Ruddlesden-Popper phases Sr3Ni2–xAlxO7–δ and some doped derivatives: Synthesis, oxygen nonstoichiometry and electrical properties

The Ruddlesden-Popper phases Sr3Ni2–xAlxO7–δ (0.5 < х ≤ 0.75) were synthesized in the system Sr–Al–Ni–O for the first time. They show 2P/RS structure, in which two perovskite layers (P) are stacked in between rock-salt layers (RS). The composition Sr3Ni2–xAlxO7–δ with x = 0.5 was stabilized by partial substitution of Sr2+ for Ba2+ to give Sr2.8Ba0.2Ni1.5Al0.5O7–δ. This substitution allowed to reduce the synthesis duration and to lower its temperature. The study of specific electrical resistivity of Sr3Ni2–xAlxO7–δ (0.5 < х ≤ 0.75) and Sr2.8Ba0.2Ni1.5Al0.5O7–δ (x = 0.5) showed that it decreased with decreasing aluminum content. The specific resistivity of yttrium-modified phases Sr2.8–yYyBa0.2Ni1.5Al0.5O7–δ was found to decrease with increasing y and reaches a minimum value for у = 0.1. Oxygen nonstoichiometry of limiting representatives in single phase range 0.5 < x ≤ 0.75 was estimated by iodometric titration at 20 °C, which gave the oxygen deficiency δ of 1.02 (x = 0.75) and 0.78 (x = 0.5), showing the decrease of δ with decreasing aluminum content. The temperature range of semiconductor-to-metal transition (350–450 °C) was determined by thermal analysis and electrical measurements. All synthesized compounds were analyzed by X-ray powder diffraction for identification of crystalline phases.