tuerdi1982 Hi, When I use "Topas Academic" to carry out a multiphase Rietveld refinement I found some thing bit peculiar. When I refine the atomic coordinates of my target phase I did get a very decent fit, but after I fix the refined coordinates everything was destroyed. Just wondering why it behaves like that? Is it because my refinement wasn't stable or there is some other reasons for it? My second question is: is there any way to define the ratio of two isotopes in a phase and assign an atomic coordinates for them (any one isotope can randomly occupy the given atomic site, but they will not come together)? It will be greatly appreciated if any one can help with my problems. Thank you in advance! Sincerely
ahhill Could you do it as... neutron_data prm U_beq 0.8 site U x 0 y 0 z 0 occ 235U u_occ 1 min=0; max=1; beq = U_beq; occ 238U = 1-u_occ; beq + U_beq; Not 100% sure it will work.
tuerdi1982 Hi, Thank you for your response, I am very much appreciated. When I tried I got an error message, but after I change the "beq + U_beq" to "beq 0.2" it worked. I did understand the definition for occupancy and this is what I want, but I didn't understand the purpose of defining the "beq". The "beq" is isotropic temperature factor, why we have to define it? how it related with atomic ratio of two isotops in the phase? Sincerely
ahhill Sorry, my code got slightly jumbled. The two beq's should be the same. neutron_data prm U_beq 0.8 site U x 0 y 0 z 0 occ 235U u_occ 1 min=0; max=1; beq = U_beq; occ 238U = 1-u_occ; beq = U_beq;