Centrosymmetric Positions Research Articles

Dynamic porous property in a new heterometallic supramolecular compound

Supramolecular compounds have attracted considerable interest to chemists, physicists and materials scientists due to their fascinating structures and potential applications as porosity [1-3] but one of the most appealing aims today, is to build multifunctional compounds. We are interested to rationalize the synthesis of porous heterometallic compounds by self- assembly via hydrogen bonds. In this communication, we present a stacked 2D Catena-{Co(amp)3Cr(ox)3.6H2O} (amp = 2-picolylamine, ox=oxalate). It is built by layers in which both (Co(amp)3+ (D) and Cr(ox)3- ( A)) ionic units are linked in a repeating DADADA...pattern along both the a and c axis with four and two hydrogen bonds respectively. These layers host very well resolved dodecameric discrete ring water clusters (R12) built by six independent molecules located around the centrosymmetric Wyckoff position's of the P21/n space group in which the compound crystallizes. The clusters are ranged along the [001] direction, occupy 807.6 Å3 (23.9%) of the unit cell and have a chair conformation via 10 hydrogen bonds. The dehydration process of the compound occurs in one step around 77oC and the dehydrated compound remains crystalline although all framework atoms move by 2.25 Å along a mean vector (dm= 2.21 i + 0.17 j + 0.37 k) during the process. The unit cell is then reduced by 12.34 % along a, 12.22% along b and 2. 03% along c reducing V by 22.03%. By exposure to air, the regeneration of the compound needs 90 min, following the first kinetic Avrami's model.

Synthesis and Luminescence Properties of Ba<sub>3</sub>Y<sub>1</sub><sub>-</sub><i><sub>x</sub></i>Eu<i><sub>x</sub></i>B<sub>3</sub>O<sub>9 </sub>(0.05≤<i>x</i>≤0.35) under UV Excitation

Single phase of Ba3Y1-xEuxB3O9 (0.05 ≤ x≤ 0.35)samples was prepared by the solid state reaction. The crystal structure, the surface, the particle size and luminescence properties were characterized by X-ray diffractometer, scanning electron microscopy and spectrophotometer. The results showed that the obtained samples with the particle size from 0.3 to 1.5 um were hexagonal structure. Monitored by 592 nm emission, the excitation spectra of Ba3Y1-xEuxB3O9were dominated by the charge transfer band of O2-→Eu3+ peaking at about 259 nm until x= 0.15; the transition from 7F0 to 5L6 level of Eu3+ at about 395 nm was maximum after x> 0.15. The strongest emission peak due to the 5D0→7F1transition of Eu3+ was all located at about 592 nm in the emission spectra of Ba3Y1-xEuxB3O9, and the maximum emission was obtained at x= 0.15 and 0.3 under 254 and 395 nm excitation, respectively. The excitationprocess under the two excitation energies was analyzed. It indicated that only the centrosymmetric lattices could be excited under 254 nm excitation; both the centrosymmetric and noncentrosymmetric positions could be excited by 395 nm, accordingly, the ratio of the red emission (5D0→7F2) to the orange one (5D0→7F1) increased, leading to a better chromaticity of the phosphors.

High-Pressure Synthesis, Crystal Structure, and Phase Stability Relations of a LiNbO3-Type Polar Titanate ZnTiO3and Its Reinforced Polarity by the Second-Order Jahn–Teller Effect

