Doubling Of Unit Cell Research Articles

Magneto-elastic coupling and Peierls-like phase transition of one-dimensional magnetic nanoparticle chains

The fine structure of magnetic nanoparticle periodic chains is studied by considering the competition between magnetic dipolar interaction and elastic interaction. A Peierls phase with a doubled unit cell length can be formed under proper conditions and a Peierls transition can be induced by temperature and by external magnetic field. Due to the coupling between the magnetic state and the structural state, the nanoparticle chains would possess distinct properties and exhibit interesting behaviour.

Boron‐Induced Hydrogen Localization in the Novel Metal Hydride LaNi3BHx (x = 2.5—3.0).

AbstractFor Abstract see ChemInform Abstract in Full Text.

Chemically ordered fcc (1 1 1) Mn–(Fe xNi 1− x) alloy ultrathin films

Ultrathin Mn deposits (0.25–2.0 ML equivalent) on an fcc (1 1 1) Fe–Ni substrate layer grown on W(1 1 0) have been annealed to form Mn alloys. Low energy electron diffraction (LEED) and directional Auger electron spectroscopy (DAES) were used to assess the long-range and short-range order of the alloy films, respectively. Neither structural nor chemical order was detected before annealing. However, after annealing to 600 K for 2 min, DAES showed that the Mn, Ni and Fe atoms all occupied fcc (1 1 1) structural positions, and a p(2 × 2) LEED pattern indicated a doubling of the surface unit cell through chemical ordering of the Mn atoms. The dependence of the half-integral LEED spot intensity on the amount of Mn deposited, and the annealing sequences required to observe the p(2 × 2) pattern are inconsistent with the formation of a surface alloy. Rather, the ordered alloy is formed after diffusion of Mn into the film, and extends to the surface. Since no ordered bulk alloy exists for the Mn–Fe–Ni system, it is argued that the close-packed fcc (1 1 1) surface stabilizes its formation in these ultrathin films.

New aspects in the (x-P-T) phase diagram of Pb(Zr1-xTix)O3materials (PZT)

PZT solid solutions were studied as a function of pressure and temperature by neutron and x-ray diffraction, Raman spectroscopy and dielectric measurements. Pressure was found to favour low-symmetry structures (Cm, Cc, F1, F1) characterized by a rotation and a reduction of the spontaneous polarisation and by the tilting of (Zr,Ti)O 6 octahedra leading to unit cell doubling (except Cm space group). Additionally, our results show that for Ti-rich PZT. high pressures stabilize the ferroelectric monoclinic phases responsible for the high piezoelectric properties of the MPB composition.

Boron-Induced Hydrogen Localization in the Novel Metal Hydride LaNi3BHx(x= 2.5−3.0)

The crystal structure and hydrogenation properties of the intermetallic boride LaNi(3)B were investigated. The hydrogen-free compound has a novel structure with orthorhombic symmetry, space group Imma, a = 4.9698(8) A, b = 7.1337(8) A, c = 8.3001(9) A, and V = 294.26(7) A(3). Thermal gravimetrical analysis reveals a hydride phase that forms near ambient conditions within the compositional range LaNi(3)BH(2.5)(-)(3.0). Single-crystal X-ray diffraction on both the alloy and the hydride, using the same crystal, shows an expansion in the a-c plane (by up to approximately 8%) and a contraction along b (by approximately 3%), while the symmetry changes from Imma to Bmmb (Cmcm) and the unit cell doubles along a and b. The cell parameters for the composition of LaNi(3)BD(2.73(4)) are a = 10.7709(7) A, b = 16.0852(10) A, c = 7.6365(5) A, V = 1323.03(15) A(3), and space group Cmcm. Four nearly fully occupied interstitial hydrogen sites were located by neutron powder diffraction on deuterides and found to have tetrahedral, La(2)Ni(2) (D1,D2), trigonal-prismatic, La(3)Ni(3) (D3), and trigonal-bipyramidal, La(2)Ni(3) (D4), metal environments. The structure can also be described in terms of alternating quasi two-dimensional [NiD](-) slabs (Ni-D = 1.62-1.97 A) and La-B sheets for which bond-valence sums suggest the limiting formula La(3+)B(0)[Ni(3)D(3)](3)(-). The La-B planes do not accommodate deuterium; the B-D and D-D interactions appear to be repulsive. The shortest B-D and D-D contacts are 2.52(2) and 2.33(2) A, respectively.

