Phase behaviour of the discotic mesogen 5,10,15,20-tetrakis (4-n-dodecylphenyl)porphyrin under pressure

The phase behaviour of the discotic mesogen 5,10,15,20-tetrakis(4-n -dodecylphenyl)porphyrin (C12TPP) was investigated under hydrostatic pressures up to 300MPa by high pressure DTA and wide angle X-ray diffraction methods. The typical enantiotropic phase transitions of C12TPP, low- to high-temperature crystal (Cr2-Cr1), Cr1-discotic lamellar phase (DL), and DL-isotropic liquid (I) are observed at pressures up to 10MPa. Application of hydrostatic pressure to the sample generates a pressure-induced crystal polymorph (Cr3) between the Cr2 and Cr1 phases, and the phase transitions Cr2-Cr3-Cr1-DL-I occur reversibly in the pressure region between 10 and 180MPa. On heating at higher pressures above 180MPa, the fourth crystal polymorph (Cr4) is formed between the Cr2 and Cr3 phases at lower temperatures, and at the same time the fifth crystal polymorph (Cr5) appears abruptly between the Cr1 and DL phases at high temperatures. The Cr2-Cr4-Cr3-C1-(Cr5)-DL-I transition processes were observed at 180 200MPa. Further increasing the pressure above 270MPa induces entirely different thermal behaviour: only two peaks for the pressure-induced transition between the sixth and fifth polymorphs (Cr6-Cr5) and the Cr5-I transitions are detected at low and high temperatures on heating, while both the DTA and WAXD experiments on cooling show the formation of the DL phase as a monotropic phase between the I and Cr5 phases, indicating the I DL Cr5 Cr6 process. The thermal behaviour was ambiguous and complex in the pressure region between 200 and 260MPa because the peaks for the intermediate crystal transitions were too small to detect with confidence. The two different sequences of the Cr2-Cr4-Cr3-Cr1-DL-I and Cr6-Cr5-(DL)-I processes seems to occur competitively. The T vs. P phase diagram of a sample cooled at 300MPa was studied to determine the triple point of the DL phase and to investigate the phase stability of the pressure-induced crystal polymorphs. The Cr6-Cr5-I transition process was observed on heating at 200 and 300MPa, while the Cr6-Cr5-DL-I process was detected at lower pressures below 100MPa. Since the Cr5-DL transition temperature changes linearly with a slope dT/dP 40 degrees C/100 MPa, while the DL-I transition temperature changes slightly (dT/dP 5.5 degrees C/100MPa), the DL phase forms a triangle in the T vs. P diagram. The triple point of the DL phase was found to be 240.8MPa and 168.8 C. The Cr6 polymorph reorganized to the stable Cr2 form under atmospheric pressure on annealing at room temperature overnight.     

Crystal structures of three intermetallic phases in the Mo-Pt-Si system

The crystal structures of three ternary Mo-Pt-Si intermetallic compounds have been determined ab initio from powder X-ray diffraction data. All three structures are representative of new structure types. Both the X (MoPt₂Si₃, Pmc2₁, oP12, a=3.48438(6), b=9.1511(2), c=5.48253(8) Å) and Y (MoPt₃Si₄, Pnma, oP32, a=5.51210(9), b=3.49474(7), c=24.3090(4) Å) phases derive from PtSi (FeAs type) structure while the Z phase (ideal composition Mo₃₂Pt₂₀Si₁₆, refined composition Mo_29.9(2)Pt_21.0(3)Si_17.1(1), Cc, mC68, a=13.8868(3), b=8.0769(2), c=9.6110(2) Å, β=100.898(1)°) present similarities with the group of Frank-Kasper phases.     

Partial Sn‐atom ordering in Sm3Ga0.80–2.48Sn4.20–2.52

Trisamarium digallide tristannide crystallizes with a partially ordered Pu₃Pd₅‐type structure in space group Cmcm. In a single crystal of Sm₃Ga_1.89(4)Sn_3.11(4), the 8g position is mostly occupied by Sn atoms (93% Sn and 7% Ga), while the 4c and 8f positions are occupied by a Ga/Sn statistical mixture. The evolution of the structure as a function of the Ga content has been studied by X‐ray powder diffraction on ten Sm₃Ga_5−xSn_x samples. It is shown that the 8g position remains occupied essentially exclusively by Sn atoms within the whole homogeneity range, with x ranging from 2.52 to 4.20.     

