Even though the Earth's Inner core is hotter than the liquid outer core, it is still solld because -

The inner core is solid because it is made of very dense, or heavy, materials - like iron and nickel. Even though it is very hot, these materials don't "melt" very easily, so they stay solid. Answer 3: It turns out that many materials can be a solid at a higher temperature if the pressure is also higher.

Explanation: