystal structure and magnetic properties of TbFe0.4Ge2 compound

Neutron diffraction and magnetic measurements were performed on polycrystalline TbFe_0.4Ge₂ which crystallizes with the orthorhombic structure of the CeNiSi₂-type. Despite the presence of some clear anomalies in the low-temperature magnetic susceptibility, the neutron diffraction experiment did not reveal any long-range magnetic ordering of the Tb magnetic moments down to 1.57 K.     

Electrical conductivity of the solid system AgI-Sb2S3

Electrical conductivity of the solid system AgI-Sb₂S₃ and its dependence on composition within the temperature range from room temperature to 280°C was investigated. The temperature of phase transition β→α AgI divides the conductivity region into low-temperature and high-temperature parts. Within the high-temperature region the conductivity increases monotonously with an increase in mole fraction of AgI, while within the low-temperature one its maximum appears at Agl mole fraction of 0.6. This composition is glassy and was investigated in detail with regard to the conductivity and the double layer capacitance. The electric conductivity course was explained in the light of structure investigations.     

High-temperature crystal structure and electronic properties of cesium niobate Cs2Nb4O11

The antiferroelectric material Cs₂Nb₄O₁₁ transforms at 165 °C from a low-temperature, antiferroelectric phase in space group Pnna to a high-temperature, paraelectric phase in space group Imma; the latter structure has been determined by single-crystal X-ray diffraction. The high-temperature lattice is comprised of niobium-centered tetrahedra and octahedra connected through shared vertices and edges; cesium atoms occupy channels afforded by the three-dimensional polyhedral network. Calculated band structures for both phases predict a bandgap of 3.1-3.2 eV, which is similar to that found experimentally through photoluminescence. The calculated band structure is also conducive to its observed photocatalytic properties.     

CRYSTAL STRUCTURE AND MAGNETIC PROPERTIES OF La1.2Sr1.8-xCaxMn2O7

The effect of divalent cation substitution on the structure and magnetic properties of La_1.2Sr_1.8-xCa_xMn₂O₇ (x = 0－0.900) is investigated in this paper. Partly replacing divalent cation Sr²⁺ by Ca²⁺ ions results in the weakening and then disappearance of long-range ferromagnetic ordering, and the formation of spin canting and low-temperature spin-glass. Based on structural analysis by Rietveld profile fitting, we suggest that this variation of magnetic property be related to a Jahn－Teller-type attice distortion of MnO₆ octahedra due to the introduction of the smaller sized Ca²⁺ ions.     

First-principles study on the electronic structure and magnetic properties of metallic antiferromagnet C7H3S2N2

First-principles full potential linearized augmented plane wave (FPLAPW) calculations have been performed to study the electronic structure and the magnetic properties of 3-Cyanobenzo-1,3,2-dithiazolyl,C₇H₃S₂N₂. The density of states (DOS), the total energy of the cell, and the spontaneous magnetic moment of C₇H₃S₂N₂ were all calculated. The calculations reveal that the low-temperature phase of the compound C₇H₃S₂N₂ has a stable metal-antiferromagnetic ground state, and there exists an antiferromagnetically coupled interactions between the dithiazolyl radical(1), which is in good agreement with experiment.     

Thermodynamic properties of LiCu2O2 multiferroic compound

A spin model of quasi-one-dimensional LiCu₂O₂ compound with ground state of ellipsoidal helical structure has been adopted. The helical axis is along the diagonal of CuO₄ squares. By taking into account the interchain coupling and exchange anisotropy, the exotic magnetic properties and ferroelectricity induced by spiral spin order have been studied by performing Monte Carlo simulation. The simulation results qualitatively reproduce the main characters of ferroelectric and magnetic behaviors of LiCu₂O₂ compound and confirm the low-temperature noncollinear spiral ordering. Furthermore, by performing the calculations of spin structure factor, we systematically investigate the effects of different exchange couplings on the lower-temperature magnetic transition, and find that the spiral spin order depends not only on the ratio of nearest and next-nearest neighbor inchain spin coupling but also strongly on the exchange anisotropy.     

On the ground-state magnetic structure in Er2Ni2Pb

We have studied the unusual low-temperature magnetic phase of Er₂Ni₂Pb using powder neutron diffraction measurements in zero field down to 460 mK. Our previous neutron diffraction experiments down to 1.5 K showed that magnetic Bragg reflections seen in Er₂Ni₂Pb can be indexed by several propagation vectors that partially coexist. All the incommensurate propagation vectors seemed to disappear in the low temperature limit. The present study, however, shows that reflections belonging to the propagation vector q’ = (0.47 0 1/2) do not disappear but remain present down to 460 mK. This highly unexpected result suggests that the magnetic structure described by this propagation vector might not be a simple sine-wave modulation. One interesting possibility here is a spin-slip structure as the ground state.     

Raman scattering and X-ray diffraction study of structural phase transitions in the perovskite-type layer compound (C3H7NH3)2CdCl4

The structural phase transitions of (C₃H₇NH₃)₂CdCl₄ (PACC) have been studied by means of Raman scattering. X-ray diffraction and DSC measurements It is shown that the order-disorder phase transition Abma ? Pbca occurs at 156 K and not at 183 K as previously proposed by Chapuis (Acta CrystB34, 1506 (1978)) Apparently, the transition at 183 K does not change the Abma space group; it is suggested that it could be related to the occurrence of an incommensurate phase Another structural transformation of PACC is detected at 114 K, but no conclusion can be given as far as the structure of the low-temperature phase is not determined.     