A polar LiNbO3-type (LN-type) titanate ZnTiO3 has been successfully synthesized using ilmenite-type (IL-type) ZnTiO3 under high pressure and high temperature. The first principles calculation indicates that LN-type ZnTiO3 is a metastable phase obtained by the transformation in the decompression process from the perovskite-type phase, which is stable at high pressure and high temperature. The Rietveld structural refinement using synchrotron powder X-ray diffraction data reveals that LN-type ZnTiO3 crystallizes into a hexagonal structure with a polar space group R3c and exhibits greater intradistortion of the TiO6 octahedron in LN-type ZnTiO3 than that of the SnO6 octahedron in LN-type ZnSnO3. The estimated spontaneous polarization (75 μC/cm(2), 88 μC/cm(2)) using the nominal charge and the Born effective charge (BEC) derived from density functional perturbation theory, respectively, are greater than those of ZnSnO3 (59 μC/cm(2), 65 μC/cm(2)), which is strongly attributed to the great displacement of Ti from the centrosymmetric position along the c-axis and the fact that the BEC of Ti (+6.1) is greater than that of Sn (+4.1). Furthermore, the spontaneous polarization of LN-type ZnTiO3 is greater than that of LiNbO3 (62 μC/cm(2), 76 μC/cm(2)), indicating that LN-type ZnTiO3, like LiNbO3, is a candidate ferroelectric material with high performance. The second harmonic generation (SHG) response of LN-type ZnTiO3 is 24 times greater than that of LN-type ZnSnO3. The findings indicate that the intraoctahedral distortion, spontaneous polarization, and the accompanying SHG response are caused by the stabilization of the polar LiNbO3-type structure and reinforced by the second-order Jahn-Teller effect attributable to the orbital interaction between oxygen ions and d(0) ions such as Ti(4+).

Specific heat of Ce x La1 − xB6 in the low cerium concentration limit (x ≤ 0.03)

The specific heat of high-quality CexLa1 − xB6 (x = 0, 0.01, 0.03) single crystals is studied in the temperature range 0.4–300 K. LaB6 samples with various boron isotope compositions (10B, 11B, natB) are analyzed to estimate the effect of boron vacancies. The experimental data are used to take into account the electron component correctly under the renormalization of the density of states at T < 8 K, the contribution of the quasi-local vibrational mode of a rare-earth ion with the Einstein temperature ΘE ≈ 152 K, the Debye contribution from the rigid cage of boron atoms with the Debye temperature ΘD ≈ 1160 K, and the low-temperature Schottky contribution related to the presence of 1.5−2.3% boron vacancies in the rare-earth hexaborides. The detected low-temperature anomalies in the specific heat are shown to be interpreted in terms of the formation of two-level systems with an energy ΔE = 92–98 K caused by the displacement of rare-earth ions from their centrosymmetric positions. A scenario of heavy fermion formation that is alternative to the Kondo mechanism is proposed for the systems with a magnetic impurity.

Effects of disorder and isotopic substitution in the specific heat and Raman scattering in LuB12

Precision measurements of the specific heat and spectral intensity I(ω) of Raman scattering for LuNB12 single crystal samples with various boron isotopes (N = 10, 11, nat) have been performed at low and intermediate temperatures. A boson peak in the low-frequency part of the I(ω) spectrum has been observed for the first time for lutetium dodecaboride at liquid nitrogen temperatures. It has been shown that low-temperature anomalies in the specific heat, along with the features of Raman spectra, can be interpreted in terms of the transition to a cageglass state at T* = 50−70 K, which appears when Lu3+ ions are displaced from the centrosymmetric position in cavities of a rigid covalent boron sublattice towards the randomly located boron vacancies. The concentrations of various two-level systems that correspond to two types of vibrational clusters with correlation lengths of 12–15 and 18–22 A, respectively, have been estimated. The vibrational density of states of LuB12 has been calculated from Raman spectra in the model of soft atomic potentials. An approach has been proposed to explain the dielectrization of the properties of the YbB12 compound at T < T*, as well as the features of the formation of magnetic structures in RB12 antiferromagnets (R = Tb, Dy, Ho, Er, Tm) and the suppression of superconductivity in LuB12.

