Y_0.76Ho_0.24FeGe₂O₇: a new member of thortveitite-like layered compounds

Y_0.76Ho_0.24FeGe₂O₇ (yttrium holmium iron digermanate) was synthesized by solid-state reaction at 1573 K. This thortveitite-like compound presents a crystallographic group–subgroup isotranslational (klassengleiche) relation with some other pyrogermanates, such as FeInGe₂O₇, In_1.08Gd_0.92Ge₂O₇ and InYGe₂O₇, which are configurationally isotypic with the Sc₂Si₂O₇ thortveitite structure first reported by Zachariasen [(1930). Z. Kristallogr. 73, 1–6]. Holmium cations share with yttrium the 4f Wyckoff position at the center of a seven-coordinated pentagonal bipyramid, while Fe atoms also occupy one site with Wyckoff position 4f at the center of the octahedron. All these sites have the point symmetry C₁. Two types of Ge₂O₇ diorthogroups with point symmetry C_1h are present in the structure, each one of them defining a layer type which alternates with the other. These diorthogroups have their tetrahedral groups in an eclipsed conformation.

Related literature

The method of preparation was based on work published by Cascales et al. (1998b). For related structures, see: Zachariasen (1930); Cascales et al. (1998a,b, 2002); Bucio et al. (2001); Redhammer et al. (2007).

Experimental

Crystal data

• Y_0.76Ho_0.24FeGe₂O₇

• M_r = 420.17

• Monoclinic, P 2₁ /m

• a = 9.6496 (2) Å

• b = 8.5073 (2) Å

• c = 6.6712 (2) Å

• β = 100.621 (1)°

• V = 538.27 (2) ų

• Z = 4

• Cu Kα radiation

• T = 295 K

• Specimen shape: flat sheet

• 20 × 20 × 0.2 mm

• Specimen prepared at 1573 K

• Particle morphology: spherical, brown

Data collection

• Bruker Advance D8 diffractometer

• Specimen mounting: packed powder sample container

• Specimen mounted in reflection mode

• Scan method: step

• 2θ_min = 10, 2θ_max = 80.0°

• Increment in 2θ = 0.02°

Refinement

• R_p = 0.07

• R_wp = 0.09

• R_exp = 0.06

• R_B = 0.03

• S = 1.53

• Wavelength of incident radiation: 1.54175 Å

• Profile function: pseudo-Voigt modified by Thompson et al. (1987)

• 843 reflections

• 60 parameters

Table 1
Selected geometric parameters (Å, °)

Ge1—O4^i	1.70 (3)
Ge1—O9^ii	1.82 (3)
Ge1—O10	1.78 (3)
Ge1—O10^iii	1.78 (3)
Ge2—O1	1.69 (3)
Ge2—O1^iv	1.69 (3)
Ge2—O4	1.98 (3)
Ge2—O8	1.78 (4)
Ge3—O3^v	1.77 (4)
Ge3—O5	1.82 (3)
Ge3—O5^iv	1.82 (3)
Ge3—O6^vi	1.76 (6)
Ge4—O2^vii	1.67 (3)
Ge4—O3^vii	1.73 (4)
Ge4—O7	1.87 (3)
Ge4—O7^iv	1.87 (3)

Ge1^viii—O4—Ge2	127.7 (16)
Ge3^ix—O3—Ge4^vi	165 (2)
Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) x, y, z-1; (vi) x+1, y, z; (vii) x-1, y, z; (viii) ; (ix) x, y, z+1.

Data collection: DIFFRAC/AT (Siemens, 1993); cell refinement: DICVOL91 (Boultif & Louër, 1991); data reduction: FULLPROF (Rodríguez-Carvajal, 2006); program(s) used to solve structure: coordinates taken from an isotypic compound (Cascales et al., 2002) show [/query]>; program(s) used to refine structure: FULLPROF ; molecular graphics: ATOMS (Dowty, 2000); software used to prepare material for publication: ATOMS.

