Patent Publication Number: US-4322500-A

Title: Potassium fluoroborate silicate glasses

Description:
This invention relates to fluorine containing glasses. 
     Certain sodium silicate and sodium borosilicate glasses in which some of the oxygen is replaced by fluorine are known, and some use has been made of a limited number of compositions in order to obtain particular refractive index/dispersion relationships. In such glasses it is believed that the extent of replacement of oxygen with fluorine atoms has been limited to about 10%. 
     According to the present invention there is provided a potassium fluoroborate, potassium alumino-fluoroborate, potassium fluoroborate silicate, or potassium alumino-fluoroborate-silicate glass, which glass contains at least 20 mole % potassium computed on the basis of the stoiciometric relationship K 2  (O,F 2 ), contains not more than 5 mole % in aggregate of other glass modifying oxides and fluorides and not more than 5 mole % in aggregate of other glass forming oxides and fluorides, and wherein the proportion of fluorine atoms to oxygen atoms in the glass is at least 5%. 
     There follows a description of illustrative embodiments of the invention, and of related glass compositions. The description refers to the accompanying drawing which is a graph showing the relationship between refractive index and fluorine content. 
     One characteristic of these glasses is that they tend to have low refractive indices, and hence in the case of clear homogeneous glasses, can be used in optical elements such as lenses and the like. Another use is in optical waveguide elements. One material commonly used as the high index core material of a step-index waveguide is silica. Since silica is itself a low index glass, the lower index cladding material commonly used in conjunction with a silica cored waveguide is a plastics material. However, for some applications such as for instance in optical fibre connectors it is often desirable to employ a dimensionally more stable cladding material. Such a material may be provided by one of the compositions having a refractive index less than that of silica. 
     Another characteristic of at least some of these glasses, particularly those that do not readily if at all form clear homogeneous glasses, is that they tend to have relatively large coefficients of thermal expansion that are comparable with those of many metals including aluminum, copper and brass. These compositions hence also find application as expansion coefficient matched glasses for metal-to-glass seals. 
     The partial replacement of oxygen with fluorine has been found generally to produce a reduction in refractive index approximately in proportion to the magnitude of the replacement. This replacement is at least 5%, is preferably 10% or more, and for glasses having a refractive index, n D , comparable with or lower than that of silica, there are more fluorine atoms present than oxygen atoms. 
     A low refractive index is also provided by selecting for the main cation constituent those ions with a low effective atomic refraction such as potassium rather than other alkali metals, and aluminum rather than alkaline earths. Preferably the mole percentage of potassium computed as K 2  (O,F 2 ) does not exceed 45 mole %, and that of aluminum computed as Al 2  (O 3 ,F 6 ) does not exceed 20 mole %. 
     The molar percentage of boron computed as B 2  (O 3 ,F 6 ) is typically selected to lie in the range 20 to 60 mole %. Silica may be included in the composition in order to stabilize the glass against devitrification and phase separation, and also against hydrolytic instability effects. For this purpose the molar percentage of silicon computed as Si(O 2 ,F 4 ) is typically at least 5 mole % to provide a significant stabilization effect, and not more than 35 mole % so as not to raise the melting point unduly high. 
     Even within these preferred ranges it is of course necessary, if an optical quality glass is required, to select regions of the composition diagram for which a person skilled in the art would expect neither liquid-liquid nor solid-liquid phase separation to be an insurmountable problem. As with most glass systems, minor additions of other glass modifiers may be included, such as oxides and fluorides of calcium and magnesium, up to an amount not exceeding 5 mole % in aggregate. Similarly minor additions to titanium and germanium may be included up to an amount not exceeding 5 mole % in aggregate. 
     The use of boric oxide and silica as glass forming oxides tends to set a potentially inconveniently low upper limit to the fluorine/oxygen atomic ratio in the glass when the fluorine atoms are introduced into the system solely by using conventional fluorides such as potassium fluoride and aluminum trifluoride. This limit can however be exceeded by adding part or all of the potassium and fluorine atoms to the system in the form of potassium fluoroborate. Some boron fluoride is lost by volatilization during melting, but under standardized conditions this loss can be compensated by altering the proportions of the constituents of the batch from which the glass is made. 
     The batch consisting of dry powdered constituents is well mixed and melted, preferably while being stirred. The melting may be carried out in a platinum crucible heated in dry air or other dry inert gas such as nitrogen, argon or oxygen, preferably in electrically heated furnace, at a temperature in the range 600° C. to 1200° C. depending upon composition. It will be clear that the relatively higher silica content glasses will in general require a higher melting temperature than the relatively lower silica content glasses. 
    
    
     Examples of typical compositions are given in the following table in which constituent quantities are expressed in mole %. The majority of these examples lie within the claimed composition range, but for the purpose of comparison related examples are included which lie beyond the borders of this range. 
     
                                           TABLE                                   
__________________________________________________________________________
Example                                                                   
      1   2   3   4   5   6   7   8   9   10  11  12   13  14             
__________________________________________________________________________
K.sub.2 O                                                                 
      37.3                                                                
          33.3                                                            
              46.8                                                        
                  44.9                                                    
                      38.3                                                
                          32.7                                            
                              27.2                                        
                                  22.1                                    
                                      21.6                                
                                          17.5                            
                                              21.8                        
                                                  25.7 23.5               
                                                           31.9           
      (44.5)*                                                             
          (37.7)                                                          
              (54.9)                                                      
                  (51.7)                                                  
                      (44.0)                                              
                          (37.3)                                          
                              (30.9)                                      
                                  (24.9)                                  
                                      (24.4)                              
                                          (19.6)                          
                                              (24.6)                      
                                                  (29.3)                  
                                                       (26.5)             
                                                           (37.8)         
B.sub.2 O.sub.3                                                           
      62.7                                                                
          48.3                                                            
              44.2                                                        
                  41.6                                                    
                      44.1                                                
                          45.7                                            
                              47.4                                        
                                  48.3                                    
                                      47.8                                
                                          48.3                            
                                              47.2                        
                                                  47.3 46.2               
                                                           50.2           
      (55.5)                                                              
          (40.7)                                                          
              (38.5)                                                      
                  (35.6)                                                  
                      (37.6)                                              
                          (38.7)                                          
                              (40.0)                                      
                                  (40.4)                                  
                                      (40.2)                              
                                          (40.3)                          
                                              (39.6)                      
                                                  (40.1)                  
                                                       (38.7)             
                                                           (44.2)         
SiO.sub.2                                                                 
      --  6.4 9.0 9.7 9.5 9.5 9.4 9.6 10.9                                
                                          10.8                            
                                              11.0                        
                                                  10.9 10.8               
                                                           11.8           
          (4.6)                                                           
              (6.8)                                                       
                  (7.2)                                                   
                      (7.0)                                               
                          (6.9)                                           
                              (6.8)                                       
                                  (6.9)                                   
                                      (7.9)                               
                                          (7.8)                           
                                              (7.9)                       
                                                  (8.0)                   
                                                       (7.8)              
                                                           (9.0)          
2(AlF.sub.3)                                                              
      --  --  --  --  --  --  --  --  --  --  --  --   --  --             
2(KF) --  12.1                                                            
              0   3.9 8.05                                                
                          12.1                                            
                              15.9                                        
                                  20.0                                    
                                      19.7                                
                                          23.4                            
                                              20.0                        
                                                  16.1 19.5               
                                                           6.13           
          (16.9)  (5.5)                                                   
                      (11.4)                                              
                          (17.0)                                          
                              (22.3)                                      
                                  (27.8)                                  
                                      (27.5)                              
                                          (32.4)                          
                                              (27.8)                      
                                                  (22.6)                  
                                                       (27.1)             
                                                           (9.0)          
2(KBF.sub.4)                                                              
      --  --  --  --  --  --  --  --  --  --  --  --   --  --             
ΣF/ΣO                                                         
      0   0.13                                                            
              0   0.04                                                    
                      0.08                                                
                          0.13                                            
                              0.17                                        
                                  0.21                                    
                                      0.21                                
                                          0.25                            
                                              0.22                        
                                                  0.17 0.21               
                                                           0.06           
K.sub.2 (O,F.sub. 2)                                                      
      37.3                                                                
          45.4                                                            
              46.8                                                        
                  48.8                                                    
                      46.4                                                
                          44.8                                            
                              43.1                                        
                                  42.1                                    
                                      41.3                                
                                          40.9                            
                                              41.8                        
                                                  41.8 43.0               
                                                           38.0           
Index 1.51                                                                
          1.489                                        1.465              
Appear-       Crys-                                    Opal-              
                                                           Good           
ance          tal-                                     escent             
                                                           glass          
              line                                     after              
                                                           after          
                                                       anneal             
                                                           anneal         
__________________________________________________________________________
Example                                                                   
      15  16  17  18  19  20  21  22  23  24  25  26   27  28             
__________________________________________________________________________
K.sub.2 O                                                                 
      26.75                                                               
          19.45                                                           
              41.2                                                        
                  35.9                                                    
                      42.3                                                
                          40.2                                            
                              29.4                                        
                                  24.5                                    
                                      28.8                                
                                          29.9                            
                                              15.0                        
