Compounds are generally provided, which may be particularly used to form a layer in a coating system. In one embodiment, the compound may have the formula: AxBbLn1-x-bHf1-t-dTitDdMO6, where: A is Al, Ga, In, Sc, Y, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Fe, Cr, Co, Mn, Bi, or a mixture thereof; x is about 0.01 to about 0.99; b is 0 to about 0.5, with 1-x-b being 0 to about 0.99 such that Ln is present in the compound; Ln is a rare earth or a mixture thereof that is different than A; t is 0 to about 0.99; D is Zr, Ce, Ge, Si, or a mixture thereof; d is 0 to about 0.5; the sum of t and d is less than 1 such that Hf is present in the compound; and M is Ta, Nb, or a mixture thereof.