Most of materials exhibit pressure induced phase transformations which can induce strong modifications of their properties (electronic, magnetic...). These properties are often related to the interaction between the neighboring atoms and therefore the knowledge of the local order in the new phases is required to understand what happens to the material. When long range order does not exist (amorphous material, glasses or liquid) or is only an average of the local order local investigations, like X-ray Absorption Spectroscopy (XAS) are needed to follow the modifications of the short range order. Moreover, in case of dilute species in a matrix, the local probes become unique tools to determine the effect of the phase transformation on the impurity.

After a short introduction on high pressure technology, the limitations of measuring XAS specially at low energy when using a pressure set-up will be presented. Thanks to the combination of new high pressure cells with perforated diamonds and preindented gasket and of the highly focused XAS LUCIA beamline (SLS-PSI, Switzerland), low energy edges (2-8 keV) are now accessible to high pressure experiments above 20 GPa. Some examples illustrating these possibilities will be given.

- Phase transformation on the dilute magnetic semiconductor Zn_1-xMn_xO [1] for x =0.25 and x=0.05. For both compositions, Mn atoms substitute to Zn in the low pressure phase wurtzite structure. Under pressure the phase transition from wurtzite to rocksalt structure occurs above 7 GPa for the two compositions studied. The transition is shown to be reversible for x=0.05 and irreversible for x=0.25.

- LiV₂O₄ is a mixed-valence compound with equal ratio of V³⁺ and V⁴⁺. The observed pressure induced phase transition can be attributed to a charge ordering as it was observed in the isostructural compound, AlV₂O₄ [2].

[1] Pellicer-Porres J. et al , Appl. Phys.Lett., 2006, 89, 231904

[2] Pinsard-Gaudart L. et al, to be published in PRB

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