Second Harmonic Generation in the Polar A′SmNb₂O₇ (A′ = Rb, Cs) Dion–Jacobson Layered Perovskites

Abstract

Members belonging to the Dion−Jacobson (DJ) family of layered perovskites have time and again proven to be the preferred materials of choice for the exploration and identification of polar structures. We report two noncentrosymmetric n = 2 DJ members, A′SmNb 2 O 7 (A′ = Rb, Cs), prepared by hightemperature solid-state reactions, thus expanding the polar n = 2, DJ series as A′ANb 2 O 7 (A′ = Rb, Cs; A = La, Pr, Nd, Sm, Bi). Optical second harmonic generation measurements utilizing 1064 nm substantiated the existence of Type I phase matchable compounds, and the efficiencies were 4 to 6 times higher than the standard KDP. Structural refinements by the Rietveld method confirm the polar orthorhombic space groups of I2cm and P2 1 am, respectively, for RbSmNb 2 O 7 [a = 5.4368(3) Å, b = 5.3944(3) Å, c = 21.9051(1) Å] and CsSmNb 2 O 7 [a = 5.4669(5) Å, b = 5.4158(4) Å, c = 11.1437(1) Å]. The orthorhombic distortions and symmetry lowering were further supported by the Raman spectroscopy measurements. The difference in the orthorhombic a and b parameters (∼0.05 Å) possibly originates from the chemical pressure induced by the smaller ionic size of Sm 3+ (1.24 Å, XII) as compared to other Ln 3+ (La, Pr, Nd) ions. Ferroelectric switching of A′SmNb 2 O 7 (A′ = Rb, Cs) oxides is demonstrated by the polarization hysteresis loops observed at room temperature with remanent polarization values of 5.3 and 2.0 μC/cm 2 corresponding to the Rb and Cs analogues. Variable-temperature powder X-ray diffraction measurements confirmed the phase transition from polar to anti-polar transitions around temperatures ∼1273 and ∼1173 K, respectively, for the Rb and Cs analogues. Structural analysis results in the identification of prototype tetragonal (a ∼ 3.88 Å, c ∼ 11.20 Å) centrosymmetric structures relating the transition temperatures to potential Curie temperatures. The transitions coincide with the observed Curie point of 1113 K for CsSmNb 2 O 7 , from the temperature-dependent relative permittivity measurements, and a much higher temperature is expected for the Rb analogue.

Three-dimensional porous borocarbonitride composed of pentagonal motifs as a high-performance pyroelectric material

“…For instance, the paraelectric-to-ferroelectric phase transition in perovskite oxides reduces the crystal symmetry from centrosymmetric cubic to polar tetragonal. 1 Such symmetry breaking leads to the emergence of many physical effects, including piezoelectricity, 2 pyroelectricity, 3 second harmonic generation (SHG), 4 and bulk photovoltaic effect (BPVE), 5 which are forbidden in pristine non-polar materials. In addition, polar symmetry can also be achieved by strain gradient, 6 doping processes, 7 electric field, 8 and layer stacking, 9 etc.…”