Supporting information

S1. Comment

The crystal structure of the original thortveitite was first reported by Zachariasen (1930) and has the formula Sc₂Si₂O₇. In this silicate, the substitution of silicon has given rise to germanates, phosphates, arsenates and vanadates which present layered structures.

The ionic substitution from Si to Ge and Sc to trivalent metals and rare earths in thortveitite, give often germanates with thortveitite-like crystal structure RMGe₂O₇, where R and M represent cations of rare earths, transition metals, divalent or trivalent elements in octahedral coordination. The frameworks of these phases are built up from corner-sharing octahedra along ab planes forming a hexagonal disposition on layers interspersed with layers of Ge₂O₇ groups in staggered conformation (in Fig. 1a the octahedra appear in dark cyan, while the Ge₂O₇ group in yellow color).

Some ionic substitutions give rise to seven-coordinated cations occupying the half of octahedral sites in the thortveitite structure. In such case, the generalized formula can be written as MRX₂O₇ where X₂O₇ is the same diorthogroup mentioned before presenting almost the same features as in thortveitite. The octahedral sites split in two new sites: half for cation M and other half for cation R, such as the cases for R = Y, Tb–Yb (Cascales et al., 1998a,b, 2002). M remains with octahedral coordination while R changes its coordination to seven. In the present work we present the crystal structure of the new compound Y_1-xHo_xFeGe₂O₇ with x = 0.24.

Fig. 1b show the crystal structure of Y_0.76Ho_0.24FeGe₂O₇ in which the bridging O atoms at the middle of the Ge₂O₇ diorthogroup are displaced up (u) or down (d) along a direction normal to the cb plane. RO₇ polyhedra are connected alternately by either a vertex or an edge into chains along the b axis, Fig. 1b (medium slate blue colored polyhedra). In the same direction only isolated pairs of associated MO₆ octahedra exist, as can be seen in the same figure (light gray octahedra). Flattened chains of RO₇ polyhedra (in yellow) are linked in the c direction through pairs of MO₆ octahedra with which they share edges forming layers running parallel to the bc crystal plane.

The most important feature in the structure previously described is the presence of Ge—O—Ge angles in the Ge₂O₇ group different from 180° giving rise to seven-coordinated cations in a half of the octahedral sites in the idealized thortveitite structure. The results of the Rietveld refinement established the presence of two crystalline phases for the method of synthesis used. The quantitative analysis gave 91.2 (8)% for Y_0.76Ho_0.24FeGe₂O₇ and 8.8 (3)% for Y₂Ge₂O₇. With these results, the chemical reaction compatible with the quantitative analysis is:

4Y₂O₃ + Ho₂O₃ + 5Fe₂O₃ + 30GeO₂ → 8.43Y_0.76Ho_0.24FeGe₂O₇ + 0.79Y₂Ge₂O₇ + 11.56 GeO₂ + 0.78Fe₂O₃.

Related literature

The method of preparation was based on work published by Cascales et al. (1998b). For related structures, see: Zachariasen (1930); Cascales et al. (1998a,b, 2002); Bucio et al. (2001).

S2. Experimental

The reactive mixture was prepared from Y₂O₃ (Aldrich.99.99%), Ho₂O₃ (Aldrich.99.9%), Fe₂O₃ (Aldrich.99.99%) and GeO₂ (CERAC 99.999%) according to the method reported by Cascales et al. (1998b). This mixture was first powdered using an agate mortar; and then was heated in air in a tube furnace at 1573 K for 5 d with intermediate regrinding. At the end of the reaction, some vitreous phase impregnated and segregated at the bottom of the crucible was attributed to the presence of amorphous GeO₂. Small amount of Fe₂O₃ was also detected as trace phase. The characterization of the bulk material by conventional X-ray powder diffraction data indicated two phases well crystallized. One of them showed reflections that were explained matching the isostructural phase YFeGe₂O₇ (PDF 01-072-6099) and the other one was identified as Y₂Ge₂O₇ (PDF 38-288).

