Twinning as a clue to a CDW-like behavior and to a lack of SME in plutonium metal

Abstract

The twinning systems of alpha phase Pu metal have been reported by Zocco et al. (Proc. XII Int. Congr. for Electron Micros., 1990, p. 1014). Twinning modes and charge-density wave (CDW) states have both been reported for the CDW materials 2HTaSe 2 and TiNi alloys. The latter alloys also show shape memory effect (SME). The similarities in properties of α-Pu and TiNi alloys have recently been pointed out by Sandenaw (J. Nucl. Mater., 189 (1992) 343). A possible clue to a CDW-like behavior and a lack of SME in α-Pu may be in the twinning systems observed. Microtwinning has been suggested as interfering with flow of itinerant electrons and thus causing a rise in electrical resistivity as reported for the TiNi alloys (Wang et al., J. Appl. Phys., 43 (1972) 97). The latter workers report a one-to-one correspondence between mechanical “memory” effect and electrical resistivity. α-U exists with many twinning modes (Frank, Acta Metall., 1 (1953) 71) but does not show any CDW behavior except at low temperatures. Therefore, twinning alone cannot be the source of a CDW-like behavior in α-Pu metal. Bak ( Rep. Prog. Phys., 45 (1982) 587) explained that the conduction electron density in a distorted phase is spacially modulated to form the CDW which accompanies the periodic lattice distortion (PLD). There appear to be competing periodicities (a PLD) in the complex α-Pu structure because the eight crystallographically different Pu atoms of the unit cell are seen to occur in a different but reproducible pattern in each of three directions on the a-c plane of the distorted pseudo hexagonal structure (Smith et al., ACS Symp. Series, No. 216, Plutonium Chemistry (1983) Fig. 1). It is suggested that the modulation of the conduction electrons, in three competing periodicities (chains) of the α-Pu lattice, should result in three different competing CDW-like states. The interference between three different CDW-like states may lead to the inability to see any one CDW-like state in α-Pu by neutron diffraction. The α-Pu structure is more complex than those of the CDW materials 2HTaSe 2 and TiNi alloys which also show twinning and a hexagonal structure. The twinning planes and twinning (shear) directions of single-crystal TiNi have been described with only the number one and zeros. (Miyazaki and Wayman, Acta Metal., 36 (1988) 181). The twinning planes and twinning directions of α-Pu were described by Zocco et al. with the numbers 1, 2, 3, 5 and 7 (and zeros). The greater complexity of twinning systems in α-Pu metal may explain an inability to see CDW states by TEM. This complexity could explain the lack of SME in Pu metal.