Abstract: A ceramic including a first phase having a general formula R1+(x/2)Zr4P6-xSixO24 where R is selected from the group consisting of Ba, Ca, and Sr and 0≦x≦2, wherein the first phase has a volumetric heat capacity (Cp1), and at least 10 weight percent of a second phase having a volumetric heat capacity (Cp2), wherein Cp2>Cp1. The ceramic has a coefficient of thermal expansion from 22° to 1000° C. of −15×10−7/° C. to +15×10−7/° C., a permeability of at least 0.25×10−12 m2, a total porosity of at least 35% by volume, and a median pore diameter of at least 6 micrometers, and a volumetric heat capacity of the solid Cp(solid) of at least 3.15 J/cm3 K.