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New materials with surprising and sometimes useful new properties are often found by modifying the structures of known materials without altering their composition. One approach is to subject materials to very high pressures of up to 100 GPa (100GPa = 1,000,000 times atmosphere pressure). The classic example is graphite, a soft and opaque material with a loose layered structure, which can be turned into ultra-hard, transparent diamond, a crystal with cubic symmetry.
Unlike diamond, which retains its structure even after the pressure is released, most high pressure structures are metastable, and return to their unpressurized form when pressure is released. Thus, for example, sulfur, which is superconducting under pressure, is not useful as a superconductor because this property is lost when the pressure is removed.
A long term goal of many materials scientists is to make high pressure materials that retain their properties even at ambient pressure. Recently, researchers from the chemistry and materials science departments of the State University of New York at Binghamton and Cornell University have joined forces to make progress toward this goal. They demonstrated for the first time that the semiconductor PbTe has a high-pressure-tuned metastable structure that can be retained at ambient conditions. This material also shows, for the first time, a reversal of a so-called Hall-Petch relation relating the structural stability to particle size. In addition, they went a step further to show that by improving the protocol for synthesis, useful forms of PbTe can be created without any pressure at all. This important result raises the possibility that PbTe semiconductor materials could someday serve a host of useful technological applications, such as thermo-electronics, energy conversion, etc.
Fig 1: Configuration for x-ray diffraction measurement through a DAC cell (center) and the x-ray powder diffraction patterns of starting materials at ambient conditions (left) and under high-pressure (right).
Published as a report in Nano Letters (see reference below), Jiye Fang and his group, including leading scientist Zewei Quan and others from SUNY Binghamton, and Zhongwu Wang from the Cornell High Energy Synchrotron Source (CHESS) studied the formation of a dense amorphous phase of PbTe as a function of pressure. The experimenters pressurized PbTe nanoparticles in a diamond anvil cell (DAC) while monitoring the structural changes in-situ with Wide Angle X-ray (WAXS) diffraction at the B2 station at CHESS (figure 1). They systematically observed pressure induce changes as a function of particle size from 3 to 13 nanometers. The WAXS patterns showed the consequences of pressure-tuning on phase transformations; specifically, pressure transforms the rocksalt-type structure of PbTe either to a dense orthorhombic phase or an amorphous phase (Figure 2).
Fig 2: Schematic of the relationships between the first-order transition pressure and particle size (A); TEM image (B) and corresponding WAXS image (C) of 3 nm PbTe nanoparticles without compression.
The investigators found that nanoparticle size strongly affected the structure of the high pressure phase (i.e. orthorhombic and amorphous) and the transition pressure. The result differs from the well-known Hall-Petch relation, which has previously been observed to hold true for all metal and alloy nanocrystals. In traditional systems, the pressure needed to cause the structural transition goes up as the particle size decreases – thus the slope of the line on the graph in figure 2 would be consistently negative. Instead, in PbTe, a reversal of Hall-Petch relation - an unexpected weakening of structural stability upon decrease of particle size down to 9 nm – was discovered. For comparison, the transition to the orthorhombic phase of macroscopic bulk samples of PbTe occurs at ~6 GPa. With 13 nm particles, PbTe transforms to an orthorhombic phase at 8 GPa. With smaller 10 nm particles, the orthorhombic phase nucleates at 10 GPa, and at 9 nm jumps to 16 GPa. When the particle size is smaller than 9 nm, PbTe instead turns amorphous. As the particle becomes smaller, the amorphization pressure displays a dramatic drop.
Using the measured correlation between particle size and amorphization pressure, the researchers projected that when the particle size is reduced to 3 nm, the amorphization pressure should likewise reduce to 0 GPa – that is, the amorphous phase becomes stable at ambient conditions. They concluded that PbTe prefers to nucleate into an amorphous form at an early stage that remains constant and stable to 3 nm. This suggests that if materials scientists modify their synthesis protocols to control particle size, they could succeed in making stable amorphous PbTe without need for high-pressure.
Discovering that there is a reversal of structural stability at a critical size, and seeing the strong relationship between particle size and stability, offers promise that other materials systems might be found that bring the unique properties of high-pressure down to ambient conditions. It was also clear from this study that in-situ x-ray characterization yields direct physical information about structural stability and new phases and nucleation mechanisms. Such tools provide researchers with insights to design and fabricate novel materials at low-pressure or ambient conditions, considerations highly relevant to realizing new materials for industrial applications.
Z.W. Quan, Y. Wang, I.T. Bae, W.S. Loc, C. Wang, Z. Wang, and J. Fang; Nano Letters 11(12), 5531-5536 (2011)
Submitted by: Zhongwu Wang, CHESS, Cornell University