Session J2: PC-1: Physics at High Pressure

Pressure induced phase transitions in Vacancy-doped nano-crystalline manganites through High-pressure M\"{o}ssbauer spectroscopy

Nanocrystalline perovskite Manganites ABO$_{3}$ (A=RE, and B= Mn) are interesting materials due to their colossal magneto resistive behaviour. The effect of vacancy-doping at A and B sites show changes in their behaviour as compared to the stoichiometric counterpart. We report here the effect of pressure on the vacancy-doped nanocrystalline manganites synthesized by sol-gel nitrate technique up to 10 GPa. The M\"{o}ssbauer measurements on nano-crystalline La-deficient sample La$_{0.9}$Mn$_{0.8}$Fe$_{0.2}$O$_{3.15}$ at ambient condition show distribution of Fe$^{3+}$ ions at two different environments. Interesting features are observed with variation in pressure on the sample -- an isostructural high spin Fe$^{3+}$ to low spin Fe$^{3+}$ transition (at 2.1 GPa), reversal to high spin again (at 2.8 GPa) and an orthorhombic to monoclinic structural transition (at 4.9 GPa). However nanocrystalline Mn-deficient sample La$_{0.8}$Sr$_{0.2}$Mn$_{0.8}$Fe$_{.16}$O$_{2.95}$ behaves differently. Unlike La-deficient sample, it retains isostructural high spin Fe$^{3+}$ configuration up to 4.2 GPa. The structural transition from orthorhombic to monoclinic seems to be still incomplete even at 6.3 GPa. Decrease in isomer shift of one of the site indicates strong covalent interaction between Mn and Fe ions. Low temperature M\"{o}ssbauer measurements at 80 K show appearance of magnetic sextet in Mn deficient sample while La-deficient remains paramagnetic.   

Measuring the volume of a fluid in a diamond anvil cell using a confocal microscope

Confocal microscopy is a potentially powerful technique for obtaining equation-of-state (EOS) data for fluids in a diamond anvil cell. Unlike conventional microscopy, a confocal microscope scans the cell in three dimensions. From the intensity profile of the reflected laser light, we calculated the index of refraction and optical thickness of the sample contained in the cell. These measurements, combined with the cross-sectional area of the sample, enabled us to calculate the volume. As a test of the experimental technique and analysis, we produced a pressure-volume curve for liquid water at 300K. The results agree with published EOS data within experimental error. We have also applied the technique to measure the pressure-volume curve for fluid argon at 300K.   

Melting curve of molecular hydrogen

More than 70 years ago Wigner and Huntington predicted that at sufficiently high pressures hydrogen will become an atomic metallic solid. Metallic hydrogen has not yet been observed at pressures exceeding 3 Mbar at low temperatures. Recent calculations predict a maximum in the melting line of hydrogen. Extrapolations to higher pressures suggest that metallic hydrogen may be a liquid at T=0 K with interesting quantum properties. Confining hydrogen at elevated temperatures is challenging as hydrogen tends to diffuse out of the cell. Combination of static pressure techniques with dynamic temperature variations can be used to suppress the diffusion of the sample out of the pressure cell. We have extended the melting line of hydrogen and observed the predicted peak and shall discuss this, the unusual properties of hydrogen, and it's various phases.   

A study on cavity collapse for utilizing green implosion energy

The mechanical energy generated by laser-induced implosion and the dynamics between non-condensable gas and liquid are studied experimentally and numerically. We have designed a micro implosion piston (MIP) to utilize the energy of implosion for inducing a piston motion. The MIP has the shape of a cone and is filled with liquid at room temperature and a high pressure ($\sim $ 6 bars). Focusing of a high power laser pulse inside the MIP leads to creation of several bubbles that expand and collapse with successive rebounds. The bubble-liquid interaction develops a micro- jet that destroys the symmetry of the bubble. This bubble implosion motion, induced by the pressure gradient across the cavity wall produces high pressures wave within a few nanoseconds. These pressure waves are affected by different conditions such as the distance between the bubble and piston head, the dimension of the MIP, and the pressure at which the MIP is driven. The radius of the bubble is measured by double exposure photography, while pressure histories are measured by hydrophones. We investigate the relationship between the radius of the bubble, the overpressure of the secondary shock wave and the motion of the micro piston, and compare it to numerical simulations. The aim is to reach a state inside the MIP that would cause a sustainable and efficient motion of piston through cavity collapse induced high pressure pulses.   

Effect of shear stress on the high-pressure behaviour of nitromethane: Raman spectroscopy in a shear diamond anvil cell

A detailed description of the reaction mechanisms occurring in shock-induced decomposition of condensed energetic materials is very important for a comprehensive understanding of detonation. Besides pressure and temperature effects, shear stress has also been proposed to play an important role in the initiation and decomposition mechanisms. In order to study this effect, a Shear Diamond Anvil Cell (SDAC) has been developed. It is actually a classical DAC with the upper diamond anvil rotating about the compression axis relative to the opposite anvil. In this paper, we present a Raman spectroscopy study of the effect of shear stress on the high-pressure behaviour of nitromethane. Two major effects of shear stress are observed in our experiments. The first one is a lowering of the pressures at which the different structural modifications that nitromethane undergoes are observed. The second effect is observed at 28 GPa where sudden decomposition of the sample occurs just after shear application. Observation of the sample after decomposition shows the presence of a black residue which is composed of carbon as indicated by the Raman spectrum. [1] Manaa, M. R., Fried, L. E., and Reed, E. J., \textit{Journal of Computer-Aided Materials Design}, 10, pp 75-97, 2003.