Redetermination of Nd2Ti2O7: a non-centrosymmetric structure with perovskite-type slabs.

Single crystals of dineodymium(III) dititanium(IV) hepta-oxide, Nd2Ti2O7, were synthesized by the flux method and found to belong to the family of compounds with perovskite-type structural motifs. The asymmetric unit contains four Nd, four Ti and 14 O-atom sites. The perovskite-type slabs are stacked parallel to (010) with a thickness corresponding to four corner-sharing TiO6 octa-hedra. The Nd and Ti ions are displaced from the geometrical centres of respective coordin-ation polyhedra so that the net polarization occurs along the c axis. The investigated crystals were all twinned and have a halved monoclinic unit cell in comparison with the first structure determination of this compound [Scheunemann & Müller-Buschbaum (1975 ▶). J. Inorg. Nucl. Chem. 37, 2261-2263].

On the origin of the solid-state thermochromism and thermal fatigue of polycyclic overcrowded enes.

Aimed at unraveling the relative contribution of the folding, twisting and bending in the mechanism of the solid-state thermochromism of overcrowded polycyclic aromatic enes (PAEs), the structures of two typical heteromeric and homomeric representatives, 2-(thioxanthen-9-ylidene)indane-1,3-dione (1) and 9,9'-bi-9(10H)-anthracenylidene-10,10'-dione (bianthrone, 2), were studied by temperature-resolved single crystal X-ray diffraction (120-530 K) and solid-state UV-visible spectroscopy. Aside from negligibly small unfolding of the tricyclic moiety of 1, this first direct diffraction study of the high-temperature structures of solid PAEs did not unravel any significant and detectable changes in the time- and space-averaged intramolecular structures, thus, showing that the PAE-type thermochromism is not due to phase transitions or to major and permanent molecular distortions of a large portion of the material that would be caused by folding, twisting and/or bending. Instead, the experimental observations and theoretical modeling indicated that the color change is probably due to a dynamic process, where the absorption spectrum changes as a result of enhanced thermal oscillations of the two halves of the molecules around the central bridge. In addition to the reversible coloration, we also observed irreversible processes of thermal fatigue that afford stable chemical products that absorb in the visible region. We showed that the stable products are conductive and they act electrocatalytically toward oxidation of several biomarkers.

Sr(3)(Al(3+x)Si(13-x))(N(21-x)O(2+x)):Eu (x ∼ 0): a monoclinic modification of Sr-sialon.

The structure of the title compound, Sr-bearing oxonitrido-aluminosilicate (Sr-sialon), contains two types of channels running along the a axis, with the three unique Sr atoms (coordinatioon number seven) residing in the larger one. The channels cross a three-dimensional Si-Al-O-N network, in which the Si and Al atoms are in a tetra-hedral coordination with N and O atoms. The chemical composition of the crystal is close to Sr(3)Al(3)Si(13)N(21)O(2) (tris-trontium trialuminium trideca-silicon henicosa-nitride dioxide), which can be expressed as a mixture of SrSiN(2), Si(3)N(4), AlN, and SiO(2) components in the molar ratio 3:3:3:1. The crystal studied was metrically orthorhombic, consisting of four twin components related by metric merohedry.

Apatite-type SrPr(4)(SiO(4))(3)O.

Single crystals of the title compound, strontium tetra-praseo-dymium tris-(silicate) oxide, SrPr(4)(SiO(4))(3)O, have been grown by the self-flux method using SrCl(2). The structure is isotypic with the apatite supergroup family having the generic formula (IX)M1(2) (VII)M2(3)((IV)TO(4))(3)X, where M = alkaline earth and rare earth metals, T = Si and X = O. The M1 site (3.. symmetry) is occupied by Pr and Sr atoms with almost even proportions and is surrounded by nine O atoms forming a tricapped trigonal prism. The M2 site (m.. symmetry) is almost exclusively occupied by Pr and surrounded by seven O atoms, forming a distorted penta-gonal bipyramid. The Si atom (m.. symmetry) is surrounded by two O (m.. symmetry) and two O atoms in general positions, forming an isolated SiO(4) tetra-hedron. Another O atom at the inversion centre (.. symmetry) is surrounded by three M2 sites, forming an equilateral triangle perpendicular to the c axis.

(Ca(x)Nd(11-x))Ru(4)O(24) (x = 4.175).

