X-RAY-DIFFRACTION AND INFRARED INVESTIGATION OF RBA(2)CU(3)O(7) AND R(0.5)PR(0.5)BA(2)CU(3)O(7) COMPOUNDS (R=Y AND LANTHANIDES)

RBa(2)Cu(3)O(7) and [R(0.05)Pr(0.5)]Ba2Cu3O7 compounds with R=Y and la nthanides have been prepared and studied at ambient temperature by mea ns of XRD and infrared-reflectance spectroscopy. In Pr free compounds, increasing the Shannon radius of R, r(R), leaves the puckering of the Cu(2)-O(2, 3) sheet unaffected. In the Pr containing compounds, the P r-O coordination polyhedron is typical of R(+3) ions in eight-fold coo rdination and argues against differentiating Pr in terms of valence or hybridization. Instead, a repositioning of Ba within the cuprate bloc k is observed, leading to the flattening of the sheet and the anomalou sly high frequency of the Ba and Cu(7)-O(2, 3) B-1u modes. In R-Pr com pounds the c-axis and the interplanar distances related to the positio n of Pa and O(2, 3) are found to exhibit a sigmoidal dependence on r(R ). It is argued that this sigmoidal dependence is related to the two-m ode behavior exhibited by several phonon modes of these compounds.