High-temperature modifications

The structure of the low-temperature modification of the compound, Na2Ta205F2 (I), has not yet been determined. The high-temperature modification, Na2Ta205F2 (II), can be conceived as two sub-lattices Tai6X52, which is composed of TaX6 octahedrons and Na Xt, which contains Na X2 tetrahedrons [192]. Fig. 38 shows the structure of Na2Ta205F2 (II). Two additional sodium atoms occupy the centers of two bi-pyramids with distorted hexagonal bases. 

Fig. 44. IR absorption spectra of Li3Ta04 (1), Li4Ta04F (2), LiJJbOfr (3), Li3Ti03F (4)- rock-salt-type structures with disordered ionic arrangement and high-temperature modifications of Li3Ta04 (5), Li4Ta04F (6), Li3Nb04 (7), LiMO.F (8).

High temperature modification, measured at 230°C (after Agulyansky et al. [148]). 

AgaSI, is prepared at 210-215°C. In both cases, it takes several weeks to complete the reaction. The high-temperature modification a-AgaSI, which is stable above 235°C, can be obtained either by a treatment at 280°C instead of 210-215°C, as for /3-AgaSI, or by annealing of the latter for some hundreds of hours at 280°C and quenching to room temperature (317). a-AgaSI forms in only 17 hours if the reaction takes place at 550°C under various sulfur pressures (370). 

Cupric sulfide, CuS, occurring as the mineral covellite (also known as covelline), exhibits a very unusual structure, in which the Cu is again partly 3-coordinate and partly 4-coordinate, with two-thirds of the sulfur atoms existing as S2 groups like those in pyrites. The low-temperature form of CuSe has also a covellite structure, the high-temperature modification (P-CuSe) being orthorhombic. All CuX2 compounds assume pyrite-type structures. 

Orientational disordering of the carbonate groups in CaC03 above 1260 K may serve as an example of application of Landau theory. Below the transition temperature, alternate layers of planar CO3 groups point in opposite directions. In the high-temperature modification they are free to rotate and become equivalent. The sym- ... 

Concluding this section, we may mention a paper by Daams and Villars (1993) concerning an atomic environment classification of the chemical elements. Critically evaluated crystallographic data for all element modifications (and recommended atomic volumes) have been reported. Special structural stability diagrams were used to separate AET stability domains and to predict the structure (in terms of environment types) of hitherto unknown high-pressure and high-temperature modifications. Reference to the use of AET in thermodynamic (CALPHAD) modelling and calculation has been made by Ferro and Cacciamani (2002). 

High-temperature modifications generally have lower coordination numbers, just as low-pressure modifications do. Structures of certain metals such as Zn, Cd and Hg do not conform to this pattern. Mercury has a rhombohedral structure in which each atom is six-coordinated following the (8 — N) rule indicating the presence of the covalent... 

A model of thermal pain. Mice or rats are placed on a heated metal plate of variable temperature. Depending on the temperature (48-58 °C), a weak or strong pain stimulus is induced. Animals respond either by licking their paws or by jumping. Analgesics increase the latency for this pain reaction. The low temperature hot plate detects a broader spectrum of less efficacious analgesics in comparison to its high temperature modification or the tail flick test (Eddy and Leimbach J. Pharmacol. 1953, 107, 385). 

As we can see from Eq. (26), the main effect coming from chain segment torsions and rotations is that higher orders of the Bessel functions Jn drop out It depends, however, essentially on the symmetry of the molecule how this effect influences F. We will discuss this important point in more detail in Sect. 4.2.1 High Temperature Modification of PTFE (Phase I) . 

HT high temperature modification P unit cell angle (in degrees)... 

The high temperature modification for the YsSbs has been reported recently (Mozharivskyj and Franzen, 2000b). 

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