Pseudohexagonal Phase Research Articles

Kinetics of a structural phase transition in Langmuir monolayers studied using x-ray diffraction

Frequently, time-dependent effects are seen in monolayers of amphiphilic molecules (Langmuir films) when they are compressed, so that the pressure after some time is different from that recorded immediately after compression. We have identified for the first time a microscopic relaxation mechanism in monolayers of heneicosanol (C21H43OH): namely, a transition from a uniaxially distorted (‘‘pseudohexagonal’’) structural, formed upon compression, to an undistorted hexagonal structure. For T>20 °C we observe only an apparently hexagonal phase, while at T=5 °C we observe only an apparently stable pseudohexagonal phase. When 10≤T≤20 °C, the monolayer structure changes with time from pseudohexagonal to hexagonal. The rate at which this transformation occurs is strongly temperature dependent. We propose that the observed temperature dependence is determined by the rate of nucleation of a hexagonal phase from a metastable shear-induced structure.

Disorder at the bilayer interface in the pseudohexagonal rotator phase of solid n-alkanes.

Molecular-dynamics calculations are used to characterize the structure and dynamics of the two solid bilayer phases of the n-alkane tricosane (${\mathrm{C}}_{23}$${\mathrm{H}}_{48}$). In the crystalline orthorhombic phase at 38\ifmmode^\circ\else\textdegree\fi{}C, chains undergo translational, rotational, and torsional motions, but are otherwise essentially all-trans and perfectly ordered with a herringbone packing. By contrast, at 42\ifmmode^\circ\else\textdegree\fi{}C, in the pseudohexagonal (rotator) phase, there is a dramatic increase in longitudinal chain motion, each chain now has four possible orientations and a significant number of conformational defects develop, predominantly at the chain ends.

Transmission electron microscopy on meteoritic troilite

Phase transitions and associated domains of meteoritic troilite (FeS) have been studied by means of transmission electron microscopy (TEM). Three polymorphs have been found, two of which can be described by superstructures of the NiAs-type structure (A, C subcell). The P\(\overline 6\)2c (√3A, 2C) polymorph, stable at room temperature, displays antiphase domains with the displacement vector 1/3 . In situ heating experiments showed that the P\(\overline 6\)2c polymorph changes at temperatures of 115°–150° C into an orthorhombic pseudohexagonal transitional phase with the probable space group Pmcn (A,√3A, C). It contains antiphase domains with the displacement vector 1/2 [110] and twins with a threefold twin-axis parallel c. When heated above 210° C the transitional phase transforms into the high-temperature modification with NiAs structure (P63/mmc). All observed phase transitions are reversible. The occurrence of antiphase and twin domains, respectively, agrees with the symmetry reductions involved in the subsolidus phase transitions. This is demonstrated by group-subgroup relationships among the space groups P63/mmc, Pmcn, and P\(\overline 6\)2c.

Structural and electrical properties of CuIn0.7Ga0.3Se2 epitaxial layers on GaAs substrates

AbstractEpitaxial layers of CuIn0.7Ga0.3Se2 could be prepared by flash evaporation onto (111)A‐oriented GaAs substrates in the substrate temperature range Tsub = 745 … 870 K. At Tsub = 745 … 820 K the films had the chalcopyrite structure, at Tsub = 820 … 845 K an additional pseudohexagonal phase was found. Indications to the presence of a sphalerite phase were found at high substrate temperatures. Films grown at Tsub ≦ 750 K were always n‐type conducting and showed a largely pronounced impurity band conduction effect. At Tsub ≧ 860 K the films were always p‐type conducting and two acceptor states with ionization energies of some 10−3 eV and of 125 … 140 meV were found.

Relationship of Copper Sulfide and Cadmium Sulfide Phases

Single-crystal CdS can be converted by cation exchange in CuCl solution to single-crystal copper sulfide with the chalcocite, djurletie, and CuxS (with 1.96>x>1.8) structures. The last is a previously undescribed hexagonal or pseudohexagonal phase with a=15.475±0.010 Å and c=13.356±0.010 Å. The structural relationship of the various phases to each other and to CdS is described with reference to two basic sulfur arrangements: hexagonal and cubic close packing. The gradual shift of phase of small crystals from high to lowered copper content is attributed to surface oxidation of copper atoms and strongly suggests that natural chalcocite crystals may also change composition and phase with time.

X-ray investigation of bredigite

The X-ray work described below has been done on crystals of bredigite obtained from two different sources, the natural mineral from Scawt Hill, County Antrim, and the synthetic mineral from spiegeleisen slags. Both types of material have been fully deseribed by Tilley and Vincent (1948). Their analysis shows that the slag mineral is a calcium orthosilicate in which Ca is partly replaced by Mg, Mn, and Ba, the extent of the replacement being represented by the formula (Cal.59Ba0.08Mg0.31Mn0.09)SiO4. Tilley and Vincent conclude that this orthorhombic (pseudo-hexagonal) phase is identical with the high-temperature form α' of pure Ca2SiO4, the existence of which was suggested by Bredig (1943a), and confirmed by TrSmel (1949) using a high-temperature powder camera. Bredig (1943a) also suggests, on the basis of a comparison of X-ray powder patterns, that the crystal structure of α'-Ca2SiO4 is similar to that of β-K2SO4. The present single-crystal work was originally undertaken in order to investigate the structure of α'-Ca2SiO4 more fully.