In_1.06Ho_0.94Ge₂O₇: a thortveitite-type compound

A new indium holmium digermanate, In_1.06Ho_0.94Ge₂O₇, with a thortveitite-type structure, has been prepared as a polycrystalline powder material by high-temperature solid-state reaction. This new compound crystallizes in the monoclinic system (space group C2/c, No. 15). The structure was characterized by Rietveld refinement of powder laboratory X-ray diffraction data. The In³⁺ and Ho³⁺ cations occupy the same octahedral site, forming a hexagonal arrangement on the ab plane. In their turn, the hexagonal arrangements of (In/Ho)O₆ octahedral layers are held together by sheets of isolated diortho groups comprised of double tetrahedra sharing a common vertex. In this compound, the Ge₂O₇ diortho groups lose the ideal D_3d point symmetry and also the C_2h point symmetry present in the thortveitite diortho groups. The Ge-O-Ge angle bridging the diortho groups is 160.2 (3)°, compared with 180.0° for Si-O-Si in thortveitite (Sc₂Si₂O₇). The characteristic mirror plane in the thortveitite space group (C2/m, No. 12) is not present in this new thortveitite-type compound and the diortho groups lose the C_2h point symmetry, reducing to C₂.

S1. Comment

Previous work on a series of isomorphous germanates MRGe₂O₇ (where M is In, Mn, Fe, Y, Sc, Ga or Al and R is a rare earth) has reported on compounds such as FeInGe₂O₇ (Bucio et al., 2001) and InYGe₂O₇ (Juarez-Arellano, Bucio et al., 2002), described by the space group C2/m (No. 12) and adopting the thortveitite structure, FeRGe₂O₇ (where R is La, Pr, Nd or Gd; Bucio et al., 1996) and NdAlGe₂O₇ (Jarchow et al., 1985), taking space group P2₁/c (No. 14), and FeRGe₂O₇ (R is Y or Tb—Yb; Cascales et al., 1998), GdMnGe₂O₇ and EuMnGe₂O₇ (Taviot-Gueho et al., 1999; Juarez-Arellano et al., 2001), with space group A222 (No.21). Interesting optical, electrical and magnetic properties have been reported in these kinds of compounds.

In recent years, compounds with rare earth cations (especially Gd³⁺, Tb³⁺, Eu³⁺ and Ho³⁺) have been developed and employed as scintillators for radiation detectors used in medical diagnostics, industrial inspection, dosimetry, nuclear medicine and high-energy physics. In each application, the scintillator is the primary radiation sensor that emits light or scintillates when it is struck by high-energy phonons Photons? (Greskovich & Duclos, 1997). Polycrystalline ceramic scintillators are a relatively new class of materials developed for quantitative detection accuracy. Ceramic scintillators have been drawing increasing attention because their complex compositions, which cannot be grown by single-crystal methods, can be synthesized by relatively inexpensive ceramic processes.

Recently, our group has reported the crystal structure of two new germanates with remarkable luminescence, namely In_1.08Gd_0.96Ge₂O₇ (Juarez-Arellano, Rosales et al., 2002), space group C2/m (No. 12), and InTbGe₂O₇ (Juarez-Arellano et al., 2003), space group C2/c (No. 15). Both compounds are potentially useful scintillators. Bearing in mind our previous results, we have been trying to expand the search for scintillation to include thousands of compounds that are not yet available as crystals (Derezo et al., 1990; Moses et al., 1997). The present work is devoted to the synthesis and crystal structure characterization of a new holmium-based compound having the stoichiometric formula In_1.06Ho_0.94Ge₂O₇. The first thortveitite-type compound reported with the space group C2/c (No. 15) was the phosphate Cu₂P₂O₇ (Robertson & Calvo, 1967). Since then, just four laminar compounds have been published (Bucio et al., 2003) with this symmetry, namely Zn₂V₂O₇ (Gopal & Calvo, 1973), Cu₂V₂O₇ (Mercurio-Lavaud & Frit, 1973), Al₂Ge₂O₇ (Agafonov et al., 1967) and InTbGe₂O₇ (Juarez-Arellano et al., 2003).

The title structure is built up of hexagonally arranged (In/Ho)O₆ octahedral layers, which are separated by intermediate layers of Ge₂O₇ diorthogroups. The average (In, Ho)—O distance of 2.27 Å, evaluated over the unique octahedral sites occupied by both In and Ho atoms, is very close to the sum of the ionic radii, r_(In3+/Ho3+) + r_O2- = 2.251 Å, where r_(In3+/Ho3+) is the average value of the In³⁺ and Ho³⁺ ionic radii. The values used for these calculations were 0.80, 0.901 and 1.40 Å, for r_In3+, r_Ho3+ and r_O2-, respectively (Shannon, 1976). The Ge—O distances range from 1.57 to 1.80 Å (mean 1.68 Å)and the GeO₄ tetrahedra are a little more irregular than those encountered in the InTbGe₂O₇ compound [mean 1.72 (2) Å; Juarez-Arellano et al., 2003]. In the latter case, the average (In/Tb)—O distance for the (In/Tb)O₆ octahedra is 2.24 (2) Å, which is in agreement with the sum of the ionic radii of In/Tb and O atoms.

In the ideal thortveitite structure, the diorthogroups possess C_{2 h} point symmetry. However, with the incorporation of Ho³⁺, distortions reduce the coordination number and the symmetry is lowered to C₂. The twofold symmetry is not broken because the bridging O atoms are displaced in a parallel direction with respect to the twofold symmetry axis. Contiguous sheets of diorthogroups have their bridging O atoms displaced in opposing directions, generating a doubled c axis with a c-glide plane replacing the mirror plane of thortveitite. The distortions reduce the diorthogroup Ge—O—Ge bridging angle from 180.0 to 160.2 (3)°.

S2. Experimental

In_1.06Ho_0.94Ge₂O₇ was prepared as a polycrystalline powder material by solid-state reaction from a stoichiometric mixture of analytical grade Ho₂O₃, GeO₂ and In₂O₃. The sample was ground and heated in air at 1423 K for 5 d with intermediate regrinding. The standard X-ray powder diffraction analysis indicated the final sample was well crystallized and appeared completely free of secondary crystalline phases. The elemental composition was determined by Rutherford backscattering spectrometry (RBS), following the procedure reported by Bucio et al. (2001). The stoichiometric values for In, Ho, Ge and O elements were 1.12, 1.0, 2.6, 7.8 (±7%), respectively. The amounts of Ge and O are slightly higher, owing to the presence of amorphous GeO₂.

S3. Refinement

The powder diffraction pattern was indexed using the TREOR program (Werner et al., 1985). Following the criteria of Cruickshank et al. (1962), we consider the novel compound In_1.06Ho_0.94Ge₂O₇ to be a thortveitite-type structure described by the space group C2/c (No. 12). The Rietveld method was used to refine the crystal structure using initial parameters obtained from the Cu₂P₂O₇ thortveitite type structure (ICSD card No. 14369, structural data from Robertson & Calvo, 1967). A pseudo-Voigt function modified by Thompson et al. (1987) was chosen to model the shape of the diffraction peaks. A total of 36 independent parameters were refined, including the zero point, scale factor, five background polynomial coefficients, unit-cell parameters, half-width and asymmetry parameters for the peak shape, atomic coordinates, occupation and isotropic atomic displacement parameters.

Computing details

Data collection: DIFFRAC/AT (Siemens, 1993); cell refinement: DICVOL91 (Boultif & Louër, 1991); data reduction: Please provide missing details; program(s) used to solve structure: Please provide missing details; program(s) used to refine structure: FULLPROF (Rodríguez-Carvajal, 1990); molecular graphics: ATOMS (Dowty, 1994).

Figures

	Fig. 1. The observed (crosses), calculated (solid line) and difference (at the bottom) X-ray powder diffraction profile for In1.06Ho0.94Ge2O7 at room temperature. Vertical marks correspond to the position of the allowed Bragg reflections.
	Fig. 2. A projection of In1.06Ho0.94Ge2O7 on the ab plane, showing the honeycomb-like arrangement of (In/Ho)O6 octahedra (in dark grey). (a) The Ge2O7 diorthogroups (light grey) link at three points to the hexagonal rings of octahedra. The displacements of bridging O atoms to the right-hand side (black arrows) are clearly seen, changing the C2 h symmetry of the Ge2O7 diorthogroups to C2. (b) (In/Ho)O6 octahedral layers (dark grey) are held together alternately along the c axis by sheets of isolated Ge2O7 diorthogroups (in light grey).
	Fig. 3. A projection of the Sc2Si2O7 thortveitite structure on the ab plane.
	Fig. 4. (a) A comparison between the C2 h point symmetry of the Ge2O7 diorthogroups in the thortveitite structure and the C2 point symmetry in the thortveitite-type structure. (b) Contiguous sheets of Ge2O7 diorthogroups have their bridging O atoms displaced in opposing directions, generating the c-glide plane.

indium holmiuium digermanate

Crystal data

In1.06Ho0.94Ge2O7	Z = 4
Mr = 536.97	F(000) = 944.0
Monoclinic, C2/c	Dx = 6.110 (2) Mg m^−3
Hall symbol: -C 2yc	Cu Kα radiation, λ = 1.540562, 1.544390 Å
a = 6.8348 (2) Å	T = 295 K
b = 8.8863 (3) Å	Particle morphology: heterogeneous particles with sizes 1-5µm
c = 9.8177 (3) Å	white
β = 101.789 (1)°	irregular, 20 × 20 mm
V = 583.71 (3) Å^3	Specimen preparation: Prepared at 1423 K

Data collection

Siemens D5000 diffractometer	Data collection mode: reflection
Radiation source: sealed X-ray tube	Scan method: step
Graphite monochromator	2θmin = 14°, 2θmax = 90°, 2θstep = 0.02°

Refinement

Refinement on Inet	3800 data points
Least-squares matrix: full with fixed elements per cycle	Profile function: pseudo-Voigt modified by Thompson et al. (1987)
Rp = 0.079	34 parameters
Rwp = 0.111	Weighting scheme based on measured s.u.'s
Rexp = 0.078	(Δ/σ)max = 0.01
RBragg = 0.030	Background function: polynomial
χ^2 = 2.045	Preferred orientation correction: none

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
In	0.0055 (7)	0.3044 (1)	0.4984 (5)	0.0110 (6)	0.5
Ho	0.0055 (7)	0.3044 (1)	0.4984 (5)	0.0110 (6)	0.5
Ge	0.2284 (3)	0.003 (1)	0.2050 (2)	0.0052 (8)
O1	0.00000	0.025 (3)	0.25	0.014 (2)
O2	0.379 (1)	−0.017 (3)	0.3557 (9)	0.014 (2)
O3	0.245 (4)	0.175 (3)	0.112 (2)	0.014 (2)
O4	0.239 (4)	−0.144 (2)	0.119 (2)	0.014 (2)

Atomic displacement parameters (Ų)

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

Geometric parameters (Å, º)

In—O2^i	2.17 (2)	In—O4^vi	2.33 (2)
In—O2^ii	2.40 (2)	Ge—O1	1.718 (4)
In—O3^iii	2.16 (2)	Ge—O2	1.63 (1)
In—O3^iv	2.30 (2)	Ge—O3	1.79 (2)
In—O4^v	2.28 (2)	Ge—O4	1.56 (2)
O2^i—In—O2^ii	81.3 (9)	O3^iii—In—O4^vi	117.0 (16)
O2^i—In—O3^iii	84.0 (14)	O3^iv—In—O4^v	109.1 (9)
O2^i—In—O3^iv	90.6 (12)	O3^iv—In—O4^vi	164 (2)
O2^i—In—O4^v	159.0 (16)	O4^v—In—O4^vi	82.2 (13)
O2^i—In—O4^vi	77.1 (11)	O1—Ge—O2	102.6 (3)
O2^ii—In—O3^iii	147.9 (16)	O1—Ge—O3	100.9 (15)
O2^ii—In—O3^iv	80.7 (11)	O1—Ge—O4	112.1 (17)
O2^ii—In—O4^v	94.3 (14)	O2—Ge—O3	117.4 (18)
O2^ii—In—O4^vi	87.2 (11)	O2—Ge—O4	108.0 (18)
O3^iii—In—O3^iv	71.0 (12)	O3—Ge—O4	114.9 (10)
O3^iii—In—O4^v	108.9 (15)	Ge—O1—Ge^iii	167.0 (3)

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