Phase transitions at low oxygen concentration in YBa2Cu3Ox

A micro-Raman systematic spectroscopic study at room temperature has been c arried out on YBa2Cu3Ox polycrystalline compounds in the whole range from t he antiferromagnetic insulating to the metallic state (6<x<7). The evolutio n of the spectral characteristics mainly in the y(cc)(y) over bar and the y (xx)(y) over bar scattering configurations was examined on several microcry stallites of each sample as a function of the oxygen content. The O-apex ph onon appears sensitive to the local chain oxygen environment as modified wi th the oxygen content. The behavior of the O-pl in-phase A(g) symmetry phon on in the range 6.00<x<6.40 can be described by steplike changes correspond ing to chain superstructure phases, usually separated by miscibility gaps, as has been previously done for 6.40<x<7.00. In analogy to the martensitic phase transformation observed in the underdoped-overdoped separation line, x congruent to6.95, from the sudden frequency shift of the O-pl in-phase ph onon, there appears a phase transition in the range 6.10<x<6.15, the existe nce of which is additionally supported by the behavior of other phonons.