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Angle-resolved x-ray diffraction patterns of Xe to 127 GPa indicate that the fcc-to-hcp transition occurs martensitically between 3 and 70 GPa in diamond-anvil cells without an intermediate phase. These data also reveal that the transition occurs by the introduction of stacking disorder in the fcc lattice at low pressure, which grows into hcp domains with increasing pressure. The small energy difference between the hcp and the fcc structures may allow the two phases to coexist over a wide pressure range. Evidence of similar stacking disorder and incipient growth of an hcp phase are also observed in solid Kr.

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We have measured the pressure-volume (P-V) relations for cubic iron-nickel alloys for three different compositions: Fe 0.64Ni (0.36), Fe 0.55Ni (0.45), and Fe 0.20Ni (0.80). It is observed that for a certain pressure range the bulk modulus does not change or can even decrease to some minimum value, after which it begins to increase under still higher pressure. In our experiment, we observe for the first time a new effect, namely, that the Fe-Ni alloys with high Ni concentrations, which show positive thermal expansion at ambient pressure, become Invar system upon compression over a certain pressure range.

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After having emerged as primary contenders in the race for highly efficient optoelectronics materials, organolead halide perovskites (OHLP) are now being investigated in the nanoscale regime as promising building blocks with unique properties. For example, unlike their bulk counterpart, quantum dots of OHLP are brightly luminescent, owing to large exciton binding energies that cannot be rationalized solely on the basis of quantum confinement. Here, we establish the direct correlation between the structure and the electronic band-edge properties of CH3NH3PbBr3 nanoparticles. Complementary structural and spectroscopic measurements probing long-range and local order reveal that lattice strain influences the nature of the valence band and modifies the subtle stereochemical activity of the Pb(2+) lone-pair. More generally, this work demonstrates that the stereochemical activity of the lone-pair at the metal site is a specific physicochemical parameter coupled to composition, size and strain, which can be employed to engineer novel functionalities in OHLP nanomaterials.

The crystal structure of Na 3 MnF 6 has been investigated at high pressures by means of single-crystal x-ray diffraction, and its Mn(III) coordination environment has been studied by means of single-crystal optical absorption spectroscopy using diamond anvil techniques. Compressibility data (unit cell parameters) were collected in the pressure range from ambient to 4.06 GPa, and structural refinements based on single-crystal diffraction data were performed at 0.12, 0.91, 2.27, and 2.79 GPa. The monoclinic space group symmetry (P2 1 /n) is retained in the entire pressure range, but, at increasing pressure, a discontinuous phase transition is observed at ∼2.2 GPa. This is interpreted as an effect of a reversible, isosymmetric phase transition with a hysteresis width of 0.5 GPa, observed when the pressure is successively lowered. The structure refinements show that the phase transition involves a reorientation of the static prolate distortion of the coordination around manganese(III). The angle between the elongation axis (z) of the MnF 6 3octahedron with [0 0 1] flips from ∼20°at ambient pressures to ∼70°at 2.79 GPa. Polarized single-crystal absorption spectra of Na 3 MnF 6 show drastic changes of the polarization of bands due to spinallowed d-d transitions in Mn(III) when passing the transition pressure, which confirm the results of the singlecrystal structure refinements. A possible explanation for this transition is discussed in terms of structure packing arguments. The isothermal bulk modulus at ambient pressure and its pressure derivative were determined to B 0 ) 47.8(1) GPa and B 0 ′ ) 1.2(1), respectively.