Structural Stability and Anharmonicity of Pr₂Ti₂O₇: Raman Spectroscopic and XRD Studies

Abstract

Herein we report results of pressure- and temperature-dependent Raman scattering studies on PrTiO. Pressure-dependent studies performed up to 23 GPa suggest a reversible phase transition above 15 GPa with subtle changes. Temperature-dependent investigations performed in the range of 77-1073 K showed anomalous temperature dependence of some of the Raman modes. Temperature-dependent X-ray diffraction data indicated no structural transition but nonlinear expansion of unit-cell parameters with increasing temperature. With increasing temperature, the structure dilates anisotropically, and volume of coordination polyhedra around all the atoms expands. Also with increasing temperature the distortions in coordination polyhedra around all the atoms decrease, and appreciable decrease is observed in Pr(1)O and Pr(3)O units. The pressure evolution of Raman-mode frequencies was analyzed for both ambient as well as high-pressure phases, and mode Grüneisen parameters for ambient pressure phase were obtained. The temperature evolution of Raman-mode frequencies was analyzed to obtain the explicit and implicit anharmonic components, and it was found that some of the modes attributable to TiO octahedra and PrO polyhedra have dominating explicit anharmonic component. Comparison of the structural data with the temperature dependence of Raman modes suggests that the anomalous behavior in Raman modes is due to phonon-phonon interaction.

Pressure-Induced Structural Behavior of Orthorhombic Mn3(VO4)2: Raman Spectroscopic and X-ray Diffraction Investigations

“…Above 10 GPa, this weak mode becomes the highest-intensity mode in the Raman spectra. Similar intensity exchange behavior is seen in the Raman spectra of Pr 2 Ti 2 O 7 across the structural phase transition . Above 15 GPa, a new mode appears at around 875 cm –1 marked with an arrow in Figure b.…”

“…Similar intensity exchange behavior is seen in the Raman spectra of Pr 2 Ti 2 O 7 across the structural phase transition. 33 Above 15 GPa, a new mode appears at around 875 cm −1 marked with an arrow in Figure 4b. This appearance of a new stretching mode could be related to distortions in VO 4 tetrahedra under high pressure.…”

Dimethyl Carbonate Synthesis via Transesterification of Propylene Carbonate Using a Titanium–Praseodymium-Based Catalyst

“…The XRD study of the Ti 100– x Pr x O 2 -type catalysts shows the presence of fluorite, cubic pyrochlore, monoclinic perovskite, tetragonal, etc . In the XRD pattern of Ti x Pr 100– x O 2 (Figure S1a of the Supporting Information), the most intense peak can be found at a diffraction angle 2θ = 25.84°, indicating the presence of anatase (TiO 2 ), with the tetragonal structure corresponding to the plane (011), which is a metastable structure very much suitable for the conversion of DMC.…”

Thermal expansion and phase transformation in the rare earth di-titanate (R2Ti2O7) system

“…The larger rare earth elements, R = La, Pr, Nd and Ce, in R 2 Ti 2 O 7 (or R 2 O 3 Á2TiO 2 ) form the perovskite-type layered structure which is monoclinic, space group P2 1 (No. 4), Z = 4, at 25 C and 1 atm (Gasperin, 1975;Ishizawa et al, 1982Ishizawa et al, , 2013Kesari et al, 2016). La 2 Ti 2 O 7 undergoes a phase transition at high temperatures to an orthorhombic structure, space group Cmc2 1 (No.…”

“…Recently, the structure of this incommensurately modulated phase and the associated phase transitions were described in detail, and reported to be present from 716 C to 807 C (Ishizawa et al, 2019). Pr 2 Ti 2 O 7 , Nd 2 Ti 2 O 7 and La 2 Ti 2 O 7 form the P2 1 structure at 25 C and 1 atm, so some have assumed that Pr 2 Ti 2 O 7 and Nd 2 Ti 2 O 7 have a phase transition to Cmc2 1 like La 2 Ti 2 O 7 , however this phase transition has not been identified in Pr 2 Ti 2 O 7 between 25 C and 1200 C (Patwe et al, 2015;Kesari et al, 2016).…”