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′SmNb2O7 (A′ = Rb, Cs), prepared by high-temperature solid-state reactions, thus expanding the polar n = 2, DJ series as A′ANb2O7 (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 P21am, respectively, for RbSmNb2O7 [a = 5.4368(3) Å, b = 5.3944(3) Å, c = 21.9051(1) Å] and CsSmNb2O7 [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 Sm3+ (1.24 Å, XII) as compared to other Ln3+ (La, Pr, Nd) ions. Ferroelectric switching of A′SmNb2O7 (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/cm2 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 CsSmNb2O7, from the temperature-dependent relative permittivity measurements, and a much higher temperature is expected for the Rb analogue.