RELATIONSHIP BETWEEN THE ELASTIC PROPERTIES AND THE ORDER-PARAMETER OF THE SPIN-DENSITY-WAVE PHASE OF (TMTSF)2PF6

We present elastic constant data of the material (TMTSF)2PF6, with par ticular emphasis on the region about the phase transition, and at low temperatures. Specific heat, resistivity, and NMR measurements have in dicated a very sharp transition at T(SDW) = 12.1K. Similarly sharp fea tures have been observed in the temperature dependence of the Young's Modulus, followed by a continual stiffening below the transition. Comp arison to recent NMR results indicate that the modulus varies as the s quare of the internal magnetic field below T(SDW), thus constraining t he form of the coupling of the order parameter to the strain field. Th ere is no evidence for anomalies in the modulus below 4K, where it has been suggested that a transition to a commensurate state occurs.