Superstructure Modulation Research Articles

Microstructure and structural modulation of lutetium dihydride LuH2 as seen via transmission electron microscopy

Structural investigations conducted using transmission electron microscopy (TEM) on LuH2 synthesized under atmospheric pressure (AP-LuH2) and nitrogen-doped LuH2 synthesized under high pressure (HP-LuH2) have revealed numerous microstructural phenomena. Both materials show a clear superstructure modulation with wave vector, q*=14(22¯0), and this modulation can be interpreted by the displacements of Lu atoms. Further investigations on the nitrogen-doped HP-LuH2 materials reveal the appearance of high-density antiphase boundaries, in particular, domain walls of a few atomic layer thickness without structural modulation can be observed, suggesting possible interface properties could be detected in this system. In-situ TEM observations of AP-LuH2 suggest that no evident structural phase transition occurs between 94 K and 673 K.

Observation of charge density wave in layered hexagonal Cu1.89Te single crystal

Abstract We report comprehensive transport, electron microscopy and Raman spectroscopy studies on the transition-metal chalcogenides Cu1.89Te single crystals. The metallic Cu1.89Te displays successive metal-semiconductor transitions at low temperatures and almost ideal linear MR when magnetic field up to 33T. Through the electron diffraction patterns, the stable room temperature phase is identified as a 3×3×2 modulated superstructure based on Nowotny hexagonal structure. The superlattice spots of Transmission electron microscopy and Scanning tunneling microscopy clearly show the structural transitions from the room temperature Commensurate Ⅰ (C-Ⅰ) phase to the low temperature Commensurate Ⅱ (C-Ⅱ) phase. All the results can be understood in term of charge density wave (CDW) instability, yielding intuitive evidences for the CDW formations in Cu1.89Te. The additional Raman modes below room temperature further reveal that the zone-folded phonon modes may play an important role on the CDW transitions. Our research sheds light on the novel electron features of Cu1.89Te at low temperature, and may provide potential applications for future nano-devices.

Observation of Charge Density Wave in Layered Hexagonal Cu1.89Te Single Crystal

We report comprehensive transport, electron microscopy and Raman spectroscopy studies on transition-metal chalcogenides Cu1.89Te single crystals. The metallic Cu1.89Te displays successive metal-semiconductor transitions at low temperatures and almost ideal linear MR when magnetic field up to 33 T. Through the electron diffraction patterns, the stable room-temperature phase is identified as a 3 × 3 × 2 modulated superstructure based on the Nowotny hexagonal structure. The superlattice spots of transmission electron microscopy and scanning tunneling microscopy clearly show the structural transitions from the room-temperature commensurate I phase, named as C-I phase, to the low temperature commensurate II (C-II) phase. All the results can be understood in terms of charge density wave (CDW) instability, yielding intuitive evidences for the CDW formations in Cu1.89Te. The additional Raman modes below room temperature further reveal that the zone-folded phonon modes may play an important role on the CDW transitions. Our research sheds light on the novel electron features of Cu1.89Te at low temperature, and may provide potential applications for future nano-devices.

Temperature-driven reorganization of electronic order in CsV3Sb5

We report x-ray diffraction studies of the electronic ordering instabilities in the kagome material ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ as a function of temperature and applied magnetic field. Our zero-field measurements between 10 and 120 K reveal an unexpected reorganization of the three-dimensional electronic order in the bulk of ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$: At low temperatures, a $2\ifmmode\times\else\texttimes\fi{}2\ifmmode\times\else\texttimes\fi{}2$ superstructure modulation due to electronic order is observed, which upon warming changes to a $2\ifmmode\times\else\texttimes\fi{}2\ifmmode\times\else\texttimes\fi{}4$ superstructure at 60 K. The electronic order-order transition discovered here involves a change in the stacking of electronically ordered ${\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ layers, which coincides with anomalies previously observed in magnetotransport measurements. This implies that the temperature-dependent three-dimensional electronic order plays a decisive role for transport properties, which are related to the Berry curvature of the V bands. We also show that the bulk electronic order in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ breaks the sixfold rotational symmetry of the underlying $P6/mmm$ lattice and perform a crystallographic analysis of the $2\ifmmode\times\else\texttimes\fi{}2\ifmmode\times\else\texttimes\fi{}2$ phase. The latter yields two possible superlattices, namely a staggered star-of-David and a staggered inverse star-of-David structure. Applied magnetic fields up to 10 T have no effect on the x-ray diffraction signal. This, however, does not rule out time-reversal symmetry breaking in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$.

