this is due to the fact that diamond has perfect lattice structure which helps it to conduct heat but due to the absence of free electrons it cannot conduct electricity.

there are two modes to conduct heat :1) thraugh free electron transfer 2) thraugh lattice vibration .in diamond lattice vibration is dominant.

for further study read conduction heat tranfer .

Heat is generally transferred in solids with the conduction mode.In conduction mode,heat is trasferred by lattice vibration.

Litium can conduct energy in the following ways

Electron movement- electrical conductivity

Electron movement- thermal conductivity

Lattice vibration (phonons) - sound

Crystal lattice. lattice

In physics, the reciprocal lattice of a lattice (usually a Bravais lattice) is the lattice in which the Fourier Transform of the spatial function of the original lattice (or direct lattice) is represented. This space is also known as momentum space or less commonly k-space, due to the relationship between the Pontryagin momentum and position. The reciprocal lattice of a reciprocal lattice is the original or direct lattice.