Dear Davide, As Bob said, you should constrain RM11, RM22 and RM33 of the tricilinc phase to be equal to RM11 of the cubic one, then you MUST fix the RM12, RM13 and RM23 to be zero (use the Hold command in EXPEDT). If you don't fix RMij to zero then the angles will not be fixed in 90.
Also be careful with damping. If you still want to use some be sure that both phases got the same damping factor all the time. And additionally if your triclinic cell is not the same volume of the cubic one you must include the adequate scaling among the RMii values and in the phase fractions. I've usually found there is a difference in the 5th or 6th digit of the cell parameters that I attribute to rounding errors, but this should not make a difference. Hope this also helps. Leo Dr. Leopoldo Suescun Argonne National Laboratory Materials Science Division Bldg 223 9700 S. Cass Ave. Argonne IL 60439 Phone: 1 (630) 252-9760 Fax: 1 (630) 252-7777 ----- Original Message ----- From: "Von Dreele, Robert B." <[EMAIL PROTECTED]> Date: Wednesday, November 15, 2006 7:42 am Subject: RE: Re: Cell parameter constrain > Hi All, > A couple of points about lattice parameter constraints: > 1) Beware of using "damping"; it isn't needed especially for > lattice parameters in Rietveld refinements and it will mess up the > intent of constraints if not very carefully used. > 2) The constraints are on the reciprocal metric tensor elements > and must only include those that are refined for the Laue > symmetry. So for cubic only RM11 is available for a constraint to > another phase. > Best, > Bob Von Dreele > > ________________________________ > > From: Andreas Leineweber [mailto:[EMAIL PROTECTED] > Sent: Wed 11/15/2006 5:35 AM > To: rietveld_l@ill.fr > Subject: Re: Cell parameter constrain > > > Hello all, > > I can confirm that problem. I recall that there were constraints > imposed, but somehow with different factors than intended... > However, the problem was not that severe that I solved it. > Best regards > Andreas > > Davide Levy wrote: > > > Hi, > I am trying to constrain the cell parameters of two phases (cubic > and triclinic) in GSAS. I have given the constrain of the > reciprocal metric tensor, but when I refine the cell parameters > are not constrained. > how con do it correctly? > Davide > > Davide Levy > Dipartimento di Scienze Mineralogiche e Petrologiche > Universita' di Torino > V. Valperga Caluso 35 > I-10125 Torino > Italy > Phone: +(39)0116705122 > > > > > > > -- > Dr. Andreas Leineweber > Max-Planck-Institut fuer Metallforschung > Heisenbergstrasse 3 > 70569 Stuttgart > Germany > Tel. +49 711 689 3365 > Fax. +49 711 689 3312 > e-mail: [EMAIL PROTECTED] > home page of department: > http://www.mf.mpg.de/de/abteilungen/mittemeijer/english/index_english.h tm > >
begin:vcard n:Suescun;Leopoldo fn:Leopoldo Suescun tel;fax:1 (630) 252-7777 tel;work:1 (630) 252-9760 org:Argonne National Laboratory;Materials Science Division adr:;;9700 S. Cass Avenue;Argonne;IL;60439;USA version:2.1 email;internet:[EMAIL PROTECTED] title:Dr. end:vcard