The Transport Properties of Quasi–One-Dimensional Ba3Co2O6(CO3)0.7

We have performed combined elastic neutron diffuse, electrical transport, specific heat, and thermal conductivity measurements on the quasi–one-dimensional Ba3Co2O6(CO3)0.7 single crystal to characterize its transport properties. A modulated superstructure of polyatomic CO32− is formed, which not only interferes the electronic properties of this compound, but also reduces the thermal conductivity along the c-axis. Furthermore, a large magnetic entropy is observed to be contributed to the heat conduction. Our investigations reveal the influence of both structural and magnetic effects on its transport properties and suggest a theoretical improvement on the thermoelectric materials by building up superlattice with conducting ionic group.

Using (3+1)D space to investigate a modulated superstructure mystery

Using (3+1)D space to investigate a modulated superstructure mystery

A new modulated crystal structure of the ANS complex of the St John's wort Hyp-1 protein with 36 protein molecules in the asymmetric unit of the supercell.

Superstructure modulation, with violation of the strict short-range periodic order of consecutive crystal unit cells, is well known in small-molecule crystallography but is rarely reported for macromolecular crystals. To date, one modulated macromolecular crystal structure has been successfully determined and refined for a pathogenesis-related class 10 protein from Hypericum perforatum (Hyp-1) crystallized as a complex with 8-anilinonaphthalene-1-sulfonate (ANS) [Sliwiak et al. (2015), Acta Cryst. D71, 829-843]. The commensurate modulation in that case was interpreted in a supercell with sevenfold expansion along c. When crystallized in the additional presence of melatonin, the Hyp-1-ANS complex formed crystals with a different pattern of structure modulation, in which the supercell shows a ninefold expansion of c, manifested in the diffraction pattern by a wave of reflection-intensity modulation with crests at l = 9n and l = 9n ± 4. Despite complicated tetartohedral twinning, the structure has been successfully determined and refined to 2.3 Å resolution using a description in a ninefold-expanded supercell, with 36 independent Hyp-1 chains and 156 ANS ligands populating the three internal (95 ligands) and five interstitial (61 ligands) binding sites. The commensurate superstructures and ligand-binding sites of the two crystal structures are compared, with a discussion of the effect of melatonin on the co-crystallization process.

Magnetic Properties of theRE2Pt6X15(RE= Y, La–Nd, Sm, Gd–Lu;X= Al, Ga) Series

The members of theRE2Pt6Al15(RE= Ce–Nd, Sm, Gd–Yb) andRE2Pt6Ga15(RE= Y, La–Nd, Sm, Gd–Lu) series have been synthesized from the elements via arc‐melting followed by annealing in an induction furnace. Isotypism of Ho2Pt6Al15with orthorhombic (3+1)D commensurately modulated Sc2Pt6Al15[Cmcm(α,0,0)0s0,α= 2/3] was observed from single‐crystal diffraction data. The diffraction pattern shows the same satellite reflections pattern as the prototype, therefore the same modulated superstructure was refined. Full ordering of the Pt and Al atoms within the [Pt6Al15]δ–polyanion was observed. The lattice parameters of the averaged hexagonal structure were refined from powder X‐ray diffraction experiments and agree well with previous reports on these compounds. The X‐ray pure polycrystalline samples of both series were studied with respect to their magnetic properties. The susceptibility data of all investigated compounds show experimental magnetic moments close to the free ion values of theRE3+cations and antiferro‐/ferrimagnetic ordering at low temperatures with Curie temperatures up toTC= 16.8(1) K for Gd2Pt6Ga15. Both, Sm2Pt6Al15and Sm2Pt6Ga15show the typicalvan Vlecktype behavior along with antiferromagnetic ordering at a Néel temperature ofTN= 4.5(1) K (Al) and ferrimagnetic ordering atTC= 6.5(1) K (Ga). Yb2Pt6Al15exhibits ytterbium in the trivalent oxidation state, while for Yb2Pt6Ga15a divalent state of the Yb atoms was observed.

