Change In Lattice Symmetry Research Articles

Strong spin-phonon coupling in the geometrically frustrated pyrochloreY2Ru2O7

We investigated temperature-dependent infrared-active transverse phonon modes of geometrically frustrated pyrochlore ${\mathrm{Y}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$, which exhibits a spin-glass-like transition at ${T}_{G}\ensuremath{\sim}80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. According to previous neutron-scattering experiments, although ${\mathrm{Y}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ does not accompany any static lattice symmetry change, it has an intriguing magnetic ground state with strong antiferromagnetic correlation [M. Ito et al., J. Phys. Soc. Jpn. 69, 888 (2000)]. In our far-infrared spectra, we found seven phonon modes; thermal effects could explain the temperature dependences of three phonons, but not the other four phonons. We found that these abnormal temperature-dependent behaviors could be attributed to a strong antiferromagnetic correlation, not to a structural distortion. Using quantitative analysis, we suggested that the strong spin-phonon coupling effects should play an important role in the intriguing magnetic state of this geometrically frustrated compound.

Hydrogen-induced phase transition in barium cerate

The action of water vapor on nominally pure BaCeO3 perovskite at 850 K causes hydrogen to be incorporated into the lattice and a thermally metastable state to form. The lattice symmetry changes as a result of an orthorhombic-pseudocubic transition which takes place at a temperature substantially lower than the known thermally initiated rhombohedral-cubic transition at 1173 K.

Structural phase sequence and modulated phases of (C3H7NH3)2CuBr4

Using single crystal X-ray methods, the structural phase sequence of (C3H7NH3)2CuBr4 (abbreviated to C3CuBr) has been studied at 100 K < T < 420 K. The perovskite-type layer structure, distorted by the Jahn-Teller effect, exhibits four phases: δ phase: Pbca, (4); ε phase: P21/b11, (2); ζ phase: IC (P21/b11); η phase; P 1, (4); the number of formula units per primitive cell is given in brackets. Remarkably, the ζ phase is incommensurate (IC), and the normal phase (ε phase) appears upon heating only: The changes in lattice symmetry at the strongly discontinuous δ → ζ and ε → δ phase transitions point to a spontaneous rearrangement of the octahedral JT-distortion pattern within every second layer. Upon cooling, this process is triggered by the onset of the modulation of the IC ζ phase. The wave vector is given by q ζ = (1/2 - δ2)b* m - δ3 c* m, with the irrational parts δ2 and δ3 in the cooling (c) and heating (h) case: δ c 2 ≈ 0.009, δh 2 ≈ 0;δ c,h 3 ≈ 0.09.δ3 decreases slightly with temperature, both δ2 and δ3 jump to zero at the ζ → η transition.

Superconducting epitaxial Tl 2Ba 2CuO 6 + δ thin films on SrTiO 3 with tetragonal lattice and continuously adjustable critical temperature

Epitaxial Tl 2Ba 2CuO 6 + ° (Tl-2201) thin films on single crytalline SrTiO 3 substrate have been synthesized by RF magnetron sputtering followed by a two-step annealing process. X-ray diffraction data show that the film is c-axis oriented with full width at half maximum of rocking curve (FWHM) of 0.265° and alignment in the ab-plane. The zero-resistance T c of the same film can be increased or decreased continually and reversibly in the 11–80 K range by annealing in argon or air, with no change in tetragonal lattice symmetry.

Structural peculiarities of graphite-like boron nitride produced by crystallization in the region of metastability

A structural examination has been performed on boron nitride (BN) graphite-like modifications obtained by crystallization from boron nitride solutions in supercritical ammonia at high pressures and temperatures. A clear tendency has been observed to a variation of the crystal cut with change in the lattice symmetry. A dislocation structure of rhombohedral BN has been studied for the first time.

Shape Evolution of a Coherent Tetragonal Precipitate in Partially Stabilized Cubic ZrO2: A Computer Simulation

The kinetics of shape evolution of a tetragonal precipitate coherently embedded in a cubic matrix are examined. Specifically, the morphology of tetragonal ZrO2 particles in partially stabilized cubic ZrO2 is discussed. A computer simulation, carried out without any a priori constraint on possible kinetic paths and particle morphologies, shows that a lenslike shape appears during growth of a tetragonal particle. Upon further coarsening, the shape relaxes into a rhombus bounded by facets. Depending on the balance between interfacial and strain energies controlled by the particle size, the facets can be smoothly curved or straight. The predicted particle morphologies are in good agreement with the experimental observations. The kinetic model proposed is quite general for simulating microstructural developments during decomposition involving a crystal lattice symmetry change where elastic strain accommodation plays an important role.

