The inner core is the very center of the Earth, and the hottest part of the planet. It is a mainly a solid ball with a radius of about 1,220 km (760 mi), according to seismological studies.
The INNER CORE is made up of solid iron and nickel and has a radius of 1300 kilometers .its temperature reaches to about 5000°C.The extreme temperature could have molten the iron and nickel but it is believed to have solidified as a result of pressure freezing, which is common to liquids subjected under tremendous pressure.
Inner core is the innermost layer of Earth. It is composed of iron-nickel alloy and some light elements. It is once discovered by Inge Lehmann in 1929. And it is also more than 5,000 kilometers below the Earth's surface. Inner core is mostly solid.