Heat capacities and entropies of Mg2SiOa, Mn2SiOa, and Co2SiOa between 5 and 380 K

The heat capacities of synthetic single crystats of Mg2Sio4 (forsterite), Mn2Sioa (tephroite), and co2Sioa (cobalt olivine), were measured between 5 and 3g0 K using an adiabatically shieldedc alorimeter. Mg2SiOais diamagnetic,a nd its heat capacity follows a normal sigmoidal curve at low temperatures. co2sioa shows a single sharp )r-type
transition at 49.85+0102 K associated with the antiferromagnetic ordering of the magnetic moments of the Co2+ ions into a collinear spin arrangement below 49.8 K. In contrast to co2sioa, Mn2Sioah ast wo transitionsi n ci, a sharpL -typet ransitiona t 47.3g10.05K and a smaller "shoulder" in Ci centered near 12 K. The upper transition corresponds to the paramagnetic (disordered) to collinear antiferromagnetic ordering of the Mn2+ moments,
whereas the shoulder near 12 K corresponds to the change from the collinear to a canted spin structure. Our calorimetric values for the antiferromagnetic-paramagnetictr ansition temperature (N6el Temperature) are in excellent agreement with those obtained by powder magnetic susceptibility measurements, 49t2 K and 5015 K for co2Sioa and Mn2Sioa respectively.

The thermal Debye temperature, 0$, of Mg2Sioa calculated from our c$ measurements between 6.3 and 13.8 K is 768+15 K and agrees well with the elastic vaiue ofi of 758 K based on the mean sound velocity calculated from the room temperature elastic stiffness constants (ci:) of Graham and Barsch.

At 298.15K (25"c) the molar heat capacitiesa re 118.6,1 28.7, and 133.4J /(mol . K) and the molar entropiesa re 94.11-r0.101, 55.910.4a nd,1 42.6-+0.J2l (mol . K) respectivelyf or Mg2SiO4M, n2SiOaa, nd Co2SiOa.