Polymorphism and photoluminescence properties of K3ErSi2O7

Dabić, P.; Nikolić, M.G.; Kovač, S.; Kremenović, A.

doi:10.1107/S2053229619011926

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Polymorphism and photoluminescence properties of K₃ErSi₂O₇

P. Dabić, M. G. Nikolić, S. Kovač and A. Kremenović

Two polymorphs of tripotassium erbium disilicate, K₃ErSi₂O₇, were synthesized by high-temperature flux crystal growth during the exploration of the flux technique for growing new alkali rare-earth elements (REE) containing silicates. Their crystal structures were determined by single-crystal X-ray diffraction analysis. One of them (denoted 1) crystallizes in the space group P6₃/mmc and is isostructural with disilicates K₃LuSi₂O₇, K₃ScSi₂O₇ and K₃YSi₂O₇, while the other (denoted 2) crystallizes in the space group P6₃/mcm and is isostructural with disilicates K₃NdSi₂O₇, K₃REESi₂O₇ (REE = Gd–Yb), K₃YSi₂O₇, K₃(Y_0.9Dy_0.1)Si₂O₇ and K₃SmSi₂O₇. In the crystal structure of polymorph 1, the Er cations are in an almost perfect octahedral coordination, while in the crystal structure of polymorph 2, part of the Er cations are in a slightly distorted octahedral coordination and the other part are in an ideal trigonal prismatic coordination environment. Sharing six corners, disilicate Si₂O₇ groups in the crystal structure of polymorph 1 link six ErO₆ octahedra, forming a three-dimensional network and nine-coordinated potassium cations are located in its holes. In the crystal structure of polymorph 2, the disilicate Si₂O₇ groups connect four ErO₆ octahedra, as well as one ErO₆ trigonal prism. Three differently coordinated potassium cations are situated between them. Different site symmetries of the erbium cations in the crystal structures of polymorphs 1 and 2 affect their photoluminescence properties. Only polymorph 2 exhibits luminescence. Intense narrow lines in the emission spectrum are a result of the 4f–4f transition. The green emission line at 560 nm is the result of the Er³⁺ transition ⁴S_3/2→⁴I_15/2, and the luminescence line at 690 nm is the result of a ⁴F_9/2→⁴I_15/2 transition. The crystal morphologies of the two polymorphs are similar. Crystals of polymorph 1 are in the form of a hexagonal prism in combination with a hexagonal base, while crystals of polymorph 2 contain a dihexagonal prism in combination with a hexagonal base, although poorly developed faces of the dihexagonal pyramid can also be noticed.

Keywords: alkali rare-earth silicates; lanthanide silicates; polymorphism; photoluminescence; crystal structure.

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Supporting information

	Crystallographic Information File (CIF) https://doi.org/10.1107/S2053229619011926/fn3325sup1.cif Contains datablocks shelx, shelx_1, global
	Structure factor file (CIF format) https://doi.org/10.1107/S2053229619011926/fn3325shelxsup2.hkl Contains datablock shelx
	Structure factor file (CIF format) https://doi.org/10.1107/S2053229619011926/fn3325shelx_1sup3.hkl Contains datablock shelx_1
	Portable Document Format (PDF) file https://doi.org/10.1107/S2053229619011926/fn3325sup4.pdf PXRD patterns

CCDC references: 1950047; 1950046

Computing details top

For both structures, data collection: CrysAlis PRO (Agilent, 2014); cell refinement: CrysAlis PRO (Agilent, 2014); data reduction: CrysAlis PRO (Agilent, 2014); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL2015 (Sheldrick, 2015); molecular graphics: VESTA (Momma & Izumi, 2011); software used to prepare material for publication: WinGX (Farrugia, 2012) and publCIF (Westrip, 2010).

Tripotassium erbium disilicate (shelx) top

Crystal data top

K₃ErSi₂O₇	D_x = 3.750 Mg m⁻³
M_r = 452.74	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P6₃/mmc	Cell parameters from 1775 reflections
a = 5.7513 (1) Å	θ = 4.1–28.7°
c = 13.9960 (3) Å	µ = 12.32 mm⁻¹
V = 400.93 (2) Å³	T = 298 K
Z = 2	Prismatic, pale-pink
F(000) = 418	0.12 × 0.06 × 0.05 mm

Data collection top

Agilent Xcalibur Sapphire3 Gemini diffractometer	218 reflections with I > 2σ(I)
ω scans	R_int = 0.035
Absorption correction: analytical (Clark & Reid, 1995)	θ_max = 29.1°, θ_min = 2.9°
	h = −7→7
2755 measured reflections	k = −7→7
237 independent reflections	l = −18→18

Refinement top

Refinement on F²	0 restraints
Least-squares matrix: full	w = 1/[σ²(F_o²) + (0.0164P)² + 0.5447P] where P = (F_o² + 2F_c²)/3
R[F² > 2σ(F²)] = 0.015	(Δ/σ)_max < 0.001
wR(F²) = 0.039	Δρ_max = 0.56 e Å⁻³
S = 1.30	Δρ_min = −0.65 e Å⁻³
237 reflections	Extinction correction: SHELXL2015 (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
20 parameters	Extinction coefficient: 0.048 (2)

Special details top

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 (Å²) top

	x	y	z	U_iso*/U_eq
K1	0.3333	0.6667	0.09128 (11)	0.0138 (3)
K2	0.0000	0.0000	0.2500	0.0188 (5)
ER1	0.0000	0.0000	0.0000	0.0058 (2)
SI1	0.6667	0.3333	0.13270 (12)	0.0054 (3)
O1	0.3592 (5)	0.1796 (3)	0.09641 (18)	0.0131 (6)
O2	0.6667	0.3333	0.2500	0.0207 (16)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
K1	0.0121 (4)	0.0121 (4)	0.0173 (7)	0.0060 (2)	0.000	0.000
K2	0.0239 (8)	0.0239 (8)	0.0085 (9)	0.0119 (4)	0.000	0.000
ER1	0.0053 (2)	0.0053 (2)	0.0068 (3)	0.00267 (11)	0.000	0.000
SI1	0.0055 (5)	0.0055 (5)	0.0051 (7)	0.0028 (2)	0.000	0.000
O1	0.0086 (12)	0.0130 (10)	0.0163 (12)	0.0043 (6)	−0.0051 (12)	−0.0026 (6)
O2	0.029 (3)	0.029 (3)	0.005 (3)	0.0143 (13)	0.000	0.000

Geometric parameters (Å, º) top

K1—O1ⁱ	2.8794 (1)	ER1—O1^xvi	2.241 (2)
K1—O1ⁱⁱ	2.8794 (1)	ER1—O1^xvii	2.241 (2)
K1—O1ⁱⁱⁱ	2.8794 (1)	ER1—O1^iv	2.241 (2)
K1—O1^iv	2.8794 (1)	ER1—O1^vii	2.241 (2)
K1—O1^v	2.8794 (1)	ER1—O1^x	2.241 (2)
K1—O1	2.8794 (1)	ER1—O1	2.241 (2)
K1—O1^vi	3.041 (3)	ER1—K2^xvi	3.4990 (1)
K1—O1^vii	3.041 (3)	ER1—K1^xvi	3.5578 (6)
K1—O1^viii	3.041 (3)	ER1—K1^xiii	3.5578 (6)
K1—SI1^viii	3.135 (2)	ER1—K1^viii	3.5578 (6)
K1—SI1	3.3707 (4)	SI1—O1	1.613 (3)
K1—SI1ⁱⁱ	3.3708 (4)	SI1—O1^xviii	1.613 (3)
K2—O1	2.797 (3)	SI1—O1^v	1.613 (3)
K2—O1^ix	2.797 (3)	SI1—O2	1.6418 (17)
K2—O1^x	2.797 (3)	SI1—K1^viii	3.135 (2)
K2—O1^xi	2.797 (3)	SI1—K1^xix	3.3708 (4)
K2—O1^iv	2.797 (3)	SI1—K1^xx	3.3708 (4)
K2—O1^xii	2.797 (3)	SI1—K2^xx	3.7042 (7)
K2—O2^xiii	3.3205 (1)	SI1—K2^xxi	3.7042 (7)
K2—O2	3.3205 (1)	O1—K1^xix	2.8795 (1)
K2—O2^xiv	3.3205 (1)	O1—K1^viii	3.041 (3)
K2—ER1	3.4990 (1)	O2—SI1^ix	1.6418 (17)
K2—ER1^xv	3.4990 (1)	O2—K2^xx	3.3205 (1)
K2—SI1^xiii	3.7042 (7)	O2—K2^xxi	3.3205 (1)

