A heat-insulating material has a first phase with the stoichiometric composition of 0.1 to 10 mol-% M1.sub.2O.sub.3, 0.1 to 10 mol-% Li.sub.2O, and as the remainder M2.sub.2O.sub.3 with possible impurities. M1 is selected from the elements lanthanum, neodymium, gadolinium, or a mixture thereof, and M2 is selected from the elements aluminum, gallium, iron, or a mixture thereof. The first phase is present in a magnetoplumbite structure.