Tetragonal high-pressure structures

In the AnX compounds, the B1 B2 transition is accompanied by a volume decrease of the order of 10%. Antimony and bismuth as anions favor tetragonal high-pressure structures. Monocarbides, -nitrides, -phosphides, and -sulfides of thorium and uranium either did not transform in the pressure range studied (ThC, ThN) or transformed to structures of relatively low symmetry (UC, UN, UP, US, ThS), which could possibly point to an f contribution in bonding. In this group of Th and U compounds with light anions, only ThP shows the Bl B2 structural transition. 

Figure 15.11 Structure refinement using full diffraction profiles of a tetragonal high pressure modification of caesium (unpublished results).

P. Bouvier, E. Djurado, G. Lucazeau, and T. Le Bihan, High-pressure structural evolution of undoped tetragonal nanocrystaUine zirconia, Phys. Rev. B 62(13), 8731-8737 (2000). 

Table 2.1 shows the crystal structure data of the phases existing in the Mg-H system. Pnre Mg has a hexagonal crystal structure and its hydride has a tetragonal lattice nnit cell (rutile type). The low-pressure MgH is commonly designated as P-MgH in order to differentiate it from its high-pressure polymorph, which will be discussed later. Figure 2.2 shows the crystal structure of p-MgH where the positions of Mg and H atoms are clearly discerned. Precise measurements of the lattice parameters of p-MgH by synchrotron X-ray diffraction yielded a = 0.45180(6) mn and c = 0.30211(4) nm [2]. The powder diffraction file JCPDS 12-0697 lists a = 0.4517 nm and c = 0.30205 nm. The density of MgH is 1.45 g/cm [3]. 

Figure 2.11 Very high pressure (0.3-2.1 GPa) structural changes of gas hydrates at room temperature. Numerical values (adjacent to square boxes) indicate transition pressures. Hexagonal (sH ) and tetragonal (sT ) hydrate phases are distinct from sH and sT hydrate structures found at normal pressures. (Modified and redrawn from Hirai, H., Tanaka, H., Kawamura, K., Yamamoto, Y., Yagi, T., J. Phys. Chem. Solids, 65, 1555 (2004). With permission from Elsevier.)...

The monophosphides REP (RE = La, Ce, Pr, Nd, Sm, Gd, Tb, Tm, Yb) crystallize in the NaCl-type structure at ambient pressure. All of them undergo phase transitions, some into a tetragonally distorted CsCl related structure (Ea, Pr, Nd) and the others into a more complicated unknown high-pressure phase (Sm, Gd, Tb, Tm, Yb) at pressures from 35 to 51 GPa, respectively. It might be that a disproportionation occurs, similar to that in Eu2As2. ... 

---