Rhombohedral phase

Kohs W, Santner H.J., Hofer F., Schrottner H., Doninger J., Barsukov I., Albering J.H., Moller K.-C., Besenhard J.O., and Winter M. A study on electrolyte interactions with graphite anodes exhibiting structures with various amounts of rhombohedral phase. J. 

Fig. 21 Cyclic voltammogram and corresponding in situ STM images of 3 mM 4,4 -bipyridine (44-BP) on Au(lll) in 0.05 M KCIO4, scan rate 10 mV s-1. The sizes of the STM images are 10 x 10 nm2. The following 44-BP adlayer structures have been observed in the potential regions I, II, and III (a) high coverage densely packed phase, (b) striped structure, and (c) rhombohedral phase. The corresponding molecular orientations as derived from in situ IR studies on Au(l 11) are shown in panels (d-f) [303], The pairs of peaks Pl/Pl, P2/P2, and P3/P3 indicate first-order phase transitions between the respective adlayers [304]...

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],... 

Lei and Zhu [63] found that adding 2.0 mol% Mn203 to llScSZ can inhibit the cubic-rhombohedral phase transformation in both oxidation and reduction atmospheres, and the codoped zirconia can reach nearly full density when sintered at temperatures as low as 850°C. The conductivity of 2Mn203-l IScSZ sintered at 900°C is 0.1 Scm-1 at 800°C. Figure 1.11 illustrates the conductivity of some zirconia-based ternary systems [32,42,57,63-67],... 

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]...

Fig. 20 Schematic models of the gold nanoparticle strings (a) rhombohedral phase of 14 and (b) hexagonal columnar phase of 13. Gold nanoparticles are in yellow, mesogens in green [542]. (Copyright Wiley-VCH Verlag GmbH Co. KGaA, reproduced with permission)...

The atomic layers from c-BN (ABCABC) have to rearrange into an ABAB stacking sequence of h-BN during the solid state phase conversion. A possible mechanism would be the intermediate formation of the rhombohedral BN phase (r-BN) with ABCABC stacking. The r-BN phase is structurally related to the hexagonal phase, but only differs in the d-values (h-BN d = 6.66 A r-BN d = 10.0 A) of the layers (Fig. 5b). Subsequently the rhombohedral phase is transformed into the hexagonal modification at the reaction temperatures [10]. 

The manganites with general formula Laj vSrvMn03 have structures of distorted perovskite. Depending on the types of distortions, the structure could be called orthorhombic O with space group Pnma, orthorhombic O with space group Pnma and rhombohedral phase with the space group R3c. The first structure... 

Besides orthorhombic f/ and O phases, the defected LaMn03 is observed in monoclinic and rhombohedral phases [5]. The monoclinic one is observed at low temperatures and with an increase of temperature it transforms to rhombohedral structure. 

However, the monoclinic phase differs strongly in the distortions of oxygen MnO ] octahedra with respect to ideal perovskite structure. The observed monoclinic phase is much closer to the rhombohedral phase, which could be achieved by the R point BZ rotation of the MnOel octahedra around (1 1 1) direction. 

The rhombohedral structure, obtained via the optimization of crystal energy [equation (1)] without the JT term inclusion or setting IVj = 0, is stable with respect to deformations of the crystal. The experimental and calculated structure parameters are listed in Table 2. As the temperature decreases down to about 400 K [5,8] the crystal symmetry changes to monoclinic phase. The major difference between the monoclinic and rhombohedral phases is the presence of JT distortions and a doubled primitive cell. The local values of JT distortions on different Mn3+ ions could be obtained via the projection of oxygen ions coordinates onto the normal local Eg modes of each [Mn06] octahedra (the numbering of Mn3+ is carried out according to Fig. 1). 

Fig. 3. The dependence of intensity of highest A2g (in rhombohedral phase notation) mode on the value of I Ve I. The frequency and intensity were averaged on i> for the value of p, which corresponds to the energy minimum. For each value of

The temperature of transition is small relative to the value of splitting of the Mn3+ 5E ground state. That is why we suppose that the transition from monoclinic to rhombohedral phase is a transition from static to dynamic JT effect. Because of the strong correlation in motion of different [Mn06] octahedra we suppose that there is a cooperative dynamical effect. 

Table 1. O (c < b/y/2 < a) and O (b/y/2 < c < a) orthorhombic phases have both Pnma space group symmetry. R is the rhombohedral phase with R3c space group symmetry.

These considerations lead us to propose the tentative phase diagram of fig. 28. The orthorhombic-rhombohedral transition has been tracked by Wold and Arnott (1959). The two-phase region 0.05 < 5 < 0.10 observed by room-temperature X-ray diffraction has been shown from the spin-glass behavior below 7 to extend over a wider compositional range below 200 K, which indicates that a spinodal phase segregation separates the O -orthorhombic (c/a < x/2) from an O -orthorhombic (c/a J2) or R-rhombohedral phase. With neutron powder diffraction, Huang et al. (1997) have shown the two-phase region at 300 Kin the inter-... 

The compositional dependence of the structural O -O transition at 7jt and the 0 -R transition at Tor can be clearly followed by monitoring the temperature dependence of the resistance (Mandal et al., 2001) monitoring the variation with x of the higher-order transition at T from the resistance curve R(T) is more subtle and has been accomplished with further aid from the thermoelectric power a(T) measured on single crystals (Zhou and Goodenough, 2000). The transition from polaronic to itinerant electronic behavior in the paramagnetic R-rhombohedral phase has not been studied. 

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... 

Figure 2.4 Strain-field curves for < 001 > oriented 0.91PbZn1/3Nb2/303-0.09PbTi03 single crystals. The sample in (a) was poled at room temperature, where the resulting domain state is unstable (due to induction of tetragonal material associated with the curved morphotropic phase boundary), yielding substantial hysteresis. In (b) the crystal was poled at low temperatures to keep it in the rhombohedral phase. When measured at room temperature, the piezoelectric response is much more linear and non-hysteretic, due to the improved stability of the ferroelectric domain state. Data courtesy of S. E. Park.

---