Single crystals of the title compound, calcium neodymium ruthenate, (Ca(x)Nd(11-x))Ru(4)O(24) (x = 4.175), have been grown by the flux method. The structure consists of two crystallographically independent RuO(6) octa-hedra, which are isolated from each other and embedded in a matrix composed of the Ca and Nd atoms. There are seven M sites which accommodate the Ca and Nd atoms with different populations. Four M sites at general positions are enriched with Nd, whereas the remaining three M sites on twofold rotation axes are enriched with Ca. The coordination numbers of O atoms to the M sites range from 6 to 9. The mean oxidation state of Ru was estimated at +4.79 from the composition analysis. The title compound is non-centrosymmetric and potentially multiferroic.

New type of dual solid-state thermochromism: modulation of intramolecular charge transfer by intermolecular pi-pi interactions, kinetic trapping of the aci-nitro group, and reversible molecular locking.

When heated above room temperature, some crystalline polymorphs of the 1,3-bis(hydroxyalkylamino)-4,6-dinitrobenzenes (BDBn, n = 2-5), bis(hydroxyalkyl) analogues of the intramolecular charge-transfer molecule 1,3-diamino-4,6-dinitrobenzene, exhibit "dual" thermochromism: gradual color change from yellow to orange at lower temperatures, and sharp color change from orange to red at higher temperatures. These two thermochromic changes are related to different solid-state processes. When allowed to cool to room temperature, the yellow color of the thermochromic molecules with different alkyl length (n) is recovered with unexpectedly different kinetics, the order of the respective rate constants ranging from 10(-7)-10(-6) s(-1) for BDB2 to about 0.1 s(-1) in the case of BDB3. The thermochromic mechanism and the reasons behind the different kinetics were clarified on the basis of detailed crystallographic characterization, kinetic thermoanalysis, and spectroscopic study of eight crystalline forms (seven polymorphs and one solvate). It was found that the polymorphism is due to the possibility of "locking" and "unlocking" of the alkyl arms by formation of a strong intramolecular hydrogen bond between the hydroxyl groups at their hydroxyl termini. The locking of BDB2, with shortest alkyl arms, is reversible and it can be controlled thermally; either of the two conformations can be obtained in the solid state by proper thermal treatment. By use of high temperature in situ single crystal X-ray diffraction analysis of BDB3, direct evidence was obtained that the gradual thermochromic change is related to increased distance and weakened pi-pi interactions between the stacked benzene rings: the lattice expands preferably in the stacking direction, causing enhanced oscillator strength and red shift of the absorption edge of the intramolecular charge transfer transition. The second, sharp thermochromic change had been assigned previously to solid-solid phase transition triggered by intramolecular proton transfer of one amino proton to the nitro group, whereupon an aci-nitro form is thermally populated. Contrary to the numerous examples of solid thermochromic molecules based on either pericyclic reactions or keto-enol tautomerism, this system appears to be the first organic thermochromic family where the thermochromic change appears as an effect of intermolecular pi-pi interactions and thermal intramolecular proton transfer to aromatic nitro group.

Structural evolution of La2Ti2O7 at elevated temperatures.

Structural evolution of a La2Ti2O7 ferroelectric compound possessing perovskite-type slabs at elevated temperatures was investigated using the single-crystal X-ray diffraction technique. The monoclinic low-temperature phase (L) transformed into the orthorhombic high-temperature phase (H) via an incommensurately modulated phase (IC) between ∼989 and ∼1080 K. The L-IC transition was considered to be of the first order, with the L+IC two-phase co-existing region between ∼989 and ∼1027 K. The structure of IC was determined from the (3+1)-dimensional superspace representation with a modulation vector q = αao (α ≃ 0.49), where ao is the a-axis vector of the basic cell. The structural modulation originated from the variation of the tilt angle of the TiO6 octahedra in the perovskite-type slab in association with small positional displacements of La atoms. The IC-H transition took place at ∼1080 K and was close to the second order. During the IC-H transition, nanoscale flat plate domains having either a cell twin of the L-type structural modules or a cell twin of the alternating H- and L-type structural modules began to appear in the approximant structure of IC. The thickness of the flat plate domains then grew rapidly along the modulation vector in proportion to (½ - α)-1 as α approached ½ with decreasing temperature. In the two-phase L+IC co-existing region, the IC phase consisting of the two types of cell twins was gradually replaced with the low-temperature monoclinic phase L, which is not cell twinned but rather twinned macroscopically by the L-type structural modules.

