One-dimensional ceramic oxide superconductors

Abstract

This application describes ceramic oxide superconductors, their precursors and methods of preparation. The superconductors contain no copper, but comprise the transition metals Nickel or Cobalt (M T ) together with bulky cations of Groups 2 and 1 of the Periodic Table (M 2+ and M 1+ ) such as Ba 2+ and K 1+ . Their hexagonal crystal structure comprises parallel chains of highly covalent, octahedrally coordinated oxygen around the transition metal, which exhibits its highest valence +4. The octahedra are facet-linked forming polyacid chains or macroanions that run the full length of crystallites or single crystals along the c axis. The chains are separated by distances that prevent covalency or orbital overlap among them. They are held together by coulombic forces from the cations located between them that also run parallel to the c axis. Electrical current, carried by holes, can only flow within the chains. Thus the materials are one-dimensional superconductors of composition M 2+ (1-x) M 1+ x M T O 3-δ .

1 . A crystalline superconducting composition having the formula  
         M 2+   (1-x) M 1+   x M T O 3-δ   
       where 
 M 2+  is a cation of Group 2 of the Periodic Table,  
 M 1+  is a cation of Group 1 of the Periodic Table,  
 M T  is the transition metal Ni or Co,  
 0.04<x≦0.30,  
 0≦δ≦0.20, and  
 having hexagonal symmetry.  
 
     
     
         2 . A composition according to  claim 1 , wherein 0≦δ≦0.10.  
     
     
         3 . A composition according to  claim 1 , wherein the hexagonal symmetry possesses space group P6 3 /mmc.  
     
     
         4 . A composition according to  claim 1 , wherein M 2+  is Ba 2+  or Sr 2+ .  
     
     
         5 . A composition according to  claim 1 , wherein M 1+  is Na 1+ , K 1+ , or Rb 1+ .  
     
     
         6 . A composition according to  claim 1 , wherein 0.05<x≦0.20.  
     
     
         7 . A composition according to  claim 1 , wherein the crystalline material is a single crystal.  
     
     
         8 . A composition according to  claim 7 , wherein the single crystal is larger than about 1 mm in diameter and about 4 mm in length.  
     
     
         9 . A composition having the formula  
         M 2+   (1-x) M 1+   x M T O 2+y    
       where 
 M 2+  is a cation of Group 2 of the Periodic Table,  
 M 1+  is a cation of Group 1 of the Periodic Table,  
 M T  is the transition metal Ni or Co,  
 0.04<x≦0.30, and  
 0≦y<0.90.  
 
     
     
         10 . A composition according to  claim 9 , wherein y is >0.75 and having hexagonal symmetry.  
     
     
         11 . A method of making a superconducting composition of the formula  
         M 2+   (1-x) M 1+   x M T O 3-δ   
       according to  claim 1 , comprising heating metal oxides of M 1+  and M T  with the peroxides of M 2+  in the required proportions under oxygen at temperatures ranging from about 450° C. to about 700° C.  
     
     
         12 . A method according to  claim 11 , wherein the metal oxides of M 1+  and M T  are higher oxides than M 1+   2 O and M T O.  
     
     
         13 . A method according to  claim 11 , wherein the reaction takes place at an absolute pressure of oxygen of about 2 bars±1 bar.  
     
     
         14 . A method of making a superconducting composition of the formula  
         
       M 2+   (1-x) M 1+   x M T O 3-δ   
         
       according to  claim 1 , comprising oxidizing a composition having the formula M 2+   (1-x) M 1+   x M T  O 2+y  under an absolute pressure of oxygen greater than 1000 bars and at a temperature of at least 500° C.  
     
     
         15 . A method of making superconducting single crystals of the formula  
         M 2+   (1-x) M 1+   x M T O 3-δ   
       according to  claim 1 , comprising placing a polycrystalline powder of M 2+   (1-x) M 1+   x M T O 2+y  where 0≦y<0.90 next to a mixture of M 2+ (OH) 2 .8H 2 O and M 1+ OH and heating the mixture to a temperature above about 450° C. to create a flux with a molar proportion which is in equilibrium at said temperature with said solid powder and at an absolute pressure of O 2  of greater than 500 bars for sufficient time to form single crystals of M 2+   (1-x) M 1+   x M T O 3-δ .  
     
     
         16 . The method of  claim 15 , wherein the absolute pressure of O 2  is greater than 500 bars.  
     
     
         17 . The method of  claim 15 , wherein a temperature gradient of at least 5° C. per cm is maintained within the flux in contact with the polycrystalline powder.  
     
     
         18 . A method of making a composition of the formula  
         M 2+   (1-x) M 1+   x M T O 2+y    
       according to  claim 9 , comprising mixing compounds of the three reactant metals M 2+ , M 1+ , and M T  in the required proportions and thermally treating the mixture in flowing air or oxygen at sufficient temperature to decompose and oxidize if necessary the compounds to form the composition of the formula  
         M 2+   (1-x )M 1+   x M T O 2+y .