Two competing soft modes and an unusual phase transition in the stuffed tridymite-type oxideBaAl2O4

Giant thermal vibrations in the framework compoundsBa1−xSrxAl2O4

“…The T C values determined in this study were plotted together with the previously reported data. 10,22 As shown in this figure, T C becomes largely suppressed as x increases, in agreement with previous reports. 22,26 Synchrotron XRD experiments using single crystals 10 revealed a continuous variation of the superlattice intensities at T C , indicating a second-order phase transition.…”

“…However, the B iso of the O br atom in this system with x ≥ 0.1 does not follow this general trend; instead, it exhibits fairly large values even at 15 K. According to our previous studies, this system possesses structural instabilities at the M-and K-points, leading to two energetically competing soft modes. 10 Mode analyses have revealed that the O br atom shows a remarkable in-plane vibration around the ideal 2d site. In addition, strong thermal diffuse scattering because of these soft modes has been observed in the electron diffraction patterns for x ≥ 0.1 over a wide range of temperatures between 100 and 800 K. 22 Clearly, these soft modes are responsible for the unusually large B iso observed in this study.…”

“…Below this line, the M-point mode fluctuates with short-range correlations in nanoscale regions. 10 The anomalously large and temperature-independent B iso even at low temperatures can be attributed to this fluctuating M-point mode.…”

“…This compound undergoes a structural phase transition from a high-temperature phase with a space group of P 6 3 22 to a low-temperature phase with P 6 3 at approximately 450 K. [10][11][12] This transition is accompanied by the significant tilting of the Al-O-Al bond angle along the c-axis, giving rise to the enlargement of the cell volume to 2a × 2b × c. The crystal structures of the hightemperature and low-temperature phases are shown in Figs. 1(a) and (b), respectively.…”

“…10 According to the calculations, this compound possesses two unstable phonon modes at the M-and K-points with nearly the same energies. Both of these unstable phonon modes gave rise to strong diffuse scattering intensities in a wide range of temperatures below 800 K. Interestingly, their intensities sharply increase towards T C , indicating that the two modes soften simultaneously.…”