Pressure dependence of room-temperature structural properties of CaAl₂Si₂

We investigated the pressure dependence of the crystal structure of CaAl₂Si₂by means ofab initiocalculations and room-temperature synchrotron x-ray powder diffraction.Ab initiocalculations reproduce satisfactorily the experimentally observed pressure-dependent structural evolution up to 3 GPa where the title system remains in the trigonalP3¯m1phase. In the pressure range 3-8 GPa, pressure evolution of the calculated in-plane lattice parameters is steeper than the observed.Ab initiocalculations predict a structural phase transition to a tetragonal phase (P3¯m1to I4/mmm) near 7.5 GPa for zero (or room) temperature. Temperature effects are included through calculation of vibrational properties (phonon spectra). These calculations confirm that both phases are either globally or locally stable (metastable) and allow for the construction of aP-Tphase diagram for this system. However, our experiments show no sign of such a transition up to 12 GPa. Such a discrepancy can be explained if one considers the trigonal (P3¯m1) structure to be metastable above the critical pressure, but is separated from the predicted tetragonal (I4/mmm) structure by a relatively high energy barrier. The applied pressure alone may not be able to surpass the energy-barrier; rather a joint high-pressure and high-temperature (HPHT) treatment may lead to it. However, empirical verification of such a hypothetical transition may be hampered by the chemistry of CaAl₂Si₂system which shows tendency to decompose peritectically into Ca₂Al₃Si₄and aluminum under HPHT treatment.