X-ray and FT-IR studies of structures of cyclic oxaalkyl diamide of o-phthalic acid and its complex with lead(II) perchlorate

The crystals of cyclic oxaalkyl diamide of o-phthalic acid (CPhDA) and its complex with lead(II) perchlorate (CPhDA–Pb(ClO 4) 2–H 2O (6:2:3)) were obtained from acetonitrile and were studied using X-ray crystallography and FT-IR spectroscopy. CPhDA crystallises in the centrosymmetric space group ( P2 1/ n) of monoclinic system, while its complex with Pb(ClO 4) 2 in the space group R 3 ¯ of rhombohedral system. The conformation of CPhDA does not change significantly upon coordination of the Pb 2+cation. Within the complex structure all six carbonyl oxygen atoms of the three CPhDA ligand molecules coordinate the Pb 2+ cation with comparable bond lengths forming a cationic complex [Pb(CPhDA) 3] 2+. One water molecule occupies the centro-symmetric position in the cationic [(CPhDA) 3⋯Pb 2+⋯(H 2O)⋯Pb 2+⋯(CPhDA) 3] dimeric structure. The FT-IR spectrum of the complex shows the characteristic shifts of amide I and amide II bands as all oxygen atoms of the carbonyl group are involved in complexation.

X-ray absorption spectroscopy: a powerful tool to probe the Zr shift towards the centrosymmetric position in PZT (PbZr0.52Ti0.48O3) under high pressure and high temperature

X-ray absorption spectroscopy: a powerful tool to probe the Zr shift towards the centrosymmetric position in PZT (PbZr_0.52Ti_0.48O₃) under high pressure and high temperature

Local atomic structure of niobates and titanates from X-ray absorption spectroscopic data

The local atomic structure of PbTiO3, BaTiO3, and KNbO3 perovskite-type crystals and K x Na1 − x NbO3 solid solutions in different phases is investigated using the angular dependence of the pre-edge structure of the Ti and Nb K X-ray absorption spectra and the EXAFS data. In noncubic phases, a considerable deviation of the local structure from the structure determined from diffraction data is observed only for the tetragonal phase of the BaTiO3 crystal. It is revealed that, in the cubic phase of niobates, the niobium atoms are characterized by significant displacements from the centrosymmetric positions along the threefold axes, so that they are close in the magnitude and the direction to the displacements in the low-temperatures rhombohedral phases.

Microscopic mechanisms for improper ferroelectricity in multiferroic perovskites: atheoretical review

Two microscopic mechanisms helping us to understand the multiferroic behavior ofdistorted rare-earth manganites are here briefly reviewed. The original work was carriedout by means of Hamiltonian modeling and first-principles density functionalsimulations. Our first topic concerns the link between the Dzyaloshinskii–Moriyainteraction and ferroelectricity in incommensurate magnets. We argue thatthe Dzyaloshinskii–Moriya interaction may play a key role since (i) it inducesferroelectric displacements of oxygen atoms and (ii) it favors the stabilization of ahelical magnetic structure at low temperatures. Our second topic concerns theprediction, based on Landau theory, that the symmetry of the zigzag spin chainsin the AFM-E (E-type antiferromagnetic) orthorhombic manganites (such asHoMnO3) allows a finitepolarization along the c axis. The microscopic mechanism at the basis of ferroelectricity is interpreted through a gain in band energyof the eg electrons within the orbitally degenerate double-exchange model. Related Monte Carlosimulations have confirmed that the polarization can be much higher than what is observedin spiral magnetic phases. Density functional calculations performed on orthorhombicHoMnO3 quantitatively confirm a magnetically induced ferroelectric polarization up to∼6 µC cm−2, the largest reported so far for improper magnetic ferroelectrics. We find inHoMnO3, in addition to the conventional displacement mechanism, a sizable contribution arising from thepurely electronic effect of orbital polarization. The relatively large ferroelectric polarization,present even with centrosymmetric atomic positions, is a clear sign of a magnetism-inducedelectronic mechanism at play, which is also confirmed by the large displacements ofthe Wannier function centers with respect to the corresponding ions in AFM-EHoMnO3.The final polarization is shown to be the result of competing effects, as shown by the opposite signs ofthe eg and t2g contributions to the ferroelectric polarization.

