X-RAY DIFFRACTION ON (KCN)_X(KBr)_1-X MIXED CRYSTALS. Y. Yoshimura, T. Tsuruta, T. Okazaki, N. Tsuda and H. Iwasaki, Department of Physics, Ritsumeikan University, Kusatsu, Shiga 525-77, Japan

The phase transitions in the title compound (X=0.95, 0.90, 0.80, 0.70) were studied by X-ray precession method using single crystals at the temperature range between 85K and 300K. At 300K all the mixed crystals have the NaCl type cubic structure, with a disorder of linear CN^- ion orientation. As the temperature is decreased, the crystals undergo an order-disorder transition associated with the change in the orientation of the CN^- ions. The observed transition temperature for each crystal was 148.5K for x=0.95, 137.0K for x=0.90 and 108.5K for x=0.80. For x=0.70, no transition was observed till down to the 85K. A variety of diffraction photos indicating a coexistence of various forms were obtained in the low temperature phase. Paying attention to the presence of new reflections and splitting reflections, a coexistence of monoclinic, triclinic I and triclinic II forms was found in this phase. The lattice constants of each form for x=0.95 at 135.5K were shown in the following table.

         a ( Å )   b ( Å )  c ( Å   (  )     (  )     (  )    
                            )                                 
monocli  4.56(1)   4.57(1)  7.54(1           121.9(1          
.                           )                )                
tricli.  9.37(1)   4.41(1)  4.55(1  120.0(1  119.2(1  90.0(1  
I                           )       )        )        )       
tricli.  4.65(1)   4.75(1)  6.40(1  136.6(1  136.0(1  59.9(1  
II                          )       )        )        )       
                                                              

Crystallographic relationships between each form and those and between each

form and the cubic form are presented. The results are compared with the

findings in the related compounds KCN and RbCN.