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We report on high-pressure powder synchrotron x-ray diffraction studies on MFe2As2 (M = Ba, Ca) over a range of temperatures and pressures up to about 56 GPa using a membrane diamond-anvil cell. Our data indicate a phase transition to a collapsed tetragonal phase in both compounds upon compression. The data at 300 K are measured in both pressure-increasing and -decreasing cycles. Our measurements show that at 300 K in the Ba compound, the transition occurs at 27 GPa, which is much higher than the transition pressure of 1.7 GPa in the Ca compound. At low temperature, we could obtain data only in the pressure-increasing cycle, therefore a precise transition pressure is not identified. At a low temperature of 33 K, the transition to the tetragonal phase in the Ba compound starts, upon compression, at about 29 GPa, which is much higher than the transition pressure of 0.3 GPa at 40 K as known in the case of the Ca compound. The much higher transition pressure in the Ba compound may be due to its larger unit-cell volume at ambient pressure. It is important to note that the transition in both compounds occurs when they are compressed to almost the same value of the unit-cell volume and attain similar c(t)/a(t) ratios. We also show that the FeAs4 tetrahedra are much less compressible and more distorted in the collapsed tetragonal phase than their nearly regular shape in the ambient pressure phase. We present a detailed analysis of the pressure dependence of the structures as well as equations of state in these important BaFe2As2 and CaFe2As2 compounds.

Extensive inelastic neutron scattering measurements of phonons on a single crystal of CaFe2As2 allowed us to establish a fairly complete picture of phonon dispersions in the main symmetry directions. The phonon spectra were also calculated by density functional theory (DFT) in the local density approximation (LDA). There are serious discrepancies between calculations done for the optimized structure and experiment, because the optimised structure is not the ambient pressure structure but is very close to the "collapsed" structure reached at p = 3.5 kbar. However, if the experimental crystal structure is used the calculation gives correct frequencies of most phonons. The most important new result is that linewidths/frequencies of certain modes are larger/softer than predicted by DFT-LDA. We also observed strong temperature dependence of some phonons near the structural phase transition near 172 K. This behavior may indicate anomalously strong electron phonon coupling and/or anharmonicity, which may be important to the mechanism of superconductivity.

We measured phonon dispersions of CaFe2As2 using inelastic neutron scattering and compared our results to predictions of density functional theory in the local density approximation. The calculation gives correct frequencies of most phonons if the experimental crystal structure is used, except observed linewidths/frequencies of certain modes were larger/softer than predicted. Strong temperature dependence of some phonons near the structural phase transition near 172 K may indicate strong electron-phonon coupling and/or anharmonicity, which may be important for superconductivity.

We have investigated the temperature and pressure variation of the M-3 phonon mode of ReO3 by inelastic neutron scattering on a single crystal. The energy of the M-3 phonon mode decreases with decreasing temperature contrary to the normal hardening behavior. The energy of this mode also decreases with increasing pressure and tends to zero at about 3.5 kbar at T=150 K. The M-3 mode softening drives the pressure-induced phase transition in ReO3. The temperature-induced softening of the M-3 mode on lowering the temperature can be understood from the known negative thermal-expansion behavior and also from the fact that the high-pressure phase can also be approached at ambient pressure by cooling. In order to understand the M-3 phonon softening in ReO3 we have done lattice-dynamical calculations that support the qualitative arguments given above.

We have investigated the temperature variation of the unit-cell volume of ReO3 by neutron diffraction. The lattice parameter and the unit-cell volume decrease continuously as the temperature is increased from T=2 K to about T=200 K. After exhibiting minima at about 200 K they increase with increasing temperature linearly up to about 305 K, the maximum temperature investigated during the present experiment. We attribute the negative thermal expansion of ReO3 to be the results of rigid antiphase rotations of the neighboring ReO6 octahedra. We have done model lattice dynamical calculation of phonon dispersions, thermal expansion, and atomic displacement parameters in ReO3. The results of these calculations are in agreement with the experimental results.

We report inelastic neutron scattering measurements of the phonon density of states of superconducting Sr0.6K0.4Fe2As2 (T-c=32 K) and Ca0.6Na0.4Fe2As2 (T-c=21 K). Compared with the parent compound BaFe2As2 doping affects mainly the lower and intermediate frequency part of the vibrations. Mass effects and lattice contraction cannot solely explain these changes. Softening of phonon modes below 10 meV has been observed in both samples on cooling from 300 to 140 K. In the Ca-doped compound the softening amounts to about 1 meV while for the Sr-doped compound the softening is about 0.5 meV. There is no appreciable change in the phonon density of states on crossing T-c.

Physical Review Letters

Physical Review Letters

Lattice dynamical calculations using a shell model are carried out for the geophysically important mineral zircon, ZrSiO4. The model parameters are adjusted so that the model satisfactorily reproduces the literature data on structure, elastic constants and optically active modes. Phonon dispersion relations are measured on a single crystal using coherent-inelastic neutron scattering on the PRISMA spectromter at the UK ISIS facility. The one-phonon structure factors are calculated to preselect the Bragg points for the measurements and to assign measured phonon signals to different branches. The phonon dispersion relations measured are in good agreement with the calculations.