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We have investigated the high-pressure behavior of the 'lone-pair' compound SnO using monochromatic synchrotron X-ray diffraction, synchrotron far-and mid-infrared reflection measurements, and Raman spectroscopy. The litharge-type ambient pressure structure is observed up to at least 20 GPa, though with indications for a small distortion above 15 GPa. Changes in interatomic distances are determined via full Rietveld refinements of diffraction patterns. SnO is found to undergo a semiconductor to metal transition near 5(1) GPa; the transition is attributed to the closure of the indirect fundamental gap. The mode Grüneisen parameters of zone-center Raman mode frequencies are reported. The results are discussed in light of related experimental and theoretical studies of structural, dynamical, and electronic properties of SnO.

Electronic conduction in GaM4Se8 (M=Nb,Ta) compounds with the fcc GaMo4S8-type structure originates from hopping of localized unpaired electrons (S=1 / 2) among widely separated tetrahedral M4 metal clusters. We show that under pressure these systems transform from Mott insulators to a metallic and superconducting state with T(C)=2.9 and 5.8 K at 13 and 11.5 GPa for GaNb4Se8 and GaTa4Se8, respectively. The occurrence of superconductivity is shown to be connected with a pressure-induced decrease of the MSe6 octahedral distortion and simultaneous softening of the phonon associated with M-Se bonds.

The effect of pressure on the zone-center optical phonon modes of antimony in the A7 structure has been investigated by Raman spectroscopy. The Ag and Eg frequencies exhibit a pronounced softening with increasing pressure, the effect being related to a gradual suppression of the Peierls-like distortion of the A7 phase relative to a cubic primitive lattice. Also, both Raman modes broaden significantly under pressure. Spectra taken at low temperature indicate that the broadening is at least partly caused by phonon-phonon interactions. We also report results of ab initio frozenphonon calculations of the Ag and Eg mode frequencies. Presence of strong anharmonicity is clearly apparent in calculated total energy versus atom displacement relations. Pronounced nonlinearities in the force versus displacement relations are observed. Structural instabilities of the Sb-A7 phase are briefly addressed in the Appendix.

The crystal structure of the layered, perovskite-related LaTiO3.41 (La5Ti5O 17+δ ) has been studied by synchrotron powder x-ray diffraction under hydrostatic pressure up to 27 GPa (T = 295 K). The ambient-pressure phase was found to remain stable up to 18 GPa. A sluggish, but reversible phase transition occurs in the range 18-24 GPa. The structural changes of the low-pressure phase are characterized by a pronounced anisotropy in the axis compressibilities, which are at a ratio of approximately 1 : 2 : 3 for the a, b, and c axes. Possible effects of pressure on the electronic properties of LaTiO3.41 are discussed.

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The structural evolution of recently characterized sodium nitride Na 3 N as a function of pressure was investigated at room temperature by the angle-dispersive powder x-ray diffraction in a diamond-anvil cell up to 36 GPa. The rather open cubic anti-ReO 3 -type structure stable at ambient pressure is followed by a series of four high-pressure modifications. Along the route, the coordination number for the nitride anion increases from 6 in Na 3 N-I to 8 in hexagonal Li 3 N-type Na 3 N-II, 9 in orthorhombic anti-YF 3 -type Na 3 N-III, 11 in hexagonal Cu 3 P-type Na 3 N-IV, and finally 14 in cubic Li 3 Bi-type Na 3 N-V structures. The experimental data are compared to the results of total-energy calculations and are discussed with regard to the structural details of the five phases and their equations of state.

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