IN DEVICES USED HITHERTO FOR THE DIRECT CONVERSION OF HEAT INTO ELECTRICITY, COMMONLY KNOWN AS "THERMOELECTRIC ENERGY CONVERTERS," THE EFFICIENCY OF CONVERSION IS APPRECIABLY LOWER THAN THAT OF CONVENTIONAL RECIPROCATING OR ROTARY HEAT ENGINES. THE BASIC REASON FOR THIS LOW EFFICIENCY IS INHERENT IN THE PHYSICAL PROPERTIES OF THE MATERIALS SELECTED FOR THE MANUFACTURE OF THESE DEVICES. THE MATERIALS THAT HAVE BEEN AND ARE CURRENTLY BEING USED FOR THIS PURPOSE ARE INTERMETALLIC COMPOUNDS AND ALLOYS OF SILICON AND GERMANIUM. IN THIS INVENTION AN ENTIRELY NEW MATERIAL IS DEVELOPED. IT IS COMPOSED OF AN ALLOY OR SOLID SOLUTION OF THE THREE INTERMETALLIC COMPOUNDS: MAGNESIUM STANNIDE, MAGNESIUM GERMANIDE AND MAGNESIUM SILICIDE, AND DEFINED BY THE CHEMICAL FORMULA: MG2SIXGEYSN1-X-Y. THIS MATERIAL, WHEN PROPERLY DOPED, POSSESSES A FIGURE OF MERIT AND, CONSEQUENTLY, AN EFFICIENCY OF DIRECT CONVERSION OF THERMAL ENERGY INTO ELECTRICAL ENERGY FAR EXCEEDING THAT OF ANY OTHER MATERIAL PREVIOUSLY KNOWN OR USED.