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Poisson's ratio in chiral Cosserat elastic solids is considered. Chirality allows the Poisson's ratio to exceed classical bounds, even if the material is directionally isotropic and all elastic moduli are within thermodynamic limits based on strain energy density. Poisson's ratio in chiral rods depends on the chiral elastic constants as well as on the shear and bulk moduli, assumed positive. Poisson's ratio can be greater than 0.5 or smaller than −1 for slender chiral specimens. [ABSTRACT FROM AUTHOR]

The noncentrosymmetric structure is one prerequisite of a nonlinear optical (NLO) material. Here, a sulfate Cs2Ca2(SO4)3 with a noncentrosymmetric structure was synthesized by high‐temperature solid‐state reaction, and its crystals were grown by spontaneous crystallization. Single‐crystal X‐ray diffraction indicates that Cs2Ca2(SO4)3 features a three‐dimensional framework constituted by corner‐sharing CaO6 octahedra and SO4 tetrahedra. Optical characterizations suggest that Cs2Ca2(SO4)3 is transparent down to 200 nm and exhibits a moderate second‐order NLO response of about 0.6 × KH2PO4. Electronic band structure calculations were performed to elucidate the optical origin. Furthermore, the title compound has good thermal stability of up to about 1000 K, which is greatly higher than those of NLO sulfates reported recently. This work may shed useful light on the development of new NLO sulfates with high thermal stability. [ABSTRACT FROM AUTHOR]

The noncentrosymmetric structure is one prerequisite of a nonlinear optical (NLO) material. Here, a sulfate Cs2Ca2(SO4)3 with a noncentrosymmetric structure was synthesized by high‐temperature solid‐state reaction, and its crystals were grown by spontaneous crystallization. Single‐crystal X‐ray diffraction indicates that Cs2Ca2(SO4)3 features a three‐dimensional framework constituted by corner‐sharing CaO6 octahedra and SO4 tetrahedra. Optical characterizations suggest that Cs2Ca2(SO4)3 is transparent down to 200 nm and exhibits a moderate second‐order NLO response of about 0.6 × KH2PO4. Electronic band structure calculations were performed to elucidate the optical origin. Furthermore, the title compound has good thermal stability of up to about 1000 K, which is greatly higher than those of NLO sulfates reported recently. This work may shed useful light on the development of new NLO sulfates with high thermal stability. [ABSTRACT FROM AUTHOR]

The loss of centrosymmetry via oxygen octahedral rotations is demonstrated in the n = 2 Dion-Jacobson family of layered oxide perovskites, A′LaB2O7 (A′ = Rb, Cs; B = Nb, Ta). Ab initio density functional theory calculations predict that all four materials should adopt polar space groups, in contrast to the results of previous experimental studies that have assigned these materials to the centrosymmetric P4/ mmm space group. Optical second harmonic generation experiments confirm the presence of a noncentrosymmetric phase at ambient temperature. Piezoresponse force microscopy experiments also show that this phase is piezoelectric. To elucidate the symmetry-breaking and assign the appropriate space groups, the crystal structure of CsLaNb2O7 is refined as a function of temperature from synchrotron X-ray diffraction data. Above 550 K, CsLaNb2O7 adopts the previously determined centrosymmetric P4/ mmm space group. Between 550 and 350 K, the symmetry is lowered to the noncentrosymmetric space group A mm2. Below 350 K, additional symmetry lowering is observed as peak splitting, but the space group cannot be unambiguously identified. [ABSTRACT FROM AUTHOR]

A series of noncentrosymmetric sulfides Ln8Sb2S15 (Ln = La, Pr, Nd) were prepared from stoichiometric mixtures of the elements at 1223 K in an evacuated silica tube. The compounds Ln8Sb2S15 with Ln = La and Nd are isostructural to Pr8Sb2S15 and crystallize in the tetragonal noncentrosymmetric space group I41cd. Their structure contains discrete [SbS3]3- trigonal pyramids separated by Ln3+ cations and S2- anions. La8Sb2S15 shows second harmonic generation with intensities 1.2 times that of the commercially used IR NLO (nonlinear optics) material AgGaS2 (at 2.05 μm laser). It exhibits excellent thermal stability up to 663 °C. Studies with UV/Vis-NIR diffuse reflectance spectroscopy show that La8Sb2S15 has an optical gap of around 2.3 eV, and a DFT study indicates a direct band gap with an electronic transfer excitation of S 3p electrons to a La 5d orbital. [ABSTRACT FROM AUTHOR]

Herein, we investigate human single hydroxyapatite crystals (enamel and dentine) by convergent-beam electron diffraction (CBED) and automated electron-diffraction tomography (ADT). The CBED pattern shows the absence of the mirror plane perpendicular to the c axis leading to the P63 space group instead of the P63/ m space group considered for larger-scale crystals, this is confirmed by ADT. This experimental evidence is of prime importance for understanding the morphogenesis and the architectural organization of calcified tissues. [ABSTRACT FROM AUTHOR]

The noncentrosymmetric borate CsCdBO3 was synthesized by a solid-state reaction, and its structure was determined by single-crystal X-ray diffraction. The crystal structure is a complicated three-dimensional network composed of CdO6 octahedra, CsO6 octahedra, and BO3 triangles. Interestingly, CsCdBO3 crystallizes in the cubic space group P213, which is quite rare for borate-containing anisotropic polarized planar BO3 groups. Furthermore, the IR spectrum confirms the presence of BO3 groups. Second harmonic generation (SHG) measurements on powder samples reveal that the SHG response of CsCdBO3 is approximately 0.5 × KDP (KH2PO4). Thermal analysis and UV/Vis/NIR diffuse reflectance spectroscopy data, as well as electronic band structure calculations, are also reported. [ABSTRACT FROM AUTHOR]

Tables of alphanumeric data concerning crystals twinned by merohedry are presented, assembled into a spreadsheet. The twin table is structured on the 230 space groups and contains the following information: centrosymmetric/chiral indicator, polar axes, twin index, twin operators, some physical property relations, symmetry-related reflections, inversion techniques and a pointer to software use. The software table contains the preceding information translated into command lines for SHELXL [Sheldrick (2008). Acta Cryst. A 64, 112-122]. The macromolecular table contains a reduced form of the twin table applicable to enantiomerically pure compounds of known absolute configuration. [ABSTRACT FROM AUTHOR]