The combination 3 wherein {TAIL ~ said rare earth material} is selected from the group consisting of yttrium, lanthanum and {HEAD ~ hafnium} 
The combination 3 wherein { TAIL ~ said rare earth material } is selected from the group consisting of yttrium , lanthanum and { HEAD ~ hafnium }
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD