Competing Polar and Antipolar Structures in the Ruddlesden-Popper Layered Perovskite Li2SrNb2O7.

Over the past few years, several studies have reported the existence of polar phases in n = 2 Ruddlesden-Popper layer perovskites by trilinear coupling of oxygen octahedral rotations (OOR) and polar distortions, a phenomenon termed as hybrid improper ferroelectricity. This phenomenon has opened an avenue to expand the available compositions of ferroelectric and piezoelectric layered oxides. In this study, we report a new polar n = 2 Ruddlesden-Popper layered niobate, Li2SrNb2O7, which undergoes a structural transformation to an antipolar phase when cooled to 90 K. This structural transition results from a change in the phase of rotation of the octahedral layers within the perovskite slabs across the interlayers. First-principles calculations predicted that the antipolar Pnam phase would compete with the polar A 2 1 a m phase and that both would be energetically lower than the previously assigned centrosymmetric Amam phase. This phase transition was experimentally observed by a combination of synchrotron X-ray diffraction, powder neutron diffraction, and electrical and nonlinear optical characterization techniques. The competition between symmetry breaking to yield polar layer perovskites and hybrid improper antiferroelectrics provides new insight into the rational design of antiferroelectric materials that can have applications as electrostatic capacitors for energy storage.

Random anion distribution in MS x Se2-x (M = Mo, W) crystals and nanosheets.

The group VIb dichalcogenides (MX2, M = Mo, W; X= S, Se) have a layered molybdenite structure in which M atoms are coordinated by a trigonal prism of X atoms. Ternary solid solutions of MS x Se2-x were synthesized, microcrystals were grown by chemical vapor transport, and their morphologies and structures were characterized by using synchrotron X-ray diffraction, Rietveld refinement, DIFFaX simulation of structural disorder, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Double aberration corrected scanning transmission electron microscopy was used to determine the anion distributions in single-layer nanosheets exfoliated from the microcrystals. These experiments indicate that the size difference between S and Se atoms does not result in phase separation, consistent with earlier studies of MX2 monolayer sheets grown by chemical vapor deposition. However, stacking faults occur in microcrystals along the layering axis, particularly in sulfur-rich compositions of MS x Se2-x solid solutions.

Reinvestigation of electric field-induced optical activity in α-quartz: application of a polarimeter with four photoelastic modulators.

Linear electrogyration (electric field-induced optical activity) and electro-optic effects in x-cut and z-cut right-handed (RH) α-quartz were measured using a complete Mueller matrix polarimeter. The polarimeter used in the analysis was equipped with four photoelastic modulators operating at different frequencies. This configuration is especially sensitive due to the fidelity of the modulators and the fact that all the Mueller matrix elements can be delivered without any moving optical elements. The linear electrogyration coefficient γ11 as a function of the wavelength of incident light was remeasured. The coefficient γ33 , required by symmetry to be zero, was evaluated as a control. γ11 was much smaller than values obtained previously using devices dependent on mechanical light modulation. Electrogyration measurements have often been confounded by the much larger linear electro-optic effect. The Mueller calculus used herein is well suited to the separation of the competing changes to the polarization state of light. Quadratic electrogyration associated with elements ß(11) and ß(33) was not detectable.