Evidence for low-temperature internal dynamics in Cu12As4S13 according to copper NQR and nuclear relaxation

^63,65Cu nuclear quadrupole resonance (NQR) was applied to study the natural mineral Cu₁₂As₄S₁₃ (tennantite) in the temperature range 4.2–210 K. The obtained results point to the presence of field fluctuations caused by internal motions in tennantite. Consistently with the crystal structure, the experimental data can be described by an occurrence of a magnetic phase transition, which takes place near 65 K. The low-temperature phase is characterized by Cu(II) electron magnetic moments freezing in the form of a spin-glass-like constitution.     

The effect of Ag substitution in the mixed conductor Cu7PSe6

Cu₇PSe₆ is a mixed conductor that crystallizes in the simple cubic structure at room temperature. Structural transitions above and below room temperature are accompanied by step-like changes in electrical conductivity. The substitution of Ag⁺ for Cu⁺ in Cu₇PSe₆ stabilized the simple cubic structure over a wider range of temperatures than is observed for the pure compound. A disproportionate decrease in electrical conductivity accompanies modest levels of silver substitution. The prominent step in electrical conductivity associated with the low-temperature crystallographic phase transition disappears in (Ag_xCu_1−x)₇PSe₆ solid solutions for a composition parameter x=0.20, replaced by two distinct changes in the slope of conductivity below room temperature.     

Crystal structure and dielectric properties of ordered perovskites Ba2BiSbO6 and BaSrBiSbO6

Neutron powder diffraction studies showed that the ordered perovskites Ba₂BiSbO₆ (BBS) and BaSrBiSbO₆ (BSBS) crystallize in a rhombohedral structure with the space group R3¯. The room-temperature lattice parameters are a=6.0351(2) Å; α=60.202(1)° and a=5.9809(2) Å; α=60.045(2)°, respectively. BBS exhibits a dielectric anomaly near room temperature which may be related to structural transition from the R3¯ to low-temperature monoclinic I2/m symmetry. BSBS shows a dielectric anomaly near 723 K which coincides with a phase transition from the rhombohedral to cubic (Fm3¯m) structure. In contrast to BBS, BSBS does not undergo structural transition below room temperature.     

Controlled synthesis and relationship between luminescent properties and shape/crystal structure of Zn2SiO4:MN phosphor

Mn-doped Zn₂SiO₄ phosphors with different morphology and crystal structure, which show different luminescence and photoluminescence intensity, were synthesized via a low-temperature hydrothermal route without further calcining treatment. As-synthesized zinc silicate nanostructures show green or yellow luminescence depending on their different crystal structure obtained under different preparation conditions. The yellow peak occurring at 575 nm comes from the β-phase zinc silicate, while the green peak centering at 525 nm results from the usual α-phase zinc silicate. From photoluminescence spectra, it is found that Zn₂SiO₄ nanorods have higher photoluminescence intensity than Zn₂SiO₄ nanoparticles. It can be ascribed to reduced surface-damaged region and high crystallinity of nanorods.     

Polarisation remanente dans les varietes de basse temperature de (NH4) 3AlF6 ET (NH4) 3FeF6

Dielectric and thermocurrent measurements have been carried out on (NH₄) ₃AlF₆ and (NH₄) ₃FeF₆ ceramic samples. A maximum of permittivity is observed close to the transition temperature (T_{T(NH4) 3AlF6} = 217K; T_T(NH43FeF6 = 264K. In the low-temperature phase a polarization current of about 10^-9A is obtained and can be reversed when the sign of the polarization field is changed, a property which could correspond to a ferroelectric behavior. However, no pyroelectric current is detected when the temperature decreases from T_T. Another hypothesis, based on a field-induced polarization, has been considered : the depolarization current could be due to charge displacements from potential minima favored by rising temperature. In any way, the low-temperature phase is characterized by a remanent polarization.     

A re-evaluation of the heat capacity of cerium zirconate (Ce2Zr2O7)

The heat capacity of cerium zirconate pyrochlore, Ce₂Zr₂O₇, was measured from 0.4 to 305 K by hybrid adiabatic relaxation method for various magnetic field strengths. Magnetisation measurements were performed on the sample also. The results revealed a low-temperature anomaly that showed Schottky-type characteristics with increasing magnetic field strength. The estimated entropy due to the magnetic ordering of the two Ce³⁺ moments is 1.37R, close to the theoretical value for a doublet ground state (1.39R). The enthalpy increments relative to 298.15 K were measured by drop calorimetry from 531 to 1556 K. The obtained results significantly differ from those reported in the literature; the origin of the discrepancy is due to the probable oxidation of the pyrochlore structure into fluorite.     

Novel photoluminescence properties of InAlO3（ZnO）m superlattice nanowires

One-dimension InAlO 3 (ZnO) m superlattice nanowires were successfully synthesized via chemical vapor deposition.Transmission electron microscopy measurements reveal that the nanowires have a periodic layered structure along the 0001 direction.The photoluminescence properties of InAlO 3 (ZnO) m superlattice nanowires are studied for the first time.The near-band-edge emissions exhibit an obvious red shift due to the formation of the localized tail states.The two peaks centered at 3.348 eV and 3.299 eV indicate a lever phenomenon at the low-temperature region.A new luminescence mechanism is proposed,combined with the special energy band structure of InAlO 3 (ZnO) m.