How can high pressure make potassium, rubidium, and cesium behave more like transition metals?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — alkali metal Modern version of the periodic table of the elements (printable).

When potassium, rubidium, and cesium are subjected to extreme pressure—reaching up to one-half million atmospheres or more—they undergo a series of phase transitions. During these transitions, the occupation of a d-type orbital becomes energetically preferred over the s-orbital that typically holds their valence electron. This shift in electronic configuration causes these alkali metals to exhibit properties characteristic of transition metals, such as the ability to form alloys with metals like iron, which does not occur under standard atmospheric conditions.1