Doubling Of Unit Cell Research Articles

Synthesis and structural details of [formula omitted] compounds with M = Al, Nb, Fe, Ru, Ga & Co and RE = Eu, Y

Synthesis and structural details of MSr2RECu2Oz (M-1212) compounds with M=Ga, Nb, Fe, Al and Co) and RE=Eu, Y are reported. Reitveld refinement of X-ray diffraction (XRD) patterns shows that all compounds are crystallized in single phase. Nb-, Fe- and Al-1212 possess tetragonal P4/mmm space group structure while the Ga-1212 and Co-1212 are crystallized in orthorhombic Ima2 space group. The change of space group from P4/mmm to Ima2 indicating towards the doubling of unit cell. The buckling angle [Cu(2)–O(2)–Cu(2) angle] shows that most of the studied samples are heavily under doped and hence they could not exhibit superconductivity. Thermogravimetric (TGA) analysis shows the M-1212 compounds to be more stable than widely studied 90K superconductor Cu-1212 (RE-123).

Structural features of phases (Na0.5R0.5)MO4 and (Na0.5R0.5)MO4:R′ (R = Gd, La; R′ = Er, Tm, Yb; M = W, Mo) of the scheelite family

The structural data for single crystals of (Na0.5R0.5)MO4 and (Na0.5R0.5)MO4:R′ (R = La, Gd; R′ = Er, Tm, Yb; M = W, Mo) grown by the Czochralski method were studied by X-ray diffraction and analyzed. The structural characteristics of these compounds depend on the sort of cations M and R. The formation of superstructures was found in the scheelite structure, and distortion of the scheelite structure depending on the composition and preparation conditions was established (with unit cell rotation by 45° and triclinic distortion of the scheelite structure for (Na0.5Gd0.5)WO4:Tm, with doubled unit cell compared to the scheelite type structure for (Na0.5Gd0.5)WO4 and (Na0.5Gd0.5)WO4:Yb). In the case of overstoichiometric oxygen content in the crystal, the unit cell symmetry increases to space group I41/amd or (Na0.5Gd0.5)WO4:Yb without considerable change in the cell parameters. On the basis of experimental data, a transformation scheme for the structures in the system Na 2 + M6+O4−-“R3+M5+O4” was proposed.

Phonon anomalies in Pb1−xLax(Zr0.9Ti0.1)O3 ceramics

The lattice dynamics of Pb1−xLax(Zr0.9Ti0.1)O3 (PLZT X/90/10) with X=0, 2, 4, 10% of La was studied by means of Far Infrared, Raman and terahertz spectroscopies in the temperature range of 20–800 K. Infrared active soft phonons with anomalies near the paraelectric-ferroelectric phase transition were found for all the samples. Some Raman active phonons show anomalies ∼200 K below TC, due to another phase transition to a ferroelectric state with doubled unit cell. Samples with higher La content (X=4,10) display nonclassical phonon softening described by ω2=a(T−TC)γ with γ=2/3.

Counterion dimerisation effects in the two-chain compound (Per)2[Co(mnt)2]: structure and anomalous pressure dependence of the electrical transport properties

The crystal structure of (Per)2[Co(mnt)2], was investigated by single crystal X-ray diffraction. Its refinement shows a strong chemical dimerization of the anions [Co(mnt)2]22−. This is responsible for a unit cell doubling along the staking axis b when compared with other members of the α-(Per)2[M(mnt)2] family of the low dimensional molecular conductors, with M = Ni, Cu, Au, Pt or Pd. The dimerized [Co(mnt)2]22− stacks appear however as poorly correlated in the ac plane. The electrical conductivity of this compound measured in single crystals exhibits anomalous pressure dependence, where under increasing hydrostatic pressure the conductivity decreases while the metal insulator transition temperature is enhanced. This effect is ascribed to the dimeric nature of the counterions which imposes a sliding of the stacked perylene molecules along alternated directions in their planes.

