Legut, Friák, and Šob reply: Relativistic effects are important in solid-state physics and chemistry. However, in our experience, they are usually not strong enough to promote a phase transition to the simple cubic structure under pressure. The figure below shows the energy–volume curves of tellurium and polonium in their ground-state structures and of Te in the simple cubic structure, calculated with relativistic effects included (the energies are given relative to the ground-state energy E0). If we compare it with figure 2 in our paper, 1 in which the relativistic effects were ignored, we see that the transition pressure needed to transform the trigonal spiral to the simple cubic structure, proportional to the slope of the common tangent of both energy–volume curves, is about twice as high in the nonrelativistic Te as in the relativistic case. Also, the energy difference between the two structures of Te is considerably higher in the nonrelativistic Te. Hence, the relativistic effects somewhat facilitate that phase transition in Te but are not the driving force. In Po, the relativistic effects are strong enough to reverse the order of the two structures. 1  

Those considerations, however, are hypothetical since simple cubic Te has never been found experimentally. (For a recent review, see reference 2.) Although simple cubic may be considered a special case of the trigonal spiral structure, it is not found in either selenium or Te as the pressure increases. 2,3 That contradicts Pavel Karen’s penultimate statement.

Phosphorus, calcium, and arsenic exhibit the simple cubic structure under pressure, 2 but we doubt that relativistic effects are the reason. Since the atomic numbers of those elements are not high, we expect that both the relativistic and nonrelativistic total energy calculations will provide similar results and, almost certainly, the same order of structures. Furthermore, application of pressure to simple cubic Po induces a transformation to trigonal (rhombohedral) structure, 1 which also contradicts Karen. Therefore, most elements do not become simple cubic under pressure, and in those that do, the transformation is not governed by relativistic effects.

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D.
Legut
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M.
Friák
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M.
Šob
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Phys. Rev. Lett.
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M. I.
McMahon
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R. J.
Nelmes
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Chem. Soc. Rev.
35
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943
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2006
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3.
C.
Hejny
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S.
Falconi
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L. F.
Lundegaard
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M. I.
McMahon
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Phys. Rev. B
74
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174119
(
2006
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