Crystal structures at room-temperature

The term crystal structure in essence covers all of the descriptive information, such as the crystal system, the space lattice, the symmetry class, the space group and the lattice parameters pertaining to the crystal under reference. Most metals are found to have relatively simple crystal structures body centered cubic (bcc), face centered cubic (fee) and hexagonal close packed (eph) structures. The majority of the metals exhibit one of these three crystal structures at room temperature. However, some metals do exhibit more complex crystal structures. 

According to the chemical analysis and coordination distances, the Rietveld refinement of the crystal structure at room temperature revealed 1.2 Co2+ atoms per unit cell at the Col and Co2 sites, whereas the 1.4 Ag+ cations are spread over the Co3 site, from now on referred to as Ag5 for clarity, and two new sites, Ag2 and Ag3, located near Co2 in the 10-membered ring (Fig. 3). In addition, for this catalyst the presence of Ag° clusters outside the zeolite structure was recognized by the detection of a strong reflection at about 40° 28. In agreement with the lower Ag content, in Ag2.7Co2.8AF the Ag3 site... 

The temp>erature of the Jahn-Teller phase transition is very high for CuFj, CuClj, and KjCuF among the examples mentioned above [6-31]. Therefore, at room temperature their crystal structures display distortions. Other compounds have symmetric crystal structures at room temperatures as their Jahn-Teller phase transition occurs at lower temperatures. Cupric chelate compounds and [CuCNOj), ] compounds, such as KjPbCuCNOj) and TljPbCuCNO ), can be mentioned as examples [6-33]. Further cooling, however, may make even these structures distorted [6-33]. 

Figure 1. Crystal structures at room temperature of (a) (TTF)5Pt(CN)4.2 CH3CN,...

Figure 1. The p-terphenyl crystal structure at room temperature and below the phase transition at 193 K. Shown is the projection of the short axes M of the molecules onto the a/h-plane (respectively a jb for the low temperature phase).

The rare-earth compounds RMO4 have a zircon-type crystal structure at room temperatures with space group I4i/amd (D J) and local symmetry E)2d at the rare-earth lattice sites. This structure is shown in fig. 6. Coordinates of ions in an elementary cell are as follows ... 

As mentioned, the rare earths exhibit five different crystal structures at room temperature. Three of these are common metallic structures, hexagonal close packed (hep), cubic close packed (cep), also called face-centered cubic fee, and body-centered cubic. The first two are pictured in Fig. 4. The bcc structure is not pictured but consists of cubes of atoms surrounding another atom inserted into the geometric center or body-center of the cube. The two structures, unique to the rare earths are the double-hexagonal close packed structure (dhep) found for La, Pr, Nd and Pm and the complex structure found for Sm. Both of these are variants of the hep structure. Their occurrence for the early members of the series can be explained by postulating that the 4/ electrons, which have relatively large radial extensions for the early elements, participate in the metallic bonding. 

Since the relaxor PLZT compositions used in this study have a cubic crystal structure at room temperature, the expectation is that the strain generated should be mostly quadratic, especially if no dc bias is applied. Figure 5 shows the Fourier-transformed displacement amplitude as a function of frequency for a PLZT (9.5/65/35) ceramic under a combination of both ac and dc bias electric fields. As expected, several frequency peaks are seen this indicates that the material is vibrating at harmonic frequencies to the applied field. 

Structure nium and ASTM grades 7,11, and 12 are typically 100% a-crystal structures at room temperature. As levels of impurities (primarily iron) increase, small but increasing amoimts of spheroidal are observed metallographically, usually at the grain boundaries. Larger amounts of spheroidal P are more likely in ASTM grade 12 (Ti-0.3Mo-0.8Ni) than in unalloyed titanium. 

Metal Natural Crystal Structure at Room Temperature Other Crystal Structures at Different Temperatures and Pressures... 

PerCo(mnt)2 0.5CH2Cl2 presents a rare and interesting structural type in which stacks of perylene coexist with a true polymeric arrangement of Co(mnt)2 units [72]. Its crystal structure at room temperature, i.e. well above the phase transitions undergone at lower temperatures, presents already an incommensurate modulation with wave vector q = (0.22a, —0.13b, —0.36c ) as denoted by first- and second-order X-ray satellite reflections. The modulated structure was recently solved [101] but for sake of clarity we will refer first to the average structure that is triclinic space group PT a = 6.551(2) k, b= 11.732(2) A, c = 16.481(2) A, a = 92.08(1)°, )8 = 95.30(1)°, y = 94.62(1)°, Z = 2 and V = 1248.6(3) A [72]. TTie crystal structure... 

Another common metal that experiences an al-lotropic change is tin. White (or p) tin, having a body-centered tetragonal crystal structure at room temperature, transforms, at 13.2°C (55.8°F), to gray... 

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