Magnetic and electronic Co states in the layered cobaltateGdBaCo2O5.5−x

We have performed nonresonant x-ray diffraction, resonant soft and hard x-ray magnetic diffraction, soft x-ray absorption, and x-ray magnetic circular dichroism measurements to clarify the electronic and magnetic states of the ${\text{Co}}^{3+}$ ions in ${\text{GdBaCo}}_{2}{\text{O}}_{5.5}$. Our data are consistent with a ${\text{Co}}_{\text{Py}}^{3+}$ high-spin (HS) state at the pyramidal sites and a ${\text{Co}}_{\text{O}c}^{3+}$ low-spin (LS) state at the octahedral sites. The structural distortion with a doubling of the $a$ axis ($2{a}_{p}\ifmmode\times\else\texttimes\fi{}2{a}_{p}\ifmmode\times\else\texttimes\fi{}2{a}_{p}$ cell) shows alternating elongations and contractions of the pyramids, and indicates that the metal-insulator transition is associated with orbital order in the ${t}_{2g}$ orbitals of the ${\text{Co}}_{\text{Py}}^{3+}$ HS state. This distortion corresponds to an alternating ordering of $xz$ and $yz$ orbitals along the $a$ and $c$ axes for the ${\text{Co}}_{\text{Py}}^{3+}$. The orbital ordering and pyramidal distortion lead to deformation of the octahedra but the ${\text{Co}}_{\text{Oc}}^{3+}$ LS state does not allow an orbital order to occur for the ${\text{Co}}_{\text{Oc}}^{3+}$ ions. The soft x-ray magnetic diffraction results indicate that the magnetic moments are aligned in the $ab$ plane but are not parallel to the crystallographic $a$ or $b$ axes. The orbital order and the doubling of the magnetic unit cell along the $c$ axis support a noncollinear magnetic structure. The x-ray magnetic circular dichroism data indicate that there is a large orbital magnetic contribution to the total ordered Co moment.

Raman spectroscopy and effective dielectric function in PLZTx/40/60

The Raman spectra of the ceramics PLZTx/40/60 have been measured in the temperature range 20–800 K. An assignment of the Ramanmodes is presented using the results of the effective medium approximation for the IR data.Factor-group analysis is presented for the case of off-site position of cations or unit celldoubling, which allows a higher number of active phonons in the IR and Ramanspectra. The presence of Raman-active modes in the cubic phase is associated withlocal unit cell doubling and symmetry breakdown due to the off-site cations.

Hidden Degrees of Freedom in Aperiodic Materials

Numerous crystalline materials, including those of bioorganic origin, comprise incommensurate sublattices whose mutual arrangement is described in a superspace framework exceeding three dimensions. We report direct observation by neutron diffraction of superspace symmetry breaking in a solid-solid phase transition of an incommensurate host-guest system: the channel inclusion compound of nonadecane/urea. Strikingly, this phase transition generates a unit cell doubling that concerns only the modulation of one substructure by the other-an internal variable available only in superspace. This unanticipated pathway for degrees of freedom to rearrange leads to a second phase transition, which again is controlled by the higher dimensionality of superspace. These results reveal nature's capacity to explore the increased number of phases allowed in aperiodic crystals.

Structural Phase Transition of Gd3RuO7

Structural phase transition of trigadorinium ruthenium heptaoxide, Gd(3)RuO(7), has been investigated by in situ high-temperature single-crystal X-ray diffraction. A small shrinkage in b-length and an expansion in c-length were observed between 363 and 383 K with increasing temperature. No significant change occurred in a-length within experimental errors. The changes were essentially reversible against temperature. Structures of the high-temperature modification have been determined at 423, 773, and 1223 K assuming the orthorhombic Cmcm symmetry. The structure of the low-temperature modification has been determined at 293 K, assuming the orthorhombic P2(1)nb symmetry with doubled unit cell along the b-axis of the high-temperature modification. The transition from high- to low-temperature modification can be structurally characterized by tilts about axes close to the c-axis of the unit cell occurring on half of the RuO(6) octahedra. These octahedral tilts couple with a reduction in coordination number of the Gd atom bridging the adjacent RuO(6) single chains along the b-axis. The present study also revealed the presence of structural disorder in the high-temperature Cmcm modification that had not been reported for the archetypal Cmcm structures of lanthanide ruthenates (Ln(3)RuO(7)) and osmates (Ln(3)OsO(7)) in the literature. The disorder includes a dynamical or static distribution of one-third of Gd atoms in the unit cell, which is presumably linked to the libration of the octahedral tilts about the axes close to c.

Monoclinic Ferroelastic Domains in LaCoO3‐Based Perovskites

In addition to the well known ferroelastic domains formed by deformation twinning in rhombohedral perovskites, a monoclinic structure is observed by transmission electron microscopy in LaCoO3-based materials. The monoclinic domains consist of a one-dimensional superstructure corresponding to doubling or tripling of the pseudo cubic unit cell due to twinning at the atomic scale, as shown in the figure.

Enantiotropic Polymorphism in Di-indenoperylene

The enantiotropic polymorphic phase transformation of di-indenoperylene (DIP) at 403 K has been structurally characterized, using single-crystal X-ray diffraction. Both the low temperature α- and the high-temperature β-phase have a herringbone-type structure, with the α-phase being triclinic with doubled unit cell volume compared to the monoclinic β-phase. In the latter, the molecules have a more upright orientation in the herringbone plane. The epitactic transformation from β- to α-phase involves strong shearing displacements as well as bending and torsional deformations of the DIP molecules.