                                                  30.0 26.5               
                                                           10.4           
      (31.8)                                                              
          (23.3)                                                          
              (43.6)                                                      
                  (36.5)                                                  
                      (42.0)                                              
                          (48.1)                                          
                              (32.2)                                      
                                  (25.8)                                  
                                      (32.8)                              
                                          (31.0)                          
                                              (15.0)                      
                                                  (29.3)                  
                                                       (26.8)             
                                                           (10.1)         
B.sub.2 O.sub.3                                                           
      52.7                                                                
          56.8                                                            
              39.1                                                        
                  38.5                                                    
                      31.9                                                
                          50.1                                            
                              51.3                                        
                                  51.3                                    
                                      47.5                                
                                          44.8                            
                                              44.4                        
                                                  45.0 42.1               
                                                           42.3           
      (46.6)                                                              
          (50.6)                                                          
              (30.7)                                                      
                  (29.1)                                                  
                      (23.6)                                              
                          (44.5)                                          
                              (41.8)                                      
                                  (40.2)                                  
                                      (40.2)                              
                                          (34.5)                          
                                              (33.0)                      
                                                  (32.6)                  
                                                       (31.6)             
                                                           (30.5)         
SiO.sub.2                                                                 
      12.2                                                                
          12.8                                                            
              9.5 10.3                                                    
                      10.0                                                
                          9.7 9.5 9.6 9.3 10.4                            
                                              10.5                        
                                                  5.2  10.1               
                                                           10.4           
      (9.3)                                                               
          (9.8)                                                           
              (6.4)                                                       
                  (6.7)                                                   
                      (6.4)                                               
                          (7.4)                                           
                              (6.7)                                       
                                  (6.5)                                   
                                      (6.8)                               
                                          (6.9)                           
                                              (6.7)                       
                                                  (3.2)                   
                                                       (6.5)              
                                                           (6.5)          
2(AlF.sub.3)                                                              
      --  --  10.2                                                        
                  15.3                                                    
                      15.8                                                
                          --  9.85                                        
                                  14.65                                   
                                      --  14.9                            
                                              15.0                        
                                                  20.0 15.8               
                                                           15.8           
              (19.3)                                                      
                  (27.8)                                                  
                      (28.0)  (19.3)                                      
                                  (27.6)  (27.6)                          
                                              (26.8)                      
                                                  (34.8)                  
                                                       (28.5)             
                                                           (27.4)         
2(KF) 8.38                                                                
          11.01                                                           
              --  --  --  --  --  --  14.45                               
                                          --  15.1                        
                                                  --   5.2 21.4           
      (12.3)                                                              
          (16.3)                      (20.3)  (18.6)   (6.5)              
                                                           (25.6)         
2(KBF.sub.4)                                                              
      --  --  --  --  --  --  --  --  --  --  --  --   --  --             
ΣF/ΣO                                                         
      0.08                                                                
          0.10                                                            
              0.34                                                        
                  0.53                                                    
                      0.60                                                
                          0   0.30                                        
                                  0.45                                    
                                      0.15                                
                                          0.49                            
                                              0.71                        
                                                  0.68 0.62               
                                                           0.88           
K.sub.2 (O,F.sub.2)                                                       
      35.1                                                                
          30.5                                                            
              41.2                                                        
                  35.9                                                    
                      42.3                                                
                          40.2                                            
                              29.4                                        
                                  24.5                                    
                                      43.3                                
                                          29.9                            
                                              30.1                        
                                                  30.0 31.7               
                                                           31.8           
Index                     1.507.sub. 5                                    
                              1.484.sub.0                                 
                                  1.481.sub.0                             
                                      1.491.sub.3                         
                                          1.483.sub.5                     
                                              1.468                       
                                                  --   1.479              
                                                           1.461          
Appear-                                                                   
      Good                                                                
          Good            Fluid                                           
                              V.visc.                                     
                                  Visc.                                   
                                      Fluid                               
                                          V.visc.                         
                                              V.visc.                     
                                                  Diff-                   
                                                       Visc.              
                                                           Fluid          
ance  glass                                                               
          glass                                   icult                   
      after                                                               
          after           good                                            
                              good                                        
                                  good                                    
                                      good                                
                                          good                            
                                              good                        
                                                  to melt                 
                                                       good               
                                                           good           
      anneal                                                              
          anneal          glass                                           
                              glass                                       
                                  glass                                   
                                      glass                               
                                          glass                           
                                              glass                       
                                                  1100° C.         
