We report on pressure-induced phase transitions in the Sb2Te and Ag11In6Sb55Te28 (AIST) phase change alloys used in optical recording media over a pressure range from ambient pressure to 40 GPa. The results clearly demonstrate the crucial role of dopants in sequences of high-pressure structural transformations and in the degree of reversion.We also demonstrate that prolonged exposure of AIST to high pressures leads to an additional phase transition that may have fatal consequences for process reversibility in repeated compression/decompression cycles.

Perovskite (ABO3) ferroelectric materials exhibit a Ferroelectric-Paraelectric (Ferro-Para) phase transition at a critical temperature TC (and a critical pressure PC), which can be defined as a phase transition from a non-centrosymmetric to a centrosymmetric structure due to the disappearance of the spontaneous polarization (PS) characterizing these materials. Neutron diffraction is particularly useful to investigate this Ferro-Para phase transition in pressure-temperature space due to the relatively strong scattering of oxygen compared to XRD. It enables 1) to calculate PS with the dipole equation moment and also 2) to detect phase transitions involving octahedral tilting. In this study, high-temperature and high-pressure neutron powder diffraction patterns of typical perovskites PbTiO3 and PbZr0.52Ti0.48O3 contain specific reflections, (h, k, 2l or h, 2k, 2l), strongly linked to polar shifts. The progressive disappearance of these reflections is due to the anti-phase scattering of A (Pb), B (Ti, Zr) and O atoms. In other words, this disappearance is related to the decrease in the imaginary part of the form factor corresponding to these reflections which becomes zero at the Ferro-Para phase transition. In addition, the temperature- and pressure- dependence of the calculated PS is different from the behavior of the c/a ratio which should be proportion to √(P_C-P) in the case of purely displacive transition.

Ferroelectric materials with the perovskite structure have drawn considerable attention for many years. Among them, PbZr1−xTixO3 (PZT) materials are particularly interesting due to their high piezoelectric response in the so-called “morphotropic phase boundary” (MPB) region. Recently, synchrotron X-ray powder diffraction experiments done by Noheda et al. showed the existence of a monoclinic (M) phase (near x ~ 0.5) between the tetragonal and rhombohedral phases1,2. The effect of high pressure on the physical properties of PZT has been studied recently3. Experiments based on electron diffraction4 and neutron diffraction studies5, Raman scattering results6, as well as theoretical calculations7 have indicated the existence of an intrinsic short range dynamical disorder over nearly the entire PZT solid solution. Rouquette et al. observed the presence of diffuse X-ray/neutron scattering in PbZr0.52Ti0.48O3 diffraction patterns which is an evidence of local lattice deformations with respect to the average structure. Additional high-pressure high-temperature investigations showed the disappearance of the polar phonons A1(TO) (which is the origin of the ferroelectric instability) upon compression around 5 GPa in agreement with neutron diffraction data. High-pressure high-temperature EXAFS experiments were carried out on BM29 at the ESRF to follow the corresponding changes in short-range order around Zr at the ferro-to-paraelectric phase transition. The EXAFS results at 300 K isotherm show a change in the fine structure that starts at 5.3 GPa and this is in consistent with the value of the curie pressure Pc obtained by neutron diffraction3 and Resonance Raman Spectroscopy3 experiments. By fitting the first coordination shell (Zr,Ti)O6 octahedra using a cubic model, the values of the Debye-Waller (DW) factor at different pressure points were obtained. A noticeable drop in the DW factor above 6 GPa was observed, and these values agree with the value of the DW for BaZrO38 that has a centro-symmetric structure with no shift in the position of the Zr atom. This drop at an isotherm can be considered as a representative of the polar displacement and therefore of the ferro-to-paraelectric phase transition. Below 6 GPa, the obtained values of DW are in good agreement with what was obtained for PZT9. 1B. Noheda, D. E. Cox, G. Shirane, J. A. Gonzalo, L. E. Cross, and S.-E. Park, Appl. Phys. Lett. 74, 2059 (1999). 2B. Noheda, J. A. Gonzalo, L. E. Cross, R. Guo, S.-E. Park, D. E. Cox, and G. Shirane, Phys. Rev. B 61, 8687 (2000). 3J. Rouquette, J. Haines, V. Bornand, M. Pintard, Ph. Papet, R. Astier, J. M. Léger, and F. Gorelli, Phys. Rev. B 65, 2141021 (2002); J. Rouquette, J. Haines, V. Bornand, M. Pintard, Ph. Papet, W. G. Marshall, and S. Hull, Phys. Rev. B 71, 024112 (2005). 4 A.M. Glazer, P.A. Thomas, K. Z. Baba-Kishi, G. K. H. Pang, and C. W. Tai, Phys. Rev. B 70, 184123 (2004). 5 D. L. Corker, A. M. Glazer, R. W. Whatmore, A. Stallard, and F. J. Fauth, J. Phys.: Condens. Matter 10, 6251 (1998). 6 J. Frantti, S. Ivanov, J. Lappalainen, S. Eriksson, V. Lantto, S. Nishio, K. Kakihana, and H. Rudlöf, Ferroelectrics 266, 73 (2002). 7 I. Grinberg, V. R. Cooper, and A. M. Rappe, Nature 419, 909 (2002). 8 C.Laulhe, F.Hippert, J.Kreisel, M.Maglione, A.Simon, J.L.Hazemann, and V.Nassif, Phys. Rev. B 74, 014106, (2006). 9 D.Cao, I.-K.Jeong, R.H.heffner, T.Darling, J.-K.Lee, F.Bridges, J.-S.Park, and K.-S.Hong , Phys. Rev. B 70, 224102, (2004).