Gadolinium Aluminate Garnet (Gd3Al5O12): Crystal Structure Stabilization Via Lutetium Doping and Properties of the (Gd1−xLux)3Al5O12 Solid Solutions (x = 0-0.5) (Gadolinium Aluminate Garnet (Gd3Al5O12): Crystal Structure Stabilization via Lutetium Doping and Properties of the (Gd1-xLux)3Al5O12 Solid Solutions (x=0-0.5))