Calcium Debye temperature

The free energy due to harmonic lattice vibrations (or equivalently the Debye temperature) is approximately the same for bcc, fee, and hep structures but with a significant tendency for the bcc value to be a few percent lower. The more open bcc structure has a transverse phonon mode with a particularly low frequency which causes a more rapid decrease in the free energy with temperature. On cooling, sodium and lithium transform partially from bcc to hep at very low temperatures (0.1-0.2 Tm). Calcium, strontium, beryllium, and thallium transform to a bcc phase at high temperatures (0.66-0.98 Tm) when there is a considerable anharmonic contribution to the free energy. 

For calcium carbonate, which has 15 degrees of freedom, six will be acoustic vibrations with two Debye temperatures and nine degrees of freedom will be optical vibrations, with an Einstein temperature for each one. The partition function (and the resulting internal energy) contains two Debyean terms and nine Einsteinian terms. 

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