J.M. Igartua

High-pressure Raman study of the Sr2CaWO6 double perovskite

An in situ Raman spectroscopic study was conducted to explore the pressure-induced phase transformation of Sr2CaWO6 to pressures of 50.8 GPa and room temperature. Group theory yields 24 Raman active modes for Sr2CaWO6, of which 17 are observed at ambient conditions. Upon elevation of pressure to 3.2 GPa, the results suggest some structural changes, possibly a phase transition. At 10.4 GPa, new modes at high frequencies (824 and 903 cm−1) come into existence. As the material is compressed further, the intensities of the bands, especially the W–O stretching modes, observed at 824 and 903 cm−1 become stronger, while those centred at 863 and 937 cm−1 (at 10.4 GPa) decrease significantly, until disappearance. These changes in the spectrum indicate clearly that Sr2CaWO6 undergoes a phase transition at 9.7 ± 0.7 GPa. Upon release of pressure to ambient conditions, a new phase is observed with ten bands compared to the 17 observed in the original phase.

Structural study of SrPrZnRuO6, SrPrCoRuO6, SrPrMgRuO6 and SrPrNiRuO6 double perovskite oxides by symmetry-adapted mode analysis

Powder SrPrZnRuO6, SrPrCoRuO6, SrPrMgRuO6 and SrPrNiRuO6 double perovskites were synthesized by the solid-state reaction method, and their crystal structure was investigated by Rietveld analysis (using the symmetry-mode procedure) by x-ray, synchrotron and neutron powder diffraction data. SrPrMRuO6 materials are monoclinic perovskites at room temperature, adopting the space group P21/n, [Formula: see text], c ≈ 2ap, Z = 2. The unit cell parameters increase through the series as the B-cation size increases. The tilting of the octahedra is associated with irreducible representations GM(+)4 and X(+)3. In the studied series, Sr(2+)/Pr(3+) are in eight-fold coordination and are displaced from the center of the Sr/PrO8 polyhedron along [010] by the X(+)5(A10) mode. The size of the first coordination sphere of Sr/Pr increases and the second coordination sphere reduces with temperature, suggesting an actual change in coordination. While changing the interatomic distances, the distortion of the structure diminishes, as observed in distortion mode amplitudes and octahedral tilt angles. The temperature driven [Formula: see text] phase transition was observed for Ni and Mg materials at ≈1025 K and 950 K, respectively. Tilting of the octahedra in the trigonal phase is associated with GM(+)4, which is the unique active mode associated with the not experimentally observed [Formula: see text] phase transition.

Symmetry‐mode analysis of the phase transitions in SrLaZnRuO6 and SrLaMgRuO6 ordered double perovskites

The crystal structures of double perovskites SrLaMRuO6 (M = Zn and Mg) have been studied using X-ray and neutron diffraction data. Both compounds have monoclinic structure at room temperature with space group P21/n. A first-order structural phase transition P21/n → R{\overline 3} has been observed in SrLaZnRuO6 and SrLaMgRuO6, at about 800 and 500 K, respectively, with the two phases coexisting in a temperature interval of 150 K. The temperature- and tolerance-factor-dependent phase diagram of the SrLnMRuO6 (Ln = La, Nd and Pr; M = Zn, Co, Cu, Mg and Ni) family shows two different phase transition sequences depending on the A-site cation. The structures of the title compounds at room and high temperatures have been analyzed using the symmetry-mode decomposition approach.