Specific Heat of Gadolinium and Ytterbium Metals between 0.4 and 4°K

The heat capacities of gadolinium and ytterbium metals have been measured between 0.4 and 4^∘{}K in a He³ cryostat. For gadolinium, in qualitative agreement with earlier results, anomalous humps were observed in Cₚ at 1.1, 1.6, and 3.7^∘{}K. The ground state of the Gd³⁺ ion is ⁸S7/2 and consequently, due to zero orbital angular momentum of the $4f$ electrons, the nuclear specific heat is small. The observed Cₚ is much larger than one would expect for the metal alone; the excess entropy could be attributed to magnetic ordering of Gd³⁺ ions in the Gd₂{O}₃impurity (0.54% of oxygen in our sample). The specific heat of ytterbium, between 0.4 and 4^∘K, can be expressed accurately by{C}ₚ({mJ}{mole^∘{}K})=1.180{T}³+2.90T+0.012{T}^{{-}2}$. The lattice specific heat corresponds to a Debye ${θ}=118.1^∘K, which is considerably lower than the value{θ}{~}160^∘K observed for other higher rare earths. The electronic specific heat of ytterbium{C}E=2.90T$ is also much smaller than the usual value ${C}E{~}10T$. In the nonmagnetic ytterbium metal the $5d$ electron, normally in the conduction band, is added to the $4f$ shell which thereby becomes full. This explains the differences between the specific heat of ytterbium and of the other rare earths. The small term in ${C}ₚ$ proportional to ${T}^{{-}2}$, is probably caused by long-range exchange-type coupling between the electronic moments of rare-earth impurities in our ytterbium sample.