S3. Refinement

The structural model for YFeGe₂O₇ (ICSD 95935) was taken for start the Rietveld refinement of Y_1-xHo_xFeGe₂O₇ with x = 1/5, while for the secondary phase, the data used for Y₂Ge₂O₇ (ICSD 240989) was those reported by Redhammer et al. (2007). The Rietveld refinement was made using the Fullprof program (Rodríguez-Carvajal, 2006). A pseudo-Voigt function modified by Thompson et al. (1987) was chosen to generate the peak shape of the diffraction reflections. The following parameters were refined: zero point and scale factors, cell parameters, half-width profile parameters, overall temperature factors, preferred orientation, atomic coordinates, and asymmetries. For the Y₂Ge₂O₇ phase no preferred orientation was considered, and the atomic coordinates were fixed to their starting values and an overall temperature factor was considered. The background was refined first by mean of a linear interpolation between 55 background points with adjustable heights. At the end of the refinement, the values for all of these heights of the background were fixed. The final Rietveld refinement of conventional diffraction pattern is shown in Figure 21.

Computing details

Data collection: DIFFRAC/AT (Siemens, 1993); cell refinement: DICVOL91 (Boultif & Louër, 1991); data reduction: FULLPROF (Rodríguez-Carvajal, 2006); program(s) used to solve structure: coordinates taken from an isotypic compound (Redhammer et al., 2007); program(s) used to refine structure: FULLPROF (Rodríguez-Carvajal, 2006); molecular graphics: ATOMS (Dowty, 2000); software used to prepare material for publication: ATOMS (Dowty, 2000).

Figures

Fig. 1. (a) View of a layer in the thortveitite structure (ab projection). Diorthogroups X2O7 are represented as yellow tetrahedra and MO6 octahedra in dark cyan. (b) View of thortveitite-like structure of Y0.76Ho0.24FeGe2O7 (ac projection). Diorthogroups Ge2O7 are represented as yellow tetrahedra, while FeO6 octahedra appear in light gray, and (Y,Ho)O7 polyhedra (medium slate blue). The bridging O atoms of Ge2O7 groups are displaced up (u) and down (d) normal to the cb plane from its original position in thortveitite.

Fig. 2. Rietveld refinement for Y0.76Ho0.24FeGe2O7 X-ray diffraction data. Observed (crosses), calculated (solid line) and difference (bottom trace) plots are represented; vertical marks correspond to the allowed Bragg reflections for Y0.76Ho0.24FeGe2O7 (top) and Y2Ge2O7 (bottom).

yttrium holmium iron digermanate

Crystal data

Y0.76Ho0.24FeGe2O7	Z = 4
Mr = 420.17	F(000) = 768.0
Monoclinic, P21/m	Dx = 5.186 Mg m^−3
Hall symbol: -P 2yb	Cu Kα radiation, λ = 1.54175 Å
a = 9.6496 (2) Å	T = 295 K
b = 8.5073 (2) Å	brown
c = 6.6712 (2) Å	flat sheet, 20 × 20 mm
β = 100.621 (1)°	Specimen preparation: Prepared at 1573 K
V = 538.27 (2) Å^3

Data collection

Bruker Advance D8 diffractometer	Data collection mode: reflection
Radiation source: sealed X-ray tube, Cu Kα	Scan method: step
Graphite monochromator	2θmin = 10°, 2θmax = 80.00°, 2θstep = 0.02°
Specimen mounting: packed powder sample container

Refinement

Least-squares matrix: full with fixed elements per cycle	3501 data points
Rp = 0.07	Profile function: pseudo-Voigt modified by Thompson et al. (1987)
Rwp = 0.09	60 parameters
Rexp = 0.06	Weighting scheme based on measured s.u.'s
RBragg = 0.03	(Δ/σ)max = 0.02
R(F^2) = 0.03	Background function: linear interpolation between a set background points with refinable heights
χ^2 = 2.341

