Rhombohedral-cubic phase

P, which can be switched by an electric field, E, as illustrated in the P vs. E hysteresis loops in Figure 2) in favor of the non-ferroelectric cubic and antiferroelectric (AFE) phases. At a 65/35 ratio of PbZr03 to PbTi03, a concentration of 9.5% lanthanum is sufficient to reduce the rhombohedral-cubic phase transi-... 

Figure 16.35 (Plate 1). All strontium myristate. Top left lOOx crossed polars - room-temperature lamellar. Top right lOOx crossed polars 90°C - lamellar. Middle left lOOx crossed polars, gypsum plate in, heated to 218°C - rhombohedral. Middle right 200 x crossed polars, gypsum plate in, cooled from rhombohedral-cubic phase boundary, oscillated near 210-215°C - rhombohedral (bright) and cubic (dark). Bottom left lOOx parallel polars, cooled to 210 from 218°C - cubic to rhombohedral transition. Bottom right 200 x crossed polars, gypsum plate in, cooled from 290°C and oscillated near 260° C - hexagonal...

Fig. 3.6 Band stmcture and band-decomposed charge density plots for ihombohedral (a, b) and intermediate (e, f) phase AgBiSe2, respectively. Partial chaige density plots for c the VBM (valance band maximum) and d the CBM (conduction band minimum) of Ag-Bi-Se chain in the rhombohedral phase AgBiSe2. g The partial chaige density plots of Ag-Bi-Se chain for AgBiSe2 after the Ag/Bi atoms exchange during the rhombohedral-cubic phase transition...

Fig. 3.7 Projected density of states (PDOS) for rhombohedral left column) and intermediate right column) phase of AgBiSc2 during the rhombohedral-cubic phase transition...

Relational structures at different temperatures in the rhombohedral-cubic phase transition. As temperature increases, AgBiSea crystallized in the hexagonal phase is observed to undergo continuous phase transition to rhombohedral phase around 410 K and then to cubic phase around 580 K. Also the phase transitions take place reversibly as temperature decreases, that is, the cubic phase undergoes the continuous phase transition to rhombohedral phase around 560 K and then to the hexagonal phase around 390 K during cooling process. Our optimized lattice... 

Another issue with ScSZ is that there is a decrease in conductivity at around 580°C as indicated by broken lines in two of the curves for higher scandia contents, 10.0 to 12.0 mol%. With high scandia contents, the cubic phase transforms to a lower conductivity rhombohedral phase, the p phase, at lower temperatures [25], The phase change can be avoided by limiting the scandia content to 8 mole% [25] or by codoping with other oxides, such as those of bismuth [36] or ytterbium [37],... 

Fig. 10 Medium-range PDF of PMN. Temperatm-e dependence (upper panel) is well explained by the transition from rhombohedral phase to the cubic phase (lower panel). The two PDF peaks at 8.5 and 9.09 A indicate 300 K as a characteristic temperature for crossover [18]...

Fig. 11 PDF of PMN at a 500, b 250, and c 50 K, fit well by the calculated PDF assuming a mixture of the rhombohedral and the cubic phase with varying fraction, d Volume fraction of the rhombohedral phase (fraction of PNR) [18]...

Measurements of NMR for Ti, Ti [33], and Sr [34,35] were carried out for STO 16 and STO 18-96. Ti and Sr nuclear magnetic resonance spectra provide direct evidence for Ti disorder even in the cubic phase and show that the ferroelectric transition at Tc = 25 K occurs in two steps. Below 70 K, rhomb ohedral polar clusters are formed in the tetragonal matrix. These clusters subsequently grow in concentration, freeze out, and percolate, leading to an inhomogeneous ferroelectric state below Tc. This shows that the elusive ferroelectric transition in STO 18 is indeed connected with local symmetry lowering and impHes the existence of an order-disorder component in addition to the displacive soft mode [33-35]. Rhombohedral clusters, Ti disorder, and a two-component state are found in the so-called quantum paraelectric... 

The optical properties of ferroelectric materials are characterized by birefringence. Barium titanate is isotropic only in the cubic phase. The tetragonal and the rhombohedral phases are... 

It was clear that if the formulation O2+ [PtF,]- were true, then platinum hexafluoride should oxidize molecular oxygen. A sample of platinum hexafluoride was therefore prepared, and since it has a vapor pressure of 80 mm. at room temperature, the interaction with molecular oxygen was followed tensimetrically. The deep red (bromine colored) platinum hexafluoride vapor reacted instantly with the oxygen in a 1 1 molar ratio to give the familiar red compound. Interestingly, the solid made by this method at low temperatures, is iso-morphous and almost isodimensional with rhombohedral potassium hexafluoroplatinate (V). At higher temperatures transition to the cubic phase occurs. 

Exotic phases are found at compositions between lamellae and hexagonal cylinders (see Figs. 12-19 and 12-20). Some examples of the morphologies of these phases are shown in Fig. 12-22 these include cubic strut phases, tetragonal and rhombohedral mesh phases, and rectangular ribbon phases. 

A central issue in the field of surfactant self-assembly is the structure of the liquid crystalline mesophases denoted bicontinuous cubic, and "intermediate" phases (i.e. rhombohedral, monoclinic and tetragonal phases). Cubic phases were detected by Luzzati et al. and Fontell in the 1960 s, although they were believed to be rare in comparison with the classical lamellar, hexagonal and micellar mesophases. It is now clear that these phases are ubiquitous in surfactant and Upid systems. Further, a number of cubic phases can occur within the same system, as the temperature or concentration is varied. Luzzati s group also discovered a number of crystalline mesophases in soaps and lipids, of tetragonal and rhombohedral symmetries (the so-called "T" and "R" phases). More recently, Tiddy et al. have detected systematic replacement of cubic mesophases by "intermediate" T and R phases as the surfactant architecture is varied [22-24]. The most detailed mesophase study to date has revealed the presence of monoclinic. 

The relative stability of mesh and IPMS structures is still unclear. For example, the Ri mesophase (of rhombohedral symmetry) in the SDS-water system transforms continuously into the neighbouring bicontinuous cubic phase (Fig. 4.14) [20]. This suggests that this mesophase is a hyperbolic (reversed) bilayer Ijring on a rhombohedral IPMS. Indeed, the rhombohedral rPD surface is only marginally less homogeneous than its cubic counterparts, the P- and D-svu-faces. 

C.J. Howard, B.J. Kennedy, and B.C. Chakoumakos, Neutron powder diffraction study of rhom-bohedral rare-earth aluminates and the rhombohedral to cubic phase transition. J. Phys.-Conden. Matter. 12(4), 349-365 (2000). 

The cubic phases of zirconium- and hafnium nitride display properties quite similar to those of TiN. These compositions have been studied to some extent, however, the number of reports dealing with the CVD of ZrN and HfN is considerably lower than those describing the CVD of TiN. Zirconium and hafnium nitride can be prepared by the decomposition of the appropriate metal tetrakis(dialkylamide) compounds [155]. Zirconium and hafnium nitrides of the stoichiometry M3N4 also have been prepared by CVD. The compounds are formed when M(NEt2)4 is reacted with NH3 at 200-450°C [144]. The films thus produced are crystalline, yellow, transparent and insulating. It has been suggested that these phases are related to the MN phase by a rhombohedral distortion. 

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