Polonium structure

Under pressure black phosphorus transforms first to a modification that corresponds to gray arsenic. At an even higher pressure this is converted to the a-polonium structure. Then follows a hexagonal-primitive structure, which has also been observed for silicon under pressure (p. 122), but that hardly ever occurs otherwise. Above 262 GPa phosphorus is body-centered cubic this modification becomes superconducting below 22 K.. 

Arsenic also adopts the a-polonium structure at 25 GPa and becomes body-centered cubic at the highest pressures. The rather unusual Bi-III structure appears intermediately between these two modifications. 

The interesting polymorphism observed in sulfur is consistent with previous predictions. Bridgman (19) named sulfur as a possible candidate for new irreversibly created high pressure and high temperature modifications. Von Hippel (I) showed that the structures of selenium and tellurium are interrelated to the structure of polonium and can be developed by displacing the atoms of the octahedral planes or sliding the (111) planes in the polonium structure. He reported that at ambient conditions... 

At high pressure and low temperature tellurium, and perhaps even selenium and sulfur, may undergo a transition to the a-polonium structure. This probably is not observed experimentally without great difficulty because of the probable extreme p-T conditions associated with formation of the phase and the expectation that it could not be recovered to be analyzed at ambient conditions thus requiring a sophisticated high pressure low temperature x-ray camera. 

The metal polonium (which was named by Marie Curie after her homeland, Poland) crystallizes in a primitive cubic structure, with an atom at each corner of a cubic unit cell. The atomic radius of polonium is 167 pm. Sketch the unit cell and determine (a) the number of atoms per unit cell (b) the coordination number of an atom of polonium (c) the length of the side of the unit cell. 

The trend toward more metallic character of the elements in Group 16 is complete at polonium, which has two allotropes, both with typically metallic structures a-cubic, which converts at 36 °C to P-rhombohedral (m.p. 254 "C). 

In a cubic-primitive structure (a-polonium, Fig. 2.4, p. 7) the situation is similar. By stacking square nets and considering how the orbitals interact at different points of the Brillouin zone, a qualitative picture of the band structure can be obtained. 

Two modifications are known for polonium. At room temperature a-polonium is stable it has a cubic-primitive structure, every atom having an exact octahedral coordination (Fig. 2.4, p. 7). This is a rather unusual structure, but it also occurs for phosphorus and antimony at high pressures. At 54 °C a-Po is converted to /3-Po. The phase transition involves a compression in the direction of one of the body diagonals of the cubic-primitive unit cell, and the result is a rhombohedral lattice. The bond angles are 98.2°. 

Under normal conditions an atom in elemental tellurium has coordination number 2 + 4. It has been known for a long time that pressure causes the interatomic distances to approximate each other until finally every tellurium atom has six equidistant neighboring atoms at 297 pm the structure (now called Te-IV) corresponds to /3-polonium. However, before this is attained, two other modifications (Te-II and Te-III) that are out of the ordinary appear at 4 GPa and 7 GPa. Te-II contains parallel, linear chains that are mutually shifted in such a way that each Te atom has, in addition to its two neighboring atoms within the chain... 

Sb LSb, yields the primitive-cubic polonium-type structure, which is S / known as a high-pressure modification of arsenic. Therefore, five electrons... 

J per atom are needed for this structure. Remarkably, polonium itself has one... 

Marie Sklodowska Curie (1867—1934) and Pierre Curie (1859—1906) are credited with discovering polonium as they sought the source of radiation in pitchblende after they removed the uranium from its ore. Their discovery in 1898 led to the modern concepts of the nucleus of the atom, its structure, and how it reacts. 

The simple cubic crystal structure we discussed above is the simplest crystal structure to visualize, but it is of limited practical interest at least for elements in their bulk form because other than polonium no elements exist with this structure. A much more common crystal stmcture in the periodic table is the face-centered-cubic (fee) structure. We can form this structure by filling space with cubes of side length a that have atoms at the corners of each cube and also atoms in the center of each face of each cube. We can define a supercell for an fee material using the same cube of side length a that we used for the simple cubic material and placing atoms at (0,0,0), (0,g/2,g/2), (g/2,0,g/2), and (g/2,g/2,0). You should be able to check this statement for yourself by sketching the structure. 

The simplest of the cubic structures is the primitive cubic structure. This is built by placing square layers like the one shown in Figure 1.1 (a), directly on top of one another. Figure 1.9(a) illustrates this, and you can see in Figure 1.9(b) that each atom sits at the corner of a cube. The coordination number of an atom in this structure is six. The majority of metals have one of the three basic structures hep, cep, or bcc. Polonium alone adopts the primitive structure. The distribution of the packing types among the most stable forms of the metals at 298 K is shown in Figure 1.10. As we noted earlier, a very few metals have a mixed hcp/ccp structure of a more complex type. The structures of the actinides tend to be rather complex and are not included. 

The nickel-polonium system resembles the analogous tellurium system in that compounds of composition varying continuously between NiPo and NiPo2 are formed, with structures between the NiAs and Cd(OH)2 types (134). 

The concept of close packing is particularly useful in describing the crystal structures of metals, most of which fall into one of three classes hexagonal close packed, cubic close packed (i.e., fee), and body-centered cubic (bcc). The bcc unit cell is shown in Fig. 4.8 its structure is not close packed. The stablest structures of metals under ambient conditions are summarized in Table 4.1. Notable omissions from Table 4.1, such as aluminum, tin, and manganese, reflect structures that are not so conveniently classified. The artificially produced radioactive element americium is interesting in that the close-packed sequence is ABAC..., while one form of polonium has... 

The formation of linear chains can be extended to two dimensions. Parallel LSh" chains lying side by side can be joined to a square net. One more singly occupied p orbital per Sb atom is needed. Formally, an oxidation, tSIv ISb , has to take place. Six valence electrons per atom are needed for the square net. Nets of this kind occur, for example, in YbSb2 (with Yb ). Starting from the square nets, another formal oxidation, iSb ISb, yields the primitive-cubic polonium-type structure, which is known as a high-pressure modification of arsenic. Therefore, five electrons per atom are needed for this structure. Remarkably, polonium itself has one electron per atom too many for its structure. 

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