Theory for displacive excitation of coherent phonons.

We report femtosecond time-resolved pump-probe reflection experiments in semimetals and semiconductors that show large-amplitude oscillations with periods characteristic of lattice vibrations. Only ${\mathit{A}}_{1}$ modes are detected, although modes with other symmetries are observed with comparable intensity in Raman scattering. We present a theory of the excitation process in this class of materials, which we refer to as displacive excitation of coherent phonons (DECP). In DECP, after excitation by a pump pulse, the electronically excited system rapidly comes to quasiequilibrium in a time short compared to nuclear response times. In materials with ${\mathit{A}}_{1}$ vibrational modes, the quasiequilibrium nuclear ${\mathit{A}}_{1}$ coordinates are displaced with no change in lattice symmetry, giving rise to a coherent vibration of ${\mathit{A}}_{1}$ symmetry about the displaced quasiequilibrium coordinates. One important prediction of the DECP mechanism is the excitation of only modes with ${\mathit{A}}_{1}$ symmetry. Furthermore, the oscillations in the reflectivity R are excited with a cos(${\mathrm{\ensuremath{\omega}}}_{0}$t) dependence, where t=0 is the time of arrival of the pump pulse peak, and ${\mathrm{\ensuremath{\omega}}}_{0}$ is the vibrational frequency of the ${\mathit{A}}_{1}$ mode. These predictions agree well with our observations in Bi, Sb, Te, and ${\mathrm{Ti}}_{2}$${\mathrm{O}}_{3}$. The fit of the experimental \ensuremath{\Delta}R(t)/R(0) data to the theory is excellent.

Ionic conductivity of tetragonal PbSnF 4 prepared by solid state reaction in HF atmosphere

Tetragonal PbSnF 4 prepared by a solid state reaction under streaming gaseous HF has been characterized by X-ray Rietveld analysis, differential scanning calorimetry, differential thermal analysis and ac conductivity measurement. The room-temperature tetragonal phase is composed of the cationic framework made up of layers perpendicular to the c-axis with a sequence of PbPbSnSn. The conductivity vs. temperature curves between 170 and 573 K showed a gradual slope change from a region of high activation energy ( E ⋍ 30 kjmol −1 ) to that of low activation energy ( E ⋍ 10 kjmol −1 ) around room temperature. Such a behavior accompanied no lattice symmetry change, corresponding to a so-called diffuse phase transition.

Effect of addition of lead on the superconductivity and structure of Bi 2Sr 2Ca 2Cu 3O 10 ceramics

Optimum sintering treatment of and the amount of lead needed in Bi 2Sr 2Ca 2Cu 3O 10 to obtain the best superconducting properties are reported. On the basis of X-ray diffraction studies it has been established that the presence of a large fraction of high- T c phase does not necessarily imply the best superconducting behaviour. A model has been proposed to explain the changes in lattice symmetry and the variations in (i) superconducting behaviour, (ii) fraction of high- T c phase and (iii) c-parameter with increase in lead content.

Thermal decomposition of energetic materials: 20. A comparison of the structural properties and thermal reactivity of an acyclic and cyclic tetramethylenetetranitramine pair

The linear secondary nitramine, 1,7-dimethyl-1,3,5,7-tetranitrotrimethylenetetramine CH 3[N(NO 2)CH 2] 3N(NO 2)CH 3 (OHMX) and its cyclic analog, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, (CH 2NNO 2) 4 (HMX) are compared. By IR spectroscopy and DTA, OHMX is found to exist in five polymorphs between 273 K and its melting point at 490 K. The IR spectra suggest that the structural differences involve mostly the NO 2 and CH 3 groups and changes in lattice symmetry without a significant alteration in the conformation of the backbone. The kinetic parameters for the OHMX(III → IV) phase transition are first order ( E a = 50.8 kcal mol −1) and are comparable to those of HMX. The nucleation process is probably being measured. The values of Δ H for the phase transitions of OHMX (0.8–2.1 kcal mol −1) are also similar to those of HMX. The molecular structure of OHMX(II) was determined: monoclinic, P2 1/ c, a = 20.952(7), b = 9.477(2), c = 6.521(2) Å, β = 96.29(3)°, Z = 4, D calc = 1.598 g cm −3, R F = 3.84, R wF = 4.68%. It consists of a helical chain of CH 2NNO 2 units staggered at approximately 120° intervals. A pseudo-C 2 axis is present. OHMX is slightly less thermally stable than HMX. High-rate thermolysis (d T /d t =110 K s −1) of OHMX liberates products that are the same, with a few exceptions, as for HMX. Competitive N-N and C-N bonds fission occurs as predicted by the IR data and molecular structure. The pressure dependence of the decomposition process indicates that the heterogeneous gas-condensed phase reactions occur to a greater extent as the pressure increases.

