A thermal barrier coating (106, 606, 706, 806) comprising: a first layer (108, 708) selected to provide thermal cycling resistance deposited over a substrate, wherein the first layer (108, 708) comprises: a base oxide comprising at least one of zirconia and hafnia, a primary dopant comprising from 2 mol. % to 10 mol. % ytterbia, a first co-dopant comprising from 0.1 mol. % to 5 mol. % samaria, and a second co-dopant comprising from 0.1 mol. % to 5 mol. % of at least one of lutetia, scandia, ceria, gadolinia, neodymia and europia, and the balance base oxide; wherein the first layer (108, 708) has a metastable tetragonal low k phase constitution; and a second layer (110, 710, 814) selected to provide low thermal conductivity, deposited over the first layer (108, 708), wherein the second layer comprises: a base oxide comprising at least one of zirconia and hafnia, a primary dopant comprising from 4 mol. % to 20 mol. % ytterbia, a first co-dopant comprising from 1 mol. % to 10 mol. % samaria, and a second co-dopant comprising from 1 mol. % to 10 mol. % of at least one of lutetia, scandia, ceria, gadolinia, neodymia and europia, and the balance base oxide; wherein the second layer (110, 710, 814) has a cubic low k phase constitution; and wherein the composition of the first layer (108, 708) is different from the composition of the second layer (110, 710, 814). a first layer (108, 708) selected to provide thermal cycling resistance deposited over a substrate, wherein the first layer (108, 708) comprises: a base oxide comprising at least one of zirconia and hafnia, a primary dopant comprising from 2 mol. % to 10 mol. % ytterbia, a first co-dopant comprising from 0.1 mol. % to 5 mol. % samaria, and a second co-dopant comprising from 0.1 mol. % to 5 mol. % of at least one of lutetia, scandia, ceria, gadolinia, neodymia and europia, and the balance base oxide; a base oxide comprising at least one of zirconia and hafnia, a primary dopant comprising from 2 mol. % to 10 mol. % ytterbia, a first co-dopant comprising from 0.1 mol. % to 5 mol. % samaria, and a second co-dopant comprising from 0.1 mol. % to 5 mol. % of at least one of lutetia, scandia, ceria, gadolinia, neodymia and europia, and the balance base oxide; wherein the first layer (108, 708) has a metastable tetragonal low k phase constitution; and a second layer (110, 710, 814) selected to provide low thermal conductivity, deposited over the first layer (108, 708), wherein the second layer comprises: a base oxide comprising at least one of zirconia and hafnia, a primary dopant comprising from 4 mol. % to 20 mol. % ytterbia, a first co-dopant comprising from 1 mol. % to 10 mol. % samaria, and a second co-dopant comprising from 1 mol. % to 10 mol. % of at least one of lutetia, scandia, ceria, gadolinia, neodymia and europia, and the balance base oxide; a base oxide comprising at least one of zirconia and hafnia, a primary dopant comprising from 4 mol. % to 20 mol. % ytterbia, a first co-dopant comprising from 1 mol. % to 10 mol. % samaria, and a second co-dopant comprising from 1 mol. % to 10 mol. % of at least one of lutetia, scandia, ceria, gadolinia, neodymia and europia, and the balance base oxide; wherein the second layer (110, 710, 814) has a cubic low k phase constitution; and wherein the composition of the first layer (108, 708) is different from the composition of the second layer (110, 710, 814).