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The crystal structure of the high-pressure phase of cesium (IV) was determined to be a tetragonal lattice with Z = 4. The space group is D^i^^ "lA^^^/amd and the lattice parameters are a = 3.349 A and c = 12.487 A at 8.0 GPa. From the structure it is deduced that the atomic radius of cesium decreases dramatically at the III-IV transition, which suggests a discontinuous s~d electronic transition. PACS numbers: 61.55.Fe, 64.70.KbCesium is well known to show many polymorphic transitions under pressure. The crystal structure of Cs, whic is bcc at normal pressure, changes to fee (Cs II) at 2.3 GPa and to another fee (Cs III) at 4.2 GPa.^ The transition from Cs II to Cs III is a rare case of isomorphic transitions. It goes further to Cs IV at 4.3 GPa^»2 and Cs V at about 10 GPa.^*^ The characteristic behavior of Cs under pressure is interpreted in terms of s-d electronic transition. At normal pressure, the conduction band of Cs is almost of s character, but the lowest M band including the X^ state already touches the Fermi level. As pressure is increased, the M subbands near the X point go down relative to the 6s band near the T point. ^'^ As the X^ subband has the same symmetry as the 65 band, there occurs hybridization between the two bands. The movement of the X^ subband downwards in energy with pressure facilitates the electron transfer from the 6s band to the more localized ^d band, This causes the unusually soft bulk modulus in the low-pressure region (phases I and 11)."^ On the other hand, the isostructural II-III transition is related to the higher energy M states of X^ symmetry.® These states do not have the right symmetry to hybridize with the 6s band and therefore the X3 subband abruptly shifts down through the Fermi level with increasing pressure. Recent relativistic calculation of pressure isotherms of Cs shows that the isostructural transition does not occur at T = 0 and that the thermal effect plays an important role in the transition at room temperature.^ This calculation well accounts for the

Metallic iodine in the high-pressure phase above P t = 21 GPa is found to form a bodycentered orthorhombic Bravais lattice. Hence, pressure-induced molecular dissociation takes place at P t .

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