Phase tetragonal-orthorhombic transition

Figure 7. A linear variation of the sqnare of the symmetry breaking strain with pressnre implies second order character for the tetragonal orthorhombic transition in stishovite. Circles represent strains calcnlated nsing flo = (abf and the data of Andranlt et al. (1998) crosses represent strains calcnlated nsing the alternative variation of Oq shown by crosses in Fignre 6. Open sqnares are from single crystal X-ray diffraction data of Mao et al. (1994) D designates measnrements of the orthorhombic phase made dnring decompression.

The nonstoichiometric monohydrides formed by V, Nb and Ta have structures that are determined by T and the H content . Order-disorder transitions involving the H atoms lead to structural complexity in the ordered phases the bcc metal lattice distorts to tetragonal, orthorhombic or monoclinic. In addition, V and Nb, but not Ta, at >0.1 MPa Hj form dihydrides that have the fluorite structure. Hence, the phase diagrams of these elements with H are complex. 

Figure 12. Variation of frequency squared for the soft optic mode through the tetragonal o orthorhombic transition in stishovite (data from Kingma et al. 1995). Straight lines through the data intersect at P = 51.6 GPa the straight line for tetragonal stishovite extrapolates to zero at = 102.3 GPa. A broken line for the orthorhombic phase includes the pressure dependence of b but is barely distinguishable from the straight line fit to the data (solid line). After Carpenter et al. (2000a).

Fig. 2. Phase diagram for Laj. Sr CuO showing the phase boundaries for the tetragonal-orthorhombic, paramagnetic-antiferTomagnetic, and normal-supeiconducting phase transitions as a function of temperature and doping. From Keimer et al. (1992).

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