O1ⁱ—K1—O1ⁱⁱ	65.11 (10)	O1^xvii—ER1—O1^x	92.51 (10)
O1ⁱ—K1—O1ⁱⁱⁱ	119.938 (6)	O1^iv—ER1—O1^x	87.49 (10)
O1ⁱⁱ—K1—O1ⁱⁱⁱ	54.85 (10)	O1^vii—ER1—O1^x	180.00 (14)
O1ⁱ—K1—O1^iv	54.85 (10)	O1^xvi—ER1—O1	180.0
O1ⁱⁱ—K1—O1^iv	119.938 (6)	O1^xvii—ER1—O1	92.51 (10)
O1ⁱⁱⁱ—K1—O1^iv	174.13 (10)	O1^iv—ER1—O1	87.49 (10)
O1ⁱ—K1—O1^v	174.13 (10)	O1^vii—ER1—O1	92.51 (10)
O1ⁱⁱ—K1—O1^v	119.938 (6)	O1^x—ER1—O1	87.49 (10)
O1ⁱⁱⁱ—K1—O1^v	65.11 (10)	O1^xvi—ER1—K2	127.02 (7)
O1^iv—K1—O1^v	119.938 (6)	O1^xvii—ER1—K2	127.02 (7)
O1ⁱ—K1—O1	119.938 (6)	O1^iv—ER1—K2	52.98 (7)
O1ⁱⁱ—K1—O1	174.13 (10)	O1^vii—ER1—K2	127.02 (7)
O1ⁱⁱⁱ—K1—O1	119.939 (6)	O1^x—ER1—K2	52.98 (7)
O1^iv—K1—O1	65.11 (10)	O1—ER1—K2	52.98 (7)
O1^v—K1—O1	54.85 (10)	O1^xvi—ER1—K2^xvi	52.98 (7)
O1ⁱ—K1—O1^vi	66.25 (8)	O1^xvii—ER1—K2^xvi	52.98 (7)
O1ⁱⁱ—K1—O1^vi	66.25 (8)	O1^iv—ER1—K2^xvi	127.02 (7)
O1ⁱⁱⁱ—K1—O1^vi	89.94 (6)	O1^vii—ER1—K2^xvi	52.98 (7)
O1^iv—K1—O1^vi	89.94 (6)	O1^x—ER1—K2^xvi	127.02 (7)
O1^v—K1—O1^vi	117.93 (6)	O1—ER1—K2^xvi	127.02 (7)
O1—K1—O1^vi	117.93 (6)	K2—ER1—K2^xvi	180.0
O1ⁱ—K1—O1^vii	89.94 (6)	O1^xvi—ER1—K1^xvi	53.928 (12)
O1ⁱⁱ—K1—O1^vii	117.93 (6)	O1^xvii—ER1—K1^xvi	53.929 (12)
O1ⁱⁱⁱ—K1—O1^vii	117.93 (6)	O1^iv—ER1—K1^xvi	126.071 (12)
O1^iv—K1—O1^vii	66.25 (8)	O1^vii—ER1—K1^xvi	121.93 (7)
O1^v—K1—O1^vii	89.94 (6)	O1^x—ER1—K1^xvi	58.07 (7)
O1—K1—O1^vii	66.25 (8)	O1—ER1—K1^xvi	126.071 (12)
O1^vi—K1—O1^vii	51.72 (8)	K2—ER1—K1^xvi	111.04 (2)
O1ⁱ—K1—O1^viii	117.93 (6)	K2^xvi—ER1—K1^xvi	68.96 (2)
O1ⁱⁱ—K1—O1^viii	89.94 (6)	O1^xvi—ER1—K1^xiii	58.07 (7)
O1ⁱⁱⁱ—K1—O1^viii	66.25 (8)	O1^xvii—ER1—K1^xiii	126.071 (12)
O1^iv—K1—O1^viii	117.93 (6)	O1^iv—ER1—K1^xiii	53.929 (12)
O1^v—K1—O1^viii	66.25 (8)	O1^vii—ER1—K1^xiii	126.071 (12)
O1—K1—O1^viii	89.94 (6)	O1^x—ER1—K1^xiii	53.929 (12)
O1^vi—K1—O1^viii	51.72 (8)	O1—ER1—K1^xiii	121.93 (7)
O1^vii—K1—O1^viii	51.72 (8)	K2—ER1—K1^xiii	68.96 (2)
O1ⁱ—K1—SI1^viii	91.43 (6)	K2^xvi—ER1—K1^xiii	111.04 (2)
O1ⁱⁱ—K1—SI1^viii	91.43 (6)	K1^xvi—ER1—K1^xiii	72.15 (2)
O1ⁱⁱⁱ—K1—SI1^viii	91.43 (6)	O1^xvi—ER1—K1^viii	121.93 (7)
O1^iv—K1—SI1^viii	91.43 (6)	O1^xvii—ER1—K1^viii	53.929 (12)
O1^v—K1—SI1^viii	91.43 (6)	O1^iv—ER1—K1^viii	126.071 (12)
O1—K1—SI1^viii	91.43 (6)	O1^vii—ER1—K1^viii	53.929 (12)
O1^vi—K1—SI1^viii	30.24 (5)	O1^x—ER1—K1^viii	126.071 (12)
O1^vii—K1—SI1^viii	30.24 (5)	O1—ER1—K1^viii	58.07 (7)
O1^viii—K1—SI1^viii	30.24 (5)	K2—ER1—K1^viii	111.04 (2)
O1ⁱ—K1—SI1	145.66 (5)	K2^xvi—ER1—K1^viii	68.96 (2)
O1ⁱⁱ—K1—SI1	145.66 (5)	K1^xvi—ER1—K1^viii	107.85 (2)
O1ⁱⁱⁱ—K1—SI1	92.28 (5)	K1^xiii—ER1—K1^viii	180.00 (5)
O1^iv—K1—SI1	92.28 (5)	O1^xvi—ER1—K1	126.071 (12)
O1^v—K1—SI1	28.56 (5)	O1^xvii—ER1—K1	126.071 (12)
O1—K1—SI1	28.56 (5)	O1^iv—ER1—K1	53.929 (12)
O1^vi—K1—SI1	130.14 (7)	O1^vii—ER1—K1	58.07 (7)
O1^vii—K1—SI1	84.29 (4)	O1^x—ER1—K1	121.93 (7)
O1^viii—K1—SI1	84.29 (4)	O1—ER1—K1	53.929 (12)
SI1^viii—K1—SI1	99.90 (4)	K2—ER1—K1	68.96 (2)
O1ⁱ—K1—SI1ⁱⁱ	92.28 (5)	K2^xvi—ER1—K1	111.04 (2)
O1ⁱⁱ—K1—SI1ⁱⁱ	28.56 (5)	K1^xvi—ER1—K1	180.0
O1ⁱⁱⁱ—K1—SI1ⁱⁱ	28.56 (5)	K1^xiii—ER1—K1	107.85 (3)
O1^iv—K1—SI1ⁱⁱ	145.66 (5)	K1^viii—ER1—K1	72.15 (3)
O1^v—K1—SI1ⁱⁱ	92.28 (5)	O1—SI1—O1^xviii	110.57 (11)
O1—K1—SI1ⁱⁱ	145.66 (5)	O1—SI1—O1^v	110.57 (11)
O1^vi—K1—SI1ⁱⁱ	84.29 (4)	O1^xviii—SI1—O1^v	110.57 (11)
O1^vii—K1—SI1ⁱⁱ	130.14 (7)	O1—SI1—O2	108.35 (11)
O1^viii—K1—SI1ⁱⁱ	84.29 (4)	O1^xviii—SI1—O2	108.35 (11)
SI1^viii—K1—SI1ⁱⁱ	99.90 (4)	O1^v—SI1—O2	108.35 (11)
SI1—K1—SI1ⁱⁱ	117.11 (2)	O1—SI1—K1^viii	71.65 (11)
O1—K2—O1^ix	100.46 (10)	O1^xviii—SI1—K1^viii	71.65 (11)
O1—K2—O1^x	67.28 (8)	O1^v—SI1—K1^viii	71.65 (11)
O1^ix—K2—O1^x	142.69 (4)	O2—SI1—K1^viii	180.0
O1—K2—O1^xi	142.69 (4)	O1—SI1—K1	58.557 (11)
O1^ix—K2—O1^xi	67.28 (8)	O1^xviii—SI1—K1	151.75 (13)
O1^x—K2—O1^xi	142.69 (4)	O1^v—SI1—K1	58.557 (11)
O1—K2—O1^iv	67.28 (8)	O2—SI1—K1	99.90 (4)
O1^ix—K2—O1^iv	142.69 (4)	K1^viii—SI1—K1	80.10 (4)
O1^x—K2—O1^iv	67.28 (8)	O1—SI1—K1^xix	58.558 (11)
O1^xi—K2—O1^iv	100.46 (10)	O1^xviii—SI1—K1^xix	58.557 (11)
O1—K2—O1^xii	142.69 (4)	O1^v—SI1—K1^xix	151.75 (13)
O1^ix—K2—O1^xii	67.28 (8)	O2—SI1—K1^xix	99.90 (4)
O1^x—K2—O1^xii	100.46 (10)	K1^viii—SI1—K1^xix	80.10 (4)
O1^xi—K2—O1^xii	67.28 (8)	K1—SI1—K1^xix	117.11 (2)
O1^iv—K2—O1^xii	142.69 (4)	O1—SI1—K1^xx	151.75 (13)
O1—K2—O2^xiii	108.65 (2)	O1^xviii—SI1—K1^xx	58.557 (11)
O1^ix—K2—O2^xiii	108.65 (2)	O1^v—SI1—K1^xx	58.557 (11)
O1^x—K2—O2^xiii	50.23 (5)	O2—SI1—K1^xx	99.90 (4)
O1^xi—K2—O2^xiii	108.65 (2)	K1^viii—SI1—K1^xx	80.10 (4)
O1^iv—K2—O2^xiii	108.65 (2)	K1—SI1—K1^xx	117.11 (2)
O1^xii—K2—O2^xiii	50.23 (5)	K1^xix—SI1—K1^xx	117.10 (2)
O1—K2—O2	50.23 (5)	O1—SI1—K2^xx	124.40 (4)
O1^ix—K2—O2	50.23 (5)	O1^xviii—SI1—K2^xx	44.66 (10)
O1^x—K2—O2	108.65 (2)	O1^v—SI1—K2^xx	124.40 (4)
O1^xi—K2—O2	108.65 (2)	O2—SI1—K2^xx	63.69 (2)
O1^iv—K2—O2	108.65 (2)	K1^viii—SI1—K2^xx	116.31 (2)
O1^xii—K2—O2	108.65 (2)	K1—SI1—K2^xx	163.59 (6)
O2^xiii—K2—O2	120.0	K1^xix—SI1—K2^xx	68.573 (16)
O1—K2—O2^xiv	108.65 (2)	K1^xx—SI1—K2^xx	68.573 (16)
O1^ix—K2—O2^xiv	108.65 (2)	O1—SI1—K2^xxi	124.40 (4)
O1^x—K2—O2^xiv	108.65 (2)	O1^xviii—SI1—K2^xxi	124.40 (4)
O1^xi—K2—O2^xiv	50.23 (5)	O1^v—SI1—K2^xxi	44.66 (10)
O1^iv—K2—O2^xiv	50.23 (5)	O2—SI1—K2^xxi	63.69 (2)
O1^xii—K2—O2^xiv	108.65 (2)	K1^viii—SI1—K2^xxi	116.31 (2)
O2^xiii—K2—O2^xiv	120.0	K1—SI1—K2^xxi	68.574 (16)
O2—K2—O2^xiv	120.0	K1^xix—SI1—K2^xxi	163.59 (6)
O1—K2—ER1	39.77 (5)	K1^xx—SI1—K2^xxi	68.574 (16)
O1^ix—K2—ER1	140.23 (5)	K2^xx—SI1—K2^xxi	101.85 (3)
O1^x—K2—ER1	39.77 (5)	O1—SI1—K2	44.66 (10)
O1^xi—K2—ER1	140.23 (5)	O1^xviii—SI1—K2	124.40 (4)
O1^iv—K2—ER1	39.77 (5)	O1^v—SI1—K2	124.40 (4)
O1^xii—K2—ER1	140.23 (5)	O2—SI1—K2	63.69 (2)
O2^xiii—K2—ER1	90.0	K1^viii—SI1—K2	116.31 (2)
O2—K2—ER1	90.0	K1—SI1—K2	68.574 (16)
O2^xiv—K2—ER1	90.0	K1^xix—SI1—K2	68.574 (16)
O1—K2—ER1^xv	140.23 (5)	K1^xx—SI1—K2	163.59 (6)
O1^ix—K2—ER1^xv	39.77 (5)	K2^xx—SI1—K2	101.85 (3)
O1^x—K2—ER1^xv	140.23 (5)	K2^xxi—SI1—K2	101.85 (3)
O1^xi—K2—ER1^xv	39.77 (5)	SI1—O1—ER1	161.33 (17)
O1^iv—K2—ER1^xv	140.23 (5)	SI1—O1—K2	111.42 (13)
O1^xii—K2—ER1^xv	39.77 (5)	ER1—O1—K2	87.25 (8)
O2^xiii—K2—ER1^xv	90.0	SI1—O1—K1	92.88 (5)
O2—K2—ER1^xv	90.0	ER1—O1—K1	87.09 (5)
O2^xiv—K2—ER1^xv	90.0	K2—O1—K1	89.46 (6)
ER1—K2—ER1^xv	180.0	SI1—O1—K1^xix	92.88 (5)
O1—K2—SI1^xiii	86.91 (4)	ER1—O1—K1^xix	87.09 (5)
O1^ix—K2—SI1^xiii	128.858 (17)	K2—O1—K1^xix	89.46 (6)
O1^x—K2—SI1^xiii	23.92 (6)	K1—O1—K1^xix	174.13 (10)
O1^xi—K2—SI1^xiii	128.857 (17)	SI1—O1—K1^viii	78.11 (12)
O1^iv—K2—SI1^xiii	86.91 (4)	ER1—O1—K1^viii	83.22 (7)
O1^xii—K2—SI1^xiii	76.54 (6)	K2—O1—K1^viii	170.47 (10)
O2^xiii—K2—SI1^xiii	26.31 (2)	K1—O1—K1^viii	90.06 (6)
O2—K2—SI1^xiii	116.629 (6)	K1^xix—O1—K1^viii	90.06 (6)
O2^xiv—K2—SI1^xiii	116.628 (6)	SI1^ix—O2—SI1	180.0
ER1—K2—SI1^xiii	63.69 (2)	SI1^ix—O2—K2^xx	90.0
ER1^xv—K2—SI1^xiii	116.31 (2)	SI1—O2—K2^xx	90.0
O1^xvi—ER1—O1^xvii	87.49 (10)	SI1^ix—O2—K2^xxi	90.0
O1^xvi—ER1—O1^iv	92.51 (10)	SI1—O2—K2^xxi	90.0
O1^xvii—ER1—O1^iv	180.00 (10)	K2^xx—O2—K2^xxi	120.0
O1^xvi—ER1—O1^vii	87.49 (10)	SI1^ix—O2—K2	90.0
O1^xvii—ER1—O1^vii	87.49 (10)	SI1—O2—K2	90.0
O1^iv—ER1—O1^vii	92.51 (10)	K2^xx—O2—K2	120.0
O1^xvi—ER1—O1^x	92.51 (10)	K2^xxi—O2—K2	120.0

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

Tripotassium erbium disilicate (shelx_1) top

Crystal data top

K₃ErSi₂O₇	D_x = 3.771 Mg m⁻³
M_r = 452.74	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P6₃/mcm	Cell parameters from 2298 reflections
a = 9.8338 (2) Å	θ = 3.7–28.9°
c = 14.2843 (4) Å	µ = 12.39 mm⁻¹
V = 1196.28 (6) Å³	T = 298 K
Z = 6	Prismatic, pink
F(000) = 1254	0.16 × 0.11 × 0.08 mm

Data collection top

Agilent Xcalibur Sapphire3 Gemini diffractometer	512 reflections with I > 2σ(I)
ω scans	R_int = 0.023
Absorption correction: analytical (Clark & Reid, 1995)	θ_max = 29.0°, θ_min = 3.7°
	h = −13→11
5150 measured reflections	k = −13→12
578 independent reflections	l = −18→19

Refinement top

Refinement on F²	0 restraints
Least-squares matrix: full	w = 1/[σ²(F_o²) + (0.0156P)² + 2.7206P] where P = (F_o² + 2F_c²)/3
R[F² > 2σ(F²)] = 0.021	(Δ/σ)_max < 0.001
wR(F²) = 0.046	Δρ_max = 0.59 e Å⁻³
S = 1.23	Δρ_min = −0.82 e Å⁻³
578 reflections	Extinction correction: SHELXL2015 (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
41 parameters	Extinction coefficient: 0.00223 (11)

Special details top

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
K1	0.33132 (11)	0.0000	0.09214 (9)	0.0170 (2)
K2	0.3333	0.6667	0.2500	0.0148 (4)
K3	0.0000	0.0000	0.0000	0.0178 (6)
ER1	0.3333	0.6667	0.0000	0.00505 (13)
ER2	0.0000	0.0000	0.2500	0.00584 (15)
SI1	0.65794 (12)	0.0000	0.14248 (9)	0.0050 (2)
O1	0.6785 (3)	0.5233 (3)	0.90838 (16)	0.0115 (5)
O2	0.8232 (3)	0.0000	0.1469 (2)	0.0103 (7)
O3	0.5932 (4)	0.0000	0.2500	0.0117 (10)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
K1	0.0147 (4)	0.0102 (5)	0.0245 (6)	0.0051 (2)	0.0045 (4)	0.000
K2	0.0169 (6)	0.0169 (6)	0.0107 (10)	0.0084 (3)	0.000	0.000
K3	0.0195 (9)	0.0195 (9)	0.0145 (15)	0.0097 (4)	0.000	0.000
ER1	0.00503 (14)	0.00503 (14)	0.0051 (2)	0.00252 (7)	0.000	0.000
ER2	0.00548 (17)	0.00548 (17)	0.0066 (3)	0.00274 (9)	0.000	0.000
SI1	0.0047 (4)	0.0054 (6)	0.0053 (5)	0.0027 (3)	−0.0001 (5)	0.000
O1	0.0133 (11)	0.0092 (11)	0.0130 (12)	0.0064 (9)	−0.0001 (10)	0.0024 (10)
O2	0.0079 (11)	0.0136 (16)	0.0113 (17)	0.0068 (8)	−0.0003 (13)	0.000
O3	0.0107 (16)	0.020 (3)	0.007 (2)	0.0102 (13)	0.000	0.000

Geometric parameters (Å, º) top

K1—O1ⁱ	2.742 (2)	ER1—K1^xx	3.5416 (7)
K1—O1ⁱⁱ	2.742 (2)	ER1—K1^xvii	3.5416 (7)
K1—O2ⁱⁱⁱ	2.9302 (13)	ER1—K1^xvi	3.5416 (7)
K1—O2^iv	2.9302 (13)	ER1—K1^xxix	3.5416 (7)
K1—O1^v	2.962 (2)	ER1—K1^xxiv	3.5416 (7)
K1—O1^vi	2.962 (2)	ER1—K1^iv	3.5416 (7)
K1—O1^vii	3.014 (2)	ER2—O2^xxx	2.279 (3)
K1—O1^viii	3.014 (2)	ER2—O2ⁱⁱⁱ	2.279 (3)
K1—SI1	3.2915 (15)	ER2—O2^xxi	2.279 (3)
K1—SI1^ix	3.3529 (15)	ER2—O2^iv	2.279 (3)
K1—SI1ⁱⁱⁱ	3.3893 (9)	ER2—O2^xxxi	2.279 (3)
K1—SI1^iv	3.3893 (9)	ER2—O2^xxxii	2.279 (3)
K2—O1^x	2.899 (2)	ER2—K3^xxxiii	3.5711 (1)
K2—O1^xi	2.899 (2)	ER2—K1^xxv	3.9623 (11)
K2—O1^xii	2.899 (2)	ER2—K1^xxxiv	3.9623 (11)
K2—O1^xiii	2.899 (2)	ER2—K1^xviii	3.9623 (11)
K2—O1^xiv	2.899 (2)	ER2—K1^xxxv	3.9623 (11)
K2—O1^xv	2.899 (2)	SI1—O1^vi	1.609 (2)
K2—O3^xvi	2.9830 (14)	SI1—O1^v	1.609 (2)
K2—O3^xvii	2.9830 (14)	SI1—O2	1.626 (3)
K2—O3^iv	2.9830 (14)	SI1—O3	1.663 (2)
K2—ER1^xviii	3.5711 (1)	SI1—K1^ix	3.3530 (15)
K2—ER1	3.5711 (1)	SI1—K1^xxxvi	3.3893 (9)
K2—SI1^xvi	3.5819 (7)	SI1—K1^xxxvii	3.3893 (9)
K3—O2^xix	2.726 (3)	SI1—K2^xxxviii	3.5819 (7)
K3—O2^iv	2.726 (3)	SI1—K2^xxxix	3.5819 (7)
K3—O2^xx	2.726 (3)	SI1—K3^xl	3.9315 (12)
K3—O2ⁱⁱⁱ	2.726 (3)	O1—SI1^x	1.609 (2)
K3—O2^ix	2.726 (3)	O1—ER1^xiv	2.236 (2)
K3—O2^xxi	2.726 (3)	O1—K1^xli	2.742 (2)
K3—K1^xxii	3.5139 (11)	O1—K2^xiv	2.899 (2)
K3—K1^xvi	3.5139 (11)	O1—K1^x	2.962 (2)
K3—K1^xxiii	3.5139 (11)	O1—K1^xlii	3.014 (2)
K3—K1^xxiv	3.5139 (11)	O2—ER2^xl	2.279 (3)
K3—K1^xxv	3.5139 (11)	O2—K3^xl	2.726 (3)
ER1—O1^xxvi	2.236 (2)	O2—K1^xxxvii	2.9302 (13)
ER1—O1^x	2.236 (2)	O2—K1^xxxvi	2.9302 (13)
ER1—O1^xxvii	2.236 (2)	O3—SI1^xviii	1.663 (2)
ER1—O1^xiii	2.236 (2)	O3—K2^xxxviii	2.9830 (14)
ER1—O1^xxviii	2.236 (2)	O3—K2^xxxix	2.9830 (14)
ER1—O1^xiv	2.236 (2)	O3—K1^xviii	3.423 (4)

O1ⁱ—K1—O1ⁱⁱ	173.67 (10)	O1^xxvi—ER1—K1^xx	50.74 (6)
O1ⁱ—K1—O2ⁱⁱⁱ	56.12 (8)	O1^x—ER1—K1^xx	57.71 (6)
O1ⁱⁱ—K1—O2ⁱⁱⁱ	117.97 (8)	O1^xxvii—ER1—K1^xx	56.39 (6)
O1ⁱ—K1—O2^iv	117.97 (8)	O1^xiii—ER1—K1^xx	128.78 (6)
O1ⁱⁱ—K1—O2^iv	56.12 (8)	O1^xxviii—ER1—K1^xx	122.26 (6)
O2ⁱⁱⁱ—K1—O2^iv	61.86 (12)	O1^xiv—ER1—K1^xx	124.09 (6)
O1ⁱ—K1—O1^v	119.66 (3)	O1^xxvi—ER1—K1^xvii	57.71 (6)
O1ⁱⁱ—K1—O1^v	66.66 (9)	O1^x—ER1—K1^xvii	50.74 (6)
O2ⁱⁱⁱ—K1—O1^v	163.64 (9)	O1^xxvii—ER1—K1^xvii	128.78 (6)
O2^iv—K1—O1^v	120.06 (7)	O1^xiii—ER1—K1^xvii	56.39 (6)
O1ⁱ—K1—O1^vi	66.66 (9)	O1^xxviii—ER1—K1^xvii	124.09 (6)
O1ⁱⁱ—K1—O1^vi	119.66 (3)	O1^xiv—ER1—K1^xvii	122.26 (6)
O2ⁱⁱⁱ—K1—O1^vi	120.06 (7)	K1^xx—ER1—K1^xvii	72.51 (4)
O2^iv—K1—O1^vi	163.64 (9)	O1^xxvi—ER1—K1^xvi	124.09 (6)
O1^v—K1—O1^vi	53.00 (8)	O1^x—ER1—K1^xvi	56.39 (6)
O1ⁱ—K1—O1^vii	64.65 (8)	O1^xxvii—ER1—K1^xvi	57.71 (6)
O1ⁱⁱ—K1—O1^vii	116.60 (6)	O1^xiii—ER1—K1^xvi	122.26 (6)
O2ⁱⁱⁱ—K1—O1^vii	100.81 (8)	O1^xxviii—ER1—K1^xvi	128.78 (6)
O2^iv—K1—O1^vii	129.67 (9)	O1^xiv—ER1—K1^xvi	50.74 (6)
O1^v—K1—O1^vii	89.75 (7)	K1^xx—ER1—K1^xvi	73.44 (3)
O1^vi—K1—O1^vii	66.68 (8)	K1^xvii—ER1—K1^xvi	107.03 (2)
O1ⁱ—K1—O1^viii	116.60 (6)	O1^xxvi—ER1—K1^xxix	56.39 (6)
O1ⁱⁱ—K1—O1^viii	64.65 (8)	O1^x—ER1—K1^xxix	124.09 (6)
O2ⁱⁱⁱ—K1—O1^viii	129.67 (9)	O1^xxvii—ER1—K1^xxix	122.26 (6)
O2^iv—K1—O1^viii	100.81 (8)	O1^xiii—ER1—K1^xxix	57.71 (6)
O1^v—K1—O1^viii	66.68 (8)	O1^xxviii—ER1—K1^xxix	50.74 (6)
O1^vi—K1—O1^viii	89.75 (7)	O1^xiv—ER1—K1^xxix	128.78 (6)
O1^vii—K1—O1^viii	52.03 (9)	K1^xx—ER1—K1^xxix	107.03 (2)
O1ⁱ—K1—SI1	93.12 (5)	K1^xvii—ER1—K1^xxix	73.44 (3)
O1ⁱⁱ—K1—SI1	93.12 (5)	K1^xvi—ER1—K1^xxix	179.44 (3)
O2ⁱⁱⁱ—K1—SI1	137.49 (7)	O1^xxvi—ER1—K1^xxiv	122.26 (6)
O2^iv—K1—SI1	137.49 (7)	O1^x—ER1—K1^xxiv	128.78 (6)
O1^v—K1—SI1	29.22 (4)	O1^xxvii—ER1—K1^xxiv	50.74 (6)
O1^vi—K1—SI1	29.22 (4)	O1^xiii—ER1—K1^xxiv	124.09 (6)
O1^vii—K1—SI1	88.54 (6)	O1^xxviii—ER1—K1^xxiv	56.39 (6)
O1^viii—K1—SI1	88.54 (6)	O1^xiv—ER1—K1^xxiv	57.71 (6)
O1ⁱ—K1—SI1^ix	89.95 (6)	K1^xx—ER1—K1^xxiv	107.03 (2)
O1ⁱⁱ—K1—SI1^ix	89.95 (6)	K1^xvii—ER1—K1^xxiv	179.44 (3)
O2ⁱⁱⁱ—K1—SI1^ix	107.02 (7)	K1^xvi—ER1—K1^xxiv	72.51 (4)
O2^iv—K1—SI1^ix	107.02 (7)	K1^xxix—ER1—K1^xxiv	107.03 (2)
O1^v—K1—SI1^ix	88.24 (6)	O1^xxvi—ER1—K1^iv	128.78 (6)
O1^vi—K1—SI1^ix	88.24 (6)	O1^x—ER1—K1^iv	122.26 (6)
O1^vii—K1—SI1^ix	28.65 (4)	O1^xxvii—ER1—K1^iv	124.09 (6)
O1^viii—K1—SI1^ix	28.65 (4)	O1^xiii—ER1—K1^iv	50.74 (6)
SI1—K1—SI1^ix	100.81 (4)	O1^xxviii—ER1—K1^iv	57.71 (6)
O1ⁱ—K1—SI1ⁱⁱⁱ	27.96 (5)	O1^xiv—ER1—K1^iv	56.39 (6)
O1ⁱⁱ—K1—SI1ⁱⁱⁱ	146.42 (6)	K1^xx—ER1—K1^iv	179.44 (3)
O2ⁱⁱⁱ—K1—SI1ⁱⁱⁱ	28.66 (6)	K1^xvii—ER1—K1^iv	107.03 (2)
O2^iv—K1—SI1ⁱⁱⁱ	90.34 (7)	K1^xvi—ER1—K1^iv	107.03 (2)
O1^v—K1—SI1ⁱⁱⁱ	143.15 (5)	K1^xxix—ER1—K1^iv	72.51 (4)
O1^vi—K1—SI1ⁱⁱⁱ	91.90 (5)	K1^xxiv—ER1—K1^iv	73.44 (3)
O1^vii—K1—SI1ⁱⁱⁱ	84.87 (5)	O2^xxx—ER2—O2ⁱⁱⁱ	135.13 (6)
O1^viii—K1—SI1ⁱⁱⁱ	131.65 (6)	O2^xxx—ER2—O2^xxi	135.13 (6)
SI1—K1—SI1ⁱⁱⁱ	114.05 (3)	O2ⁱⁱⁱ—ER2—O2^xxi	82.75 (12)
SI1^ix—K1—SI1ⁱⁱⁱ	103.10 (4)	O2^xxx—ER2—O2^iv	80.51 (16)
O1ⁱ—K1—SI1^iv	146.42 (6)	O2ⁱⁱⁱ—ER2—O2^iv	82.75 (12)
O1ⁱⁱ—K1—SI1^iv	27.96 (5)	O2^xxi—ER2—O2^iv	82.75 (12)
O2ⁱⁱⁱ—K1—SI1^iv	90.34 (7)	O2^xxx—ER2—O2^xxxi	82.75 (12)
O2^iv—K1—SI1^iv	28.66 (6)	O2ⁱⁱⁱ—ER2—O2^xxxi	80.51 (16)
O1^v—K1—SI1^iv	91.90 (5)	O2^xxi—ER2—O2^xxxi	135.13 (6)
O1^vi—K1—SI1^iv	143.15 (5)	O2^iv—ER2—O2^xxxi	135.13 (6)
O1^vii—K1—SI1^iv	131.65 (6)	O2^xxx—ER2—O2^xxxii	82.75 (12)
O1^viii—K1—SI1^iv	84.87 (5)	O2ⁱⁱⁱ—ER2—O2^xxxii	135.13 (6)
SI1—K1—SI1^iv	114.05 (3)	O2^xxi—ER2—O2^xxxii	80.51 (16)
SI1^ix—K1—SI1^iv	103.10 (4)	O2^iv—ER2—O2^xxxii	135.13 (6)
SI1ⁱⁱⁱ—K1—SI1^iv	118.52 (5)	O2^xxxi—ER2—O2^xxxii	82.75 (12)
O1^x—K2—O1^xi	143.56 (4)	O2^xxx—ER2—K3	130.25 (8)
O1^x—K2—O1^xii	102.59 (9)	O2ⁱⁱⁱ—ER2—K3	49.75 (8)
O1^xi—K2—O1^xii	65.58 (7)	O2^xxi—ER2—K3	49.75 (8)
O1^x—K2—O1^xiii	65.58 (7)	O2^iv—ER2—K3	49.75 (8)
O1^xi—K2—O1^xiii	102.59 (9)	O2^xxxi—ER2—K3	130.25 (8)
O1^xii—K2—O1^xiii	143.56 (4)	O2^xxxii—ER2—K3	130.25 (8)
O1^x—K2—O1^xiv	65.58 (7)	O2^xxx—ER2—K3^xxxiii	49.75 (8)
O1^xi—K2—O1^xiv	143.56 (4)	O2ⁱⁱⁱ—ER2—K3^xxxiii	130.25 (8)
O1^xii—K2—O1^xiv	143.56 (4)	O2^xxi—ER2—K3^xxxiii	130.25 (8)
O1^xiii—K2—O1^xiv	65.58 (7)	O2^iv—ER2—K3^xxxiii	130.25 (8)
O1^x—K2—O1^xv	143.56 (4)	O2^xxxi—ER2—K3^xxxiii	49.75 (8)
O1^xi—K2—O1^xv	65.58 (7)	O2^xxxii—ER2—K3^xxxiii	49.75 (8)
O1^xii—K2—O1^xv	65.58 (7)	K3—ER2—K3^xxxiii	180.0
O1^xiii—K2—O1^xv	143.56 (4)	O2^xxx—ER2—K1^xxv	174.43 (8)
O1^xiv—K2—O1^xv	102.59 (9)	O2ⁱⁱⁱ—ER2—K1^xxv	47.04 (2)
O1^x—K2—O3^xvi	52.90 (5)	O2^xxi—ER2—K1^xxv	47.04 (2)
O1^xi—K2—O3^xvi	116.39 (6)	O2^iv—ER2—K1^xxv	105.06 (8)
O1^xii—K2—O3^xvi	52.91 (5)	O2^xxxi—ER2—K1^xxv	93.09 (6)
O1^xiii—K2—O3^xvi	116.39 (6)	O2^xxxii—ER2—K1^xxv	93.09 (6)
O1^xiv—K2—O3^xvi	99.13 (7)	K3—ER2—K1^xxv	55.313 (18)
O1^xv—K2—O3^xvi	99.13 (7)	K3^xxxiii—ER2—K1^xxv	124.687 (18)
O1^x—K2—O3^xvii	99.13 (7)	O2^xxx—ER2—K1^xxxiv	47.04 (2)
O1^xi—K2—O3^xvii	52.90 (5)	O2ⁱⁱⁱ—ER2—K1^xxxiv	174.43 (8)
O1^xii—K2—O3^xvii	99.13 (7)	O2^xxi—ER2—K1^xxxiv	93.09 (6)
O1^xiii—K2—O3^xvii	52.91 (5)	O2^iv—ER2—K1^xxxiv	93.09 (6)
O1^xiv—K2—O3^xvii	116.39 (6)	O2^xxxi—ER2—K1^xxxiv	105.06 (8)
O1^xv—K2—O3^xvii	116.39 (6)	O2^xxxii—ER2—K1^xxxiv	47.04 (2)
O3^xvi—K2—O3^xvii	120.0	K3—ER2—K1^xxxiv	124.687 (18)
O1^x—K2—O3^iv	116.39 (6)	K3^xxxiii—ER2—K1^xxxiv	55.313 (18)
O1^xi—K2—O3^iv	99.13 (7)	K1^xxv—ER2—K1^xxxiv	131.448 (11)
O1^xii—K2—O3^iv	116.39 (6)	O2^xxx—ER2—K1^xviii	47.04 (2)
O1^xiii—K2—O3^iv	99.13 (7)	O2ⁱⁱⁱ—ER2—K1^xviii	93.09 (6)
O1^xiv—K2—O3^iv	52.90 (5)	O2^xxi—ER2—K1^xviii	174.43 (8)
O1^xv—K2—O3^iv	52.90 (5)	O2^iv—ER2—K1^xviii	93.09 (6)
O3^xvi—K2—O3^iv	120.000 (1)	O2^xxxi—ER2—K1^xviii	47.04 (2)
O3^xvii—K2—O3^iv	120.000 (1)	O2^xxxii—ER2—K1^xviii	105.06 (8)
O1^x—K2—ER1^xviii	141.30 (4)	K3—ER2—K1^xviii	124.687 (18)
O1^xi—K2—ER1^xviii	38.70 (4)	K3^xxxiii—ER2—K1^xviii	55.313 (18)
O1^xii—K2—ER1^xviii	38.70 (4)	K1^xxv—ER2—K1^xviii	131.448 (11)
O1^xiii—K2—ER1^xviii	141.30 (4)	K1^xxxiv—ER2—K1^xviii	90.81 (2)
O1^xiv—K2—ER1^xviii	141.30 (4)	O2^xxx—ER2—K1^xxxv	105.06 (8)
O1^xv—K2—ER1^xviii	38.70 (4)	O2ⁱⁱⁱ—ER2—K1^xxxv	93.09 (6)
O3^xvi—K2—ER1^xviii	90.0	O2^xxi—ER2—K1^xxxv	93.09 (6)
O3^xvii—K2—ER1^xviii	90.0	O2^iv—ER2—K1^xxxv	174.43 (8)
O3^iv—K2—ER1^xviii	90.0	O2^xxxi—ER2—K1^xxxv	47.04 (2)
O1^x—K2—ER1	38.70 (4)	O2^xxxii—ER2—K1^xxxv	47.04 (2)
O1^xi—K2—ER1	141.30 (4)	K3—ER2—K1^xxxv	124.687 (18)
O1^xii—K2—ER1	141.30 (4)	K3^xxxiii—ER2—K1^xxxv	55.313 (18)
O1^xiii—K2—ER1	38.70 (4)	K1^xxv—ER2—K1^xxxv	69.37 (4)
O1^xiv—K2—ER1	38.70 (4)	K1^xxxiv—ER2—K1^xxxv	90.81 (2)
O1^xv—K2—ER1	141.30 (4)	K1^xviii—ER2—K1^xxxv	90.81 (2)
O3^xvi—K2—ER1	90.0	O1^vi—SI1—O1^v	110.45 (17)
O3^xvii—K2—ER1	90.0	O1^vi—SI1—O2	111.46 (11)
O3^iv—K2—ER1	90.0	O1^v—SI1—O2	111.46 (11)
ER1^xviii—K2—ER1	180.0	O1^vi—SI1—O3	106.47 (12)
O1^x—K2—SI1^xvi	26.13 (5)	O1^v—SI1—O3	106.47 (12)
O1^xi—K2—SI1^xvi	130.89 (5)	O2—SI1—O3	110.3 (2)
O1^xii—K2—SI1^xvi	76.79 (5)	O1^vi—SI1—K1	63.98 (9)
O1^xiii—K2—SI1^xvi	89.16 (5)	O1^v—SI1—K1	63.98 (9)
O1^xiv—K2—SI1^xvi	84.75 (5)	O2—SI1—K1	169.62 (14)
O1^xv—K2—SI1^xvi	125.29 (5)	O3—SI1—K1	80.10 (15)
O3^xvi—K2—SI1^xvi	27.45 (3)	O1^vi—SI1—K1^ix	63.88 (9)
O3^xvii—K2—SI1^xvi	107.29 (7)	O1^v—SI1—K1^ix	63.88 (9)
O3^iv—K2—SI1^xvi	126.17 (7)	O2—SI1—K1^ix	90.43 (13)
ER1^xviii—K2—SI1^xvi	115.393 (18)	O3—SI1—K1^ix	159.29 (15)
ER1—K2—SI1^xvi	64.608 (18)	K1—SI1—K1^ix	79.19 (4)
O2^xix—K3—O2^iv	180.00 (15)	O1^vi—SI1—K1^xxxvi	140.04 (10)
O2^xix—K3—O2^xx	67.08 (10)	O1^v—SI1—K1^xxxvi	53.04 (9)
O2^iv—K3—O2^xx	112.92 (10)	O2—SI1—K1^xxxvi	59.79 (4)
O2^xix—K3—O2ⁱⁱⁱ	112.92 (10)	O3—SI1—K1^xxxvi	113.08 (7)
O2^iv—K3—O2ⁱⁱⁱ	67.08 (10)	K1—SI1—K1^xxxvi	116.94 (3)
O2^xx—K3—O2ⁱⁱⁱ	180.00 (13)	K1^ix—SI1—K1^xxxvi	76.81 (4)
O2^xix—K3—O2^ix	67.08 (11)	O1^vi—SI1—K1^xxxvii	53.04 (9)
O2^iv—K3—O2^ix	112.92 (11)	O1^v—SI1—K1^xxxvii	140.04 (10)
O2^xx—K3—O2^ix	67.08 (11)	O2—SI1—K1^xxxvii	59.79 (4)
O2ⁱⁱⁱ—K3—O2^ix	112.92 (10)	O3—SI1—K1^xxxvii	113.08 (7)
O2^xix—K3—O2^xxi	112.92 (11)	K1—SI1—K1^xxxvii	116.94 (3)
O2^iv—K3—O2^xxi	67.08 (11)	K1^ix—SI1—K1^xxxvii	76.81 (4)
O2^xx—K3—O2^xxi	112.92 (10)	K1^xxxvi—SI1—K1^xxxvii	112.71 (5)
O2ⁱⁱⁱ—K3—O2^xxi	67.08 (11)	O1^vi—SI1—K2^xxxviii	52.51 (8)
O2^ix—K3—O2^xxi	180.00 (13)	O1^v—SI1—K2^xxxviii	133.90 (9)
O2^xix—K3—K1^xxii	54.253 (17)	O2—SI1—K2^xxxviii	114.61 (7)
O2^iv—K3—K1^xxii	125.747 (17)	O3—SI1—K2^xxxviii	55.79 (4)
O2^xx—K3—K1^xxii	54.253 (17)	K1—SI1—K2^xxxviii	70.76 (2)
O2ⁱⁱⁱ—K3—K1^xxii	125.747 (17)	K1^ix—SI1—K2^xxxviii	116.25 (2)
O2^ix—K3—K1^xxii	107.65 (7)	K1^xxxvi—SI1—K2^xxxviii	166.57 (5)
O2^xxi—K3—K1^xxii	72.35 (7)	K1^xxxvii—SI1—K2^xxxviii	69.70 (2)
O2^xix—K3—K1^xvi	125.747 (17)	O1^vi—SI1—K2^xxxix	133.90 (9)
O2^iv—K3—K1^xvi	54.253 (17)	O1^v—SI1—K2^xxxix	52.51 (8)
O2^xx—K3—K1^xvi	72.35 (7)	O2—SI1—K2^xxxix	114.62 (7)
O2ⁱⁱⁱ—K3—K1^xvi	107.65 (7)	O3—SI1—K2^xxxix	55.79 (4)
O2^ix—K3—K1^xvi	125.747 (17)	K1—SI1—K2^xxxix	70.76 (2)
O2^xxi—K3—K1^xvi	54.253 (17)	K1^ix—SI1—K2^xxxix	116.25 (2)
K1^xxii—K3—K1^xvi	73.17 (2)	K1^xxxvi—SI1—K2^xxxix	69.70 (2)
O2^xix—K3—K1^xxiii	54.253 (17)	K1^xxxvii—SI1—K2^xxxix	166.57 (5)
O2^iv—K3—K1^xxiii	125.747 (17)	K2^xxxviii—SI1—K2^xxxix	104.85 (3)
O2^xx—K3—K1^xxiii	107.65 (7)	O1^vi—SI1—K3^xl	93.70 (9)
O2ⁱⁱⁱ—K3—K1^xxiii	72.35 (7)	O1^v—SI1—K3^xl	93.70 (9)
O2^ix—K3—K1^xxiii	54.253 (17)	O2—SI1—K3^xl	33.42 (12)
O2^xxi—K3—K1^xxiii	125.747 (17)	O3—SI1—K3^xl	143.69 (15)
K1^xxii—K3—K1^xxiii	106.83 (2)	K1—SI1—K3^xl	136.21 (5)
K1^xvi—K3—K1^xxiii	180.00 (4)	K1^ix—SI1—K3^xl	57.02 (3)
O2^xix—K3—K1^xxiv	107.65 (7)	K1^xxxvi—SI1—K3^xl	56.79 (3)
O2^iv—K3—K1^xxiv	72.35 (7)	K1^xxxvii—SI1—K3^xl	56.79 (3)
O2^xx—K3—K1^xxiv	54.253 (17)	K2^xxxviii—SI1—K3^xl	126.367 (16)
O2ⁱⁱⁱ—K3—K1^xxiv	125.747 (17)	K2^xxxix—SI1—K3^xl	126.368 (16)
O2^ix—K3—K1^xxiv	54.253 (17)	SI1^x—O1—ER1^xiv	167.49 (14)
O2^xxi—K3—K1^xxiv	125.747 (17)	SI1^x—O1—K1^xli	98.99 (11)
K1^xxii—K3—K1^xxiv	106.83 (2)	ER1^xiv—O1—K1^xli	90.12 (8)
K1^xvi—K3—K1^xxiv	73.17 (2)	SI1^x—O1—K2^xiv	101.35 (11)
K1^xxiii—K3—K1^xxiv	106.83 (2)	ER1^xiv—O1—K2^xiv	87.12 (7)
O2^xix—K3—K1^xxv	72.35 (7)	K1^xli—O1—K2^xiv	89.90 (7)
O2^iv—K3—K1^xxv	107.65 (7)	SI1^x—O1—K1^x	86.81 (10)
O2^xx—K3—K1^xxv	125.747 (17)	ER1^xiv—O1—K1^x	84.66 (7)
O2ⁱⁱⁱ—K3—K1^xxv	54.253 (17)	K1^xli—O1—K1^x	173.33 (9)
O2^ix—K3—K1^xxv	125.747 (17)	K2^xiv—O1—K1^x	85.71 (6)
O2^xxi—K3—K1^xxv	54.253 (17)	SI1^x—O1—K1^xlii	87.47 (10)
K1^xxii—K3—K1^xxv	73.17 (2)	ER1^xiv—O1—K1^xlii	83.45 (7)
K1^xvi—K3—K1^xxv	106.83 (2)	K1^xli—O1—K1^xlii	93.27 (8)
K1^xxiii—K3—K1^xxv	73.17 (2)	K2^xiv—O1—K1^xlii	170.05 (9)
K1^xxiv—K3—K1^xxv	180.00 (3)	K1^x—O1—K1^xlii	90.25 (7)
O2^xix—K3—K1	125.747 (17)	SI1—O2—ER2^xl	142.0 (2)
O2^iv—K3—K1	54.253 (17)	SI1—O2—K3^xl	127.41 (18)
O2^xx—K3—K1	125.746 (17)	ER2^xl—O2—K3^xl	90.61 (10)
O2ⁱⁱⁱ—K3—K1	54.254 (17)	SI1—O2—K1^xxxvii	91.55 (7)
O2^ix—K3—K1	72.35 (7)	ER2^xl—O2—K1^xxxvii	98.27 (6)
O2^xxi—K3—K1	107.65 (7)	K3^xl—O2—K1^xxxvii	76.73 (7)
K1^xxii—K3—K1	180.0	SI1—O2—K1^xxxvi	91.55 (7)
K1^xvi—K3—K1	106.83 (2)	ER2^xl—O2—K1^xxxvi	98.27 (6)
K1^xxiii—K3—K1	73.17 (2)	K3^xl—O2—K1^xxxvi	76.73 (7)
K1^xxiv—K3—K1	73.17 (2)	K1^xxxvii—O2—K1^xxxvi	148.71 (13)
K1^xxv—K3—K1	106.83 (2)	SI1—O3—SI1^xviii	135.0 (3)
O1^xxvi—ER1—O1^x	87.29 (12)	SI1—O3—K2^xxxviii	96.756 (16)
O1^xxvi—ER1—O1^xxvii	89.20 (8)	SI1^xviii—O3—K2^xxxviii	96.756 (15)
O1^x—ER1—O1^xxvii	94.55 (12)	SI1—O3—K2^xxxix	96.757 (15)
O1^xxvi—ER1—O1^xiii	94.55 (12)	SI1^xviii—O3—K2^xxxix	96.757 (15)
O1^x—ER1—O1^xiii	89.20 (8)	K2^xxxviii—O3—K2^xxxix	144.22 (16)
O1^xxvii—ER1—O1^xiii	174.83 (12)	SI1—O3—K1	71.31 (10)
O1^xxvi—ER1—O1^xxviii	89.20 (8)	SI1^xviii—O3—K1	153.73 (19)
O1^x—ER1—O1^xxviii	174.83 (12)	K2^xxxviii—O3—K1	76.64 (7)
O1^xxvii—ER1—O1^xxviii	89.20 (8)	K2^xxxix—O3—K1	76.64 (7)
O1^xiii—ER1—O1^xxviii	87.29 (12)	SI1—O3—K1^xviii	153.73 (19)
O1^xxvi—ER1—O1^xiv	174.83 (12)	SI1^xviii—O3—K1^xviii	71.31 (10)
O1^x—ER1—O1^xiv	89.20 (8)	K2^xxxviii—O3—K1^xviii	76.64 (7)
O1^xxvii—ER1—O1^xiv	87.29 (12)	K2^xxxix—O3—K1^xviii	76.64 (7)
O1^xiii—ER1—O1^xiv	89.20 (8)	K1—O3—K1^xviii	82.41 (11)
O1^xxviii—ER1—O1^xiv	94.55 (12)

O1^vi—SI1—O2—ER2^xl	−118.04 (11)	K1^xxxvi—SI1—O3—SI1^xviii	−64.81 (7)
O1^v—SI1—O2—ER2^xl	118.04 (11)	K1^xxxvii—SI1—O3—SI1^xviii	64.81 (7)
O3—SI1—O2—ER2^xl	0.000 (1)	K2^xxxviii—SI1—O3—SI1^xviii	106.60 (9)
K1—SI1—O2—ER2^xl	180.000 (1)	K2^xxxix—SI1—O3—SI1^xviii	−106.60 (9)
K1^ix—SI1—O2—ER2^xl	180.0	K3^xl—SI1—O3—SI1^xviii	0.000 (1)
K1^xxxvi—SI1—O2—ER2^xl	105.57 (7)	O1^vi—SI1—O3—K2^xxxviii	14.46 (16)
K1^xxxvii—SI1—O2—ER2^xl	−105.57 (7)	O1^v—SI1—O3—K2^xxxviii	132.33 (11)
K2^xxxviii—SI1—O2—ER2^xl	−60.66 (6)	O2—SI1—O3—K2^xxxviii	−106.60 (9)
K2^xxxix—SI1—O2—ER2^xl	60.66 (6)	K1—SI1—O3—K2^xxxviii	73.40 (9)
K3^xl—SI1—O2—ER2^xl	180.000 (1)	K1^ix—SI1—O3—K2^xxxviii	73.40 (9)
O1^vi—SI1—O2—K3^xl	61.96 (11)	K1^xxxvi—SI1—O3—K2^xxxviii	−171.41 (5)
O1^v—SI1—O2—K3^xl	−61.96 (11)	K1^xxxvii—SI1—O3—K2^xxxviii	−41.79 (15)
O3—SI1—O2—K3^xl	180.000 (1)	K2^xxxix—SI1—O3—K2^xxxviii	146.79 (18)
K1—SI1—O2—K3^xl	0.000 (1)	K3^xl—SI1—O3—K2^xxxviii	−106.60 (9)
K1^ix—SI1—O2—K3^xl	0.000 (1)	O1^vi—SI1—O3—K2^xxxix	−132.33 (11)
K1^xxxvi—SI1—O2—K3^xl	−74.43 (7)	O1^v—SI1—O3—K2^xxxix	−14.46 (16)
K1^xxxvii—SI1—O2—K3^xl	74.43 (7)	O2—SI1—O3—K2^xxxix	106.60 (9)
K2^xxxviii—SI1—O2—K3^xl	119.34 (6)	K1—SI1—O3—K2^xxxix	−73.40 (9)
K2^xxxix—SI1—O2—K3^xl	−119.34 (6)	K1^ix—SI1—O3—K2^xxxix	−73.40 (9)
O1^vi—SI1—O2—K1^xxxvii	−12.47 (15)	K1^xxxvi—SI1—O3—K2^xxxix	41.79 (15)
O1^v—SI1—O2—K1^xxxvii	−136.39 (11)	K1^xxxvii—SI1—O3—K2^xxxix	171.42 (5)
O3—SI1—O2—K1^xxxvii	105.57 (7)	K2^xxxviii—SI1—O3—K2^xxxix	−146.79 (18)
K1—SI1—O2—K1^xxxvii	−74.43 (7)	K3^xl—SI1—O3—K2^xxxix	106.60 (9)
K1^ix—SI1—O2—K1^xxxvii	−74.43 (7)	O1^vi—SI1—O3—K1	−58.93 (10)
K1^xxxvi—SI1—O2—K1^xxxvii	−148.86 (14)	O1^v—SI1—O3—K1	58.93 (10)
K2^xxxviii—SI1—O2—K1^xxxvii	44.91 (12)	O2—SI1—O3—K1	180.000 (1)
K2^xxxix—SI1—O2—K1^xxxvii	166.23 (5)	K1^ix—SI1—O3—K1	0.000 (1)
K3^xl—SI1—O2—K1^xxxvii	−74.43 (7)	K1^xxxvi—SI1—O3—K1	115.19 (7)
O1^vi—SI1—O2—K1^xxxvi	136.39 (11)	K1^xxxvii—SI1—O3—K1	−115.19 (7)
O1^v—SI1—O2—K1^xxxvi	12.47 (15)	K2^xxxviii—SI1—O3—K1	−73.40 (9)
O3—SI1—O2—K1^xxxvi	−105.57 (7)	K2^xxxix—SI1—O3—K1	73.40 (9)
K1—SI1—O2—K1^xxxvi	74.43 (7)	K3^xl—SI1—O3—K1	180.000 (1)
K1^ix—SI1—O2—K1^xxxvi	74.43 (7)	O1^vi—SI1—O3—K1^xviii	−58.93 (10)
K1^xxxvii—SI1—O2—K1^xxxvi	148.86 (14)	O1^v—SI1—O3—K1^xviii	58.93 (10)
K2^xxxviii—SI1—O2—K1^xxxvi	−166.23 (5)	O2—SI1—O3—K1^xviii	180.000 (1)
K2^xxxix—SI1—O2—K1^xxxvi	−44.91 (12)	K1—SI1—O3—K1^xviii	0.000 (1)
K3^xl—SI1—O2—K1^xxxvi	74.43 (7)	K1^ix—SI1—O3—K1^xviii	0.000 (1)
O1^vi—SI1—O3—SI1^xviii	121.07 (10)	K1^xxxvi—SI1—O3—K1^xviii	115.19 (7)
O1^v—SI1—O3—SI1^xviii	−121.07 (10)	K1^xxxvii—SI1—O3—K1^xviii	−115.19 (7)
O2—SI1—O3—SI1^xviii	0.000 (1)	K2^xxxviii—SI1—O3—K1^xviii	−73.40 (9)
K1—SI1—O3—SI1^xviii	180.000 (1)	K2^xxxix—SI1—O3—K1^xviii	73.40 (9)
K1^ix—SI1—O3—SI1^xviii	180.000 (1)	K3^xl—SI1—O3—K1^xviii	180.000 (1)

