Dear colleague,
Without making proper analysis of reasons for lower productivity, to think that adopting 6-day work week, that too to punish teams , is more likely to boomrang with adverse effect on team morale.
You need to remove kink in your thinking that you want do this to punish them.
On the contrary, if you take your teams into confidence and get their buy in for the need for improving productivity supported by data, it would give a lot more push to the proposal of 6-day working and it may change into win-win situation.
Regards,
Vinayak Nagarkar
HR-Consultant