Form of silicon and method of making the same
The invention relates to a new phase of silicon, Si 24 , and a method of making the same. Si 24 has a quasi-direct band gap, with a direct gap value of 1.34 eV and an indirect gap value of 1.3 eV. The invention also relates to a compound of the formula Na 4 Si 24 and a method of making the same. N a 4Si 24 may be used as a precursor to make Si 24 .
1. A method of producing Si 24 comprising:
forming an Na 4 Si 24 precursor by reacting a mixture of silicon and sodium at a pressure from about 7 GPa to about 15 GPa and a temperature from about 320° C. to about 1500° C.;
subjecting the Na 4 Si 24 precursor to vacuum conditions at a temperature from about 40° C. to about 500° C. to produce Si 24 .
2. The method of claim 1 , wherein the precursor Na 4 Si 24 is dynamically stable at a temperature of about 25° C. and a pressure of about 1 atmosphere.
3. The method of claim 1 wherein the Na 4 Si 24 precursor contains 24 silicon atoms and 4 sodium atoms, has a space group of Cmcm, and has the following lattice constants: a=4.106 Å, b=10.563 Å and c=12.243 Å.
4. The method of claim 1 wherein Si 24 has a quasi-direct band gap, with a direct gap value of 1.34 eV and an indirect gap value of 1.3 eV.
5. The method of claim 1 wherein the ratio of sodium to silicon used to form Na 4 Si 24 is from about 10 to about 30 mol %.
6. The method of claim 1 wherein the mixture of sodium and silicon is heated in two steps, first to about 400° C. for about 30 minutes and second to about 800° C. for about 1.5 to 24 hours.
7. A method of producing Na 4 Si 24 by reacting a mixture of silicon and sodium at a pressure of greater than about 8 GPa and a temperature of greater than about 700° C.
8. The method of claim 7 wherein Na 4 Si 24 contains 24 silicon atoms and 4 sodium atoms, has a space group of Cmcm, and has the following lattice constants: a=4.106 Å, b=10.563 Å and c=12.243 Å.
9. The method of claim 7 wherein the ratio of sodium to silicon in the reaction is about 20 mol %.
10. A compound of the formula Si 2.4 having an orthorhombic structure.
11. The compound of claim 10 wherein the compound, has a space group of Cmcm (#63).
12. The compound of claim 11 having the following lattice constants: a=3.83 Å, b=10.69 Å and c=12.63 Å.
13. The compound of claim 10 having an indirect band gap of 1.3 eV and a direct band gap of 1.34 eV.
14. A compound of the formula Na 4 Si 24 .
15. The compound of claim 14 which is dynamically stable at a temperature of about 25° C. and a pressure of about 1 atmosphere.
16. The compound of claim 14 wherein Na 4 Si 24 contains 24 silicon atoms and 4 sodium atoms; has a space group of Cmcm; and has the following lattice constants: a=4.106 Å, b=10.563 Å and c=12.243 Å.