                                                       glass              
                                                           glass          
                          after                                           
                              after                                       
                                  after                                   
                                      after                               
                                          after                           
                                              after    after              
                                                           after          
                          anneal                                          
                              anneal                                      
                                  anneal                                  
                                      anneal                              
                                          anneal                          
                                              anneal   anneal             
                                                           anneal         
__________________________________________________________________________
Example                                                                   
      29  30  31  32  33  34  35  36  37  38  39  40   41  42             
__________________________________________________________________________
K.sub.2 O                                                                 
      28.1                                                                
          11.0                                                            
              5.45                                                        
                  10.85                                                   
                      16.5                                                
                          21.7                                            
                              0   5.3 5.0 10.0                            
                                              0   5.0  10.0               
                                                           0              
      (29.6)                                                              
          (11.1)                                                          
              (5.1)                                                       
                  (10.3)                                                  
                      (15.7)                                              
                          (20.9)  (5.1)                                   
                                      (4.8)                               
                                          (9.7)   (4.7)                   
                                                       (9.5)              
B.sub.2 O.sub.3                                                           
      44.3                                                                
          44.6                                                            
              35.45                                                       
                  35.25                                                   
                      34.5                                                
                          34.5                                            
                              41.2                                        
                                  40.8                                    
                                      40.2                                
                                          40.2                            
                                              39.0                        
                                                  39.8 39.4               
                                                           39.4           
      (34.7)                                                              
          (33.6)                                                          
              (24.8)                                                      
                  (24.9)                                                  
                      (24.4)                                              
                          (24.7)                                          
                              (28.9)                                      
                                  (28.9)                                  
                                      (28.5)                              
                                          (28.9)                          
                                              (26.7)                      
                                                  (27.7)                  
                                                       (27.7)             
                                                           (28.2)         
SiO.sub.2                                                                 
      10.8                                                                
          10.7                                                            
              10.8                                                        
                  10.8                                                    
                      11.0                                                
                          11.0                                            
                              10.3                                        
                                  10.4                                    
                                      10.0                                
                                          10.0                            
                                              9.8 9.7  10.0               
                                                           9.8            
      (7.3)                                                               
          (6.9)                                                           
              (6.5)                                                       
                  (6.6)                                                   
                      (6.7)                                               
                          (6.8)                                           
                              (6.2)                                       
                                  (6.3)                                   
                                      (6.1)                               
                                          (6.2)                           
                                              (5.8)                       
                                                  (5.8)                   
                                                       (6.0)              
                                                           (6.0)          
2(AlF.sub.3)                                                              
      11.2                                                                
          11.1                                                            
              15.6                                                        
                  15.6                                                    
                      16.3                                                
                          16.3                                            
                              16.1                                        
                                  16.2                                    
                                      14.9                                
                                          14.8                            
                                              18.4                        
                                                  18.1 18.2               
                                                           10.2           
      (21.1)                                                              
          (20.1)                                                          
              (26.2)                                                      
                  (26.5)                                                  
                      (27.7)                                              
                          (28.1)                                          
                              (27.1)                                      
                                  (27.6)                                  
                                      (25.4)                              
                                          (25.5)                          
                                              (30.3)                      
                                                  (30.3)                  
                                                       (30.7)             
                                                           (17.5)         
2(KF) 5.6 22.6                                                            
              32.25                                                       
                  27.05                                                   
                      21.8                                                
                          16.4                                            
                              32.4                                        
                                  27.3                                    
                                      29.9                                
                                          25.0                            
                                              32.8                        
                                                  27.3 22.4               
                                                           40.55          
      (7.3)                                                               
          (28.2)                                                          
              (37.4)                                                      
                  (31.7)                                                  
                      (25.6)                                              
                          (19.5)                                          
                              (37.7)                                      
                                  (32.1)                                  
                                      (35.2)                              
                                          (29.8)                          
                                              (37.3)                      
                                                  (31.6)                  
                                                       (26.1)             
                                                           (48.2)         
2(KBF.sub.4)                                                              
      --  --  --  --  --  --  --  --  --  --  --  --   --  --             
ΣF/ΣO                                                         
      0.42                                                                
          0.68                                                            
              1.19                                                        
                  1.07                                                    
                      1.00                                                
                          0.89                                            
                              1.12                                        
                                  1.02                                    
                                      1.02                                
                                          0.92                            
                                              1.29                        
                                                  1.13 1.04               
                                                           1.03           
K.sub.2 (O,F.sub.2)                                                       
      33.7                                                                
