Metastable Transitions II-V-V in NH_4No_3.I.Symmetry aspect and X-Ray Study at High Pressure

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Crystallographical and group-theoretical analysis of the metastable phase transilions II-V'-V in NH.N0, is performed. The phase transitions are explained as a se-quence 71):.(II)-71)fi,;(V')-D3:(V). The order parameter of the D';.-+D'.) phase transi-tion transforms as the irreducible representation r.-X-r7 at the A#-point of theBrillouin-zone boundary. The representation responsible for the transition D'.)-+D'.')is r. at the Z-point. Itfollows that II=V' is driven by an ordering of N0j Tons andV'=V is driven by an ordering of NH." Tons. With X-ray diffraction method, it is alsofound that the occurrence of the metastable transitions is facilitated by pressure. Thetemperature dependence of the intensity of superlattice reflection shows that the Iran-sition II-eV' is of second order type and the critical temperature is 5 1 'C at 0. 15 GPa.
社団法人日本物理学会の論文
1989-05-15