Fine control and direct monitoring of the spin crossover properties driven by pressure at room temperature are reported for the porous three-dimensional coordination polymer {Fe(pz)[Pt(CN) 4 ]} by using a homemade pressure cell that transforms a DC voltage into pressure. The pressure induced spin state switching is steadily driven through a piezoelectric ceramic element, which transforms the voltage 1−4 kV in pressures in the 0.001−0.035 GPa range. At the same time, the spin state is easily monitored through changes in the optical spectra of the title compound. The results demonstrate that {Fe(pz)[Pt(CN)4]} responds to as small pressure variations as 0.001 GPa (10 atm), thereby proving its efficacy to work as an effective pressure sensor.
Knowledge of the structure in amorphous dioxides is important in many fields of science and engineering. Here we report new experimental results of high-pressure polyamorphism in amorphous TiO 2 (a-TiO 2 ). Our data show that the Ti coordination number (CN) increases from 7.2 ± 0.3 at ∼16 GPa to 8.8 ± 0.3 at ∼70 GPa and finally reaches a plateau at 8.9 ± 0.3 at ≲86 GPa. The evolution of the structural changes under pressure is rationalized by the ratio (γ) of the ionic radius of Ti to that of O. It appears that the CN ≈ 9 plateau correlates with the two 9-fold coordinated polymorphs (cotunnite, Fe 2 P) with different γ values. This CN−γ relationship is compared with those of a-SiO 2 and a-GeO 2 , displaying remarkably consistent behavior between CN and γ. The unified CN−γ relationship may be generally used to predict the compression behavior of amorphous AO 2 compounds under extreme conditions.
Broadband self-trapped exciton (STE) emission in two-dimensional (2D) perovskites is attracting intense attention due to its promising solid-state lighting application. However, few studies have focused on the evolution of the...
Two types of experiments
conducted to investigate the effect of pressure on the spin crossover
(SCO) properties of the 2D Fe(II) coordination polymer formulated
{Fe[bipy(ttr)2]}
n
are reported,
namely, (1) magnetic measurements performed at variable temperature
and at fixed pressure and (2) visible spectroscopy at variable pressure
and fixed temperature. The magnetic experiments carried out under
a hydrostatic pressure constraint of 0.04, 0.08, and 0.8 GPa reveal
a two-step spin transition behavior. The characteristic critical temperatures
of the spin transition are shifted upward in temperature as pressure
increases. The slope of the straight-line of the T
c vs P plot, dT
c/dP, is 775 K/GPa and 300 K/GPa, for the
high temperature and the low temperature steps, respectively. These
values are remarkably large and denote the extreme sensitivity of
the material to the application of pressure. Indeed, the visible spectroscopic
measurements performed at 293 K show that a complete spin transition
is induced at pressures as low as 0.4 GPa. Moreover, the pressure-induced
spin transition is reversible and shows an asymmetric hysteresis.
An analysis of the cooperative interactions of the thermal- and pressure-induced
spin transition in the framework of the model of elastic interactions
reveals that the elastic energy of the lattice as well as the interaction
parameter between the SCO centers change during the course of the
spin transition. Consequently, the character of the spin transition
varies from abrupt for the high temperature step to continuous for
the low temperature step.
Silicon is a long-standing photosensitive material because of its unique photoelectronic properties and mature manufacturing technology. However, silicon photodetectors are generally limited by weak photoresponse in the near-infrared region. In this work, pressure is used as an effective means of tuning the photoresponse of silicon, specifically in the near-infrared region. Silicon has two different types of photoresponse under pressure. In the pressure range from 1 atm to 10 GPa, huge pressure-enhanced photocurrent is observed under illumination by a xenon lamp and near-infrared light (1064 nm). At 10 GPa, the photocurrent density ( Jph), responsivity ( R), and external quantum efficiency are increased 40-fold from those at 1.2 GPa. Interestingly, above 10 GPa, a unique pressure-induced positive–negative photoresponse switch is found along with the phase transformation from the semiconductive phase (Si I) to the metallic phase ( β-tin). Further experiments show that the photothermal effect is the main factor for negative photoresponse. All these pressure-induced properties give silicon more possibilities in the further design of visible and infrared photodetectors.
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