          33.6                                                            
              37.7                                                        
                  37.9                                                    
                      38.3                                                
                          38.1                                            
                              32.4                                        
                                  32.6                                    
                                      34.9                                
                                          35.0                            
                                              32.8                        
                                                  32.3 32.4               
                                                           40.55          
Index 1.481                                                               
          1.460.sub.3                                                     
              --  --  1.465                                               
                          1.472                                           
                              1.447.sub.5                                 
                                  1.454                                   
                                      1.451                               
                                          1.456                           
                                              1.451                       
                                                  1.456                   
                                                       1.460              
Appear-                                                                   
      Fluid                                                               
          Fluid                                                           
              Cryst.                                                      
                  Cryst.                                                  
                      Slight                                              
                          Good                                            
                              Good                                        
                                  Good                                    
                                      Opalescent on    Visc-              
                                                           V.fluid        
ance          or  or  opal-                                               
                          glass                                           
                              glass                                       
                                  glass                                   
                                      anneal. Clear    ous opal-          
      good                                                                
          slight                                                          
              phase                                                       
                  phase                                                   
                      escent          on rapid cool        escent         
      glass                                                               
          opal-                                                           
              separ-                                                      
                  separ-                                                  
      after                                                               
          escent                                                          
              ated                                                        
                  ated                                                    
      anneal                                                              
__________________________________________________________________________
Example                                                                   
      43  44  45  46  47  48  49  50  51  52  53  54   55  56             
__________________________________________________________________________
K.sub.2 O                                                                 
      0   0   0   0   5.4 4.8 4.5 4.4 4.6 4.3 4.0 13.3 5.3 16.0           
                      (4.5)                                               
                          (4.4)                                           
                              (4.2)                                       
                                  (4.2)                                   
                                      (4.3)                               
                                          (4.1)                           
                                              (3.9)                       
                                                  (12.8)                  
                                                       (4.9)              
                                                           (15.1)         
B.sub.2 O.sub.3                                                           
      37.0                                                                
          40.6                                                            
              40.5                                                        
                  38.4                                                    
                      35.6                                                
                          31.3                                            
                              29.6                                        
                                  27.3                                    
                                      16.6                                
                                          22.9                            
                                              23.0                        
                                                  20.4 23.7               
                                                           23.2           
      (26.0)                                                              
          (28.8)                                                          
              (28.5)                                                      
                  (26.6)                                                  
                      (21.8)                                              
                          (21.2)                                          
                              (20.4)                                      
                                  (19.1)                                  
                                      (11.5)                              
                                          (16.2)                          
                                              (16.7)                      
                                                  (14.6)                  
                                                       (16.3)             
                                                           (16.3)         
SiO.sub.2                                                                 
      10.2                                                                
          9.9 9.6 9.9 10.4                                                
                          13.5                                            
                              17.7                                        
                                  21.9                                    
                                      29.6                                
                                          27.3                            
                                              30.0                        
                                                  29.9 21.2               
                                                           21.3           
      (6.2)                                                               
          (6.0)                                                           
              (5.8)                                                       
                  (5.9)                                                   
                      (5.5)                                               
                          (7.9)                                           
                              (10.5)                                      
                                  (13.2)                                  
                                      (17.6)                              
                                          (16.7)                          
                                              (18.7)                      
                                                  (18.5)                  
                                                       (12.5)             
                                                           (12.9)         
2(AlF.sub.3)                                                              
      11.9                                                                
          12.8                                                            
              14.1                                                        
                  16.1                                                    
                      19.5                                                
                          17.3                                            
                              16.5                                        
                                  15.9                                    
                                      16.9                                
                                          15.6                            
                                              14.7                        
                                                  17.6 15.9               
                                                           16.0           
      (20.1)                                                              
          (21.8)                                                          
              (23.8)                                                      
                  (26.8)                                                  
                      (28.7)                                              
                          (28.1)                                          
                              (27.3)                                      
                                  (26.8)                                  
                                      (28.1)                              
                                          (26.6)                          
                                              (25.6)                      
                                                  (30.4)                  
                                                       (26.2)             
                                                           (27.0)         
2(KF) 40.8                                                                
          36.8                                                            
              35.8                                                        
                  35.5                                                    
                      21.1                                                
                          18.7                                            
                              17.8                                        
                                  17.2                                    
                                      18.2                                
                                          16.9                            
                                              15.9                        
                                                  6.6  18.5               
                                                           8.0            
      (47.7)                                                              
          (43.3)                                                          
