What about
:
17.12. TXL - ZRH > LX 967
17.12. ZRH - BCN > LX 1950 (Arrival BCN: 22:30)
18.12. BCN - ZRH > LX 1951 (Depart BCN: 06:30)
18.12. ZRH - TXL > LX 976
That was the cheapest possible routing i found for € 418,--
As you have just to kill ~ 8 hours you can just look for a nice bar or maybe even the lounge has open (to save the money for a hotel room)