Mohammad Yousuf

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.

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.

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.

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.

Phase transformation and electrical resistivity of tetracyanoethylene under pressure

This paper reports the phase transformation behaviour of tetracyanoethylene (TCNE) under pressure as revealed by AC electrical resistivity, its time evolution and X-ray diffraction studies. An irreversible transformation from monoclinic to cubic phase occurs at 2.1±0.1 GPa and is indicated by a sharp resistivity drop at this pressure. The time evolution of resistivity studies indicate that this transformation occurs via an intermediate phase having resistivity higher than either of the two crystalline phases. Finally, the kinetics of phase transformations obtained by time evolution of resistivity is compared with the X-ray studies on the pressure quenched TCNE.

A high pressure-high temperature cell for electrical resistivity studies

A high pressure-high temperature cell which permitsin-situ pressure and temperature calibration is described. The cell is in an opposed anvil configuaration, and houses two samples with four probes each along with a miniature furnace and a thermocouple. The pressure and temperature capability of the cell are 100 kbar and 1000°C respectively. This cell was developed to study the electrical resistivity of metals and alloys at high pressure and high temperature. Bismuth was used to calibrate the cell. We report in this paper the design details and the performance of this cell. Ni has been chosen as a test problem and the observed behaviour is indicated to show the quality of data.

Charge compensation mechanism in X-ray image intensifying phosphor BaFBr:Eu

Abstract Photoluminescence and diffuse reflectance spectral studies reveal that the commercially available X-ray image intensifying phosphor BaFBr:Eu contains both Eu 2+ and Eu 3+ in contrast to the X-ray image storage phosphor BaFBr:Eu which contains only Eu 2+ . In the former, photostimulated luminscence (PSL) at room temperature (RT) and thermostimulated luminescence (TSL) above RT are quenched. The latter, however, exhibits intense PSL and TSL. The results are explained on the basis that the presence of Eu 3+ removes the anion vacancies created by the incorporation of ubiquitous oxygen impurity at fluorine ion sites.

Electronic band structure calculation and structural stability of high pressure phases of selenium

Abstract We report a detailed theoretical calculation of the electronic band structure of Se in hexagonal and monoclinic phases under pressure using tight-binding linear muffin-tin orbital method (TB-LMTO) within local density approximation (LDA). Apart from the electronic band structure and structural stability calculations, the density of states (DOS) and Fermi energies (EF) at various pressures are calculated. The calculated lattice parameters, transition pressure, bulk modulus and the pressure–volume relation are found to be in good agreement with recent experimental results. Further, we have also calculated the electronic specific heat coefficient in the monoclinic phase, which decreases with the increase in pressure.

High pressure X-ray diffraction studies on UAI2, UAI2, and UAI4 systems

Abstract The UAI2, UAI2, AND UAI4 compounds have been studied by high pressure X-ray diffraction up to a maximum pressure of ∼ 35 GPa. The compressibility behaviour of UAI2 has been found to be consistent with the itinerant 5f states, whereas that of UAI2 and UAI4, indicate a more localized nature. Further, UAI2 has been found to undergo a structural transition at ∼ 11 GPa and the structure of the high pressure phase has been identified to be of MgNi2 type with space group P63/mmc. The structure of UAI2 at ambient pressure is of MgCu2 type with space group Fd3m. From the electronic considerations, for instance, free electrons per atom ratio e/a, it is anticipated that it may transform back to MgCu2 type structure at still higher pressures. On similar considerations, it is expected that most of the AB2 type Laves phase compounds of the ‘f’ electron systems may undergo the structural sequence: MgCu2 – MgZn2 (or MgNi2) – MgCu2 due to increased delocalization of their ‘f’ electron states.