Phonon Raman spectra of polycrystalline (La1-xSrx)2NiO4 have been investigated at 500, 300, 77, and 10 K in the Sr-concentration region less than 0.6. We have observed two peaks at about 220 cm-1 (R1) and at about 460 cm-1 (R2). They are assigned to an apical-oxygen motion along the a axis with Eg symmetry and an apical-oxygen motion along the c axis with A1g symmetry, respectively. With increasing x at room temperature, the energy of R1 is constant for 0<x≤0.3, decreases by about 10 cm-1 for 0.3<x<0.4, and is constant for x0.4, while that of R2 is almost constant for x≤0.3 and increases for x>0.3. Similar concentration dependences have been observed at 500, 77, and 10 K. To clarify the concentration dependence of the observed phonon energies, we have calculated the energies of normal modes at a k=0 point by the GF-matrix method. In the calculation, six kinds of longitudinal force constants have been taken. The best fitting has been made for the data at room temperature. The concentration dependence of the observed Raman modes can be mainly explained by the change of the corresponding force constants; for R1 a force constant due to the La-O bond in the La-O layers and for R2 the sum of force constants due to the Ni-O and La-O bonds along the c axis, respectively. As a comparison, we have measured Raman spectra of (La1-xSrx)2CuO4 (x≤0.2). We have found that the mode at about 230 cm-1, which corresponds to the R1 mode in La2NiO4, decreases by 6 cm-1 for x=0.2. It can be concluded that the doping of divalent ions decreases the bond stiffness in the La-O block layers for the La-based copper oxide superconductor.
ASJC Scopus subject areas
- Condensed Matter Physics