Broken tiles should be replaced with another whole tile
Broken roof tiles should only ever be sealed with silicon sealant as an emergency measure, until the damaged roof tile can then be replaced with another whole tile. Silicon and other such sealants deteriorate rapidly when exposed (as it is) to the elements of the weather ie. the harsh hot sun in the days and the cool of the nights. Within a short period of time the break will reopen and thus allow for rainwater to once again penetrate the upper ceiling area.
See Repairs & Restoration for more information