Why Barium Titanate (BaTiO3) show the decrease in weight (accurately m) when it was heated?

the experiment was to heat barium titanate up to 1000 degree Celcius, while the heating in progress we could see its weight decreased slowly when the temperature hit 800 degree celcius. i know that there is transformation/shift in its structure, but i could not find the clear connection between the structural shift & weight decrease.