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The pressure-induced structural evolution of formamidinium-based perovskite FAPbI 3 was investigated using in situ synchrotron X-ray diffraction and laser-excited photoluminescence methods. Cubic α-FAPbI 3 ( Pm3̅ m) partially and irreversibly transformed to hexagonal δ-FAPbI 3 ( P6 3 mc) at a pressure less than 0.1 GPa. Structural transitions of α-FAPbI 3 followed the sequence of Pm3̅ m → P4/ mbm → Im3̅ → partial amorphous during compression to 6.59 GPa, whereas the δ-phase converted to an orthorhombic Cmc2 1 structure between 1.26 and 1.73 GPa. During decompression, FAPbI 3 recovered the P6 3 mc structure of the δ-phase as a minor component (∼18 wt %) from 2.41-1.40 GPa and the Pm3̅ m structure of the α-phase becomes dominant (∼82 wt %) at 0.10 GPa but with an increased fraction of δ-FAPbI 3 . The photoluminescence behaviors from both the α- and δ-forms were likely controlled by radiative recombination at the defect levels rather than band-edge emission during pressure cycling. FAPbI 3 polymorphism is exquisitely sensitive to pressure. While modest pressures can engineer FAPbI 3 -based photovoltaic devices, irreversible δ-phase crystallization may be a limiting factor and should be taken into account.

In the PDF version of this article, Eq. 5 is missing all elements after the equals sign. The correct version of Eq. 5 is given below. The HTML version of the paper was correct from the time of publication.[Formula: see text].

CsPbBr 3 is a direct-gap semiconductor where optical absorption takes place across the fundamental bandgap, but this all-inorganic halide perovskite typically exhibits above-bandgap emission when excited over an energy level, lying above the conduction-band minimum. We probe this bandgap anomaly using wavelength-dependent multiphoton absorption spectroscopy and find that the fundamental gap is strictly two-photon forbidden, rendering it three-photon absorption (3PA) active. Instead, two-photon absorption (2PA) commences when the two-photon energy is resonant with the optical gap, associated with the level causing the anomaly. We determine absolute nonlinear optical dispersion over this 3PA-2PA region, which can be explained by two-band models in terms of the optical gap. The polarization dependence of 3PA and 2PA is also measured and explained by the relevant selection rules. CsPbBr 3 is highly luminescent under multiphoton absorption at room temperature with marked polarization and wavelength dependence at the 3PA-2PA crossover and therefore has potential for nonlinear optical applications.

Reducing the dimensionality of three-dimensional hybrid metal halide perovskites can improve their optoelectronic properties. Here, we show that the third-order optical nonlinearity, n 2 , of hybrid lead iodide perovskites is enhanced in the two-dimensional Ruddlesden-Popper series, (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) n-1 Pb n I 3n+1 (n = 1-4), where the layer number (n) is engineered for bandgap tuning from E g  = 1.60 eV (n = ∞; bulk) to 2.40 eV (n = 1). Despite the unfavorable relation, [Formula: see text], strong quantum confinement causes these two-dimensional perovskites to exhibit four times stronger third harmonic generation at mid-infrared when compared with the three-dimensional counterpart, (CH 3 NH 3 )PbI 3 . Surprisingly, however, the impact of dimensional reduction on two-photon absorption, which is the Kramers-Kronig conjugate of n 2 , is rather insignificant as demonstrated by broadband two-photon spectroscopy. The concomitant increase of bandgap and optical nonlinearity is truly remarkable in these novel perovskites, where the former increases the laser-induced damage threshold for high-power nonlinear optical applications.Hybrid metal halide perovskites can exhibit improved optoelectronic properties when their dimensionality is reduced. Here, Saouma et al. study the enhancement of third-order nonlinearities in two-dimensional lead iodide perovskites in the Ruddlesden-Popper series.

Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic-scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2 FeMoO6 at unparalleled low temperatures of 150-300 (o) C. The irreversible ionic motion at ambient pressure, as evidenced by real-time powder synchrotron X-ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3 TeO6 -type to an ordered-ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Mn(2+)2 Fe(2+)W(6+)O6 , a new polar magnetic phase, adopts the corundum-derived Ni3TeO6 -type structure with large spontaneous polarization (PS) of 67.8 μC cm(-2), complex antiferromagnetic order below ≈75 K, and field-induced first-order transition to a ferrimagnetic phase below ≈30 K. First-principles calculations predict a ferrimagnetic (udu) ground state, optimal switching path along the c-axis, and transition to a lower energy udu-udd magnetic double cell., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

It is established that the multiferroics RMn(2)O(5) crystallize in the centrosymmetric Pbam space group and that the magnetically induced electric polarization appearing at low temperature is accompanied by a symmetry breaking. However, both our present x-ray study-performed on compounds with R=Pr,Nd,Gd,Tb, and Dy-and first-principles calculations unambiguously rule out this picture. Based on structural refinements, geometry optimization, and physical arguments, we demonstrate in this Letter that the actual space group is likely to be Pm. This turns out to be of crucial importance for RMn(2)O(5) multiferroics since Pm is not centrosymmetric. Ferroelectricity is thus already present at room temperature, and its enhancement at low temperature is a spin-enhanced process. This result is also supported by direct observation of optical second harmonic generation. This fundamental result calls into question the actual theoretical approaches that describe the magnetoelectric coupling in this multiferroic family.

Above-room-temperature polar magnets are of interest due to their practical applications in spintronics. Here we present a strategy to design high-temperature polar magnetic oxides in the corundum-derived A2BB'O6 family, exemplified by the non-centrosymmetric (R3) Ni3TeO6-type Mn(2+)2Fe(3+)Mo(5+)O6, which shows strong ferrimagnetic ordering with TC = 337 K and demonstrates structural polarization without any ions with (n-1)d(10)ns(0), d(0), or stereoactive lone-pair electrons. Density functional theory calculations confirm the experimental results and suggest that the energy of the magnetically ordered structure, based on the Ni3TeO6 prototype, is significantly lower than that of any related structure, and accounts for the spontaneous polarization (68 μC cm(-2)) and non-centrosymmetry confirmed directly by second harmonic generation. These results motivate new directions in the search for practical magnetoelectric/multiferroic materials., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)