Multianvil synthesis and nonlinear optical properties of high-pressure SrTeO₃

Starting from stoichiometric SrO/TeO₂ mixtures, the high-pressure/high-temperature synthesis in an 18/11 multianvil assembly at 8.8 GPa and 900 °C led to a new polymorph of SrTeO₃ (HP-SrTeO₃). Single-crystal and powder X-ray diffraction revealed the non-centrosymmetric space group Cc with cell parameters a = 891.22(6), b = 1195.14(7), c = 1320.8(2) pm, β = 108.20(1)°, supported by second harmonic generation (SHG) measurements with SHG intensities comparable to those of α-quartz. The crystal structure is highly similar to that of the recently discovered HP-BaTeO₃ having the centrosymmetric space group P2₁/c. Additional weak Te⋯O secondary contacts between adjacent [TeO₃]^2- groups distinguish HP-SrTeO₃ from HP-BaTeO₃. A detailed analysis of the differences in crystal structures, along with the associated loss of centrosymmetry is discussed. Temperature-dependent powder X-ray diffraction combined with thermal analysis measurements revealed phase transitions near 225 °C to an unknown phase, followed by transformation to the ambient-pressure, high-temperature polymorph δ-SrTeO₃ at 545 °C. UV-vis diffuse reflectance spectroscopy and Tauc plot analyses indicate a wide band gap with direct and indirect transitions of 4.2 eV and 4.0 eV, respectively. Bond-length/bond-strength and CHARDI analyses confirm chemically reasonable valence distributions and experimental IR and Raman spectra are discussed and compared to data calculated by using density functional theory, DFT.