The Dynamic Nature of CO Adlayers on Pt(111) Electrodes.

CO adlayers on Pt(111) electrode surfaces are an important electrochemical system and of great relevance to electrocatalysis. The potential‐dependent structure and dynamics of these adlayers are complex and still controversial, especially in the CO pre‐oxidation regime. We here employ in situ high‐speed scanning tunneling microscopy for studying the surface phase behavior in CO‐saturated 0.1 m H2SO4 on the millisecond time scale. At potentials near the onset of CO pre‐oxidation local fluctuations in the (2×2)‐CO adlayer are observed, which increase towards more positive potentials. Above 0.20 V (vs. Ag/AgCl), this leads to an adlayer where COad apparently reside on every top site, but still exhibit a (2×2) superstructure modulation. We interpret this observation as a dynamic effect, caused by a small number of highly mobile point defects in the (2×2)‐CO adlayer. As shown by density functional theory calculations, the CO lattice near such defects relaxes into a local (1×1) arrangement, which can rapidly propagate across the surface. This scenario, where a static (2×2) COad sublattice coexists with a highly dynamic sublattice of partially occupied top sites, explains the pronounced COad surface mobility during electrooxidation.

The Dynamic Nature of CO Adlayers on Pt(111) Electrodes

AbstractCO adlayers on Pt(111) electrode surfaces are an important electrochemical system and of great relevance to electrocatalysis. The potential‐dependent structure and dynamics of these adlayers are complex and still controversial, especially in the CO pre‐oxidation regime. We here employ in situ high‐speed scanning tunneling microscopy for studying the surface phase behavior in CO‐saturated 0.1 m H2SO4 on the millisecond time scale. At potentials near the onset of CO pre‐oxidation local fluctuations in the (2×2)‐CO adlayer are observed, which increase towards more positive potentials. Above 0.20 V (vs. Ag/AgCl), this leads to an adlayer where COad apparently reside on every top site, but still exhibit a (2×2) superstructure modulation. We interpret this observation as a dynamic effect, caused by a small number of highly mobile point defects in the (2×2)‐CO adlayer. As shown by density functional theory calculations, the CO lattice near such defects relaxes into a local (1×1) arrangement, which can rapidly propagate across the surface. This scenario, where a static (2×2) COad sublattice coexists with a highly dynamic sublattice of partially occupied top sites, explains the pronounced COad surface mobility during electrooxidation.

Rich diversity of crystallographic phase formation in 2D RexMo1 – xS2 (x < 0.5) alloy

We report on the observation of the rich variety of crystallographic phase formation in a RexMo1 – xS2 alloy for x < 0.5. For x < 0.23, no low dimensional superstructural modulation is observed and inter cation hybridization remains discrete, forming dimers to tetramers with increasing Re concentration. For x > 0.23, superstructural modulation is observed. Depending on the Re concentrations (x = 0.23, 0.32, 0.38, and 0.45) and its distributions, various types of cation hybridization result in the rich variety of low dimensional superstructural modulation as directly revealed by high resolution transmission electron microscopy. These layered alloy systems may be useful for various energy and novel device applications.

