The piezoelectric properties of the bismuth titanate niobate Bi7Ti4NbO21 have been studied. The two phase transitions at 677 degrees and 856 degrees C are analyzed based on the measured piezoelectric properties. It is shown that the Bi7Ti4NbO21 compound is a ferroelectric until 856 degrees C. Bi3TiNbO9 clusters are shown to exist within Bi7Ti4NbO21 grains. These clusters may be responsible for the small additional piezoelectric activity of the studied material at temperatures higher than 856 degrees C.