We study cosmological expansion in F (R) gravity using the trace of the field equations. High frequency oscillations in the Ricci scalar, whose amplitude increases as one evolves backward in time, have been predicted in recent works. We show that the approximations used to derive this result very quickly break down in any realistic model due to the non-linear nature of the underlying problem. Using a combination of numerical and semianalytic techniques, we study a range of models which are otherwise devoid of known pathologies. We find that high frequency asymmetric oscillations and a singularity at finite time appear to be present for a wide range of initial conditions. We show that this singularity can be avoided with a certain range of initial conditions, which we find by evolving the models forwards in time. In addition we show that the oscillations in the Ricci scalar are highly suppressed in the Hubble parameter and scale factor.

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The cesium lead bromide perovskite thin films prepared by chemical vapor deposition (CVD) have excellent optoelectronic properties， but the thin films generally have two different phase structures， CsPbBr 3 and CsPb 2 Br 5 . In our work， CVD method is considered to prepare cesium lead bromide perovskite thin films， and the effects of reaction pressure and N 2 flow on CsPb 2 Br 5 in the thin films were studied by X-ray diffraction (XRD)， scanning electron microscopy(SEM)， energy dispersive spectroscopy(EDS) and fluorescence spectrometer. The results show that the change of the reaction pressure has no effect on the CsPb 2 Br 5 phase. However， with the decrease of the N 2 flow， part of the CsPb 2 Br 5 phase in the film gradually transforms into the CsPbBr 3 phase， and its luminescence also converts from a broadband emission dominated by ~630 nm to a narrowband emission dominated by ~530 nm. Experimental results reveal that N 2 flow is an effective means to control the phase structure and luminescence properties of CsPb 2 Br 5 .Key words： chemical vapor deposition (CVD)； cesium lead bromide perovskite film； CsPb 2 Br 5 phase