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To achieve standards specifications bulk acoustic wave (BAW) resonators have to be temperature compensated. This resonators consist on multilayer stack of different materials: a piezoelectric stack composed of two electrodes at the top and bottom of a piezoelectric layer, and an acoustic isolation. The temperature compensation is usually done by increasing the thickness of a silicon dioxide layer which presents a positive temperature coefficient. In order to improve and predict the behavior of the device, temperature coefficients of the different materials of the device have to be characterized. In this paper we present measurements of the dependence of sound velocity versus temperature by picosecond ultrasonics.