Spin-Orbit Coupling and Ion Displacements in MultiferroicTbMnO3

The magnetic and ferroelectric (FE) properties of TbMnO3 are investigated on the basis of relativistic density functional theory calculations. We show that, due to spin-orbit coupling, the spin-spiral plane of TbMnO3 can be either the bc or ab plane, but not the ac plane. As for the mechanism of FE polarization, our work reveals that the "pure electronic" model by Katsura, Nagaosa, and Balatsky is inadequate in predicting the absolute direction of FE polarization. Our work indicates that to determine the magnitude and the absolute direction of FE polarization in spin-spiral states, it is crucial to consider the displacements of the ions from their centrosymmetric positions.

Anion and cation disorder in [CN 3H 6]⋅(TaF 6)

A new guanidinium fluoride tantalate, [CN 3H 6]⋅(TaF 6), crystallises at 50 °C from a solution of Ta 2O 5 in 40% aqueous HF and of guanidinium chloride. The structure is rhombohedral, R 3 ¯ m space group, with the equivalent hexagonal cell a H = 8.647 ( 1 ) Å , c H = 8.507 ( 2 ) Å , Z = 3 and R = 0.029 , R w = 0.077 for 312 reflections. The three-dimensional network is built up from parallel (0001) H layers ∞([CN 3H 6]F 6) 5− between which tantalum atoms are inserted. These tantalum atoms, in 3a sites, adopt an octahedral coordination with 〈 d Ta F 〉 = 1.894 ( 7 ) Å . (TaF 6) − anions are disordered over (0001) H mirror plane related positions while guanidinium cations are disordered over centrosymmetric positions; site occupancy of fluoride and nitrogen sites is fifty per cent.

Crystal structure of hexakis(N,N-dimethylacetamide)ytterbium(III) dodecawolframophosphate [Yb(DMAA)6]PW12O40

An X-ray diffraction study of the [Yb(DMAA)6]PW12O40 complex (DMAA is N,N-dimethylacetamide) is performed. The structure contains isolated cationic complexes [Yb(DMAA)6]3+ and heteropolyanions PW12O 40 3− . The cationic complexes and the heteropolyanions occupy centrosymmetric positions. The coordination polyhedron of the Yb atom is a slightly distorted octahedron. Unlike the Nd complex of the same composition, which was studied earlier, O atoms of the heteropolyanion are not included in the Yb coordination. All the O atoms of the heteropolyanion are disordered. Analysis of the disordering reveals that the heteropolyanion has a Keggin structure (α isomer), and its disordering results from the centrosymmetric position.

Structural characterisation of melt-spun Ti50Ni25Cu25 ribbons

The structure of melt-spun Ti 50 Ni 25 Cu 25 ribbons after various heat treatments has been studied by X-ray and TEM analyses. As-received amorphous Ti 50 Ni 25 Cu 25 ribbons, when annealed at 500°C for a few minutes, crystallise into the B2 structure, which fully transforms into the orthorhombic martensite upon cooling. The X-ray scans of Ti 50 Ni 25 Cu 25 ribbons annealed for 5 min. at 500°C show the well developed orthorhombic pattern, which was used for refinement of the orthorhombic martensitic structure of TiNi-Cu alloys. The refined structure of the orthorhombic martensite in TiNi-Cu alloys differs from the standard B19 type suggested previously. X-ray investigation reveals a shift of atoms in the (010) ORT layers from the centro-symmetric positions towards the [001] ORT direction. The longer annealing of Ti 50 Ni 25 Cu 25 ribbons at 500C causes TiCu precipitation and results in appearance of untransformed B2 grains at room temperature. Annealing at 700C causes Ti 3 Cu 4 precipitation with significant Cu depletion of the matrix resulting in the appearance of the mixed orthorhombic and monoclinic martensite.

Oxygen ordering in YBa2Cu3O7−y in terms of the Landau theory

Oxygen ordering in nonstoichiometric compositions of YBa2Cu3O7−y is considered in terms of the Landau theory of second-order phase transitions. It is shown that there can only be ten types of homogeneous long-range order, of which two types correspond to the OI and OII phases. The Landau theory predicts that the filling of vacant positions 2(f)D4h1 by oxygen must follow a scenario that is far from forming chains but compatible with known facts. Such a scenario of filling vacancies with oxygen was not considered earlier. It is demonstrated that the predicted structures can be experimentally identified from the spectra of copper ions. In the course of identification of diffraction patterns, the symmetry-allowed displacement of copper ions from centrosymmetric positions in a Cu(1)(Ox□1−x)2 layer should be taken into account.

Electronic absorption spectra of Fe3+ in andradite and epidote at different temperatures and pressures

Temperature, T, and pressure, P, behaviour of absorption bands caused by spin-forbidden dd -transitions of Fe 3+ in octahedral sites of the andradite and epidote structures were studied at T from 300 to 750 K and P up to 10 GPa. The temperature dependence of the integral intensity of the 6 A 1g → 4 T 1g ( 4 G) and 6 A 1g → 4 T 2g ( 4 G) bands resemble the behaviour of spin-allowed dd -bands of the other 3d N ions: an increase on heating in case of the centrosymmetric position of Fe 3+ in the Y-site of andradite and nearly constant values in the strongly distorted M3-site of the epidote structure. Increasing T lowers the intensity of the crystal field-independent band 6 A 1g → 4 A 1g 4 E g ( 4 G) in both minerals. The influence of P on the intensity of the dd -bands is insignificant. In andradite garnet, the local thermal expansion coefficient α loc of the Fe 3+ -centred octahedra Y, as derived from the high-T spectra, is much lower than that of Cr 3+ -centred octahedra Y in chromium-bearing garnets. On the other hand, judging from T-induced shifts of 6 A 1g → 4 T 1g ( 4 G) and 6 A 1g → 4 T 2g ( 4 G) bands, α loc of FeO 6 is much higher in epidote than in andradite, being comparable to values in AlO 6 - or CrO 6 -octahedra. This indicates that α loc values of Fe 3+ -centred octahedra may vary strongly between different structural matrices, possibly as a consequence of differences in covalent bonding of Fe 3+ compared to Cr 3+ . In andradites, T and P produce energy shifts of the crystal field-independent band 6 A 1g → 4 T 1g 4 E g ( 4 G) in opposite directions, thus showing a decrease and an increase, respectively, in the covalence of the Fe 3+ -O bonds. In epidote, the effect of both T and P on the energy of the crystal field independent transition is much smaller than in andradite. In epidote, on the other hand, P causes a strong high-energy shift of the 6 A 1g → 4 T 2g ( 4 G) transition, indicating a strong increase of the crystal-field strength Dq, and thus significant compressibility of the Fe 3+ -centred M3-octahedra with k ≅ 75 GPa. Up to ca. 10 GPa, there appear no additional bands in the NIR range of the high-P spectra of both minerals. This is indicative for the absence of P-induced reduction of Fe 3+ .

Theory of size-driven transitions in displacive and order-disorder ferroelectrics

We present a simple theory for structural transitions in displacive ferroelectrics of the perovskite type. As in earlier approaches, our model is based on the competition between the elastic energy cost for the displacement of the B-ion from the centrosymmetric position, and the energy gain due to ferroelectric ordering of the dipoles. We use an effective single-site approximation which leads naturally to a first-order transition. This transition takes place at a certain temperature as the temperature is varied, and at a certain size as the size of the system is varied. The transition temperature is suppressed as the sample size is reduced, and vanishes for samples below a certain size. This is in accordance with recent observations on nanocrystalline perovskites. For systems with a continuous bulk transition, which can also be treated within our theory, we show that size effects are highly suppressed, a result that is also borne out by experiments on order-disorder ferroelectrics with a continuous transition.