Ab Initio Electronic Structure Calculation for Single-Component Molecular Conductor Au(tmdt)2 (tmdt = Trimethylenetetrathiafulvalenedithiolate)

We have investigated the electronic structure of Au(tmdt) 2 (tmdt = trimethylenetetrathiafulvalenedithiolate), which is a single-component conductor showing a magnetic phase transition around 100 K, by ab initio plane-wave pseudopotential calculations. A single band crosses the Fermi level. This band and the next band below are a result of the strong hybridization between the two neighboring molecular levels near the Fermi level (SOMO and HOMO-1) and the system is more properly described as quarter-filled rather than half-filled in the strong correlation regime. The Fermi surface has corrugated-sheet-like parts nearly parallel to each other. Intraband generalized susceptibility suggests the presence of a nesting vector a * /2 ( a * is one of the reciprocal lattice vectors). Spin-polarized calculation on the doubled unit cell along the a axis ( a is a lattice vector) results in an antiferromagnetic order. The nesting is not perfect and Fermi-surface pockets remain in the magnetic phase. The implications of the present calculations with regard to experimental results are discussed.

Pressure-induced rotation of spontaneous polarization in monoclinic and triclinicPbZr0.52Ti0.48O3

The morphotropic composition of lead zirconate titanate (PZT), PbZr{sub 0.52}Ti{sub 0.48}O{sub 3}, was studied as a function of temperature (47-310 K) and hydrostatic pressure (from 0.1 MPa to 7.6 GPa) by neutron powder diffraction. Series of monoclinic (space groups Cm and Cc) and triclinic phases (space groups F1 and F1) were observed at various points in the P-T phase diagram of PbZr{sub 0.52}Ti{sub 0.48}O{sub 3}. The high-pressure behavior is characterized by two mechanisms, a rotation and reduction of the spontaneous polarization and the onset of octahedral tilting leading to unit cell doubling. The results show that pressure, as in the case of temperature and external electric fields, can induce polarization rotation.

X-ray scattering evidence of a new short range order or correlation in the high Tc superconductor Bi2Sr2CaCu2O8+δ

Synchrotron X-ray diffuse scattering experiments performed on a single domain optimally doped crystal of Bi 2 Sr 2 CaCu 2 O 8+δ (Tc # 91 °K) have revealed a new displacive short range order corresponding to a local doubling of the unit cell along the orthorhombic a 0 direction, thus perpendicular to the commonly reported q b = 0.21 super-structure modulation. The analysis of the X-Ray diffuse structure shows that it has 2D (Dimensional) character, the planar domains being parallel to the a-b conduction plane with an in plane size of about 60 A at room temperature. Experiments as a function of the temperature (down to 37 K) show that this local order is still present in the superconducting state with no significant change in domain size.

Novel Phase Transition in the Cd6M Intermetallics.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Ba8Ge43 revisited: a 2a′×2a′×2a′ Superstructure of the Clathrate‐I Type with Full Vacancy Ordering

AbstractThe reinvestigation of the crystal structure of Ba8Ge43□3 (space group $Ia{\bar 3}d$, no. 230; a = 21.3123(5) Å; Z = 8; Pearson symbol cI408) shows a full ordering of the vacancies (□) in the germanium framework. This ordered crystal structure can be considered as a derivative of an ideal “Ba8Ge46” clathrate‐I type structure ($Pm{\bar 3}n$, a′ = a/2) in which three Ge vacancies (per formula unit) are allowed to order in a cubic superstructure with a doubled unit cell parameter (□ at the 24c site, space group $Ia{\bar 3}d$). In the resulting Ge framework, each vacancy □ is surrounded by four three‐bonded (3b)Ge‐ species. The ordering in crystals of as‐cast samples (cooled in argon atmosphere, non‐annealed) is partially disrupted. For the “as‐cast” crystals, a short‐range model is proposed based on the partial distribution of Ge on the 24c and 24d sites. From the electron balance, Ba8Ge43 can be considered as a metallic Zintl phase with four excess electrons per formula unit. The Ba8Ge43 phase is stable in the temperature range 770 ‐ 810 °C and exists in equilibrium with Ba6Ge25 and Ge. By decomposition of undercooled (metastable) Ba8Ge43, a new metastable binary BaGe˜5 phase is formed.

