Low-temperature orthorhombic phase

Figure 10 Unit cells of the two phases of AC o compounds, (a) high temperature face centered cubic phase (b) low temperature orthorhombic phase, in which the polymeric chains form in the a direetion [6].

Nuclear Magnetic Resonance. Previous NMR studies have been concerned with broad band spectra at subambient temperatures (7). These data indicated a large amount of free rotation in the low temperature orthorhombic phase. Figure 5 shows a series of rapid high resolution scans... 

Molecular packing in the low-temperature orthorhombic phase of perdeutero-acetylene shows a layer structure with a T-shaped interaction between pairs of molecules within a layer. The D—C distances form an isosceles triangle with edges of 2.738, 2.737 A and a D to C=C midpoint distance of 2.672 A. The D—C contacts are some 0.2 A shorter than those expected from van der Waals radii considerations. The geometrical arrangement clearly implies a donor-acceptor interaction between D and the triple-bond TT-electrons, a situation described as long ago as 1972 to account for close C=C—I interactions in crystals of diiodo-acetylene. More recently five O—H—C C and five NH—C=C TT-interactions with mean H—C distances of 2.69 (for O—H) and 2.61 (for N—H) have been located in the CSD. 

XRD MFI structure orthorhombic (Pnma space group at room temperature) to monoclinic (P2j/n space group at low temperatures) structural phase transition... 

Whereas the first microscopic theory of BaTiOs [1,2] was based on order-disorder behavior, later on BaTiOs was considered as a classical example of displacive soft-mode transitions [3,4] which can be described by anharmonic lattice dynamics [5] (Fig. 1). BaTiOs shows three transitions at around 408 K it undergoes a paraelectric to ferroelectric transition from the cubic Pm3m to the tetragonal P4mm structure at 278 K it becomes orthorhombic, C2mm and at 183 K a transition into the rhombohedral low-temperature Rm3 phase occurs. 

Take, for example, the plot of G versus temperature for elemental sulfur, represented by the bottom diagram in Figure 2.1. We know from experiments and observation that there are four phases we have to consider for sulfur two solid forms (a low-temperature orthorhombic form, R, and high-temperature monoclinic form, M), liquid (L), and vapor (or gas, V). The lines of G versus T for each phase, which are partially solid and continue on as dashed lines, are constructed at constant pressure using Eq. (2.10),... 

Further X-ray diffraction work revealed a second-order phase transition at 150 K, attributed to an orientational order-disorder transition of the K+-NH3 pair at the octahedral site [36]. The low-temperature orthorhombic structure (space group Fddd) is derived by doubling the lattice constants of the high-tem-perature phase along all three axes, with the K+-NH3 pairs orienting along the [110] direction in an antiferroelectric fashion. 

The phase transitions are fairly complicated and have been discussed by several authors.91-93 In addition to the stable low-temperature orthorhombic form II which transforms at 401 K to the rhombohedral I, there exists a metastable form III which can be obtained by cooling I to 396 K III then transforms to II on further cooling to 386 K. 

Vaiiable-temperamre Raman spectra for the 2.4-nm-thick BaTi03 film is shown in Fig. 21.10. Other films exhibit similar temperature evolution. As can be seen, the peak positions and lineshapes remain nearly unchanged with increasing temperature. Hence, the films remain in the single ferroelectric phase (tetragonal) and the low-temperature orthorhombic and rhombohedral phases characteristic for bulk... 

Siegel and Northrop provide X-ray powder evidence to show that the phase transition, observed for each of the solid hexafluorides of the second and third transition series, involves a low temperature orthorhombic form, evidently isomorphous with orthorhombic OsOF and a cubic high temperature from isomorphous with cubic OsOF. The equivalence of die Bravais lattices and the close similarity of the unit cell dimensions implies close structural similarity of the low temperature phases. The higher temperature, cubic phases, are on the X-ray evidence, body-centred cubic. 

The Raman spectra of the low-temperature (orthorhombic) and high-temperature (cubic) phases of MBF4 (M = K, Rb, or Cs), and that of molten LiBF4... 

AnRe2 compounds are known for An = Th, U, Np and Pu. All of them can be classified as hexagonal C14 Laves phases. URe2, however, keeps this structure only above 450 K. Below this temperature it distorts into a low-temperature orthorhombic structure of the Cmcm space group (Hatt 1961). 

In contrast, the non-reinforced material predominantly exhibited the formation of orthorhombic, 6-phase Y2S1207, although evidence for the monoclinic, X2-phase of Y2SiO<, was also found within the interior regions of some creep samples (in particular, non-preannealed samples that failed rapidly). In comparing the main products of devitrification, it should be noted that the orthorhombic 6-phase found in the non-reinforced material corresponds the most stable, high temperature phase of Y2Si20y, while the monoclinic y-phase found in the whisker reinforced material represents a low temperature variant phase, which is conceivably metastable at temperatures above 1200°C. 

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