A material represented by the following formula (M') 8 (M''M''') 6 O 24 (X,X') 2:M'''' formula wherein M' represents a monoatomic cation of Li, or a monoatomic cation of K, or a monoatomic cation of Rb; or a combination of at least two monoatomic cations of different alkali metals selected from Group 1 of the IUPAC periodic table of the elements, wherein the combination comprises 0 - 98 mol-% of the monoatomic cation of Na; M'' represents a trivalent monoatomic cation of an element selected from Group 13 of the IUPAC periodic table of the elements, or of a transition element selected from any of n Groups 3 - 12 of the IUPAC periodic table of the elements, or any combination of such cations; M''' represents a monoatomic cation of an element selected from Group 14 of the IUPAC periodic table of the elements, or of an element selected from any of Groups 13 and 15 of the IUPAC periodic table of the elements, or of Zn, or any combination of such cations; X represents an anion of an element selected from Group 17 of the IUPAC periodic table of the elements, or any combination of such anions, or wherein X is absent; X' represents an anion of an element selected from Group 16 of the IUPAC periodic table of the elements, or any combination of such anions, or wherein X' is absent; and M'''' represents a dopant cation of an element selected from transition metals of the IUPAC periodic table of the elements, or any combination of such cations; with the proviso that at least one of X and X' is present. M' represents a monoatomic cation of Li, or a monoatomic cation of K, or a monoatomic cation of Rb; or a combination of at least two monoatomic cations of different alkali metals selected from Group 1 of the IUPAC periodic table of the elements, wherein the combination comprises 0 - 98 mol-% of the monoatomic cation of Na; a monoatomic cation of Li, or a monoatomic cation of K, or a monoatomic cation of Rb; or a combination of at least two monoatomic cations of different alkali metals selected from Group 1 of the IUPAC periodic table of the elements, wherein the combination comprises 0 - 98 mol-% of the monoatomic cation of Na; M'' represents a trivalent monoatomic cation of an element selected from Group 13 of the IUPAC periodic table of the elements, or of a transition element selected from any of n Groups 3 - 12 of the IUPAC periodic table of the elements, or any combination of such cations; M''' represents a monoatomic cation of an element selected from Group 14 of the IUPAC periodic table of the elements, or of an element selected from any of Groups 13 and 15 of the IUPAC periodic table of the elements, or of Zn, or any combination of such cations; X represents an anion of an element selected from Group 17 of the IUPAC periodic table of the elements, or any combination of such anions, or wherein X is absent; X' represents an anion of an element selected from Group 16 of the IUPAC periodic table of the elements, or any combination of such anions, or wherein X' is absent; and M'''' represents a dopant cation of an element selected from transition metals of the IUPAC periodic table of the elements, or any combination of such cations; with the proviso that at least one of X and X' is present.