Critical Behavior of Adiabatic and Isothermal Elastic Compliances of NaNO2

The adiabatic elastic compliances, s i i S ( i =1, 2, 3), of NaNO 2 were measured near T N (165.3°C) and T c (164.0°C) by a composite-oscillator method. The isothermal compliances, s i i T , were calculated from s i i S and the data on specific heat at constant pressure, C p , and thermal expansion coefficients, β i . The isothermal compliances tend to diverge toward T N , while the adiabatic compliances exhibit only small cusps at this temperature. It was found that the Pippard equations hold among s i i T , β i and C p , and the anomalous part of s i i S is proportional to - T / C p . From a thermodynamic relation between elastic compliances and specific heat, it is expected that, incontrast to a large λ-like anomaly of C p , the specific heat at constant volume shows only a small cusp-like anomaly at T N . This means that the growth of the long-range and short-range order is dramatically suppressed if the free thermal expansion of the crystal is inhibited.