New representatives of the linear structure series containing empty Ga/Ge cubes in the Sm-Ga-Ge system

New ternary intermetallic compounds Sm₂Ga_7−xGe_x (x=5.2-6.1) and Sm₄Ga_11−xGe_x (x=5.76-8.75) were synthesized and their crystal structures were determined by X-ray powder diffraction at compositions Sm₂Ga_1.8Ge_5.2 and Sm₄Ga_5.24Ge_5.76. Sm₂Ga_1.8Ge_5.2 crystallizes with the Ce₂(Ga_0.1Ge_0.9)₇ type of structure (space group Cmce, Pearson code oS80-8.00, a=8.46216(13), b=8.15343(13), , Z=8), while Sm₄Ga_5.24Ge_5.76 exhibits a new structure (space group Cmmm, Pearson code oS52-22.00, a=4.21038(4), b=35.8075(3), , Z=2). Both structures are the members of the linear intergrowth structure series built up from segments of BaAl₄, AlB₂ and α-Po structure types. Their Ga/Ge networks contain characteristic empty cubes with one side capped by an atom subjected to an intrinsic displacive disorder. A model of Ga/Ge localization was suggested on the basis of crystal-chemical analysis.     

Hydrogen absorption in transition metal silicides: La3Pd5Si-hydrogen system

Pressure-composition isotherm measurements show that the ternary lanthanum palladium silicide phase La3Pd5Si absorbs reversibly up to 5 hydrogen atoms per formula unit at 550 K and 14 bar hydrogen pressure. In-situ synchrotron and neutron powder diffraction reveals three phases, an alpha-phase having the limiting composition La3Pd5SiD approximately 1.6 at low deuterium pressure (at up to 9.5 bar D2 and 550 K), a beta-phase La3Pd5SiD approximately 2.30-4 at intermediate deuterium pressure (<9.5 bar D2 and 550 K), and a relatively unstable gamma-phase La3Pd5SiD approximately 5 at high deuterium pressure (obtained at 75 bar D2 and 293 K). While the alpha and beta phases retain the symmetry of the H-free La3Pd5Si (space group Imma), the gamma-phase undergoes a symmetry lowering (a(gamma) approximately a(beta), b(gamma) approximately 3b(beta) and c(gamma) approximately c(beta), V(gamma) approximately 3V(beta), space group Pmnb). The structure of the alpha-phase contains isolated [Pd-D-Pd] fragments, which are joined into polymeric (-Pd-D-Pd-)n zig-zag chains in the beta-phase. In the gamma-phase some D sites depopulate, while new D sites are occupied, thus leading to a partial interruption of the zig-zag chains and the formation of isolated [D-Pd-D-Pd] and [D-Pd-D-Pd-D] fragments. This unexpected behavior can be attributed to the onset of repulsive Si-D and D-D interactions (Si-D > 3.0 A, D-D > 2.1 A) that divide the structure into Si-poor slabs that absorb hydrogen and Si-rich slabs that do not. The competition between silicon and deuterium which act as a transition metal ligand is further underlined by the fact that Pd atoms having one Si ligand are capable of forming Pd-D bonds, whereas Pd atoms having two Si ligands are not.     

Optical Kerr effect in nematic liquid crystals: a molecular dynamics simulation study

Molecular dynamics simulations of the Gay-Berne model have been undertaken and optical response properties calculated for the isotropic and nematic phases. The components of the optical response were calculated for the coordinate system with respect to the director. Furthermore, the response for the components was separated into the orientational and collision induced contributions in order to analyse the mechanism of the optical response. It was observed, in particular, that one of the depolarized components of the response function does not vanish after long times for the nematic phase, unlike in the isotropic phase. This means that the orientation of the director can be permanently changed by instant irradiation with polarized light. The results give a microscopic picture of the optical Kerr effect in nematic liquid crystals.