Cooperative Jahn-Teller distortion inPrO2

We report neutron diffraction data on single crystal PrO2 which reveal a cooperative Jahn-Teller distortion at TD = 120 +/- 2 K. Below this temperature an internal distortion of the oxygen sublattice causes the unit cell of the crystallographic structure to become doubled along one crystal axis. We discuss several possible models for this structure. The antiferromagnetic structure below TN = 13.5 K is found to consist of two components, one of which shares the same doubled unit cell as the distorted crystallographic structure. We also present measurements of the magnetic susceptibility, the specific heat capacity and the electrical conductivity of PrO2. The susceptibility data show an anomaly at a temperature close to TD. From the specific heat capacity data we deduce that the ground state is doubly degenerate, consistent with a distortion of the cubic local symmetry. We discuss possible mechanisms for this. The conductivity shows an activated behaviour with an activation energy Ea = 0.262 +/- 0.003 eV.

Neutron scattering study of the field-dependent ground state and the spin dynamics in spin-one-half NH4CuCl3.

Elastic and inelastic neutron scattering experiments have been performed on the dimer spin system NH4CuCl3, which shows plateaus in the magnetization curve at m=1/4 and m=3/4 of the saturation value. Two structural phase transitions at T1 approximately 156 K and at T(2)=70 K lead to a doubling of the crystallographic unit cell along the b direction and as a consequence a segregation into different dimer subsystems. Long-range magnetic ordering is reported below T(N)=1.3 K. The magnetic field dependence of the excitation spectrum identifies successive quantum phase transitions of the dimer subsystems as the driving mechanism for the unconventional magnetization process in agreement with a recent theoretical model.

The magnetic structure of Cu 5Bi 2B 4O 14: a neutron scattering study

Neutron-scattering measurements are performed on powders of Cu 5Bi 2B 4O 14 to investigate their magnetic structure at temperatures ranging from 2 to 35 K. The ferrimagnetic easy-axis structure is confirmed with spin canting for one of the sublattices. No doubling of the crystal unit cell below the transition temperature T N=25 K is observed.

Novel phase transition in the Cd 6M intermetallics

X-ray and electron diffraction, electrical and thermal property experiments at low temperatures have unambiguously shown that Cd 6Yb, Cd 6Ca and Cd 6Y intermetallics, which are cubic crystals with a∼1.5 nm, undergo a phase transition at 110, 100 and 164 K, respectively. The transitions are reasonably attributed to orientational ordering of Cd 4 tetrahedra located at the center of icosahedral clusters; above the transition temperature the Cd 4 tetrahedra are completely disordered resulting in a bcc structure ( Im 3 ̄ ), whereas below the temperature they are ordered in a non-cubic structure of a doubled unit cell, i.e. 2 a×2 a×2 a or 2 a×2 a× a superlattice.

A new crystal phase of barium nitroprusside trihydrate studied by neutron diffraction at 20 K.

The crystal of barium pentacyanonitrosylferrate trihydrate [barium nitroprusside trihydrate, Ba[Fe(CN)5(NO)] x 3H2O] has been studied by neutron diffraction at 20 K. The study was performed to characterize the structural phase generated by the phase transition undergone by the crystals at 80 K, at which temperature the unit-cell volume doubles. This crystal phase still exists at 20 K. The crystal structure, in space group P1, is completely ordered. The positional changes of the water molecules in the present structure with respect to those of the compound at 105 K are presented.

Order–disorder transition in cubic Cd 6Yb and Cd 6Ca

Specific heat, X-ray diffraction and electron diffraction experiments at low temperatures have clearly shown that the Cd 6Yb and Cd 6Ca crystals, which are 1/1 cubic approximants at room temperature, undergo an order–disorder transition at 110 and 100 K, respectively. The transition is reasonably attributed to the orientational ordering of 4Cd tetrahedra located at the center of the icosahedral clusters; above 110 K (100 K) the 4Cd tetrahedra are completely disordered resulting in a bcc structure, whereas below 110 K (100 K) they are ordered in a non-cubic structure of a doubled unit cell.