Crystal and magnetic structures of the spin-trimer compoundsCa3Cu3−xNix(PO4)4(x=0,1,2)

Crystal and magnetic structures of a series of quantum spin-trimer system ${\mathrm{Ca}}_{3}{\mathrm{Cu}}_{3\ensuremath{-}x}{\mathrm{Ni}}_{x}{(\mathrm{P}{\mathrm{O}}_{4})}_{4}$ $(x=0,1,2)$ were studied by neutron powder diffraction at temperatures of $1.5--290\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The composition with one Ni per trimer $(x=1)$ has a monoclinic structure (space group $P{2}_{1}∕a$, No. 14) with the unit cell parameters $a=17.71\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, $b=4.89\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, $c=8.85\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, and $\ensuremath{\beta}=123.84\ifmmode^\circ\else\textdegree\fi{}$ at $T=290\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The $x=2$ composition crystallizes in the $C2∕c$ space group (No. 15) with the doubled unit cell along the $c$ axis. Each trimer is formed by two crystallographic positions: one in the middle and the second one at the ends of the trimer. We have found that the middle position is occupied by the ${\mathrm{Cu}}^{2+}$, whereas the end positions are equally populated with the ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Ni}}^{2+}$ for $x=1$ while in the $x=2$ the trimers were found to be of only one type Ni-Cu-Ni. Below ${T}_{N}=20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the $x=2$ compound shows an antiferromagnetic ordering with propagation vector star ${[\frac{1}{2},\frac{1}{2},0],[\ensuremath{-}\frac{1}{2},\frac{1}{2},0]}$. The magnetic structure is very well described with the irreducible representation ${\ensuremath{\tau}}_{2}$ using both arms of the star ${\mathbf{k}}$ with the magnetic moments of 1.89(1) and $0.62(2)\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$ for ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Cu}}^{2+}$ ions, respectively. We note that our powder diffraction data cannot be fitted by a model containing only one arm of the propagation vector star. The $\mathrm{Cu}∕\mathrm{Ni}$ spins form both parallel and antiparallel configurations in the different trimers, implying substantial effect of the intertrimer interaction on the overall magnetic structure.

Quantum and classical spins on the spatially distorted kagomé lattice: Applications to volborthiteCu3V2O7(OH)2∙2H2O

In volborthite, spin-$1∕2$ moments form a distorted kagom\'e lattice of corner sharing isosceles triangles with exchange constants $J$ on two bonds and ${J}^{\ensuremath{'}}$ on the third bond. We study the properties of such spin systems and show that despite the distortion, the lattice retains a great deal of frustration. Although subextensive, the classical ground state degeneracy remains very large, growing exponentially with the system perimeter. We consider degeneracy lifting by thermal and quantum fluctuations. To linear (spin wave) order, the degeneracy is found to stay intact. Two complementary approaches are therefore introduced, appropriate to low and high temperatures, which point to the same ordered pattern for ${J}^{\ensuremath{'}}gJ$. In the low-temperature limit, an effective chirality Hamiltonian is derived from nonlinear spin waves, which predicts a transition on increasing ${J}^{\ensuremath{'}}∕J$ from $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-type order to a ferrimagnetic chirality stripe order with a doubled unit cell. This is confirmed by a large-$n$ approximation on the $\mathrm{O}(n)$ model on this lattice. While the saddle point solution produces a line degeneracy, $\mathrm{O}(1∕n)$ corrections select the nontrivial wave vector of the striped chirality state. The quantum limit of spin $1∕2$ on this lattice is studied via exact small system diagonalization and compares well with experimental results at intermediate temperatures. We suggest that the very-low-temperature spin frozen state seen in NMR experiments may be related to the disconnected nature of classical ground states on this lattice, which leads to a prediction for NMR line shapes.

Crystallographic phase transition within the magnetically ordered state ofCe2Fe17

X-ray diffraction experiments were performed on polycrystalline and single-crystal specimens of Ce{sub 2}Fe{sub 17} at temperatures between 10 and 300 K. Below T{sub t}=118{+-}2 K, additional weak superstructure reflections were observed in the antiferromagnetically ordered state. The superstructure can be described by a doubling of the chemical unit cell along the c direction in hexagonal notation with the same space group R3m as the room-temperature structure. The additional antiferromagnetic satellite reflections observed in earlier neutron diffraction experiments can be conclusively related to the appearance of this superstructure.

