A new approach for the determination of multiple cation locations and ordering, using the example of natural and heat-treated columbites

A new approach is proposed to determine unambiguously the location of four cations within the crystal structure of a natural AB₂O₆ columbite-type compound and derivatives, when submitted to order-disorder transitions caused by heat treatments. This method is based on the successive use of electron microprobe analysis to determine the cation concentration, Mössbauer spectroscopy to identify the Fe occupation, and a crystal structure determination of the samples combining Rietveld refinements of both neutron and X-ray diffraction. This approach is tested successfully to investigate (Fe, Mn, Nb, Ta) natural minerals as well as oxides obtained by heat treatment of the initial AB₂O₆ columbite-type compound.

Computing details

Program(s) used to refine structure: FULLPROF.

(Columbite)

Crystal data

?	c = 5.0938 (8) Å
Mr = ?	V = 407.95 (11) Å^3
Orthorhombic, Pbcn	Z = ?
Hall symbol: -P 2n 2ab	Constant Wavelength Neutron Diffraction radiation, λ = 1.280000 Å
a = 14.097 (2) Å	?, ? × ? × ? mm
b = 5.6815 (9) Å

Data collection

Radiation source: nuclear reactor

2θmin = 8.148°, 2θmax = 87.948°, 2θstep = 0.200°

Refinement

Rp = 2.278	400 data points
Rwp = 3.185	28 parameters
Rexp = 0.612	0 restraints
RBragg = 8.472
χ^2 = NOT FOUND

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
O1	0.0899 (10)	0.110 (3)	0.0624 (14)	0.0065 (6)*
O2	0.4225 (10)	0.111 (3)	0.0878 (18)	0.0065 (6)*
O3	0.7583 (9)	0.1330 (13)	0.1017 (15)	0.0065 (6)*
FeA	0.00000	0.342 (7)	0.25000	0.0135 (8)*	0.33000
MnA	0.00000	0.342 (7)	0.25000	0.0135 (8)*	0.24 (3)
TaA	0.00000	0.342 (7)	0.25000	0.0135 (8)*	0.43 (3)
NbA	0.00000	0.342 (7)	0.25000	0.0135 (8)*	0.00000
FeB	0.166 (2)	0.167 (3)	0.750 (5)	0.0135 (8)*	0.13500
MnB	0.166 (2)	0.167 (3)	0.750 (5)	0.0135 (8)*	0.082 (16)
TaB	0.166 (2)	0.167 (3)	0.750 (5)	0.0135 (8)*	0.433 (16)
NbB	0.166 (2)	0.167 (3)	0.750 (5)	0.0135 (8)*	0.35000