              (41.8)                                                      
                  (40.8)                                                  
                      (21.5)                                              
                          (21.0)                                          
                              (20.4)                                      
                                  (20.0)                                  
                                      (20.9)                              
                                          (19.9)                          
                                              (19.1)                      
                                                  (7.9)                   
                                                       (21.1)             
                                                           (9.3)          
2(KBF.sub.4)                                                              
      --  --  --  --  8.13                                                
                          7.2 6.9 6.6 7.1 6.5 6.1 6.1  7.7 7.7            
                      (18.0)                                              
                          (17.5)                                          
                              (17.2)                                      
                                  (16.7)                                  
                                      (17.7)                              
                                          (16.6)                          
                                              (15.9)                      
                                                  (15.8)                  
                                                       (19.0)             
                                                           (19.5)         
ΣF/ΣO                                                         
      1.16                                                                
          1.06                                                            
              1.11                                                        
                  1.24                                                    
                      1.69                                                
                          1.58                                            
                              1.47                                        
                                  1.40                                    
                                      1.71                                
                                          1.40                            
                                              1.27                        
                                                  1.21 1.63               
                                                           1.36           
K.sub.2 (O,F.sub.2)                                                       
      40.8                                                                
          36.8                                                            
              35.8                                                        
                  35.5                                                    
                      34.6                                                
                          30.7                                            
                              29.2                                        
                                  28.2                                    
                                      29.9                                
                                          27.7                            
                                              26.0                        
                                                  26.0 31.5               
                                                           31.7           
Index             1.447                                                   
                      1.429.sub.1                                         
                          1.425                                           
                              1.427                                       
                                  1.428                                   
                                      1.432                               
                                          1.433                           
                                              1.434                       
                                                  1.446                   
                                                       1.423              
                                                           1.435          
α × 10.sup.6                                                  
                      21.3                                                
                          20.6                                            
                              20.2                                        
                                  21.1                                    
                                      20.2                                
                                          18.4                            
                                              19.0     22.1               
                                                           21.4           
Appear-                                                Clear but not      
ance                                                   homogeneous        
__________________________________________________________________________
Example                                                                   
       57  58   59    60  61   62  63  64  65  66  67  68  69             
__________________________________________________________________________
K.sub.2 O                                                                 
       24.0                                                               
           16.0 16.0 16.1 16.1 15.9                                       
                                   16.8                                   
                                       10.7                               
                                           5.4 0   5.1 5.5 0              
       (23.0)                                                             
           (15.1)                                                         
                (15.1)                                                    
                     (15.3)                                               
                          (15.0)                                          
                               (14.8)                                     
                                   (15.8)                                 
                                       (10.6)                             
                                           (5.1)   (5.0)                  
                                                       (6.4)              
B.sub.2 O.sub.3                                                           
       23.1                                                               
           22.5 17.2 11.6 16.8 17.8                                       
                                   13.3                                   
                                       32.0                               
                                           34.6                           
                                               37.3                       
                                                   30.7                   
                                                       32.7               
                                                           31.3           
       (16.5)                                                             
           (15.7)                                                         
                (12.1)                                                    
                     (8.2)                                                
                          (11.7)                                          
                               (12.3)                                     
                                   (9.3)                                  
                                       (23.6)                             
                                           (24.4)                         
                                               (28.2)                     
                                                   (22.3)                 
                                                       (28.4)             
                                                           (23.3)         
SiO.sub.2                                                                 
       21.4                                                               
           21.4 26.7 32.1 26.9 26.5                                       
                                   28.0                                   
                                       21.3                               
                                           21.6                           
                                               21.2                       
                                                   20.5                   
                                                       21.8               
                                                           20.8           
       (13.1)                                                             
           (12.9)                                                         
                (16.2)                                                    
                     (19.5)                                               
                          (16.1)                                          
                               (15.8)                                     
                                   (16.8)                                 
                                       (13.6)                             
                                           (13.1)                         
                                               (13.8)                     
                                                   (12.8)                 
                                                       (16.3)             
                                                           (13.3)         
2(AlF.sub.3)                                                              
       16.0                                                               
           16.0 16.0 16.1 18.8 21.2                                       
                                   14.0                                   
                                       10.7                               
                                           8.7 7.4 10.4                   
                                                       5.5 5.2            
       (27.4)                                                             
           (26.9)                                                         
                (27.0)                                                    
                     (27.3)                                               
                          (31.4)                                          
                               (35.2)                                     
                                   (23.5)                                 
                                       (19.0)                             
                                           (14.7)                         
                                               (13.5)                     
                                                   (18.1)                 
                                                       (11.5)             
                                                           (9.3)          
2(KF)  0   8.0  8.0  8.1  10.7 13.2                                       
                                   5.6 6.6 18.9                           
                                               18.9                       
                                                   17.8                   
                                                       10.0               