Determination of the local structure of NbO6 octahedra in the orthorhombic phase of a KNbO3 crystal using EXAFS

A new method proposed by us to determine the displacement direction of B atoms from centrosymmetric positions in ABO3 crystals is used to study the local atomic structure of KNbO3 in the orthorhombic phase. It is shown that the conventional treatment of the EXAFS yields serious errors in determinations of the local distortions of NbO6 octahedrons. To eliminate these errors, it is suggested that diffraction data on the average atomic displacements should be combined with the results of direct calculations of the Nb K-EXAFS of KNbO3 in the orthorhombic phase. This approach was used to establish that the preferential direction of displacement of Nb in the orthorhombic phase of KNbO3 is in the direction of the polar twofold axis.

Planar or tetrahedral coordination in copper(II) complexes induced by bromo substitution

The diffraction study of bis(5-bromo- N- m-tolylsalicylaldiminate)copper(II) shows the copper atom in a centrosymmetric position and the two ligands displayed trans to each other in a planar environment. Two additional weak interactions due to Br atoms (Jahn-Teller effect) of neighboring units complete the coordination sphere, CuBr = 3.662(2) A ̊ , so that the comple is formally a polynuclear two-dimensional species. On the contrary, bis(3,5-dibromo- N- p-tolylsalicylaldiminate(copper(II) is mononuclear and shows only the two ligands coordinated and the geometry is distorted ‘tetrahedral’. The distortion is due to intramolecular repulsion interactions. Electronic spectra of these two compounds show a single asymmetric band at 700 nm for the former, and two bands located at 650 and 850 nm for the latter. Both compounds are monoclinic, space group P2 1/c with a = 9.919(2) A ̊ , b = 10.965(2) A ̊ , c = 12.283(2) A ̊ , β = 105.88(1)°, Z = 2 for the former, and a = 13.290(2) A ̊ , b = 12.195(2) A ̊ , c = 17.891(2) A ̊ , β = 105.38(1)°, Z = 4 for the latter.

Crystal and molecular structures of new fullerides, (Ph4P)2C60Hal (Hal=Br or I) and (Ph4As)2C60Cl

New air-stable fullerides, (Ph4P)2C60Hal (Hal=Br or I) and (Ph4As)2C60Cl, were synthesized, and their crystal structure were determined. A comparative crystal-chemical analysis of the fullerides under study demonstrated that they are isostructural, described by the general formula (Ph4X)2C60Hal (X=P or As, and Hal=Cl, Br, or I), and crystallize in the triclinic system. The C60− and Hal− anions occupy special centrosymmetric positions. The (Ph4P)+ and (Ph4As)+ cations occupy general positions. The principal parameters of the molecular structures are reported.

A neutron powder investigation of the high-temperature structure and phase transition in stoichiometric LiNbO3

Abstract The structural changes of stoichiometric LiNbO3 at temperatures up to 1470 K have been investigated by neutron powder diffraction. At Tc ≈ 1460 K, LiNbO3 transforms from a paraelectric high-temperature (R[unk]c) to a ferroelectric low temperature structure (R3c). The phase transition is of coupled order-disorder and displacive type. In the high-temperature phase, Li is highly disordered over two off-centre positions, whereas Nb takes a centrosymmetric position within O6 octahedra. An analysis of anisotropic displacement parameters shows that the probability density function of Li is extended along c, that of the oxygens is more or less perpendicular to the Nb–O bonds, and that of Nb is slightly extended in the a–b plane at high temperatures. An increase of the tilt angle of the NbO6 octahedra away from an h.c.p. arrangement towards a perovskite structure was observed. The octahedra become almost regular at high temperatures. Comparison with an earlier investigation of congruent lithium niobate shows a quite similar behaviour, however, all parameters of the latter are closer to those of the perovskite structure at same temperatures. Anomalies in the behaviour of the order parameters, a more complicated disorder and a high mobility of Li at high temperatures are explained by the competition of the order-disorder and displacive character of the phase transition.