Local hardening of Raman phonons in Ba_xSr_1-xTiO₃ thin films deposited by r.f. sputtering

Thin films of nanometric order of a solid solution of barium-strontium titanate Ba_xSr_1-xTiO₃ (BSTO), in the entire 0 ≤ x ≤ 1 range, were deposited at 495 C on quartz by RF reactive co-sputtering. The composition was controlled through the RF power (P) applied to the targets. X ray diffractograms (XRD) reveal that all the samples show the cubic perovskite crystalline structure. The percentage of elements (x) in the films were evaluated by usingof electron dispersion spectroscopy, also through the change of the (110) lattice interplanar spacing determination of BSTO by means of XRD patterns. Experimental data points of concentration of Ba (x) and Sr (1-x) as a function of P describe a symmetric sigmoidal curve. The average crystal size for the whole composition is around 20 nm as estimated from XRD data. Raman measurements in the 100-900 cm^-1 range show that optic phonons experience local hardening in the x interval 0.6-0.8. Around this region of x it is, in general, expected in BSTO a crystalline cubic-tetragonal phase transition when is prepared at low substrates temperatures. Distortion of the unit cell is observed in that interval, which probably originates the local hardening of phonons.