Enhancement of thermoelectric performance through synergy of Pb acceptor doping and superstructure modulation for p-type Bi2Te3

A facile and efficient way in optimizing thermoelectric performance of Bi2Te3 alloy was reported through synergy of Pb acceptor doping and superstructure modulation. By varying the amount of Pb doping, the substitutional defect $${\text{Pb}}^{\prime}_{\text{Bi}}$$, arranging along the c-axis of PbBi2Te4 and PbBi4Te7 and acting as electron acceptor and superstructure, was formed successively in the Bi2Te3 matrix. This significantly reduced the lattice thermal conductivity and suppressed the bipolar effect. The figure of merit was enhanced and modulated, exhibiting a peak ZT of 1.06 and a broadened and optimized average ZT of 0.9 in a wide temperature range of 323–503 K.

Line-shape analysis of the Raman-spectrum from B1g bond buckling phonon in Bi2Sr2CaCu2O8+x

We performed Raman spectroscopy on two different over-doped Bi2Sr2CaCu2O8+x (BSCCO), of which superconducting transition temperatures are 89 K and 77 K. Line-shape analysis of the Raman-spectrum was done, focused on B1g bond buckling mode which have drawn a lot of attention, since photoemission studies showed an evidence for strong coupling between the mode and electron. The line-shapes show asymmetry and are well fitted by the Fano line-shape formula. Remarkably, we found that the peak line-widths from B1g bond buckling mode in BSCCO show much broader than those in YBa2Cu3O7−x. The broad line width can be attributed to the superstructure modulation of BSCCO. Our results imply that B1g bond buckling mode may have close relation to the origin of superconductivity or to boosting the superconducting transition temperature in BSCCO.

Type-I and type-II smectic-C^{*} systems: A twist on the electroclinic critical point.

We conduct an in-depth analysis of the electroclinic effect in chiral, ferroelectric liquid crystal systems that have a first-order smectic-A^{*}-smectic-C^{*} (Sm-A^{*}-Sm-C^{*}) transition, and show that such systems can be either type I or type II. In temperature-field parameter space type-I systems exhibit a macroscopically achiral (in which the Sm-C_{M}^{*} helical superstructure is expelled) low-tilt (LT) Sm-C_{U}^{*}-high-tilt (HT) Sm-C_{U}^{*} critical point, which terminates a LT Sm-C_{U}^{*}-HT Sm-^{*}C_{U} first-order boundary. Notationally, Sm-C_{M}^{*} or Sm-C_{U}^{*} denotes the Sm-C^{*} phase with or without a modulated superstructure. This boundary extends to an achiral-chiral triple point at which the macroscopically achiral LT Sm-C_{U}^{*} and HT Sm-C_{U}^{*} phases coexist along with the chiral Sm-C_{M}^{*} phase. In type-II systems the critical point, triple point, and first-order boundary are replaced by a Sm-C_{M}^{*} region, sandwiched between LT and HT Sm-C_{U}^{*} phases, at low and high fields, respectively. Correspondingly, as the field is ramped up, the type-II system will display a reentrant Sm-C_{U}^{*}-Sm-C_{M}^{*}-Sm-C_{U}^{*} phase sequence. Moreover, discontinuity in the tilt of the optical axis at each of the two phase transitions means the type-II system is tristable, in contrast to the bistable nature of the LT Sm-C_{U}^{*}-HT Sm-C_{U}^{*} transition in type-I systems. Whether the system is type I or type II is determined by the ratio of two length scales, one of which is the zero-field Sm-C^{*} helical pitch. The other length scale depends on the size of the discontinuity (and thus the latent heat) at the zero-field first-order Sm-A^{*}-Sm-C^{*} transition. We note that this type-I vs type-II behavior in this ferroelectric smectic is the Ising universality class analog of type-I vs type-II behavior in XY universality class systems. Lastly, we make a complete mapping of the phase boundaries in all regions of temperature-field-enantiomeric-excess parameter space (not just near the critical point) and show that various interesting features are possible, including a multicritical point, tricritical points, and a doubly reentrant Sm-C_{U}^{*}-Sm-C_{M}^{*}-Sm-C_{U}^{*}-Sm-C_{M}^{*} phase sequence.

