SrZnSi₃O₈, a synthetic member of the feldspar group

Strontium tecto-zincotrisilicate, SrZnSi₃O₈, is a synthetic member of the feldspar group with monoclinic symmetry (space group P2₁/n, Z = 4). In comparison with the alkali feldspars, the aluminium-richest tetra­hedral site in these minerals is completely substituted by Zn, and the alkali metal M site by strontium. The crystal structure is fully ordered, i.e. there is no mutual co-occupation of the tetra­hedral sites or positional disorder of the M site. Although the tetra­hedral site substituted by Zn exhibits significantly longer bond lengths [(Zn—O) = 1.973 Å] due to the incorporation of the larger cation, there is no noticeable distortion of the SiO₄ tetra­hedra compared to the aluminium-bearing feldspars. The coordination number of Sr²⁺ is [6 + 2], with six short distances to oxygen sites in a narrow range [ = 2.560 Å] and two significantly longer distances > 3.10 Å.

1. Chemical context

Strontium silicate, Sr₂SiO₄, is a well-known host material for luminescence applications. When lanthanides, especially Eu, are used as dopants, efficient white light-emitting diodes can be produced (Park et al., 2003; Gupta et al., 2015).

In this context, it was investigated how the luminescence properties change when the Eu-doped host material is modified by incorporating divalent cations (M = Ca, Ba, Mg, Zn) to form a possible solid solution (Sr_1.9M_0.1)SiO₄ (Wieser, 2006). In one of these experiments for the intended preparation of (Sr_1.9Zn_0.1)SiO₄:Eu³⁺, SrZnSi₃O₈ has been obtained serendipitously instead. Its feldspar-type crystal structure is presented and discussed in the present article.

2. Structural commentary

The probable existence of SrZnSi₃O₈ was predicted some time ago (Fehr & Huber, 2001), and this phase was actually obtained during investigations of solid solutions (Ba_1–xSr_x)ZnSi₃O₈ investigated for microwave dielectric properties (Song et al., 2019). Another phase in the system SrO/Al₂O₃/SiO₂ has been identified to date, viz. synthetic Sr-hardysonite, Sr₂ZnSi₂O₇ (Ardit et al., 2010). The putative crystal structure of SrZnSi₃O₈ was derived from laboratory powder X-ray diffraction data and reported as triclinic with space group P (Song et al., 2019). Although the corresponding article states: ‘The lattice parameters of [⋯] SrZnSi₃O₈ were extracted from XRD data using the least-squares method. All the peaks are indexed accordingly, and the crystal structure information is given in the supplemental file', no such data are available. Therefore, no direct comparison can be made with the results of the current single crystal X-ray diffraction data. In any case, the latter data clearly revealed that the symmetry is monoclinic rather than triclinic as previously reported. The crystal structure shows a feldspar-type arrangement of the tetra­hedral framework and the metal position.

Feldspars define a large group of aluminium silicate minerals (class of tectosilicates; Liebau, 1985) and are considered the most important rock-forming minerals in the Earth's crust, accounting for almost 60% of its composition. The frequently occurring alkali (M) feldspars can be described with the formula M[Al(Al,Si)₃O₈], or more generally M[T₄O₈] (where T is a tetra­hedrally coordinated site), and crystallize either in the triclinic or monoclinic crystal system with unit cell parameter of a ≃ 8.4, b ≃ 13.0, c ≃ 7.2 Å, α ≃ 90, β ≃ 116, c ≃ 90° (Smith, 1974). It is precisely this dimension of the unit cell that is also found in the crystal structure of SrZnSi₃O₈. This is a case of diadochy, and the tetra­hedral position T₁(0) (for atomic designations in feldspars, see: Smith, 1974) is completely occupied by Zn, and Zn also does not co-occupy the other tetra­hedral sites (Fig. 1). In feldspars, the T₁(0) position of the tetra­hedral framework is usually occupied by the majority of Al³⁺ present in the structure, i.e. the cation with a lower charge than Si⁴⁺. In simplified terms, this trend continues in SrZnSi₃O₈ with the even lower charged Zn²⁺, and electron neutrality in the overall structure is ensured by the doubly charged alkaline earth metal Sr²⁺, which occupies the M site. This substitution at the T₁(0) site with a considerably larger cation (0.60 Å for Zn²⁺ versus 0.39 Å for Al³⁺, Shannon, 1976) results in longer T₁(0)—O bonds (average 1.973 Å, Table 1). Compared with the alkaline earth homologues CaZnSi₃O₈ (Heuer et al., 1998) and BaZnSi₃O₈ (Zou et al., 2021), which also exhibit feldspar-like crystal structures, the Zn—O distances are similar. CaZnSi₃O₈: 1.916 (3), 1.941 (3), 1.943 (3), 2.047 (3) Å [P, a = 8.121 (1), b = 12.927 (1), c = 7.206 (1) Å, α = 93.76 (5), β = 116.120 (7), γ = 84.368 (7)°, Z = 2, single crystal X-ray data, distances calculated from the deposited crystallographic information file (CIF), entry 409286 in the Inorganic Crystal Structure Database (ICSD; Zagorac et al., 2019)]; BaZnSi₃O₈: 1.875 (12), 1.923 (9), 1.972 (11), 1.980 (10) Å [P2₁/a, a = 8.725 (10), b = 13.072 (20), c = 7.307 (10) Å, β = 115.85 (2)°, Z = 4, powder synchrotron X-ray data, distances taken from the publication: note that in this publication and the corresponding deposited CIF, entry 113904 in the ICSD, the fractional coordinates of the Zn site are incorrect, with x = 0.012 most likely the correct value].

Table 1
Selected bond lengths (Å)

Sr1—O3^i	2.510 (2)	Si1—O1^iii	1.597 (2)
Sr1—O7^ii	2.523 (2)	Si1—O4^vii	1.610 (2)
Sr1—O1^iii	2.533 (2)	Si1—O6	1.613 (2)
Sr1—O2^ii	2.583 (2)	Si1—O8	1.614 (2)
Sr1—O5^iv	2.593 (2)	Si2—O3	1.580 (2)
Sr1—O1^v	2.617 (2)	Si2—O6^ix	1.621 (2)
Sr1—O8	3.115 (3)	Si2—O8^x	1.624 (2)
Sr1—O4^vi	3.294 (3)	Si2—O2	1.638 (2)
Zn1—O5	1.903 (2)	Si3—O7^x	1.587 (2)
Zn1—O7	1.920 (2)	Si3—O5^iv	1.601 (2)
Zn1—O3^vii	1.969 (2)	Si3—O4	1.630 (3)
Zn1—O1^viii	2.010 (2)	Si3—O2^ii	1.665 (2)
Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) ; (vii) ; (viii) ; (ix) ; (x) .

Figure 1 The crystal structure of SrZnSi3O8 in a projection along [00]. Displacement ellipsoids are drawn at the 90% probability level; the tetra­hedral framework atoms are shown in polyhedral representation (Zn yellow, Si red). [Symmetry code: (i) −x, 1 − y, z.]

The three SiO₄ tetra­hedra in the remaining framework of SrZnSi₃O₈ are not significantly distorted and show the usual Si—O bond lengths distributions (Table 1; averaged values: Si1 1.609, Si2 1.616, Si3 1.621 Å), in very good agreement with the mean bond length of 1.62 Å for an SiO₄ tetra­hedron (Liebau, 1985).

The coordination number (CN) of Sr can be described as [6 + 2], with six closer distances in a narrow range (average 2.560 Å) and two considerably longer Sr—O distances > 3.10 Å (Table 1). The closest matching polyhedron for CN = 6 was calculated with the Polynator program (Link & Niewa, 2023) and is a twisted trigonal prism (idealized point group 32, deviation from idealized values δ = 13.690), and the two O atoms at longer distances cap opposite faces (Fig. 2).

Figure 2 Coordination environment of the Sr1 site. Displacement ellipsoids are drawn at the 90% probability level; symmetry codes refer to Table 1. The polyhedron includes the six short Sr—O bonds, and the two O atoms capping the polyhedron are shown with green bonds.

The plausibility of the SrZnSi₃O₈ structure model was verified and confirmed using bond-valence-sum calculations (Brown, 2002) performed with the ECoN21 program (Ilinca, 2022). The calculated bond-valence sums (Table 2) are close to the expected values, and the global instability index (GII) of 0.10 valence units indicates a stable and not particularly strained crystal structure (Salinas-Sanchez et al., 1992; Brown, 2009).

Table 2
Bond-valence-sum calculations (in valence units) for SrZnSi₃O₈

	Sr1	Zn1	Si1	Si2	Si3	Σ
O1	0.30, 0.25	0.43	1.07			2.05
O2	0.27			0.96	0.90	2.13
O3	0.32	0.47		1.12		1.91
O4	0.06		1.04		0.99	2.09
O5	0.27	0.56			1.06	1.89
O6			1.03	1.01		2.04
O7	0.31	0.54			1.10	1.95
O8	0.09		1.03	1.00		2.12
Σ	1.87	2.00	4.17	4.09	4.05

The same formula type, the same space group type and similar lattice parameters might suggest that SrZnSi₃O₈ and BaZnSi₃O₈ are isotypic to one another. However, a closer look at the crystal structures (under consideration of the corrected x parameter for Zn1 in the BaZnSi₃O₈ structure, see above) reveals that the two crystalline phases are isopointal (Lima-de-Faria et al., 1990). As shown in Fig. 3, the crystal structures are clearly distinct from one another, with a different arrangement of the alkaline earth metal sites and the positions of ZnO₄ tetra­hedra within the tetra­hedral framework.

Figure 3 Comparison of the crystal structures of SrZnSi3O8 (a), projection along [00]) and BaZnSi3O8 (b), projection along [100]). Alkaline earth cations are shown as blue spheres, ZnO4 tetra­hedra are yellow and SiO4 tetra­hedra are red.

3. Synthesis and crystallization

SiO₂ (Fluka, purum), SrCO₃ (Merck, pure) and ZnO were weighted according to a composition of Sr_1.9Zn_0.1SiO₄ (total 2 g). During intensive mixing of the powders in an achate mortar, small amounts of EuF₃ (Aldrich, 99+; approx. 10 mg) were added as a dopant. The mixture was then compressed to a pellet that was heated in a corundum crucible at 1543 K for one day. After the reaction, the slightly yellowish sample appeared glassy in some places. A small colourless single crystal of the title compound was extracted from this matrix.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. The non-reduced setting of the unit cell was used to emphasize the relationship to the crystal structures of other feldspar group minerals (e.g. microcline, low-albite, high-albite, reedmergernite) listed by Smith (1974). Similarly, the atomic labelling [Zn1 = T₁(0), Si1 = T₁(m), Si2 = T₂(0), Si3 = T₂(m), O1 = O_A(1), O2 = O_A(2), O3 = O_B(0), O4 = O_B(m), O5 = O_C(0), O6 = O_C(m), O7 = O_D(0), O8 = O_D(m)] and atomic coordinates were also adjusted to the atomic parameters listed there. In order to check for any possible co-occupation of Zn and Si (and vice versa), the site occupation factor (s.o.f.) of each Zn1, Si1, Si2 and Si3 site was freely refined (constraining all other sites to be fully occupied). In all cases, the free refinement converged at s.o.f. values very close to 1.0, showing no co-occupation.

Table 3
Experimental details

Crystal data
Chemical formula	SrZnSi3O8
Mr	365.26
Crystal system, space group	Monoclinic, P21/n
Temperature (K)	293
a, b, c (Å)	8.3060 (8), 13.0111 (13), 7.2454 (7)
β (°)	114.822 (2)
V (Å^3)	710.67 (12)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	11.40
Crystal size (mm)	0.06 × 0.04 × 0.03
Data collection
Diffractometer	Bruker SMART APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015)
Tmin, Tmax	0.548, 0.764
No. of measured, independent and observed [I > 2σ(I)] reflections	7658, 2057, 1745
Rint	0.035
(sin θ/λ)max (Å^−1)	0.703
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.029, 0.069, 1.03
No. of reflections	2057
No. of parameters	118
Δρmax, Δρmin (e Å^−3)	0.88, −0.67
Computer programs: APEX2 and SAINT (Bruker, 2005), SHELXS (Sheldrick, 2008), SHELXL (Sheldrick, 2015), ATOMS (Dowty, 2006) and publCIF (Westrip, 2010).

Supporting information

Computing details

Strontium tecto-zincotrisilicate

Crystal data

SrZnSi3O8	F(000) = 696
Mr = 365.26	Dx = 3.414 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 8.3060 (8) Å	Cell parameters from 2419 reflections
b = 13.0111 (13) Å	θ = 3.1–30.0°
c = 7.2454 (7) Å	µ = 11.40 mm^−1
β = 114.822 (2)°	T = 293 K
V = 710.67 (12) Å^3	Fragment, colourless
Z = 4	0.06 × 0.04 × 0.03 mm

Data collection

Bruker SMART APEXII CCD diffractometer	2057 independent reflections
Radiation source: fine-focus sealed tube	1745 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.035
ω–scans	θmax = 30.0°, θmin = 3.1°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −11→11
Tmin = 0.548, Tmax = 0.764	k = −18→18
7658 measured reflections	l = −10→9

Refinement

Refinement on F^2	118 parameters
Least-squares matrix: full	0 restraints
R[F^2 > 2σ(F^2)] = 0.029	w = 1/[σ^2(Fo^2) + (0.0377P)^2 + 0.2907P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.069	(Δ/σ)max = 0.001
S = 1.03	Δρmax = 0.88 e Å^−3
2057 reflections	Δρmin = −0.67 e Å^−3

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

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

	x	y	z	Uiso*/Ueq
Sr1	0.26316 (4)	0.98248 (2)	0.13696 (5)	0.01620 (10)
Zn1	0.02329 (5)	0.18835 (3)	0.20052 (6)	0.01043 (10)
Si1	−0.02817 (11)	0.83033 (6)	0.24716 (13)	0.00802 (17)
Si2	0.70774 (11)	0.12373 (7)	0.32678 (13)	0.00844 (17)
Si3	0.67150 (11)	0.89689 (7)	0.38177 (13)	0.00864 (17)
O1	0.0187 (3)	0.11994 (18)	0.9496 (3)	0.0128 (5)
O2	0.6002 (3)	0.01681 (16)	0.3169 (3)	0.0103 (4)
O3	0.8042 (3)	0.12069 (18)	0.1797 (4)	0.0147 (5)
O4	0.8007 (3)	0.87771 (19)	0.2669 (4)	0.0191 (5)
O5	0.0104 (3)	0.33381 (17)	0.2183 (4)	0.0132 (5)
O6	−0.0480 (3)	0.70691 (17)	0.2380 (4)	0.0137 (5)
O7	0.2234 (3)	0.10919 (19)	0.3782 (3)	0.0163 (5)
O8	0.1561 (3)	0.85914 (19)	0.4367 (4)	0.0175 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sr1	0.00884 (14)	0.01500 (16)	0.02113 (18)	0.00124 (11)	0.00275 (12)	−0.00635 (12)
Zn1	0.01131 (18)	0.00930 (18)	0.01112 (19)	0.00110 (13)	0.00514 (14)	−0.00009 (13)
Si1	0.0077 (4)	0.0086 (4)	0.0086 (4)	−0.0006 (3)	0.0042 (3)	−0.0005 (3)
Si2	0.0078 (4)	0.0080 (4)	0.0093 (4)	−0.0005 (3)	0.0034 (3)	0.0001 (3)
Si3	0.0076 (4)	0.0074 (4)	0.0112 (4)	0.0010 (3)	0.0042 (3)	0.0002 (3)
O1	0.0173 (11)	0.0135 (11)	0.0105 (11)	−0.0011 (9)	0.0088 (9)	0.0003 (8)
O2	0.0093 (10)	0.0062 (10)	0.0142 (11)	−0.0005 (8)	0.0039 (9)	0.0019 (8)
O3	0.0148 (11)	0.0156 (11)	0.0177 (12)	−0.0021 (9)	0.0107 (10)	−0.0003 (9)
O4	0.0181 (12)	0.0172 (12)	0.0294 (14)	0.0011 (10)	0.0172 (11)	−0.0038 (10)
O5	0.0118 (10)	0.0082 (10)	0.0191 (12)	0.0012 (8)	0.0061 (9)	0.0002 (9)
O6	0.0123 (11)	0.0083 (10)	0.0218 (12)	−0.0014 (8)	0.0085 (10)	−0.0011 (9)
O7	0.0157 (11)	0.0167 (12)	0.0124 (12)	0.0046 (9)	0.0019 (10)	0.0025 (9)
O8	0.0155 (11)	0.0176 (12)	0.0128 (12)	−0.0026 (9)	−0.0006 (10)	−0.0031 (9)

Geometric parameters (Å, º)

Sr1—O3^i	2.510 (2)	Si1—O1^iii	1.597 (2)
Sr1—O7^ii	2.523 (2)	Si1—O4^vii	1.610 (2)
Sr1—O1^iii	2.533 (2)	Si1—O6	1.613 (2)
Sr1—O2^ii	2.583 (2)	Si1—O8	1.614 (2)
Sr1—O5^iv	2.593 (2)	Si2—O3	1.580 (2)
Sr1—O1^v	2.617 (2)	Si2—O6^ix	1.621 (2)
Sr1—O8	3.115 (3)	Si2—O8^x	1.624 (2)
Sr1—O4^vi	3.294 (3)	Si2—O2	1.638 (2)
Zn1—O5	1.903 (2)	Si3—O7^x	1.587 (2)
Zn1—O7	1.920 (2)	Si3—O5^iv	1.601 (2)
Zn1—O3^vii	1.969 (2)	Si3—O4	1.630 (3)
Zn1—O1^viii	2.010 (2)	Si3—O2^ii	1.665 (2)
O3^i—Sr1—O7^ii	159.85 (8)	O6—Si1—O8	108.09 (13)
O3^i—Sr1—O1^iii	70.38 (7)	O3—Si2—O6^ix	113.99 (13)
O7^ii—Sr1—O1^iii	98.00 (8)	O3—Si2—O8^x	112.93 (13)
O3^i—Sr1—O2^ii	109.40 (7)	O6^ix—Si2—O8^x	109.65 (14)
O7^ii—Sr1—O2^ii	88.09 (7)	O3—Si2—O2	111.75 (13)
O1^iii—Sr1—O2^ii	155.38 (7)	O6^ix—Si2—O2	100.91 (12)
O3^i—Sr1—O5^iv	79.41 (8)	O8^x—Si2—O2	106.74 (13)
O7^ii—Sr1—O5^iv	119.40 (7)	O7^x—Si3—O5^iv	116.82 (14)
O1^iii—Sr1—O5^iv	98.38 (7)	O7^x—Si3—O4	112.02 (14)
O2^ii—Sr1—O5^iv	58.43 (7)	O5^iv—Si3—O4	112.87 (13)
O3^i—Sr1—O1^v	93.71 (7)	O7^x—Si3—O2^ii	108.85 (13)
O7^ii—Sr1—O1^v	67.31 (7)	O5^iv—Si3—O2^ii	101.33 (12)
O1^iii—Sr1—O1^v	78.22 (8)	O4—Si3—O2^ii	103.38 (13)
O2^ii—Sr1—O1^v	125.74 (7)	Si1^iii—O1—Zn1^xi	129.67 (14)
O5^iv—Sr1—O1^v	173.05 (7)	Si1^iii—O1—Sr1^iii	112.26 (11)
O3^i—Sr1—O8	110.02 (7)	Zn1^xi—O1—Sr1^iii	96.98 (9)
O7^ii—Sr1—O8	72.24 (7)	Si1^iii—O1—Sr1^xii	114.94 (12)
O1^iii—Sr1—O8	52.92 (6)	Zn1^xi—O1—Sr1^xii	96.74 (8)
O2^ii—Sr1—O8	107.66 (7)	Sr1^iii—O1—Sr1^xii	101.78 (8)
O5^iv—Sr1—O8	72.81 (7)	Si2—O2—Si3^xiii	131.35 (14)
O1^v—Sr1—O8	109.11 (7)	Si2—O2—Sr1^xiii	128.63 (11)
O3^i—Sr1—O4^vi	65.95 (7)	Si3^xiii—O2—Sr1^xiii	99.33 (10)
O7^ii—Sr1—O4^vi	103.62 (7)	Si2—O3—Zn1^xiv	130.61 (14)
O1^iii—Sr1—O4^vi	106.45 (7)	Si2—O3—Sr1^i	130.42 (13)
O2^ii—Sr1—O4^vi	95.10 (7)	Zn1^xiv—O3—Sr1^i	98.83 (9)
O5^iv—Sr1—O4^vi	126.17 (7)	Si1^xiv—O4—Si3	154.04 (19)
O1^v—Sr1—O4^vi	50.31 (6)	Si1^xiv—O4—Sr1^vi	87.21 (11)
O8—Sr1—O4^vi	156.53 (6)	Si3—O4—Sr1^vi	118.31 (12)
O5—Zn1—O7	123.07 (10)	Si3^ix—O5—Zn1	123.27 (13)
O5—Zn1—O3^vii	111.92 (10)	Si3^ix—O5—Sr1^ix	100.79 (10)
O7—Zn1—O3^vii	108.89 (10)	Zn1—O5—Sr1^ix	135.84 (11)
O5—Zn1—O1^viii	121.42 (10)	Si1—O6—Si2^iv	137.20 (16)
O7—Zn1—O1^viii	92.94 (10)	Si3^x—O7—Zn1	133.22 (15)
O3^vii—Zn1—O1^viii	93.82 (10)	Si3^x—O7—Sr1^xiii	123.71 (13)
O1^iii—Si1—O4^vii	107.47 (13)	Zn1—O7—Sr1^xiii	102.37 (10)
O1^iii—Si1—O6	114.14 (13)	Si1—O8—Si2^x	157.11 (19)
O4^vii—Si1—O6	107.82 (13)	Si1—O8—Sr1	88.43 (10)
O1^iii—Si1—O8	106.32 (13)	Si2^x—O8—Sr1	112.89 (12)
O4^vii—Si1—O8	113.13 (14)

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