Synthesis and crystal structure of a neodymium borosilicate, Nd₃BSi₂O₁₀

A lanthanide borosilicate, trineodymium borosilicate or Nd₃BSi₂O₁₀, was synthesized using a flux method with LiCl, and its structure was determined from X-ray powder diffraction (XRD) and electron probe microanalysis (EPMA). The structure is composed of layers with [SiO₄]⁴⁻ and [BSiO₆]⁵⁻ anions alternating along the c axis linked by Nd³⁺ cations between them.

1. Chemical context

Lanthanide borosilicates (i.e. Ln₃BSi₂O₁₀) crystallize as one of the major phases within the residual glass matrix in some formulations of the glass-ceramic waste form for treatment of raffinate high-level waste (Crum et al., 2012, 2014, 2016). Studies on the crystal chemistry and crystallization mechanism of lanthanide borosilicates are important in understanding the formation and durability of crystalline phases in the glass-ceramic waste forms (Crum et al., 2012, 2014, 2016). In this work, we report the synthesis method and crystal structure of Nd₃BSi₂O₁₀ solved by powder XRD and EPMA analysis.

Different compositions of lanthanide borosilicates including LnBSiO₅ (Ln = La, Ce, Pr, Nd, Sm; McAndrew & Scott, 1955; Neumann et al., 1966; Nekrasov & Nekrasova, 1971; Voronkov & Pyatenko, 1967; Burns et al., 1993; Chi et al., 1997; Shi et al., 1997), Ln₅Si₂BO₁₃ (Ln = La, Eu, Gd, Dy; Mazza et al., 2000; Yuan et al., 2007; Naidu et al., 2010), and Ln₃BSi₂O₁₀ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb; Chi et al., 1996a,b; Müller-Bunz & Schleid, 1998; Chi et al., 1998, Shvanskii et al., 2000; Müller-Bunz et al., 2001; Bräuchle & Huppertz, 2015) have been reported in the literature. LnBSiO₅ has the stillwellite structure containing single or mixed lanthanide cations with infinite helical chains composed of six-membered rings formed by two [BO₄]⁵⁻ and one [SiO₄]⁴⁻ tetra­hedral units (Chi et al., 1997; Burns et al., 1993; Voronkov & Pyatenko, 1967; Shi et al., 1997). Ln₅Si₂BO₁₃ has an apatite-like structure in which the non-tetra­hedral cation sites are occupied by trivalent rare-earth cations, and B and Si occupy the same tetra­hedral site (Mazza et al., 2000). Ln₃BSi₂O₁₀ contains layers with [SiO₄]⁴⁻ and [BSiO₆]⁵⁻ anions alternating along the c axis linked by trivalent cations between them, and Nd₃BSi₂O₁₀ in this work is isostructural to previously reported Ln₃BSi₂O₁₀ compounds (Chi et al., 1996a,b, 1998, Braeuchle & Huppertz, 2015; Shvanskii et al., 2000; Müller–Bunz et al., 2001).

2. Structural commentary

The [BSiO₆]⁵⁻ anion in Nd₃BSi₂O₁₀ is formed by [Si1O₄]⁴⁻ and [BO₃]³⁻ anions sharing an oxygen atom, with an average <Si1—O> distance of 1.613 Å and an average <B—O> distance of 1.466 Å (Fig. 1a), while the [Si2O₄]⁴⁻ ion has an average <Si2—O> distance of 1.590 Å. The [BSi1O₆]⁵⁻ and [Si2O₄]⁴⁻ anions are arranged alternately along the c axis (Fig. 2). The Nd cations occupy the inter­layer sites between the anion units. Nd1 and Nd3 are coordinated by eight oxygen atoms with average <Nd1—O> and <Nd3—O> distances of 2.477 and 2.520 Å, respectively, and Nd2 is coordinated by nine oxygen atoms with an average <Nd2—O> distance of 2.575 Å (Fig. 1b). In our previous paper (Kroll et al., 2019), we summarized the crystallographic data from the literature on other Ln₃BSi₂O₁₀ chemistries (Braeuchle & Huppertz, 2015; Chi et al., 1996a,b, 1998; Müller–Bunz et al., 2001; Shvanskii et al., 2000) as a function of the ionic crystal radii (r_c) for the VIII-coordinated Ln³⁺ constituent according to Shannon (1976) to create predictive models for the unit-cell parameters (i.e., a, b, and c), cell volume, and cell density. The measured values of a (9.7889 Å), b (7.1077 Å), c (23.0893 Å), cell volume (1606.5 ų), and density (5.4551 Mg m⁻³) all fit reasonably well with the values calculated using the r_c for Nd (1.109 Å), i.e., a (9.799 Å), b (7.111 Å), c (23.095 Å), cell volume (1608.4 ų), and cell density (5.49 Mg/m³). Detailed atomic coordinates, bond lengths, and angles are given in Tables S1 and S2 in the supporting information.

Figure 1 (a) Structure of the BSiO6 anion and (b) coordination of oxygen atoms around Nd cations (Nd1, Nd2, and Nd3).

Figure 2 Crystal structure of Nd3BSi2O10 showing alternating [BSiO6]^5− and [SiO4]^4− anions along the c axis with Nd cations between them.

3. Synthesis and crystallization

Nd₃BSi₂O₁₀ was synthesized by a LiCl flux method; more details are provided elsewhere (Kroll et al., 2019). Powdered B₂O₃ was placed into a Pt–10%Rh crucible, melted at 1273 K in air to dehydrate fully and quenched on an Inconel plate. Appropriate amounts of Nd₂O₃, SiO₂, and B₂O₃ were mixed in an agate mortar and pestle. LiCl was dried at 378 K for several hours and mixed with oxides in a 1:1 ratio by mass in a Diamonite^TM mortar and pestle. Mixed powder was placed into a fused quartz tube, covered with a quartz lid, heated to 1173 K at 5 K min⁻¹, held for 24 h at 1173 K, and then cooled down to room temperature at 1 K min⁻¹. The Nd₃BSi₂O₁₀ was recovered from the LiCl through vacuum filtration with several rinsing steps using deionized water and a Büchner funnel. The recovered heat-treated powder was ground finer in the mortar and pestle and pressed into a 20 mm diameter pellet using a cold press with 110 MPa. The pellet was sintered at 1373 K. The heating condition included ramping up at 2 K min⁻¹ to 1373 K, dwelling for 4 h, and cooling to room temperature at 2 K min⁻¹. The heat-treated pellet, which was blue–violet in color, was ground for XRD and EPMA. Two EPMA measurements were performed on the sample to verify the composition of the crystal and showed that it closely matches the calculated value (Fig. 3).

Figure 3 Comparison of oxide mass% between targeted and measured Nd3BSi2O10 from EPMA measurement.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. A Rietveld plot is shown in Fig. 4. The structure of Nd₃BSi₂O₁₀ was determined using Rietveld refinement on the initial model with a similar chemistry and structure using TOPAS (version 4.2; Bruker, 2009). Based on the fitting of peak positions and profile of experimental XRD patterns to a reference pattern, Ce₃BSi₂O₁₀ (ICSD 94423) was used as a starting model. The Ce atoms in ICSD 94423 were replaced with Nd atoms, and all the atomic positions for Nd, B, Si, and O were refined. The profile of the model was refined from 14.5° to avoid a hump around 13.5° in the fitting of the background resulting from an instrumental artifact. The displacement parameters (B_eq) were not refined and fixed to 1 Ų to avoid divergence and unreasonable error values. In addition, parameters for unit cell, scale factors, microstructure effects, and preferred orientation with spherical harmonic function (Järvinen, 1993) were refined, and the background was fitted with a Chebychev polynomial.

Table 1
Experimental details

Crystal data
Chemical formula	Nd3BSi2O10
Mr	659.7
Crystal system, space group	Orthorhombic, Pbca
Temperature (K)	295
a, b, c (Å)	9.78891 (17), 7.10774 (12), 23.0893 (4)
V (Å^3)	1606.49 (5)
Z	8
Radiation type	Cu Kα, λ = 1.54188 Å
Specimen shape, size (mm)	Flat sheet, 25 × 25
Data collection
Diffractometer	Bruker D8 Advance
Specimen mounting	Packed powder pellet
Data collection mode	Reflection
Scan method	Step
2θ values (°)	2θmin = 14.5 2θmax = 90 2θstep = 0.014
Refinement
R factors and goodness of fit	Rp = 0.03, Rwp = 0.04, Rexp = 0.011, RBragg = 0.013, χ^2 = 13.250
No. of parameters	82
Computer programs: XRD Commander (Kienle et al., 2003), TOPAS (Bruker, 2009), VESTA (Momma & Izumi, 2011) and publCIF (Westrip, 2010).

Figure 4 Observed, calculated, and difference XRD profiles of Nd3BSi2O10.

Supporting information

Computing details

Data collection: XRD Commander (Kienle et al., 2003); cell refinement: TOPAS (Bruker, 2009); program(s) used to solve structure: TOPAS (Bruker, 2009); program(s) used to refine structure: TOPAS (Bruker, 2009); molecular graphics: VESTA (Momma & Izumi, 2011); software used to prepare material for publication: publCIF (Westrip, 2010).

Trineodymium borosilicate

Crystal data

Nd3BSi2O10	Z = 8
Mr = 659.7	Dx = 5.455 Mg m^−3
Orthorhombic, Pbca	Cu Kα radiation, λ = 1.54188 Å
a = 9.78891 (17) Å	T = 295 K
b = 7.10774 (12) Å	blue_violet
c = 23.0893 (4) Å	flat_sheet, 25 × 25 mm
V = 1606.49 (5) Å^3

Data collection

Bruker D8 Advance diffractometer	Data collection mode: reflection
Radiation source: sealed X-ray tube	Scan method: step
Specimen mounting: packed powder pellet	2θmin = 14.5°, 2θmax = 90°, 2θstep = 0.014°

Refinement

Rp = 0.03	82 parameters
Rwp = 0.04	Weighting scheme based on measured s.u.'s
Rexp = 0.011	(Δ/σ)max = 0.011
RBragg = 0.013	Background function: Chebychev
5738 data points	Preferred orientation correction: spherical harmonic
Profile function: pseudo-Voigt

Special details

Refinement. background fitted from 14.5 degree to avoid a hump from instrumental artifact

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

	x	y	z	Biso*/Beq
Nd1	0.4909 (2)	0.3621 (3)	0.42810 (6)	1
Nd2	0.1338 (2)	0.3296 (4)	0.33652 (7)	1
Nd3	0.2655 (2)	0.0934 (3)	0.18257 (7)	1
B1	0.249 (4)	0.387 (7)	0.9703 (13)	1
Si1	0.3810 (10)	0.3516 (16)	0.0787 (3)	1
Si2	0.4381 (9)	0.3240 (17)	0.2814 (4)	1
O1	0.2558 (17)	0.254 (3)	0.9191 (7)	1
O2	0.1165 (18)	0.399 (3)	0.9903 (7)	1
O3	0.3697 (19)	0.348 (3)	0.0088 (6)	1
O4	0.4525 (17)	0.170 (3)	0.1055 (7)	1
O5	0.2286 (15)	0.346 (3)	0.1083 (7)	1
O6	0.4662 (17)	0.537 (3)	0.0938 (8)	1
O7	0.6028 (18)	0.293 (2)	0.2773 (7)	1
O8	0.4151 (15)	0.369 (2)	0.2120 (7)	1
O9	0.3903 (18)	0.466 (2)	0.3239 (7)	1
O10	0.3481 (15)	0.138 (3)	0.2880 (6)	1

Atomic displacement parameters (Ų)

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

Geometric parameters (Å, º)

Nd1—O1^i	2.454 (17)	Nd3—O7^vi	2.325 (16)
Nd1—O2^ii	2.460 (19)	B1—O2	1.38 (4)
Nd1—O2^iii	2.265 (17)	Si1—O3	1.618 (16)
Nd1—O4^iv	2.39 (2)	Si1—O4	1.59 (2)
Nd1—O6^v	2.40 (2)	Si1—O5	1.641 (18)
Nd2—O1^i	2.327 (17)	Si1—O6	1.60 (2)
Nd2—O8^vi	2.432 (15)	Si2—O7	1.63 (2)
Nd2—O10^vii	2.47 (2)	Si2—O8	1.649 (19)
Nd3—Nd3^viii	3.567 (3)	Si2—O9	1.483 (19)
Nd3—Nd3^vii	3.567 (3)	Si2—O10	1.60 (2)
Nd3—O5^viii	2.457 (19)
O1^i—Nd1—O2^ii	83.2 (6)	O3—Si1—O6	105.4 (11)
O1^i—Nd1—O2^iii	128.2 (6)	O4—Si1—O5	102.6 (11)
O1^i—Nd1—O4^iv	119.9 (6)	O4—Si1—O6	110.7 (11)
O1^i—Nd1—O6^v	79.8 (6)	O5—Si1—O6	113.8 (11)
O2^ii—Nd1—O2^iii	70.5 (6)	O7—Si2—O8	96.0 (9)
O2^ii—Nd1—O4^iv	69.9 (6)	O7—Si2—O9	116.3 (11)
O2^ii—Nd1—O6^v	149.5 (6)	O7—Si2—O10	116.1 (11)
O2^iii—Nd1—O4^iv	92.2 (7)	O8—Si2—O9	117.9 (11)
O2^iii—Nd1—O6^v	101.0 (7)	O8—Si2—O10	100.3 (9)
O4^iv—Nd1—O6^v	140.6 (6)	O9—Si2—O10	109.0 (10)
O1^i—Nd2—O8^vi	149.1 (6)	Nd1^ix—O1—Nd2^ix	117.7 (7)
O1^i—Nd2—O10^vii	124.2 (6)	Nd1^x—O2—Nd1^xi	109.5 (7)
O8^vi—Nd2—O10^vii	75.6 (5)	Nd1^x—O2—B1	104 (2)
Nd3^viii—Nd3—Nd3^vii	170.24 (8)	Nd1^xi—O2—B1	141.4 (17)
Nd3^viii—Nd3—O5^viii	44.7 (4)	Nd1^v—O4—Si1	135.9 (10)
Nd3^viii—Nd3—O7^vi	123.3 (4)	Nd3^vii—O5—Si1	104.6 (9)
Nd3^vii—Nd3—O5^viii	135.7 (4)	Nd1^iv—O6—Si1	147.0 (11)
Nd3^vii—Nd3—O7^vi	48.2 (4)	Nd3^xii—O7—Si2	137.5 (9)
O5^viii—Nd3—O7^vi	137.0 (5)	Nd2^xii—O8—Si2	107.8 (8)
O3—Si1—O4	113.9 (11)	Nd2^viii—O10—Si2	137.7 (9)
O3—Si1—O5	110.7 (10)

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