Microstructure evolution of nonstoichiometric ZrC0.6 with ordered carbon vacancies under ion irradiation

ZrC0.6 with ordered carbon vacancies was irradiated with a 4 MeV Au ion beam at room temperature at fluences of 2 × 1016 ions/cm2, corresponding to 130 displacements per atom (dpa). Grazing incidence X-ray diffraction and transmission electron microscopy were performed to investigate the microstructure evolution of ZrC0.6 under ion irradiation. The lattice parameter did not clearly increase (lattice expansion of 0.013%) for irradiated ZrC0.6, which indicated that the inherent carbon vacancies acted as sinks for the interstitial atoms generated by irradiation. The peaks of Zr2C-ordered phase in GIXRD patterns and reflections of ordered Zr2C in microdiffraction patterns of the damaged layer disappeared, which indicated that superstructure modulation of the ordered carbon vacancies for Zr2C was destroyed under Au ion irradiation, and demonstrated that introducing vacancy defects in ZrC enhanced the tolerance of radiation damage.

Structural phase transitions, phase separation and physical properties for the 122-system iron-based superconductors

The discovery of iron-based superconductors stimulated the second research upsurge for high-temperature superconductors, which have been considered as one of the significant research field from both academic and technological points of view. It is noted that the studies of iron-based superconductors have brought a great development of a large number of experimental technologies and theoretical researches in superconductivity physics. The remarkable correlations between structural transitions and multiple ordered states have been observed in the typical iron-based superconductors. According to the microstructure analyses and in-situ transmission electron microscopy (TEM) investigations, AFe2As2 (A=Ba, Sr and Ca) iron-based superconducting materials often show up a structural transition from tetragonal phase to orthorhombic phase from room temperature down to 20 K, resulting in visible twinning domains in the orthorhombic phase. The tetragonal SrFe2As2 samples, consistent with X-ray and neutron-diffraction data, undergo the tetragonal- orthorhombic phase transition at about 205 K and show clear twin domains in the orthorhombic phase. On the other hand, TEM observations of CaFe2As2 reveal the presence of a pseudoperiodic structural modulation with a periodicity of around 40 nm at room temperature. This quasi-periodicity structural modulation is likely related to the local structural distortions within the Ca layers. In situ cooling TEM observations of CaFe2As2 reveal the presence of complex domain structures in the low-temperature orthorhombic phase. Phase separation and structural inhomogeneity as critical structural issues have been extensively investigated in a variety of strongly correlated systems. The phase separations associated with structural domains result visibly structural alterations in K y Fe2− x Se2 system. Structural investigations by means of TEM on K0.8Fe x Se2 and KFe x Se2, with 1.5≤ x ≤1.8, have revealed a rich variety of microstructure phenomena. Materials with 1.5≤ x ≤1.6 often show a superstructure modulation along the [310] zone-axis direction, and this modulation can be well interpreted by the Fe-vacancy order, which likely yields a superstructure phase of K2Fe4Se5. The superconducting K0.8Fe x Se2 and KFe x Se2 (1.7≤ x ≤1.8) materials contain clear phase separation, in particular, along the c-axis direction, recognizable as visible parallel lamellae in the crystals; this fact suggests that the superconducting phase could have the Fe-vacancy disordered state. The main changes of physical properties in the AFe2As2 materials have also been discussed. The substitution of A-element in AFe2As2 have a significant effect on the structural properties and spin density wave (SDW), then leads to the appearance and change of superconductivities. Polycrystalline samples of Ba1− x Sr x Fe2As2 (0≤ x ≤1) and Ba1− x Sr x Fe1.8Co0.2As2 (0≤ x ≤1) were synthesized by a solid state reaction method. Structural analysis by means of X-ray diffraction shows that the lattice parameters and unit cell volume decrease monotonically with the increase of x for Ba1– x Sr x Fe2As2. The measurements of transport properties demonstrate that the average size of the Ba(Sr)-site cations could evidently influence the SDW behavior in Ba1− x Sr x Fe2As2 and superconductivity in Ba1− x Sr x Fe1.8Co0.2As2 as well. The critical temperature for SDW ( T SDW) increases with the Sr substitution for Ba in Ba1− x Sr x Fe2As2 and, on the other hand, the superconducting T c decreases with the increase of Sr content in Ba1− x Sr x Fe1.8Co0.2As2. The inhomogeneous distributions of Ba/Sr ions and structural distortions in Ba0.5Sr0.5Fe2As2 have been investigated by TEM observations.

