Hema Sankaran

Pressure induced amorphization of AlPO4

AlPO4 has been compressed to pressures of 16 GPa in a diamond anvil cell and its X-ray diffraction pattern studied by the energy-dispersive technique. The compound is observed to become amorphous at ∼ 12 GPa. This explains the loss of Raman spectrum of AlPO4 reported by Jayaraman and coworkers (1987).

An orthorhombic structure for the high temperature lock-in phase of LiKSO4

Abstract The diffraction pattern for the lock-in phase of LiKSO 4 in the temperature range 708 K to 743 K is reinterpreted on an orthorhombic structure in the space group Pnma. The proposed structure is isomorphous with the room temperature structures of LiCsSO 4 and the β-phase of K 2 SO 4 . It is also consistent with recent Brillouin scattering measurements and the observed domain structure.

High pressure phase transitions in organic solids II: X-ray diffraction study ofp-dichlorobenzene at high pressures

X-ray diffraction experiments onp-dichlorobenzene at high pressures show a transition at ∼ 0.3 GPa, to a new phase, the diffraction pattern of which cannot be indexed on the anticipated low temperature monoclinic crystal structure. We have instead found an orthorhombic cell, very closely related to the low temperature monoclinic cell, for this new phase. This structure, which also occurs inp-diiodobenzene at ambient conditions, has cell constantsa =14.02,b = 6.06,c = 7.41Å andZ = 4. The space group is Pbca. This new phase has a non-β herring-bone structure, in contrast with the initialα phase which has aβ-structure with ribbon-like arrangement of molecules, with Cl-Cl contacts of ∼ 4A between adjacent molecules. This implies that with pressure the halogen-halogen interaction in this compound plays a less dominant role in crystal engineering.

The low-temperature phase of sodium: an intermediate orthorhombic distortion?

The authors have shown through first-principles total-energy calculations on sodium that at low temperatures there is a nearly-zero barrier path of continuous orthorhombic deformations leading from the body-centred cubic (BCC) to the more stable hexagonal close-packed (HCP) structure. The observed diffraction patterns may now be explained without the aid of stacking faults that are needed to account for deviations from the calculated pattern if a 9R structure is assumed.

A search for pressure induced amorphisation in quartz-like structures

Abstract We have started a program to search for pressure-induced crystalline-to-amorphous transitions in quartz-like structures. Preliminary results of our x-ray diffraction experiments on GaPO4, to pressures of 11GPa, are reported.

Visual observation of phase transitions in LiKSO 4 under pressure

Phase transitions in LiKSO4 have been visually observed in a diamond-anvil cell up to 16 GPa. The observations confirm a symmetry change at 0·9 GPa and a solidstate crystal to amorphous transition at 12 GPa. The crystal exhibits a memory across the crystalline transitions. Interesting microstructural features were observed in the range of the amorphous phase.