New crystalline phases of an equiatomic K-Cs alloy at low temperature

Solid equiatomic K-Cs alloys have been investigated by X-ray diffraction throughout the temperature range 300-100K. The results indicate that a phase separation occurs below 185K accompanied by the appearance of an ordered phase in this range. This phase has a hexagonal lattice with parameters: a = 9.32(1) Å and c = 11.80(2) Å (at 170K). Evidence from our other studies [7] indicates that its composition is K 2Cs. Another phase transformation in this ordered crystal is observed below 120K. There is no change of lattice symmetry but the unit cell constants shrink to the values: a = 9.11(1) Å and c = 10.86(2) Å (at 100K). The transformation can be ascribed to a rearrangement of the electronic structure of Cs.

An NMR Analysis of Transitions in FexNbxV2-2xO4

Abstract The first order semiconductor - metall phase transition with the lattice symmetry change from monoclinic to tetragonal and the conductivity jump of several orders of magnitude is observed in pure vanadium dioxide and the dioxide doped by varios impurities [I]. Transition temperature Tt for pure VO2 is 68°C and depends on the impurity. For example, the impurities which enter the insulating phase as pentavalent ions Nb5+ [2] give rise to a decrease in metal-semiconductor transition temperature at low concentrations and the trivalent ions Cr3+, Fe3+ [3,4] lead to an increase in Tt. In the latter case three different monoclinic phases are stabilized at a temperature below Tt. It has been already shown [5] that for double doping of vanadium dioxide by pentavalent Nb5+ and trivalent Fe3+ or Cr3+ ions three semiconductor phases exist (MI, M2, M3) but the transition temperature decreases as the impurity concentration increases.

Some Magnetic and Crystallographic Properties of the System LaMn1−xNixO3+λ

The system LaMn1−xNixO3+λ has been prepared under varying conditions for the entire range of compositions 0≤x≤1. The saturation magnetization, lattice parameters, and chemical analysis have been measured for each sample. The origin of discrepancies in the literature on the crystal structure of LaMnO3 is shown to be due to different preparation procedures which result in the presence of differing amounts of Mn4+: the lattice symmetry changes from orthorhombic to rhombohedral as the Mn4+ concentration exceeds 21% of the total manganese. The system is ferromagnetic with magnetization only slightly less than the spin-only value over the compositional range 0.2≤x≤0.5: a ferromagnetic Ni3+–Mn3+ interaction is indicated. The system remains orthorhombic throughout the range 0≤x&amp;lt;0.5; it shows an increasing distortion to an apparently monoclinic symmetry in the range 0.5&amp;lt;x≤0.8. LaNiO3 is rhombohedral at room temperature. The lattice symmetry is compatible with Ni3+ in a low-spin state: this is also in accord with the magnetization values. Qualitative concepts used to interpret the system (La,Ca)MnO3 are applied to interpret the magnetic and crystallographic properties of this system as well as the magnetic properties of some rhombohedral perovskite-type systems rich in Mn3+ ions.

山口県熊野鉱山産マグヘマイトとその加熱変化(第一報)

The Kumano Mine is situated in Susa Town of Yamaguchi Prefecture, Japan. It is a contact metasomatic deposit of quartz-diorite with limestone. The primary ore is consisted mainly with magnetite, but suffering secondary oxidation or hydrous oxidation remarkably, almost all the ore is changed to maghemite and hematite with lepidocrocite and goethite. Taking up several specimens of maghemite showing various degree of deficiency, then heated at 300, 450, 600, 800 and 1000°c in air respectively. The heated products were studied by X-ray and chemical method, and referring the results of differential thermal analysis, the oxidation process of maghemite including magnetite is suggested as follows : 1. Genesis of vacancy in octahedral coordination position. 2. Movement of Fe3+ ions in tetrahedral coordination position to octahedral one, 3. change of lattice symmetry from cubic to rhombohedral. These three are concerned each other and at the end of oxidation, hematite only is recognizable. The higher the degree of defficieincy, the lower the temperature of the last exothermic peak shown in DTA curves.