A study of structural non-stoichiometry with respect to oxygen in RBaCo4O7+x single crystals

Impurity-free RBaCo4O7+x (R = Y, Lu) crystals are grown and the evolution of their structure with varying oxygen concentration in the range 0 ≤ x ≤ 1.3 is studied. The structural features of the studied samples required to develop an extended strategy of the X-ray crystallographic experiment, which made it possible to obtain more accurate data on unit cell parameters and additional information on the features of superstructure modulation and diffuse scattering, along with the phase composition of crystalline samples. A crystal chemical analysis of the known structural models of RBaCo4O7+x is performed and suggestions are made about possible structural changes occurring when the oxygen concentration increases up to the limit.

Structural behavior of Tl-exchanged natrolite at high pressure depending on the composition of pressure-transmitting medium

The structural evolution of Tl-exchanged natrolite with idealized formula Tl2[Al2Si3O10]·2H2O, compressed in penetrating (water:ethanol 1:1) and non-penetrating (paraffin) media, was studied up to 4 GPa. The presence of Tl+ with non-bonded electron lone pairs, which can be either stereo-chemically active or passive, determines distinctive features of the high-pressure behavior of the Tl-form. The effective volume of assemblages Tl+(O,H2O) n depends on the E-pairs activity: single-sided coordination correlates with smaller volumes. At ambient conditions, there are two types of Tl positions, only one of them having a nearly single-sided coordination as a characteristic of stereo-activity of the Tl+ E pair. Upon the compression in paraffin, a phase transition occurs: a 5% volume contraction of flexible natrolite framework is accompanied by the conversion of all the Tl+ cations into stereo-chemically active state with a single-sided coordination. This effect requires the reconstruction of all the extra-framework subsystems with the inversion of the cation and H2O positions. The compression in water-containing medium leads to the increase of H2O content up to three molecules pfu through the filling of partly vacant positions. This hinders a single-sided coordination of Tl ions and preserves the configuration of their ion-molecular subsystem. It is likely that the extra-framework subsystem is responsible for the super-structure modulation.

A layered antiferromagnetic semiconductor EuMTe3 (M = Bi, Sb)

We synthesized ternary tellurides EuMTe3 (M = Bi, Sb) using a low-temperature flux method. These compounds crystallize in the Pmmn space group with layered structure. A superstructure modulation along the b-axis was observed. Physical property measurements revealed that these compounds are antiferromagnetic semiconductors. Finally, a strong spin–orbit coupling with possible novel quantum interference between localization and weak antilocalization effects was suggested in the present system. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Structural phase transition associated with van Hove singularity in 5d transition metal compound IrTe2

We have investigated the electronic states of IrTe2 by using angle-resolved photoemission spectroscopy to elucidate the origin of the structural phase transition. Both the Ir 4f and Te 4d core level spectra exhibit dramatic splitting below the phase transition temperature Ts, suggesting that there exist two inequivalent Ir and Te sites with distinctly different electronic states in the distorted phase. The band related to the saddle points at the Fermi level (EF) is strongly reconstructed, which removes the van Hove singularity from EF below Ts. The wavevector connecting the adjacent saddle points is consistent with the in-plane superstructure modulation wavevector. These results indicate that the phase transition in IrTe2 is intimately associated with the saddle points. As the van Hove singularity mainly originates from the Te px+py orbitals, the Te 5p electronic states play a dominant role in the structural phase transition.