A Novel Ordered Phase in SrCu2(BO3)2 under High Pressure

We report results of 11 B NMR and susceptibility measurements on the quasi two-dimensional frustrated dimer spin system SrCu 2 (BO 3 ) 2 under high pressure. At 2.4 GPa and in a magnetic field of 7 T, NMR lines split with decreasing temperature in two steps. A gradual splitting below T = 30 K breaking the four-fold symmetry of magnetic response is followed by a further sudden splitting below 3.6 K. The latter indicates a magnetic phase transition, which is also marked by a kink in the susceptibility at 1.44 GPa. From the magnetic hyperfine shift data, we conclude that the low- T phase has a doubled unit cell containing two types of dimers, one in a nearly singlet state and the other with a finite magnetization down to T = 0.

Superstructure of Challis mordenite with doubled monoclinic unit cell

A new superstructure was found in mordenite (Na 5.59 Ca 1.80 Al 9.19 Si 38.81 O 96 ·nH 2 O) from Challis Valley, Idaho-a zeolite widely used in previous studies. The occurrence of the superstructure reflections were observed in 12 specimens from two rock samples, at the midpoint of layers in the oscillation photographs around the a* and b* axes, but not around the c* axis. The apparent 2a, 2b, and c axes are orthogonal. In spite of yielding an apparent orthorhombic cell, careful observation of the intensities revealed that the superstructure is monoclinic with twice the volume of the orthorhombic cell: a’ = 27.356(9), b’ = 27.356(9), c’ = 7.517(2) Å, γ′ = 97.14(4)°, and V′ = 5582(4) Å 3 . The presence of the superstructure was examined for specimens from ten other localities and a synthetic sample but there was no evidence for extra spots of the superstructure, indicating that the occurrence is not common. The large displacement factors of O8 oxygen, and its associated 180° T-O-T angle that is energetically unfavorable, are basic to the mordenite structure. In the monoclinic superstructure the O8 oxygen is decomposed into several asymmetric O atoms by the symmetry reduction. The large displacement smearing of O8 oxygen perpendicular to the straight T-O-T, are explained by the superposition of decomposed O atoms. The extra spots are predominantly observed in the reciprocal planes with l = odd in the same way as diffuse streaks. The observation of extra spots and diffuse streaks suggests that there are three kinds of domains in the mordenite, one in the ordered form and two others in the random form of c/2 linear displacements. The domain in the ordered form represents the superstructure. Thus, we propose a model for the superstructure in which the c/2 displacement occurs alternatively in the two basic unit cells slightly modified in the superstructure, yielding the periodic arrangements.

Modulated and unmodulated structures, and the transport mechanisms in the triangular lattice systemNaxCoO2 with 0.48, 0.58 and 0.65

The crystal structures of the triangular lattice systemγ-phaseNaxCoO2 with compositions of , 0.58 and 0.65 have been determined by means of x-ray four-circle diffraction.Na0.58CoO2 has latticeconstants of ah = 2.8180(5) and ch = 11.005(9) Å withspace group P63/mmc and does not exhibit structural modulation. On the other hand,Na0.65CoO2 hasa P63/mmc-type superlattice with a doubled unit cell ofash = 5.6527(5) andcsh = 10.9356(10) Å, andNa0.48CoO2 indicatesa Pmmn-typesuperlattice with aso = 5.6261(5), bso = 11.1406(10) and cso = 4.8723(5) Å. The structural modulation for is attributed to the incomplete order of Na ions without an order for Co valence, whilethat for results in almost complete Na order and partial valence order for Co. For , the metal–poor metal transition occurs at about 50 K. The electrical resistivities for and 0.65 follow a T3/2 dependence below 200 and 80 K, respectively, which may be due to antiferromagnetic spinfluctuations in three dimensions. At higher temperatures, the resistivities for theCoO2 planehave a T1 dependence, likely due to an enhancement of two dimensionality. For , the resistivity maximum appears at about 250 K, which may be attributed to therenormalization effect of the Fermi surface by the fluctuations.

Hexagon versus Trimer Formation in In Nanowires on Si(111): Energetics and Quantum Conductance

The structural and electronic properties of the quasi-one-dimensional In/Si(111) surface system are calculated from first principles. It is found that the symmetry lowering of the In chains is energetically favorable, provided neighboring nanowires are correlated, giving rise to a doubling of the surface unit cell both along and perpendicular to the chain direction. The recently suggested formation of hexagons within the In nanowires [C. González, F. Flores, and J. Ortega, Phys. Rev. Lett. 96, 136101 (2006)]--in clear contrast to the trimer formation proposed earlier-drastically modifies the electron transport along the In chains, in agreement with experiment.