Structural evolution of calcite at high temperatures: phase V unveiled.

The calcite form of calcium carbonate CaCO3 undergoes a reversible phase transition between Rc and Rm at ~1240 K under a CO2 atmosphere of ~0.4 MPa. The joint probability density function obtained from the single-crystal X-ray diffraction data revealed that the oxygen triangles of the CO3 group in the high temperature form (Phase V) do not sit still at specified positions in the space group Rm, but migrate along the undulated circular orbital about carbon. The present study also shows how the room temperature form (Phase I) develops into Phase V through an intermediate form (Phase IV) in the temperature range between ~985 K and ~1240 K.

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.

Synchrotron X-ray study of noncentrosymmetric Tb(3)RuO(7) with partial structural disorder.

Single-crystal synchrotron X-ray diffraction reveals partial structural disorder of Tb atoms at 293 K in flux-grown Tb(3)RuO(7) (triterbium ruthenium heptaoxide) crystals. The structure is noncentrosymmetric and composed of infinite single chains of corner-linked RuO(6) octahedra embedded in a Tb(3)O matrix. Two Tb atom sites out of the six crystallographically independent Tb sites are split into two positions. The split sites are separated by approximately 0.3-0.4 Angstrom, with slightly different coordination environments. The RuO(6) octahedra in the present P2(1)nb modification have two tilt systems about the a and c axes, in contrast with a single tilt about c in the other Cmcm modifications of Ln(3)RuO(7) (Ln = lanthanoid elements).

K-site splitting in KTiOPO4 at room temperature.

The room-temperature structure of potassium titanyl phosphate (KTiOPO4, KTP) with Pna2(1) symmetry has been studied by means of synchrotron radiation. Each of the two crystallographically unique K1 and K2 cations is split over two sites that are shifted along the c direction by 0.287 (13) and 0.255 (13) A for the K1a/b and K2a/b pairs, respectively. The refined populations of the minor K1b and K2b sites are 0.102 (12) and 0.132 (17), respectively. It is shown that accurate high-resolution synchrotron data (Rmerged = 0.015 for 25 010 reflections, 9456 unique, sintheta/lambda limit > 1.0) are required for the determination of a reliable structure model.

Al6Ti2O13, a new phase in the Al2O3-TiO2 system.

The compound Al6Ti2O13 (hexaaluminium dititanium tridecaoxide) has been synthesized using an arc-imaging furnace, which allows fast cooling of melted oxides. The structure consists of infinite double chains of polyhedra running along the c axis. These chains are built up by four kinds of strongly distorted oxygen octahedra randomly occupied by either Ti or Al (point symmetry m or m2m), and by trigonal bipyramids exclusively occupied by Al (point symmetry m2m).

Synchrotron X-ray study of orthorhombic Rb(3)Ta(5)O(14) with a modified pyrochlore structure.

The structure of a new orthorhombic trirubidium pentatantalum oxide (Rb(3)Ta(5)O(14)) phase with Pnma symmetry was identified from a half sphere of synchrotron X-ray data measured at a wavelength of 0.85 A. This notionally linked TaO(6) octahedral structure broadly consists of three different modifications of the pyrochlore ring motif with layer stacking normal to (205) planes. Successive pyrochlore layers do not simply stack normal to these planes but are offset along the [100] axis. An unusual aspect of this structure is the occurrence of TaO(5) trigonal bipyramids in structurally complex regions where the modified pyrochlore rings connect.

GdAlO3 perovskite.

Flux-grown gadolinium aluminate perovskite, GdAlO(3), was examined using single-crystal 0.7 angstroms-wavelength synchrotron X-ray diffraction. In the context of other well categorized rare earth aluminate (RAlO(3)) perovskite phases, the orthorhombic Pnma symmetry determined for the current compound is unsurprising. Corner-linked AlO(6) octahedra form the structural backbone of RAlO(3) perovskites and distort to accommodate the various rare earth ions in the structural voids. For GdAlO(3), the octahedral distortion, characterized by tilting of the octahedra about the shortest R-Al-R vectors, and octahedral deformation, characterized by strain of the octahedra along those axes, are in accordance with trends in the RAlO(3) series.