Geometry optimization based on ab initio electronic structure calculations is a valuable tool for predicting structures of novel compounds in the solid state. But even the most precise optimization scheme can lead to a structure which is not a genuine minimum on the potential energy surface. That is the case for elusive bulk AuF for which three types of structures were proposed previously [1]. Our DFT (GGA/PBE) calculations of the phonon spectra revealed the presence of imaginary modes for all these polymorphs [2]. Distortion of the parent structures along the normal modes of imaginary phonons, and subsequent geometry reoptimization has led us to new polymorphs of AuF, free of imaginary lattice vibrations. Phonon - guided optimizations for a handful of possible polymorphs of XeAuF have yielded inter alia a quasi-molecular polymorph (derived from a well known InOBr structure) surpassing its predecessor in enthalpy by almost 0.4 eV [3]. These findings enabled us to propose viable high pressure (p > 10 000 atm) synthetic routes for both compounds [2,3].

To further highlight the usefulness of phonon dispersion analysis we have conducted calculations of the lattice dynamics of solid compressed silane, SiH₄. The earlier theoretical studies have failed to correctly predict its crystal structure [4,5] (determined recently by x-ray diffraction [6]).

[1] T. Söhnel, H. Hermann and P. Schwerdtfeger, J. Phys. Chem. B 109 (2005) 526.

[2] D. Kurzydłowski, W. Grochala, Chem. Commun. 9 (2008) 1073.

[3] D. Kurzydłowski, W. Grochala, Z. Anorg. Allg. Chem. 634 (2008) 1082.

[4] J. Feng, W. Grochala, T. Jaroń R. Hoffmann, A. Bergara, N. W. Ashcroft, Phys. Rev. Lett. 96 (2006) 017006.

[5] C. J. Pickard, R. J. Needs, Phys. Rev. Lett. 97 (2006) 045504.

[6] O. Degtyareva, M. Martínez Canales, A. Bergara, X. J. Chen, Y. Song, V. V. Struzhkin, H. Mao, R. J. Hemley Phys. Rev. B 76 (2007) 064123.

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