EPR study of local ordering in DMAAlS and DMAGaS crystals

X-band electron paramagnetic resonance (EPR) investigations of single crystals of Cr3+-doped dimethylammonium aluminium sulphate hexahydrate (DMAAlS) and dimethylammonium gallium sulphate hexahydrate (DMAGaS) are presented from room temperature down to liquid helium temperature. Both crystals undergo an order-disorder phase transition to ferroelectric phase at T C1 = 152 K and T C1 = 134 K, respectively. Additionally, DMAGaS undergoes a first-order transition into a low temperature non-ferroelectric intermediate phase at T C2 = 116 K and into a low temperature antiferroelectric phase at about T C3 = 60 K. The spin Hamiltonian parameters have been determined for all observed in both crystal phases. Remarkable EPR line width changes with observed triplets confirm the order-disorder character of the ferroelectric phase transition and demonstrate freezing-out of dimethyl ammonium reorientations, which is a primary reason for this transition. The doubled number of EPR lines in comparison to the paraelectric and ferroelectric phases in DMAGaS indicates a unit cell doubling in antiferroelectrics phase below 60 K.

On the Verwey charge ordering in magnetite

Magnetite, a famous permanent natural magnet, is probably the oldest magnetic material known to humans [1]. Fe3O4 is a ferrimagnet with an anomalously high Curie temperature of ∼850 K which arises from the inverse spinel type crystal structure. This is formally written as AB2O4 where A and B are two crystallographically distinct tetrahedrally and octahedrally coordinated Fe sites. Both have different oxidation states due to the large difference of the corresponding average Fe–O distances, 1.876 and 2.066 A (at 300 K), respectively. Thus the former (the A site) is occupied by Fe3+ ion, whereas octahedral B sites are occupied by an equal number of randomly distributed 2+ and 3+ Fe ions, which results in an average valence value of 2.5+ per FeB ion. The B sublattice is highly frustrated and can be viewed as a pyrochlore lattice of corner-sharing FeB tetrahedra. Therefore, mixed-valent magnetite is considered to be a charge frustrated system with a highly degenerate ground state. Fe3O4 is a poor metal with an electrical resistivity of 4 m cm (at 300 K) which is remarkably higher then the resistivity of simple metals, e.g. 0.96, 1.59 or 1.7 μ cm for Hg, Ag or Cu, respectively. Upon cooling below TV ∼ 120 K magnetite undergoes a sharp first-order metal–insulator transition (the so-called Verwey transition) at which the conductivity abruptly decreases by two orders of magnitude and the symmetry of the structure lowers from the cubic one [2]. To explain this anomaly Verwey proposed a theoretical model according to which the transition is caused by the ordering of Fe2+ ions on the B sublattice with formation of charged (001) planes alternately occupied by 2+ and 3 + FeB ions. This was the first report on a charge ordering as well as an orthorhombic (Verwey) superstructure model. The Verwey charge ordering (CO) model obeys the so-called Anderson criterion for minimal electrostatic repulsion which requires the occupation of each FeB tetrahedron by an equal number of 2+ and 3+ ions, thus leading to a short-range CO pattern [3]. At first confirmed by x-ray and neutron diffraction studies [4], the Verwey CO model was disproved by further experiments. The half-integer satellite reflections (h, k, l + 2 ) clearly observed below TV indicate a doubling of the cubic unit cell along the c axis and show the symmetry to be monoclinic Cc [5]. On the other hand, observation of the magnetoelectric effect revealed even lower P1 symmetry in the low temperature phase [6]. Nevertheless, no conclusive structural model or charge ordered arrangement was identified because of the complexity of the low temperature structure and the difficulties caused by microtwinning at the

D030 Structure determination from powder diffraction data of some moisture-sensitive, network coordination compounds

A series of new com pounds was discovered during the study of a novel solid lithium- ion battery electrolyte system based on succinonitrile. The phases LiX(succinonitrile)2 (X = PF6 - , AsF6 - , ClO4 - ) were identified as undesirable phases that formed at low salt concentrations in succinonitrile. The silver analogues of these systems also exist as crystalline phases, but the lithium salts do not share the same structure. The crystal structures of the Li salts were solved from laboratory X-ray diffraction data. The AsF6 - and PF6 - adducts are isostructural, with the monoclinic space group, P2. The ClO4 - adduct has a doubled unit cell volume compared to the AsF6 - and PF6 - materials and has the orthorhombic space group Pba2. The lithium coordination in these adducts helps explain why plastic crystal electrolytes using LiPF6 and LiClO4 salts have poor ionic conductivity at room temperature.