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Y	0.7521 (11)	0.5406 (3)	0.7509 (10)	0.0120 (5)	0.76000
Ho	0.7521 (11)	0.5406 (3)	0.7509 (10)	0.0120 (5)	0.24000
Fe	0.7467 (15)	0.4473 (5)	0.2648 (16)	0.0120 (5)
Ge1	0.5263 (14)	0.75000	0.0459 (17)	0.0120 (5)
Ge2	0.5471 (13)	0.25000	0.4897 (14)	0.0120 (5)
Ge3	0.9469 (13)	0.25000	0.0252 (15)	0.0120 (5)
Ge4	0.0310 (13)	0.25000	0.5427 (18)	0.0120 (5)
O1	0.627 (3)	0.427 (3)	0.499 (5)	0.0120 (5)
O2	0.874 (4)	0.25000	0.389 (4)	0.0120 (5)
O3	0.966 (3)	0.25000	0.766 (5)	0.0120 (5)
O4	0.590 (3)	0.25000	0.792 (4)	0.0120 (5)
O5	0.845 (3)	0.070 (3)	0.028 (4)	0.0120 (5)
O6	0.130 (6)	0.25000	0.119 (6)	0.0120 (5)
O7	0.150 (3)	0.083 (3)	0.502 (3)	0.0120 (5)
O8	0.375 (5)	0.25000	0.334 (5)	0.0120 (5)
O9	0.606 (4)	0.25000	0.186 (5)	0.0120 (5)
O10	0.640 (3)	0.584 (3)	0.070 (3)	0.0120 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
?	?	?	?	?	?	?

Geometric parameters (Å, º)

Fe—O1	2.11 (4)	Ge1—O10	1.78 (3)
Fe—O2	2.15 (2)	Ge1—O10^vi	1.78 (3)
Fe—O5^i	1.99 (3)	Ge2—O1	1.69 (3)
Fe—O7^ii	2.04 (2)	Ge2—O1^i	1.69 (3)
Fe—O9	2.16 (2)	Ge2—O4	1.98 (3)
Fe—O10	1.90 (2)	Ge2—O8	1.78 (4)
Ho—O1	2.11 (3)	Ge3—O3^vii	1.77 (4)
Ho—O4	2.96 (2)	Ge3—O5	1.82 (3)
Ho—O5^iii	2.12 (3)	Ge3—O5^i	1.82 (3)
Ho—O6^ii	2.20 (3)	Ge3—O6^viii	1.76 (6)
Ho—O7^ii	2.11 (3)	Ge4—O2^ix	1.67 (3)
Ho—O8^ii	2.18 (3)	Ge4—O3^ix	1.73 (4)
Ho—O10^iv	2.59 (3)	Ge4—O7	1.87 (3)
Ge1—O4^ii	1.70 (3)	Ge4—O7^i	1.87 (3)
Ge1—O9^v	1.82 (3)
O1—Ho—O4	57.2 (1)	O1—Ho—O8^ii	87.3 (2)
O1—Ho—O5^iii	125 (2)	O1—Ho—O10^iv	117.1 (2)
O1—Ho—O6^ii	149 (2)	Ge1^x—O4—Ge2	127.7 (16)
O1—Ho—O7^ii	73.6 (2)	Ge3^iv—O3—Ge4^viii	165 (2)

Symmetry codes: (i) x, −y+1/2, z; (ii) −x+1, y+1/2, −z+1; (iii) x, −y+1/2, z+1; (iv) x, y, z+1; (v) −x+1, y+1/2, −z; (vi) x, −y+3/2, z; (vii) x, y, z−1; (viii) x+1, y, z; (ix) x−1, y, z; (x) −x+1, y−1/2, −z+1.