Lower mantle phase transitions may generally have negative pressure‐temperature slopes

Phase transitions in oxides at pressures above about 150 kbar may commonly have negative pressure‐temperature slopes. The causes of the higher entropies of denser phases include low frequency vibrations of small atoms in sites having high coordination number and long metal‐oxygen distances, decrease in directional bonding, positional disorder and possible solid‐electrolyte behavior, and semiconductor‐metal and metal‐metal transitions.