Anisotropic Structural Changes Research Articles

Interplay of metallic and ionic bonding in layered subnitrides AE2N (AE = Ca, Sr, or Ba) under high pressure

Abstract Layered alkaline earth-metal subnitrides AE2N (AE = Ca, Sr, or Ba) adopting the anti-CdCl2-type structure (space group R 3 - m) were structurally investigated under pressures up to 40 GPa using diamond-anvil cells for pressure application and synchrotron radiation for collecting powder diffraction patterns. Anisotropic compressibility and structural changes under pressure were found to be consistent with the spatial separation of ionic and metallic bonding according to the formal description as (AE2+)2N3–•e–. Further increase of pressure was found to induce several phase transitions: in the case of Ba2N, in addition to a new layered anti-CdI2-type modification (space group P 3 - m1), a cubic anti-Th3P4-type structure (I 4 - 3d), and its distorted analogue were identified. The latter appears to be isostructural to the observed high-pressure phases of Ca2N and Sr2N. To cite this article: G.V. Vajenine et al., C. R. Chimie 8 (2005).

Changes in the Fabric and Compliance Tensors of Cancellous Bone due to Trabecular Surface Remodeling, Predicted by a Digital Image-based Model

Fabric and compliance tensors of a cube of cancellous bone with a complicated three-dimensional trabecular structure were obtained for trabecular surface remodeling by using a digital image-based model combined with a large-scale finite element method. Using mean intercept length and a homogenization method, the fabric and compliance tensors were determined for the trabecular structure obtained in the computer remodeling simulation. The tensorial quantities obtained indicated that anisotropic structural changes occur in cancellous bone adapting to the compressive loading condition. There were good correlations between the fabric tensor, bone volume fraction, and compliance tensor in the remodeling process. The result demonstrates that changes in the structural and mechanical properties of cancellous bone are essentially anisotropic and should be expressed by tensorial quantities.

Boron substitution in ternary metal phosphide superconductors

Partial substitution of boron for phosphorus in the ternary phosphide superconductors ZrRuP and LaRu 2P 2 induces highly anisotropic structural changes and suppresses the superconducting transition temperatures. Undoped ZrRuP [hexagonal, a=6.445(4) Å, c=3.765(2) Å] systematically expands for ZrRuP 1− x B x ( x=0.05, 0.10, 0.15, 0.20, 0.40) in the a direction to 6.949(4) Å and shrinks in the c direction to 3.601(3) Å. Superconductivity is still present in the boron-substituted samples, although T c drops from 10.1 K in ZrRuP to 5.6 K in ZrRuP 0.6B 0.4. LaRu 2P 2 [tetragonal, a=4.032(2) Å, c=10.632(7) Å] remains essentially unchanged along the a axis [ a=4.037(1) Å] upon substitution at a starting composition of LaRu 2P 1.6B 0.4, but shrinks along the c axis to 10.477(5) Å. Accordingly, the superconducting T c drops from 4.0 K for LaRu 2P 2 to 2.0 K for LaRu 2P 1.6B 0.4.

Anisotropic structural change induced by sub-bandgap light in As–S glasses studied by quasi-elastic light scattering

Changes in the quasi-elastic light scattering spectra with illumination of linearly polarized sub-bandgap light have been investigated by monitoring simultaneously the formation of Bragg gratings in As–S glasses in order to study the mechanism of photoinduced anisotropic structural rearrangement. We have found that the quasi-elastic light scattering intensity and the configurational relaxation time increase with illumination time only in the earlier stage of the formation of Bragg gratings. On the other hand, the low-frequency Raman spectra associated with intermediate or mesoscopic vibrational excitations exhibit changes in the whole stage of Bragg grating formation. The observations indicate that a local bonding rearrangement occurs mainly in the earlier stage, while co-operative mesoscopic fragmentary rearrangements are involved in the whole stage.

Anisotropic structural change induced by sub-bandgap lights in As–S glasses studied by low-frequency Raman scattering

Changes in low-frequency vibrational modes with illumination of linearly polarized sub-bandgap light have been investigated by monitoring the formation of Bragg gratings in As–S glasses in order to determine photoinduced anisotropic structural rearrangements in a mesoscopic region. We found that the low-frequency vibrational modes show an intriguing change to us, which we suggest is some mesoscopic structural anisotropy due to fragmentary rearrangements induced by polarized sub-bandgap light. The relaxation dynamics of the anisotropic structural rearrangements have also been studied by erasing the Bragg gratings at a temperature lower than the glass transition temperature. We suggest that the thermokinetic structure of the relaxation is low-dimensional and the cooperatively rearranging regions are anisotropic.

Synthetic metals based on tetramethyltetraselenafulvalene(TMTSF): Varied anisotropy in the low temperature (125K) structures of (TMTSF) 2X, X=C10 4-, PF 6-, and AsF 6-.

The low temperature (125K) X-ray crystal structures of (TMTSF) 2X, X= C10 4 -, PF 6 -, and AsF 6 - reveal decreases in the intermolecular interand intrastack Se-Se contact distances upon cooling (298K to 125K) which are highly anisotropic and different from one salt to another. The changes in the interstack distances, which are normal to the stacking direction, are approximately twice those involving intrastack Se-Se interactions. These observations establish that the anisotropic structural changes which accompany decreased temperature are common to numerous (TMTSF) 2X radical cation conducting salts.