                                                           30.2           
           (9.3)                                                          
                (9.3)                                                     
                     (9.5)                                                
                          (12.3)                                          
                               (15.2)                                     
                                   (6.5)                                  
                                       (8.1)                              
                                           (22.1)                         
                                               (23.7)                     
                                                   (21.4)                 
                                                       (14.4)             
                                                           (37.3)         
2(KBF.sub.4)                                                              
       7.8 8.0  8.0  8.0  5.4  2.7 11.2                                   
                                       9.4 8.1 7.6 7.8 7.3 6.3            
       (20.0)                                                             
           (20.2)                                                         
                (20.3)                                                    
                     (20.3)                                               
                          (13.5)                                          
                               (6.7)                                      
                                   (28.2)                                 
                                       (25.0)                             
                                           (20.6)                         
                                               (20.7)                     
                                                   (20.4)                 
                                                       (22.9)             
                                                           (16.9)         
ΣF/ΣO                                                         
       1.16                                                               
           1.40 1.46 1.53 1.48 1.43                                       
                                   1.64                                   
                                       1.02                               
                                           1.02                           
                                               0.93                       
                                                   1.15                   
                                                       0.89               
                                                           0.86           
K.sub.2 (O,F.sub.2)                                                       
       31.8                                                               
           32.0 32.0 32.2 32.2 31.8                                       
                                   33.6                                   
                                       26.7                               
                                           32.4                           
                                               26.5                       
                                                   30.7                   
                                                       32.8               
                                                           36.5           
Index  1.443                                                              
α × 10.sup.6                                                  
       19.5                                                               
           21.1 20.1 20.7 18.0 17.5                                       
                                   21.4                                   
                                       22.0                               
                                           25.0                           
                                               26.0                       
                                                   29.0                   
                                                       24.0               
                                                           22.0           
Appearance Visc.                                                          
                Visc.                                                     
                     Visc.                                                
                          Visc.                                           
                               White                                      
                                   Very                                   
                                       Opal-                              
                                           Opal-                          
                                               Opal-                      
                                                   White                  
                                                       White              
                                                           White          
           at   at   at   at   con-                                       
                                   opal-                                  
                                       escent                             
                                           escent                         
                                               escent                     
                                                   opal-                  
                                                       opal-              
                                                           opal-          
           1100° C.                                                
                1100° C.                                           
                     1100° C.                                      
                          1050° C.                                 
                               stit-                                      
                                   escent          escent                 
                                                       escent             
                                                           escent         
                               uent                                       
                                   glass                                  
                               in                                         
                               glass                                      
__________________________________________________________________________
Example                                                                   
       70  71  72  73  74  75  76  77  78  79  80  81  82  83             
__________________________________________________________________________
K.sub.2 O                                                                 
       19.7                                                               
           18.9                                                           
               11.1                                                       
                   10.9                                                   
                       14.7                                               
                           19.6                                           
                               24.6                                       
                                   23.7                                   
                                       22.6                               
                                           4.0 8.8 4.0 4.2 4.1            
       (20.0)                                                             
           (18.4)                                                         
               (11.0)                                                     
                   (11.0)                                                 
                       (14.1)                                             
                           (18.6)                                         
                               (22.9)                                     
                                   (22.6)                                 
                                       (22.0)                             
                                           (4.0)                          
                                               (8.8)                      
                                                   (4.0)                  
                                                       (4.1)              
                                                           (4.1)          
B.sub.2 O.sub.3                                                           
       29.5                                                               
           28.4                                                           
               33.2                                                       
                   32.7                                                   
                       19.6                                               
                           19.6                                           
                               19.7                                       
                                   24.4                                   
                                       22.4                               
                                           30.8                           
                                               30.7                       
                                                   28.1                   
                                                       34.0               
                                                           30.9           
       (22.3)                                                             
           (20.5)                                                         
               (24.5)                                                     
                   (24.4)                                                 
                       (14.0)                                             
                           (13.8)                                         
                               (13.6)                                     
                                   (17.3)                                 
                                       (16.2)                             
                                           (22.7)                         
                                               (22.9)                     
                                                   (20.9)                 
                                                       (24.9)             
                                                           (22.8)         
SiO.sub.2                                                                 
       19.7                                                               
           18.9                                                           
               22.1                                                       
                   21.8                                                   
                       26.9                                               
                           24.9                                           
                               19.6                                       
                                   18.9                                   
                                       22.6                               
                                           25.7                           
                                               25.6                       
                                                   29.4                   
                                                       22.1               
                                                           25.8           
       (12.8)                                                             
           (11.8)                                                         
