Wyckoff positions for magnetic atoms

zhouxiuquan

Hello all,

I am new to magnetic refinement using TOPAS 5. I have followed the examples on the Durham tutorial site. I am trying to use TOPAS 5 to refine a magnetic structure that we have solved recently using Fullprof, but I am having difficulties finding suitable positions for a magnetic atom.

The parent phase is a ThCr2Si2-type compound with space group I4/mmm. The Mn atom at (0.5, 0.5, 0.25) is the only magnetic ion, and we can fit the neutron diffraction data with with a propagation vector of (0.5, 0.5, 0) using fullprof. We have identified the magnetic space group as CAmmm (65.490). See the link: http://cryst.ehu.es/cryst/magnext.php?radical_sub=65.490&radical_sub_subsub=&label=C_Ammm&type=4&radical_label=Cmmm&radical_og=69&sub_og=6&subsub_og=610&label_og=F_Cmmm&og_disp=&wyckoff_positions=1

I created a separate magnetic supercell with new a(mag) = 2*a(nuclear) and b(mag) = 2*a(nuclear). Everything else worked fine except for the position of the new Mn atom in the supercell. Two special positions of the new magnetic cell can describes Mn position, but neither position is magnetic and TOPAS would not refinement any moment. General positions cannot describe the Mn atom, but I tried it to check the refinement. I can see all the magnetic peaks are indexed, but there is no intensity. I don't know what should I do to resolve this.

Any suggestion will be appreciated. Thank you very much.

Sincerely,
Xiuquan

johnsoevans

It's hard to give specific advice without knowing the system more.

As a first step I'd suggest you check your magnetic spacegroup to make sure you have everything correct. One easy way of doing this is in isodistort.

Upload your cif file, select which atom is magnetic, use method 2, select the M point (if that's what you think is relevant). Look through the choice of irreps to see if you come up with the same spacegroup. Explore this with the isodistort viewer to see if you can get the moment you "want" on the correct atoms. Export a topas .str file when you have identified what you want and refine magnetic mode amplitudes.

zhouxiuquan

Hi John,

Thank you very much for the reply. I haven't used isodistort before. I am going to try your suggestions.

I am sorry that the information was not enough. Here is the pasted magnetic cif file. Here is a link to the visualized image from the Bilbo webiste:http://webbdcrista1.ehu.es/magndata/mvisualize.htm?newcif=magndata_maxmagn_5734_5734.png&is_domain=0&is_domain_msg=0 Is it possible to use the first method described in your website?

Thanks,
Xiuquan

k-maximal magnetic symmetry
;

_parent_space_group.name_H-M 'I 4/m m m '
_parent_space_group.IT_number 139
_parent_space_group.transform_to_standard_Pp_abc 'a,b,c;0,0,0'

loop_
_magnetic_propagation_vector_seq_id
_magnetic_propagation_vector_kxkykz
k1 '0.5,0.5,0'

_magnetic_space_group.standard_setting 'no'
_magnetic_space_group.transform_from_parent_Pp_abc '2a,2b,c;0,0,0'
_magnetic_space_group.transform_to_standard_Pp_abc '-c,-1/2a+1/2b,1/2a+1/2b;0,0,0'

loop_
_magnetic_atom_site_moment_symmetry_constraints_label
_atom_site_magnetic_moment_symmetry_constraints_mxmymz
Mn1 mx,mx,0

_space_group.magn_number_BNS 65.490
_space_group.magn_name_BNS "C_A m m m"
_cell_length_a 7.87950
_cell_length_b 7.87950
_cell_length_c 13.21460
_cell_angle_alpha 90.00
_cell_angle_beta 90.00
_cell_angle_gamma 90.00

loop_
_space_group_symop.magn_id
_space_group_symop.magn_operation_xyz
_space_group_symop.magn_operation_mxmymz
1 x,y,z,+1 mx,my,mz
2 -x,-y,z,+1 -mx,-my,mz
3 -y,-x,-z,+1 -my,-mx,-mz
4 y,x,-z,+1 my,mx,-mz
5 -x,-y,-z,+1 mx,my,mz
6 x,y,-z,+1 -mx,-my,mz
7 y,x,z,+1 -my,-mx,-mz
8 -y,-x,z,+1 my,mx,-mz

loop_
_space_group_symop.magn_centering_id
_space_group_symop.magn_centering_xyz
_space_group_symop.magn_centering_mxmymz
1 x,y,z,+1 mx,my,mz
2 x+1/4,y+3/4,z+1/2,+1 mx,my,mz
3 x+1/2,y+1/2,z,+1 mx,my,mz
4 x+3/4,y+1/4,z+1/2,+1 mx,my,mz
5 x,y+1/2,z,-1 -mx,-my,-mz
6 x+1/4,y+1/4,z+1/2,-1 -mx,-my,-mz
7 x+1/2,y,z,-1 -mx,-my,-mz
8 x+3/4,y+3/4,z+1/2,-1 -mx,-my,-mz

loop_
_atom_site_label
_atom_site_type_symbol
_atom_site_fract_x
_atom_site_fract_y
_atom_site_fract_z
_atom_site_occupancy
K1 K 0.00000 0.00000 0.00000 1.
Cu1 Cu 0.00000 0.25000 0.25000 0.4944118
Mn1 Mn 0.00000 0.25000 0.25000 0.5055882
S1 S 0.00000 0.00000 0.35544 1.

loop_
_atom_site_moment_label
_atom_site_moment_crystalaxis_x
_atom_site_moment_crystalaxis_y
_atom_site_moment_crystalaxis_z
Mn1 0.10000 0.10000 0.00000