TEMPERATURE-DEPENDENT PHASE-TRANSITIONS OF SODIUM-RICH AMALGAMS

Starting from thermoanalytical measurements the temperature dependent phase transitions of the microcrystalline, sodium rich amalgams Na8Hg3 (= Na2.67Hg) and Na3Hg are investigated using a fast, position sensitive X-Ray detector in combination with a modified counter Guinier system. Near the melting point (64-degrees-C) Na8Hg3 shows two phase transitions. At 54-degrees-C alpha-Na8Hg3 forms a slightly distorted monoclinic defect variant of the cubic Li3Bi-structure (beta-Na8Hg3). At 62-degrees-C this modification transforms into the corresponding undistorted cubic phase (gamma-Na8Hg3). The increase of symmetry results mainly from the fact, that small deviations of the mercury atoms from the positions of an ideal close packing vanish due to strong thermal vibrations. The "literature-melting point" of Na3Hg (33-degrees-C) corresponds to a phase transition from alpha-Na3Hg (hexagonal, modified Na3As type) into beta-Na3Hg (monoclinic distorted Li3Bi type, corresponding to beta-Na8Hg3). The actual melting point of Na3Hg is 59-degrees-C.