The cristobalitelike forms of the ternary silica analogues BPO4 and BAsO4 were investigated at high pressure by x-ray diffraction and theoretical methods. The behavior of these compounds represents an extreme case in which the tilt angle of the constituent tetrahedra increases in a spectacular way at high pressure resulting in a major change in topology from a cristobalitelike framework towards a "collapsed cristobalite" structure. These compounds provide the first examples of the collapse of a framework structure to a close-packed form in a continuous manner without an intervening phase transition.

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The 0.255 Å/kbar linear pressure shift of the intense, narrow, and well-isolated 5D0-7F0 (named 0-0) emission line of SrB4O7:Sm2+ (at 6854.1 Å in ambient conditions) is proposed as a new optical pressure gauge for the diamond anvil cell. The pressure determinations can be made with the same experimental devices as those currently used for the ruby sensor. The good accuracy results from the singlet character and a small linewidth which is no more than 2.5 Å up to 200 kbar in a 4:1 methanol:ethanol mixture, provided the linewidth is ‘‘relaxed’’ through thermal cycles. Such a process can probably be of general interest for this purpose. In nonisostatic media, a similar pressure coefficient and a broadening of ∼0.1 Å/kbar are obtained. This broadening does not greatly disturb the pressure measurements, at least up to 200 kbar. High-pressure determinations up to 400 °C are also shown to be more accurate than with the ruby, owing to the very limited temperature effect on the wavelength (−0.001 Å/°C) and linewidth (∼0.005 Å/°C) of the 0-0 line. The other characteristics of the emission spectrum of SrB4O7:Sm2+ are promising for (a) simultaneous measurements of pressure and temperature (at least up to 400 °C) using this same material and (b) further research on other rare-earth-doped compounds. The possibility of pressure coefficients larger than that of the R1 ruby line is shown by the 0.45 Å/kbar value measured for another emission line of SrB4O7:Sm2+.