Symmetry codes: (i) x−y, x−1, −z+1; (ii) −y+1, −x+1, −z+1; (iii) −y, x−y−1, z; (iv) −x+y+1, −x+1, z; (v) x, x−y, −z+1; (vi) y, −x+y, −z+1; (vii) −x+1, −x+y, z−1; (viii) −y+1, x−y, z−1; (ix) −x+1, −y, −z; (x) x−y, x, −z+1; (xi) y, −x+y+1, z−1/2; (xii) x−y, x, z−1/2; (xiii) y, −x+y+1, −z+1; (xiv) −x+1, −y+1, −z+1; (xv) −x+1, −y+1, z−1/2; (xvi) −y, x−y, z; (xvii) x, y+1, z; (xviii) x, y, −z+1/2; (xix) x−y−1, x−1, −z; (xx) y, −x+y+1, −z; (xxi) x−1, y, z; (xxii) −x, −y, −z; (xxiii) y, −x+y, −z; (xxiv) x−y, x, −z; (xxv) −x+y, −x, z; (xxvi) −x+1, −x+y+1, z−1; (xxvii) x−y, −y+1, z−1; (xxviii) y, x, z−1; (xxix) −x+1, −y+1, −z; (xxx) −x+y+1, −x+1, −z+1/2; (xxxi) −y, x−y−1, −z+1/2; (xxxii) x−1, y, −z+1/2; (xxxiii) −x, −y, z+1/2; (xxxiv) −y, x−y, −z+1/2; (xxxv) −x+y, −x, −z+1/2; (xxxvi) −y+1, x−y, z; (xxxvii) −x+y+1, −x, z; (xxxviii) x, y−1, z; (xxxix) x−y+1, −y+1, −z+1/2; (xl) x+1, y, z; (xli) y+1, −x+y+1, −z+1; (xlii) −x+y+1, −x+1, z+1.

