Anomalous Thermal Expansion Behavior Research Articles

Nuclear Magnetic Resonance, Magnetic Susceptibility, and Lattice Constants of Solid and Liquid AuGa2,Au0.95Pd0.05Ga2, and AuIn2

We report measurements of the magnetic susceptibility, Knight shift, and nuclear spin-lattice relaxation rates in the pure compounds Au${\mathrm{Ga}}_{2}$ and Au${\mathrm{In}}_{2}$ and an alloy of nominal composition ${\mathrm{Au}}_{0.95}$${\mathrm{Pd}}_{0.05}$${\mathrm{Ga}}_{2}$. Measurements were made in both the solid and liquid states over a temperature range from 4.2 to 1100 K. Lattice constants were measured from 4.2 to 300 K for Au${\mathrm{Al}}_{2}$, Au${\mathrm{In}}_{2}$, and ${\mathrm{Au}}_{0.95}$${\mathrm{Pd}}_{0.05}$${\mathrm{Ga}}_{2}$ and to 715 K for Au${\mathrm{Ga}}_{2}$. The susceptibilities, Ga Knight shifts, and magnetic relaxation rates of Au${\mathrm{Ga}}_{2}$ and ${\mathrm{Au}}_{0.95}$${\mathrm{Pd}}_{0.05}$${\mathrm{Ga}}_{2}$ exhibit complex temperature dependences which, with the exception of the Knight shift, continue to the vicinity of the melting point. The Knight shifts become constant above 400 K. At low temperatures the Knight shifts and relaxation rates in ${\mathrm{Au}}_{0.95}$${\mathrm{Pd}}_{0.05}$${\mathrm{Ga}}_{2}$ are strongly enhanced relative to Au${\mathrm{Ga}}_{2}$, but the alloy data converge to those of the pure compound as the temperature increases. The changes in magnetic properties at the melting point are small compared with their temperature-dependent variations in the solid state. Lattice-constant data for Au${\mathrm{Ga}}_{2}$ and Au${\mathrm{In}}_{2}$ suggest anomalous thermal-expansion behavior for these compounds below 100 K. The magnetic properties of Au${\mathrm{In}}_{2}$ exhibit comparatively weak temperature dependence over the entire temperature range covered. We propose a phenomenological explanation for these results in terms of temperature-dependent modification of the electronic structure toward an essentially free-electron situation in the solid near the melting point.

On the Orientational Order-Disorder Transition in Crystalline Aniline Hydrobromide

Abstract Measurements of the heat capacity (−80∼+60°C) and the thermal expansion (−40∼+60°C) of aniline hydrobromide were carried out covering the transition temperature which has been observed recently by X-ray methods. The observed heat capacity versus temperature curve was of the λ-shape, similar to that of the order-disorder phase transition of binary alloy systems. The following results were obtained ; Tc (the temperature corresponding to the heat capacity peak) 22°C, ΔH (heat of transition, −70∼+35°C) ±10cal./mol., ΔS (entropy of transition, the same temperature interval) 1.10±0.05 e. u., ΔCp (drop of heat capacity at Tc)∼9 cal./mol. deg. Also the anomalous behavior of thermal expansion was observed in the vicinity of the critical temperature. These observations showed that the transition was of higher order nature. Based on the lits of knowledge of the crystal structures above and below the critical temperature, the author has discussed the main feature of the phase transition from the sandpoint of the orientational order-disorder of the −NH3+ ionic group and obtained fairly satisfactory results.