               (14.1)                                                     
                   (14.0)                                                 
                       (16.5)                                             
                           (15.1)                                         
                               (11.7)                                     
                                   (11.5)                                 
                                       (14.1)                             
                                           (16.3)                         
                                               (16.4)                     
                                                   (18.9)                 
                                                       (14.0)             
                                                           (16.4)         
2(AlF.sub.3)                                                              
       9.8 14.2                                                           
               16.6                                                       
                   10.9                                                   
                       17.1                                               
                           19.6                                           
                               19.6                                       
                                   18.9                                   
                                       18.1                               
                                           14.7                           
                                               14.6                       
                                                   14.4                   
                                                       15.1               
                                                           14.7           
       (17.8)                                                             
           (24.7)                                                         
               (29.5)                                                     
                   (19.6)                                                 
                       (29.3)                                             
                           (33.1)                                         
                               (32.5)                                     
                                   (32.1)                                 
                                       (31.5)                             
                                           (26.0)                         
                                               (26.2)                     
                                                   (25.8)                 
                                                       (26.6)             
                                                           (26.1)         
2(KF)  18.1                                                               
           8.8 16.2                                                       
                   8.9 17.5                                               
                           12.1                                           
                               12.9                                       
                                   14.1                                   
                                       13.5                               
                                           19.1                           
                                               14.0                       
                                                   18.8                   
                                                       20.8               
                                                           19.4           
       (22.7)                                                             
           (10.6)                                                         
               (19.9)                                                     
                   (11.0)                                                 
                       (20.7)                                             
                           (14.1)                                         
                               (14.8)                                     
                                   (16.6)                                 
                                       (16.2)                             
                                           (23.3)                         
                                               (17.3)                     
                                                   (23.2)                 
                                                       (25.3)             
                                                           (23.8)         
2(KBF.sub.4)                                                              
       1.6 5.4 0.4 7.4 2.1 2.1 1.8 0   0   2.9 3.1 2.7 1.9 2.6            
       (4.4)                                                              
           (14.1)                                                         
               (1.1)                                                      
                   (19.9)                                                 
                       (5.4)                                              
                           (5.3)                                          
                               (4.5)       (7.7)                          
                                               (8.3)                      
                                                   (7.2)                  
                                                       (5.0)              
                                                           (6.9)          
ΣF/ΣO                                                         
       0.74                                                               
           1.03                                                           
               0.87                                                       
                   0.93                                                   
                       1.21                                               
                           1.24                                           
                               1.29                                       
                                   1.05                                   
                                       1.00                               
                                           1.01                           
                                               0.93                       
                                                   0.99                   
                                                       0.98               
                                                           1.00           
K.sub.2 (O,F.sub.2)                                                       
       39.4                                                               
           33.1                                                           
               27.7                                                       
                   27.2                                                   
                       34.3                                               
                           34.3                                           
                               39.3                                       
                                   37.8                                   
                                       36.1                               
                                           26.0                           
                                               25.9                       
                                                   25.5                   
                                                       26.9               
                                                           26.1           
Index          1.44                                                       
                   1.44    1.45    1.46    1.43                           
                                               1.43                       
                                                   1.43                   
                                                       1.43               
                                                           1.43           
α  × 10.sup.6                                                 
       26.0                                                               
           24.0                                                           
               19.0                                                       
                   22.0        20.5                                       
                                   19.0                                   
                                       18.0                               
                                           22.0                           
                                               19.0                       
                                                   20.5                   
                                                       22.0               
                                                           20.5           
Appearance                                                                
       Opal-                                                              
           Opal-                                                          
               Clear,                                                     
                   Nearly                                                 
                       Yel-                                               
                           Clear                                          
                               Yel-                                       
                                   Clear                                  
                                       Clear                              
                                           Clear                          
                                               Clear                      
                                                   Clear                  
                                                       Clear              
                                                           Clear          
       escent                                                             
           escent                                                         
               not clear                                                  
                       low glass                                          
                               low glass                                  
                                       glass                              
                                           glass                          
                                               glass                      
                                                   glass                  
                                                       glass              
                                                           glass          
               homog-  opal-   opal-                                      
               eneous  escent  escent                                     
__________________________________________________________________________
 *Values in parentheses are weight % corresponding to mole % immediately  
 above.                                                                   
 
    
     BRIEF DESCRIPTION OF DRAWING 
     The accompanying drawing shows in the form of a graph that there is an approximately linear relationship between the ratio of fluorine atoms to oxygen atoms ΣF/ΣO and refractive index. Only those compositions for which refractive index is quoted in the table to at least four significant figures are plotted. Those compositions which are designated in the table as being of uncertain homogeneity are represented by plots placed within parentheses. The table also gives a measure of the expansion coefficient α for some of the compositions.