R. Chidambaram

The equation of state and structural stability of titanium obtained using the linear muffin-tin orbital band-structure method

Room-temperature isotherms of titanium in HCP and omega -phases are calculated by the first-principles linear muffin-tin orbital energy band method. Comparison with experimental data shows excellent agreement. Structural phase stability analysis by the Andersen force theorem shows that the omega -phase is the lowest-energy phase at 0 K and normal volume. The possibility that the s to d electronic transition is the cause of shock discontinuity at 17.5 GPa.

Structural stability of hafnium under pressure

A theoretical analysis on the structural stability of HF under pressure, using the linear muffin-tin orbital method in conjunction with the Andersen force theorem, predicts a new phase transition to a BCC phase at high pressures, in agreement with recent diamond cell experiments.

HIGH pressure structural investigations of zirconium using LMTO method

Abstract The structural energy differences have been calculated for zirconium as a function of pressure at zero temperature using the Andersen force theorem and the linear muffin tin orbital method. The structures included are the following: α (hcp), the room temperature room pressure phase, ω- a three atom simple hexagonal, bcc and fcc. Our calculations show that the bcc structure would become energetically most favourable above 11 GPa. This results is in agreement with well known correlation between the crystal structure and the d-electron population in transition metals at normal volume. The diamond anvil cell based high pressure x-ray diffraction experiments are in progress to verify this result.

Symmetry systematics of pressure - induced phase transitions

Abstract Most pressure induced phase transitions are diffusionless. Because of this, there exists a one-to-one correspondence between the positions of atoms in the parent and the product phases, which, therefore, show interesting symmetry relationships. In this paper, we have used these for discussing a symmetry classification of pressure induced phase transitions into four categories: iso-symmetric, group-subgroup, intersection group, and order-disorder transitions. Various examples illustrating this classification scheme are discussed. The importance of this classification is in understanding the mechanism of pressure indbced phase transitions, where both theoretical calculations and experimental measurements are employed to analyse related phenomena like softening of phonon modes, elastic instabilities, diffraction patterns, and orientation relations.

A SINGLE STAGE GAS GUN FOR SHOCK WAVE STUDIES

For the last two decades, we have been investigating the equation of state and phase transitions in various materials by first principle electron theory methods. In order to confirm some of the theoretical predictions, we have now built indigenously an experimental facility for generating both static and dynamic pressures on samples. For producing shocks, we have developed a single stage 62.8 mm gas gun. It is patterned after the Washington State University gas gun. A unique feature is a remote control unit which allows firing of the projectile in manual or auto mode. Both planar and inclined impacts can be obtained. We have developed instrumentation for measuring projectile velocity, tilt of the impactor, shock velocity, particle velocity and shock pressure profiles. A comparison of the expected and measured projectile velocities shows that the gun is performing well. Some preliminary compression experiments indicating the effect of shear on the α → ω phase transition in Ti will be reported.

Annealing of vacancy-type defects and crystallisation of icosahedral Al-Mn-Si

The authors report a positron annihilation investigation of quasicrystalline-to-crystalline phase transition in the Al-Mn-Si alloy system using both Doppler broadening and lifetime measurement techniques. The results show the presence of small vacancy clusters in the as-grown icosahedral phase. Isochronal annealing studies carried out from 100 degrees C onwards show that the vacancy clusters are annealed out along with the crystallisation process. The positron lifetime results rule out the possibility of a central hole which had been postulated in the icosahedral building block called the Mackay icosahedron.

A positron annihilation study of vacancy-type defects in Al-Cu-Fe quasicrystals

A positron annihilation study of the icosahedral Al-Cu-Fe alloy system has been carried out using both Doppler broadening and lifetime measurement techniques. Isochronal annealing studies show that a nearly unchanged two-component lifetime spectrum is obtained up to the highest annealing temperature of 450 degrees C used in the present work, indicating the stability of the structure and the defect state in this quasicrystalline phase up to this annealing temperature. The vacancy defect concentration is about 8 ppm which is higher than that observed in the authors earlier study on icosahedral Al-Mn-Si, though the vacancy size is lower-approximately a monovacancy size on average in Al-Cu-Fe against a divacancy size in Al-Mn-Si. The existence of such a high concentration of vacancy defects even in quasicrystalline Al-Cu-Fe, which has little phason disorder, should be significant from the point of view of developing a structural model for quasicrystals. It appears that a two-component positron lifetime spectrum indicative of a high concentration of vacancy-type defects is characteristic of the icosahedral quasicrystalline phase. The possibility of a central hole in the icosahedral building block in this phase is ruled out from the authors positron annihilation experiments.

Crystallisation and structural relaxation of quasicrystalline Al-Mn: a positron annihilation study

The authors report here for the first time the positron annihilation study of quasicrystalline Al86Mn14 alloy. their study confirms calorimetric results reported earlier, which indicated that, before the crystallisation, the quasicrystalline phase in this alloy goes through a structural relaxation process. The underlying cause of the substantial drop in the line-shape parameter S during the crystallisation process is also considered.

High pressure physics research at BARC

High pressure physics research at BARC spans a period of more than two decades and covers the area of both static and dynamic pressures on the one hand and both theoretical and experimental aspects on the other. The experimental facilities available include Diamond Anvil Cells for X-ray diffraction and Raman spectroscopy and a light gas gun. Phase transformations investigated in metallic systems includethose in Cd-Hg, transition metals, rare earths and actinides and these investigations have been supported by detailed band structure calculations and studies of the mechanismof phase transformations. Extensive equation of state studies over a wide range of pressures include a new model for the intermediate pressure range between 0.5 and 10 TPa and interpretation of experimental shock Hugoniots.

Structural models of quasicrystals: Significance of positron annihilation studies

We discuss implications of our results of positron annihilation studies of various icosahedral quasicrystalline alloys on the structural models of this new phase. We have studied Al-Mn, Al-Mn-Si, Al-Cu-Li and Al-Cu-Fe and a two component positron annihilation lifetime spectrum seems characteristics of all these icosahedral quasicrystalline alloy phases. Analysis of the lifetime spectra yields vacancy sizes varying from monovacancies to hexavacancies with concentration of 1 to 8 ppm in various quasicrystalline alloys. These vacancy concentrations are about three orders of magnitude less than that observed in metallic glasses. We also notice that in the case of metastable quasicrystalline alloys such as Al-Mn and Al-Mn-Si, these vacancy clusters disappear during the crystallization process resulting in single lifetime spectra. On the other hand, for stable quasicrystalline alloys, two component life time spectra, which indicate the presence of vacancy clusters, continue to exist even after prolonged heat treatment. Our results seem incompatible with the space filling Penrose tiling or random tiling models and favour cluster-based models.