Determination of the crystal structure of δ-MoN by neutron diffraction

We have determined the crystal structure of ordered hexagonal δ-MoN by use of powder X-ray diffraction and time-of-flight neutron diffraction. A disordered variety of the compound was first prepared by high-temperature ammonolysis of MoCl 5. This material has hexagonal symmetry with the space group P6 3 mc with a=2.87(2) and c=2.81(1) Å. Upon high pressure annealing, the N-atoms become ordered and the unit cell doubles in size: a=5.73659(10) and c=5.61884(17) Å. The superconducting transition temperature increases from 4 K in the disordered compound to 12.1 K in the ordered phase.

Pressure-induced phase transitions and templating effect in three-dimensional organic-inorganic hybrid perovskites

Pressure-induced structural changes of conducting halide perovskites $({\mathrm{CH}}_{3}{\mathrm{NH}}_{3}){\mathrm{SnI}}_{3},$ $({\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{)}_{0.5}({\mathrm{NH}}_{2}\mathrm{CH}={\mathrm{NH}}_{2}{)}_{0.5}{\mathrm{SnI}}_{3},$ and $({\mathrm{NH}}_{2}\mathrm{CH}={\mathrm{NH}}_{2}){\mathrm{SnI}}_{3},$ have been investigated using synchrotron x-ray powder diffraction. In contrast to low-temperature structural changes, no evidence of an increased ordering of the organic cations was observed under pressure. Instead, increase in pressure results first in a ${\mathrm{ReO}}_{3}$-type doubling of the primitive cubic unit cell, followed by a symmetry distortion, and a subsequent amorphization above 4 GPa. This process is reversible and points towards a pressure-induced templating role of the organic cation. Bulk compressions are continuous across the phase boundaries. The compressibilities identify these hybrids as the most compressible perovskite system ever reported. However, the $\mathrm{Sn}\ensuremath{-}\mathrm{I}$ bond compressibility in $({\mathrm{CH}}_{3}{\mathrm{NH}}_{3}){\mathrm{SnI}}_{3}$ shows a discontinuity within the supercell phase. This is possibly due to an electronic localization.

Ferroelectric and ferromagnetic properties of BiFeO3–PrFeO3–PbTiO3 solid solutions

Solid solutions of xBiFeO3–yPrFeO3–zPbTiO3 (x+y+z=1) and (1−w)BiFeO3–wPbTiO3 have been explored for finding ferroelectromagnetic bulk material, in which ferroelectricity and ferromagnetism coexist simultaneously. The coexistence has been observed only in some ternary composition samples, that is, 0.2BiFeO3–0.2PrFeO3–0.6PbTiO3 and 0.4BiFeO3–0.2PrFeO3–0.4PbTiO3. In the ternary solid solutions, spontaneous magnetic moments disappear with the decrease of PrFeO3 content to y<0.2 independently of BiFeO3 content. When PrFeO3 content remains constant at y=0.2, the ternary solid solutions become paraelectric with the decrease of PbTiO3 content to z⩽0.2. The ferroelectromagnetic solid solutions have the noncentrosymmetric and doubled perovskite unit cell with a space group I4cm (a=b≈5.4 Å, c≈7.9 Å). Addition of Ta2O5 dopant substantially changes the polarization–electric-field and magnetization–magnetic field curves of the 0.2BiFeO3–0.2PrFeO3–0.6PbTiO3. The binary solid solutions of (1−w)BiFeO3–wPbTiO3 do not exhibit spontaneous magnetic moments down to 10 K over the entire composition range.

The magnetic structure of Cu 5Bi 2B 4O 14. Neutron scattering

The neutron scattering measurements have been carried out on powdered sample Cu 5Bi 2B 4O 14 at temperature range 2–35 K in order to investigate the low-temperature magnetic structure. The ferrimagnetic easy-axis structure was confirmed with the spin canting for one of the sublattice. No doubling of the crystal unit cell below the transition temperature T N=25 K was observed.