Characterization of structural transitions and lattice dynamics of hybrid organic–inorganic perovskite CH3NH3PbI3

By combining temperature-dependent x-ray diffraction(XRD) with temperature-dependent Raman scattering, we have characterized the structural transitions and lattice dynamics of the hybrid organic–inorganic perovskite CH3NH3PbI3.The XRD measurements cover distinct phases between 15 K and 370 K and demonstrate a general positive thermal expansion.Clear anomalies are found around the transition temperatures.The temperature evolution of the lattice constants reveals that the transition at 160 K/330 K is of the first-/second-order type.Raman measurements uncover three strong lowfrequency modes, which can be ascribed to the vibration of the Pb/I atoms.The temperature evolution of the modes clearly catches these transitions at 160 K and 330 K, and confirms the transition types, which are exactly consistent with the XRD results.The present study may set an experimental basis to understand the high conversion efficiency in methylammonium lead iodide.

Pressure-induced anomalous insulating behavior in frustrated iridate La_(3)Ir_(3)O_(11)

The geometrically frustrated iridate La_(3)Ir_(3)O_(11) with strong spin–orbit coupling and fractional valence was recently predicted to be a quantum spin liquid candidate at ambient conditions. Here, we systematically investigate the evolution of structural and electronic properties of La_(3)Ir_(3)O_(11) under high pressure. Electrical transport measurements reveal an abnormal insulating behavior rather than metallization above a critical pressure P_(c) ~ 38.7 GPa. Synchrotron x-ray diffraction(XRD)experiments indicate the stability of the pristine cubic KSbO_(3)-type structure up to 73.1 GPa. Nevertheless, when the pressure gradually increases across P_(c), the bulk modulus gets enhanced and the pressure dependence of bond length d_(Ir-Ir) undergoes a slope change. Consistent with the XRD data, detailed analyses of Raman spectra reveal an abnormal redshift of Raman mode and a change of Raman intensity around P_(c). Our results demonstrate that the pressure-induced insulating behavior in La_(3)Ir_(3)O_(11) can be assigned to the structural modification, such as the distortion of IrO_6 octahedra. These findings will shed light on the emergent abnormal insulating behavior in other 5 d iridates reported recently.

Pressure-Induced Metallization Accompanied by Elongated S–S Dimer in Charge Transfer Insulator NiS2

The insulator-metal transition triggered by pressure in charge transfer insulator NiS2 is investigated by combining high-pressure electrical transport,synchrotron x-ray diffraction and Raman spectroscopy measurements up to40-50 GPa.Upon compression,we show that the metallization firstly appears in the low temperature region at^3.2 GPa and then extends to room temperature at^8.0 GPa.During the insulator-metal transition,the bond length of S-S dimer extracted from the synchrotron x-ray diffraction increases with pressure,which is supported by the observation of abnormal red-shift of the Raman modes between 3.2 and 7.1 GPa.Considering the decreasing bonding-antibonding splitting due to the expansion of S-S dimer,the charge gap between the S-ppπ* band and the upper Hubbard band of Ni-3 d eg state is remarkabl.y decreased.These results consistently indicate that the elongated S-S dimer plays a predominant role in the insulator-metal transition under high pressure,even though the p-d hybridization is enhanced simultaneously,in accordance with a scenario of charge-gap-controlled type.

Pressure-induced enhancement of optoelectronic properties in PtS2

PtS2, which is one of the group-10 transition metal dichalcogenides, attracts increasing attention due to its extraordinary properties under external modulations as predicted by theory, such as tunable bandgap and indirect-to-direct gap transition under strain; however, these properties have not been verified experimentally. Here we report the first experimental exploration of its optoelectronic properties under external pressure. We find that the photocurrent is weakly pressuredependent below 3 GPa but increases significantly in the pressure range of 3 GPa–4 GPa, with a maximum ～ 6 times higher than that at ambient pressure. X-ray diffraction data shows that no structural phase transition can be observed up to26.8 GPa, which indicates a stable lattice structure of PtS2 under high pressure. This is further supported by our Raman measurements with an observation of linear blue-shifts of the two Raman-active modes to 6.4 GPa. The pressure-enhanced photocurrent is related to the indirect-to-direct/quasi-direct bandgap transition under pressure, resembling the gap behavior under compression strain as predicted theoretically.

Possible Tricritical Behavior and Anomalous Lattice Softening in van der Waals Itinerant Ferromagnet Fe3GeTe2 under High Pressure

We present a high-pressure study of van der Waals ferromagnetic metal Fe3GeTe2 through electrical transport and Raman scattering measurements in diamond anvil cells at pressures up to 22.4 GPa.Upon compression,the ferromagnetic transition temperature Pc manifested by a kink in resistance curve decreases monotonically and becomes undiscernable around Pc=10 GPa,indicative of suppression of the itinerant ferromagnetism.Meanwhile,by fitting the low temperature resistance to the Fermi liquid behavior of R=R0+AT2,we found that R0 shows a cusp-like anomaly and the coefficient A diverges around Pc.These transport anomalies imply a tricritical point as commonly observed in itinerant ferromagnets under pressure.Unexpectedly,the Raman-active E2g and A1g modes soften remarkably after an initial weak hardening and the peak widths of both modes broaden evidently on approaching Pc,followed by complete disappearance of both modes above this critical pressure.A possible underlying mechanism for such anomalous lattice softening near pc is discussed.