OK, here it goes: Uiso = 1/3 * [U22 + 1/sin^2(beta)*(U11 + U33 + 2U13cos(beta)] = 1/3 * [0.01562 + (1/sin^2(110.18))*(0.01937 + 0.01277 + 2* (-0.00027)*cos(110.18)] = 1/3 * [0.01562 + (1/(0.9386)^2) * (0.03214 + (-0.00054)*(-0.345)] = 1/3 * [0.01562 + (1/0.88097) * 0.0323263] = 1/3 * [0.01562 + 1.135 * 0.0323263] = 1/3 * (0.01562 + 0.03669) = 1/3 * 0.05231 = 0.01726
The given Uiso for sulphur is, meanwhile, 0.01621. I have also been trying to calculate Uiso for melanophlogite, which is cubic, co the formula goes: Uiso = 1/3* (U11 + U22 + U33). Using this formula, I have obtained the value of 0.0339, which is exactly the same as the one given in the corresponding CIF file. However, calculating the Uiso using the GENERAL formula gives the value of 0.0287. Both values (0.0339 and 0.0287) were calculated using an EXCEL spread sheet. So how is such a difference possible? Best regards! Luke Kruszewski > Dear Łukasz, > > There must be something wrong with your calculation. A quick examination > shows that S must have a higher displacement parameter than Al and that > Ueq must be approximately 0.016. > > I hope this helps. > > Bob Gould > > > On 10/02/2014 21:50, "Łukasz Kruszewski" wrote: >> Dear Rietveld friends, >> >> I'm having some problems with calculating Uiso from the anisotropic >> parameters. Here is an example - I copy first two sites of aluminite >> (monoclinic): >> >> _atom_site_aniso_label >> _atom_site_aniso_U_11 >> _atom_site_aniso_U_22 >> _atom_site_aniso_U_33 >> _atom_site_aniso_U_12 >> _atom_site_aniso_U_13 >> _atom_site_aniso_U_23 >> S 0.01937 0.01562 0.01277 -0.00248 -0.00027 0.00954 >> Al1 0.01117 0.01169 0.01295 0.00006 -0.00012 0.00789 >> >> Using the formula for Uiso for the monoclinic system, which is: >> >> >> 1/3 * [U22 + 1/sin^2(beta)*(U11 + U33 + 2U13cos(beta)] >> >> for the "S" site we obtain the value of 0,0054 >> >> for "Al": 0.01225 >> >> Meanwhile, the listed Uiso are: >> >> _atom_site_label >> _atom_site_fract_x >> _atom_site_fract_y >> _atom_site_fract_z >> _atom_site_U_iso_or_equiv >> S 0.70076 0.37376 0.93018 0.01621 >> Al1 0.65581 0.45332 0.47633 0.01229 >> >> While the author-given value for Al is close to the one calculated >> myself, >> the one for S is much different. By the way, 0.01621 * 1/3 = 0.0054. >> >> I could find it a simple mistake, I've found such a problem in case of >> many other structures. And here's my kind question: am I missing >> something? >> >> Best regards! >> > > > -- > Robert Gould > Tel.UK: +44 (0)131 667 7230 or +44 (0)796 040 3872 > Canada: +1 519 387 8223 > > > --- > This email is free from viruses and malware because avast! Antivirus > protection is active. > http://www.avast.com > >
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