K. Govinda Rajan

Effect of magnetic transition on the lattice expansion of nickel

The lattice expansion in 99.999% pure nickel has been studied to within epsilon = mod (T-Tc)/Tc mod =10-3. It is observed that during the ferromagnetic-to-paramagnetic transition, nickel undergoes anomalous expansion leading to a lambda -type anomaly in the thermal expansion coefficient, beta (T). From beta (T), the critical part of the thermal expansion coefficient beta c(T) was separated and fitted to the power-law equation beta c+or-=(A+or-/ alpha +or-) epsilon - alpha +or-(1+E+or- epsilon x+or-)+B+or-. The parameters thus obtained are in good agreement with the values predicted by the renormalisation group theory and those derived from specific heat measurements.

Kinetics of pressure induced structural phase transitions—A review

The current status of experimental as well as theoretical advances in the understanding of kinetics of structural phase transitions is reviewed. A brief outline of the classification of phase transitions and classical ideas in the theory of kinetics of phase change is presented first. High pressure experimental techniques developed for studying the kinetics of structural transitions are reviewed and the salient features of each technique is brought out. The experimental technique using the diamond anvil cell (DAC) and image processing gets special mention as it promises to impart a new direction to this field. The usefulness of kinetic parameters in understanding the mechanism of a phase transition is examined. Typical examples from the literature are provided to give a flavour for these kind of studies. In conclusion, several open questions are raised which could pave way for future work in this area.

Performance of a diamond‐anvil high‐pressure x‐ray diffractometer in the Guinier geometry

In this paper, the performance of a novel high‐pressure diamond‐anvil powder x‐ray diffractometer in Guinier geometry is reported. The diffractometer is a combination of a curved quartz‐crystal monochromator, a Mao–Bell‐type diamond‐anvil cell, a flat position sensitive detector, and a Huber goniometer with compound stages to realize this geometry. The incident x‐ray beam is from a rotating anode x‐ray generator. It is shown that the above experimental arrangement results in an enhanced S/N ratio, improvement in resolution, and reduction of the data‐acquisition time. Test results on UAl2 are presented to demonstrate the advantages of the setup in the study of phase transitions at high pressure.

Crystal structure of UAl2 above 10 GPa at 300 K

Abstract A high pressure X-ray diffraction study on UAl2 has been done up to ≈28 GPa. It undergoes a structural transition at ≈11 GPa and the structure of the high pressure phase has been identified to be of the MgNi2 type with space group P63/mmc. The structure of the ambient pressure phase is of the MgCu2 type with space group Fd3m. From the electron per atom ratio e a , it is expected that it may transform back to the MgCu2 type structure at still higher pressures. On the basis of similar arguments, it is expected that most of the AB2 type Laves phase compounds of the f electron systems with suitable e a ratios may undergo the pressure induced structural transition in the sequence MgCu2 → MgZn2(or MgNi2) → MgCu2 due to increased delocalization of their f electron states.

Luminescence behaviour of rare earth doped alkaline earth aluminates prepared by the halide route

Abstract The fluorescence and long persistent phosphorescence behaviour of alkaline earth aluminates of the type MAl 2 O 4 (M=Ba, Sr or Ca) doped with rare earth ions has been extensively studied in the literature. These phosphors are usually prepared using their oxides as the starting materials and employing temperatures in excess of 1000 °C. In this paper, we report the possibility of synthesizing the alkaline earth aluminates starting from the alkaline earth halides. The operating temperatures are around 900 °C, yet single-phase materials with high luminescence output have been realized.

Electromigration of oxygen in YBa2Cu3O7-δ

Electromigration (EM) in metals is used to produce directed particle fluxes within the material and can be used to change the composition of a given material or to purify it. EM experiments were performed on sintered YBa2Cu3O7- delta (YBCO) rods at 200 degrees C and 350 degrees C and the results were subsequently analysed by magnetic susceptibility and X-ray diffraction. Explorative trials showed that oxygen does migrate under the influence of the field in this material and motion is directed towards the cathode. Another remarkable feature during electromigration was that towards the cathode. Another remarkable feature during electromigration was that a decomposition reaction occurred at the anode, resulting in the formation of YBCO (tetragonal), CuO, Y2O3 and BaCO3 phases. It appears that the effects of electromigration will have a bearing on the stability of this material during use.

Principle of massive support in the opposed anvil high pressure apparatus

The Bridgman anvil technique offers a simple and versatile means of generating very high pressures required in solid state studies. The opposed anvil technique is based on the principle of massive support. The practical case of a gasketted anvil is considered, and an expression for the maximum pressure generated under massive support is derived in terms of the geometric parameters, the strength of the anvil material and the gasket properties. In particular, for a given maximum pressure, it is possible to calculate the taper angle, the taper height and the gasket thickness from this expression. The anvil is assumed to be in the elastic region under load. Good agreement is obtained between the calculated and the experimental values for the massive support factor (msf) for various taper angles. By choosing the proper geometry, it is possible to achieve a pressure as high as 130 kbar in an alloy steel anvil. It has been clearly found that the straight portion, where the taper ends, does not really take any part in changing the stress pattern. Thus the minimum straight portion can serve the purpose, and will result in material saving. Anvils exhibit yielding at very high pressure. It is also pointed out that a further strengthening of the anvil can extend the ultimate pressure. Several methods of further strengthening the anvils are discussed.

Stability of ThGa 2 in the tetragonal phase up to 62 GPa at 300 K

We have investigated the structural stability of ThGa 2 under high pressures up to 62 GPa by performing X-ray powder diffraction studies in a diamond-anvil cell. ThGa 2 exhibits a tetragonal ThSi 2 -type structure at room temperature and pressure. At about 0.2 GPa the unit-cell volume drops significantly (4%) without any change in the structure. The tetragonal structure remains stable for pressures up to as high as 62 GPa. Possible reasons for the structural stability of ThGa 2 are discussed from the view point of the influence of the number of valence band electrons in stabilizing various crystal structures.

Anomalous compression in - an experimental and computational study

High-pressure x-ray diffraction experiments were performed on up to 23 GPa. Anomalous compressibility behaviour of the system was observed in the pressure range 7 - 14 GPa. The experiments are compared with similar observations in and . Band structure calculations have been performed to look for a possible explanation of this behaviour through the concept of electron transfer from the f to the d orbitals.

X-ray excited optical luminescence studies on the system BaXY (X,Y=F, Cl, Br, I)

The present paper reports the experimental observations on the x-ray excited optical luminescence (XEOL) along with the afterglow and colour center features found for the barium salts, represented by the formula,BaXY, whereX andY are the halides. The system thus consists of four dihalides (BaF2,. . . ,BaI2) and six mixed halides (BaFCl,. . . ,BaBrI). To start with, it was found that on two of the binary halides of barium, BaClI and BaBrI, no literature exists, and so these were prepared for the first time and their crystal structures were determined. An x-ray generator of 3-kW rating was next coupled to a spectrometer via a high throughput fiberoptic sensor for recording the luminescence spectra under x-irradiation. Also presented in this paper are the observations on the BaXY compounds in which about 0.1 mole% of Eu2+ was doped, in order to study the efficiency between the prompt luminescence and the photostimulated luminescence in these compounds. The crystal structure varies from fluorite (BaF2), to matlockite (BaFX) and finally to orthorhombic (BaCl2,. . .,BaBrI) for these compounds. Hence searching for systematics and empirical relations in the observed XEOL behaviour of these compounds is still a challenging problem.