In Aztec mythology, what god sacrificed himself to become the fifth sun?

In Aztec mythology, Nanahuatzin sacrificed himself in fire and became the fifth sun.

The story belongs to the creation legend of the Five Suns, in which the universe passes through successive ages of creation and destruction. The gods gather at Teotihuacan and decide that someone must leap into a great fire to become the new sun. Two candidates are chosen: the wealthy and proud Tecuciztecatl and Nanahuatzin, a humble god associated with illness and sores.

Tecuciztecatl is expected to go first, but he repeatedly loses his courage. Nanahuatzin then steps forward and leaps into the flames without hesitation. Tecuciztecatl follows, and two bright bodies appear in the sky. The gods reduce Tecuciztecatl’s brilliance by throwing a rabbit at him, transforming him into the moon; Nanahuatzin remains the sun.

This is why Tonatiuh can cause confusion: Tonatiuh is the name of the sun deity or solar manifestation in some traditions, not the god who performs the sacrifice in this foundational episode. Huitzilopochtli is also associated with the sun, but he is not the sacrificial figure in this particular myth.