Page 14 - Spring 2010

Fig. 6. Not exactly the structure of ZrW2O8, but it illustrates the following point. Cold, the bonds of the elements of the above cartoon unit cell are aligned. Pressure applied to this is resisted by the straight line of stiff bonds. Hot, it can be seen that two things happen. One is that the thermal energy “tilts” the unit cells (in fact this is intended to be a snapshot of vibrating tetrahedrons) so that it is easier to compress because compression need only tilt further bonds already misaligned. The other is that the volume naturally shrinks as the vibrations increase in amplitude because of the transition from a square lattice to a rhomboid—it is just a geometry effect.
strong response to alloying, and large volume changes during transformations to adjacent phases. It also shows strong elas- tic softening with increasing temperature, an extreme behav-
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Much of the strange behavior of Pu, perhaps the second most interesting element after He, is often ascribed to the outer electron shell (con- taining the so-called f-electrons) of Pu introducing instabili- ties into the fundamental electronic properties that control all of the non-nuclear oddities. That is, these outer electrons cannot decide whether to be “localized,” making Pu less metal-like, or “itinerant,” making it more metallic. This vac- illation turns out to be sensitive to temperature and volume. Using many techniques, the structure of δ-Pu has been found to shrink on warming. Adding about 2.4 atomic % Ga to Pu changes the thermal expansion coefficient to zero above
ior among densely–packed elements.
material shrinks as it is warmed. But what does its elastic stiffness do on warming? To put it in perspective, Invar, when warmed, has no volume change. It acts as if it “tries” to expand but internal forces “compress” it at the same time. This “compression” makes Invar stiffen as it heats, just the way most materials stiffen when compressed. ZrW2O8, shrinking with temperature, behaves as if it is strongly com- pressed on warming. But measurements using RUS of the bulk modulus, the parameter that describes the resistance to hydrostatic compression, show that it decreases with increas-
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ing temperature. Thus, this solid material softens as it is
compressed. How can this be? And what would happen if the
temperature were held constant and ZrW2O8 were put under
pressure? Using pulse-echo ultrasound measurements in a
SiC pressure cell, it was found that ZrW2O8 softens when
compressed, completing the puzzling picture of this strange
Listening to plutonium
Plutonium (Pu), a fuel for nuclear energy production, and a fuel for the triggers of thermonuclear weapons, is of essential importance to the security of the United States. With the Nuclear Non-proliferation Treaty of 1968 and the Comprehensive Test Ban Treaty of 1996, the safety of our nuclear arsenal, now down to just over 5000 warheads, falls upon science-based predictive capabilities for understanding essential physical properties of Pu such as compressibility, chemistry, and radiation-induced effects on aging. With six allotropes (crystal structures) and irregular melting behavior, plutonium presents a perplexing array of physical properties, still much studied, both experimentally and theoretically.
The simplest of the six crystal structures, δ-plutonium, displays the most puzzling properties. Besides having the highest atomic volume of all Pu-structures, it displays extreme elastic anisotropy, negative thermal expansion, a
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be visualized with the cartoon in Fig. 6
solid.
The root of this behavior, revealed by ultrasound, can
10 Acoustics Today, April 2010
Fig. 7. Temperature dependence of the bulk and shear modulus for Ga-stabilized Plutonium.