V. Vijayakumar

High pressure study of borocarbide superconductor YNi2B2C at room temperature

Abstract High pressure behaviour of YNi 2 B 2 C at room temperature is investigated by electrical resistivity, thermopower and X-ray diffraction incorporating imaging plate. These studies do not indicate any structural transition up to 16.4 GPa. Bulk modulus of the material is measured to be 200 GPa and estimated to be 270 GPa from the theoretical calculations based on TB-LMTO method.

Body-centred tetragonal to HCP phase transformation in Cd-Hg alloys under pressure

Cd1-xHgx alloys (x=0.15 and 0.35) have been studied up to 8 GPa by the four-probe resistance and X-ray diffraction techniques. Cd0.65Hg0.35 shows a BCT to HCP transformation at 2 GPa while Cd0.85Hg0.15 does not show any transition up to 8 GPa. This is consistent with the structural trends expected from the pseudopotential treatment of Hafner and Heine (1983). The possible mechanism of BCT to HCP transformation is discussed.

Distorted fcc high pressure phase of rare earths in La0.8Th0.2 alloy at ambient conditions — thcp versus modulated hexagonal structure

Abstract Two structures, thcp and modulated hexagonal structure have been proposed for the distorted fcc phase of the rare earths at high pressure. We have stabilised this phase in Ls 0.8 Th 0.2 at ambient conditions and present diffraction data to rule out the thcp structure.

Effect of pressure on the electrical resistivity and the thermoelectric power of EuPd2Si2

Abstract A continuous valence transition has been found in EuPd 2 Si 2 from the measurement of the electrical resistivity and thermoelectric power upto 70 kbar.

c/a Anomalies, thermoelectric power and electronic topological transitions in Cd

Abstract Thermoelectric power (TEP) measurements up to 6.9 GPa pressure and the total energy calculations for the axial ratio ( c a ) variation with compression were carried out for cadmium. We find two anomalies as compared to a single anomaly observed in the experimental X-ray diffraction (XRD) data. The first anomaly at V V 0 = 0.95 is due to electronic topological transition (ETT) and correlates well with our TEP data, and with the anomaly observed in the existing resistivity data. The second anomaly at V V 0 = 0.85 correlates well with the recent high-precision imaging plate-based XRD data. We attribute the 4d-orbital overlap, coupled with transfer of electrons from d to s states as the cause of this anomaly. This finding is in contrast to the existing understanding in zinc, where the c a anomaly is attributed to the destruction of a giant Kohn anomaly by ETT. Non-destruction of the giant Kohn anomaly in cadmium shows that the Fermi surface of Cd is different from that of Zn at high pressures.

A tetrahedrally close packed (tcp) structure for the high pressure phase of potassium

Abstract A structural model of the high pressure phase of potassium above 20 GPa is proposed based on the idea of Pauling that the average ligancy of atoms in metals should increase under pressure. It is shown that a body centered cubic unit cell (with 81 atoms in the primitive unit) of Mg 32 (AlZn) 49 type fits the d spacings of the X-ray diffraction patterns of potassium in this phase.

High pressure studies on YNi2B2C and LuNi2B2C: ADXRD, thermoelectric power, resistivity, and electronic structure

Abstract The electronic and structural behaviour of YNi2B2C and LuNi2B2C at high pressures were investigated by electrical resistivity, thermoelectric power (TEP), and angle dispersive X-ray diffraction measurements and by electronic band structure calculations. The pressure variation of TEP shows a peak around 2xa0GPa in YNi2B2C. However, the X-ray powder diffraction does not show any structural transition up to 16.4xa0GPa. The equation of state of YNi2B2C yielded a relatively high bulk modulus of 200xa0GPa. The observed peak in TEP and the pressure variation of the superconducting transition temperature can be correlated with different components of the electronic density of states. The universal equation of state shows deviation from linearity around 1.5xa0GPa pressure and correlates well with the TEP peak.

On the structures of the ‘distorted fcc’ phase of rare earths and actinides. Some more possible structures

Abstract Recent high-pressure experiments have revealed that Y, La, Ce, Pr, Am, Cf, and Cf0.6Bk0.4 display a distorted fcc structure beyond the well-known sequence hcp-Sm type-dhcp-fcc. Six structures have been so far proposed for this phase. Here, we add two more distorted fcc structures. However, the analysis of diffraction data of Pr III, Nd III, Ce and Tb seems to suggest that these structures are merely different indexing schemes at present.

On the nature of the dhcp to fcc transition under pressure in Pr and Pr-Th alloys

The results of electrical resistance (R), thermoelectric power (TEP) and X-ray diffraction measurements on praseodymium (Pr) and its alloys with thorium under pressure are reported. The maximum inR vsP curve exhibited by Pr persists only in the dhcp phase of PrTh alloy. X-ray measurements confirmed that in the alloys also the maximum inR vsP curve is due to the dhcp → fcc transition. Thus the behaviour of Pr and Pr-Th alloys is different from that of La and its alloys with Ce and Th where the maximum in theR vsP curve is electronic in origin and is exhibited by the dhcp, fcc and dist fcc phases.

Isostructural electronic anomaly in La and LaTh alloys

Abstract We present new resistivity and X-ray diffraction high pressure data on Laue5f8Th alloys to show that the 2.5 GPa transition in La is not dependent on a particular crystal structure and is not related to the α → γ transition in Ce.