A Fluctuating State in the Framework Compounds (Ba,Sr)Al2O4

The structural fluctuation in hexagonal Ba-1-xSrxAl2O4 with a corner-sharing AlO4 tetrahedral network was characterized at various temperatures using transmission electron microscopy experiments. For x <= 0.05, soft modes of q similar to (1/2, 1/2, 0) and equivalent wave vectors condense at a transition temperature (T-c) and form a superstructure with a cell volume of 2a x 2b x c. However, T-c is largely suppressed by Sr-substitution, and disappears for x >= 0.1. Furthermore, the q similar to (1/2, 1/2, 0) soft mode deviates from the commensurate value as temperature decreases and survives in nanoscaled regions below similar to 200 K. These results strongly suggest the presence of a new quantum criticality induced by the soft mode. Two distinct soft modes were observed as honeycomb-type diffuse scatterings in the high-temperature region up to 800 K. This intrinsic structural instability is a unique characteristic of the framework compound and is responsible for this unusually fluctuating state.