Selected bond lengths (Å) for polymorph 1 top

K1—O1ⁱ	2.87943 (14)	K2—O1^xii	2.797 (3)
K1—O1ⁱⁱ	2.87943 (14)	K2—O2^xiii	3.32050 (5)
K1—O1ⁱⁱⁱ	2.87943 (14)	K2—O2	3.32051 (6)
K1—O1^iv	2.87943 (14)	K2—O2^xiv	3.32054 (6)
K1—O1^v	2.87943 (14)	Er1—O1^xv	2.241 (2)
K1—O1	2.87943 (14)	Er1—O1^xvi	2.241 (2)
K1—O1^vi	3.041 (3)	Er1—O1^iv	2.241 (2)
K1—O1^vii	3.041 (3)	Er1—O1^vii	2.241 (2)
K1—O1^viii	3.041 (3)	Er1—O1^x	2.241 (2)
K2—O1	2.797 (3)	Er1—O1	2.241 (2)
K2—O1^ix	2.797 (3)	Si1—O1	1.613 (3)
K2—O1^x	2.797 (3)	Si1—O1^xvii	1.613 (3)
K2—O1^xi	2.797 (3)	Si1—O1^v	1.613 (3)
K2—O1^iv	2.797 (3)	Si1—O2	1.6418 (17)

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

Selected bond lengths (Å) for polymorph 2 top

K1—O1ⁱ	2.742 (2)	K3—O2ⁱⁱⁱ	2.726 (3)
K1—O1ⁱⁱ	2.742 (2)	K3—O2^xix	2.726 (3)
K1—O2ⁱⁱⁱ	2.9302 (13)	K3—O2^xx	2.726 (3)
K1—O2^iv	2.9302 (13)	Er1—O1^xxi	2.236 (2)
K1—O1^v	2.962 (2)	Er1—O1^ix	2.236 (2)
K1—O1^vi	2.962 (2)	Er1—O1^xxii	2.236 (2)
K1—O1^vii	3.014 (2)	Er1—O1^xii	2.236 (2)
K1—O1^viii	3.014 (2)	Er1—O1^xxiii	2.236 (2)
K2—O1^ix	2.899 (2)	Er1—O1^xiii	2.236 (2)
K2—O1^x	2.899 (2)	Er2—O2^xxiv	2.279 (3)
K2—O1^xi	2.899 (2)	Er2—O2ⁱⁱⁱ	2.279 (3)
K2—O1^xii	2.899 (2)	Er2—O2^xx	2.279 (3)
K2—O1^xiii	2.899 (2)	Er2—O2^iv	2.279 (3)
K2—O1^xiv	2.899 (2)	Er2—O2^xxv	2.279 (3)
K2—O3^xv	2.9830 (14)	Er2—O2^xxvi	2.279 (3)
K2—O3^xvi	2.9830 (14)	Si1—O1^vi	1.609 (2)
K2—O3^iv	2.9830 (14)	Si1—O1^v	1.609 (2)
K3—O2^xvii	2.726 (3)	Si1—O2	1.626 (3)
K3—O2^iv	2.726 (3)	Si1—O3	1.663 (2)
K3—O2^xviii	2.726 (3)

Symmetry codes: (i) x-y, x-1, -z+1; (ii) -y+1, -x+1, -z+1; (iii) -y, x-y-1, z; (iv) -x+y+1, -x+1, z; (v) x, x-y, -z+1; (vi) y, -x+y, -z+1; (vii) -x+1, -x+y, z-1; (viii) -y+1, x-y, z-1; (ix) x-y, x, -z+1; (x) y, -x+y+1, z-1/2; (xi) x-y, x, z-1/2; (xii) y, -x+y+1, -z+1; (xiii) -x+1, -y+1, -z+1; (xiv) -x+1, -y+1, z-1/2; (xv) -y, x-y, z; (xvi) x, y+1, z; (xvii) x-y-1, x-1, -z; (xviii) y, -x+y+1, -z; (xix) -x+1, -y, -z; (xx) x-1, y, z; (xxi) -x+1, -x+y+1, z-1; (xxii) x-y, -y+1, z-1; (xxiii) y, x, z-1; (xxiv) -x+y+1, -x+1, -z+1/2; (xxv) -y, x-y-1, -z+1/2; (xxvi) x-1, y, -z+1/2.

Bond-valences ν_ij (valence units) for polymorph 1 top

	K1	K2	Er1	Si1	Σν_ij
O1	0.133 × 6 + 0.086 × 3	0.166 × 6	0.505 × 6	1.030 × 3	1.967
O2	-	0.040 × 3	-	0.953	2.026
Σν_ij	1.056 (6)	1.116 (12)	3.03 (1)	4.043 (1)

Bond-valences ν_ij (valence units) for polymorph 2 top

	K1	K2	K3	Er1	Er2	Si1	Σν_ij
O1	0.192 × 2 + 0.106 × 2 + 0.092 × 2	0.126 × 6	-	0.512 × 6	-	1.041 × 2	2.069
O2	0.116 × 2	-	0.201 × 6	-	0.455 × 6	0.995	1.883
O3	-	0.100 × 3	-	-	-	0.900	2.000
Σν_ij	1.012 (1)	1.056 (6)	1.206 (21)	3.072 (2)	2.73 (9)	3.977 (1)

Comparison of the unit-cell parameters for two known structure types of potassium sorosilicates containing REEs with the general formula K₃REESi₂O₇ top

Compound	Space group	a (Å)	c (Å)	V (Å³)	Reference
K₃NdSi₂O₇	P6₃/mcm	10.025 (1)	14.526 (1)	1264.1 (3)	Hwang et al. (1987)
K₃SmSi₂O₇	P6₃/mcm	9.9764 (1)	14.4844 (3)	1248.47 (4)	Latshaw et al. (2016b)
K₃EuSi₂O₇ ^a	P6₃/mcm	9.98 (1)	14.44 (2)	1245.54 (4)	Bondar et al. (1965)
K₃GdSi₂O₇	P6₃/mcm	9.9359 (2)	14.4295 (4)	1233.66 (5)	Vidican et al. (2003)
K₃TbSi₂O₇	P6₃/mcm	9.8873 (1)	14.3856 (3)	1217.91 (3)	Vidican et al. (2003)
K₃DySi₂O₇	P6₃/mcm	9.8673 (2)	14.3572 (4)	1210.59 (5)	Vidican et al. (2003)
K₃HoSi₂O₇	P6₃/mcm	9.8408 (3)	14.3206 (6)	1200.93 (7)	Vidican et al. (2003)
K₃Y_0.9Dy_0.1Si₂O₇	P6₃/mcm	9.8496 (2)	14.3292 (4)	1203.90 (6)	Latshaw et al. (2016a)
K₃YSi₂O₇	P6₃/mcm	9.8450 (2)	14.3236 (3)	1202.30 (5)	Latshaw et al. (2016a)
K₃ErSi₂O₇	P6₃/mcm	9.8212 (1)	14.2986 (2)	1194.41 (2)	Vidican et al. (2003)
K₃ErSi₂O₇	P6₃/mcm	9.8338 (2)	14.2843 (4)	1196.28 (6)	This work
K₃TmSi₂O₇	P6₃/mcm	9.8020 (1)	14.2863 (4)	1188.72 (4)	Vidican et al. (2003)
K₃YbSi₂O₇	P6₃/mcm	9.7819 (1)	14.2401 (3)	1180.02 (3)	Vidican et al. (2003)
K₃YSi₂O₇	P6₃/mmc	5.7627 (1)	14.0205 (6)	403.22 (2)	Latshaw et al. (2016a)
K₃ErSi₂O₇	P6₃/mmc	5.7513 (1)	13.9960 (3)	400.93 (2)	This work
K₃LuSi₂O₇	P6₃/mmc	5.7116 (1)	13.8883 (6)	392.37 (2)	Vidican et al. (2003)
K₃ScSi₂O₇	P6₃/mmc	5.6065 (1)	13.6420 (4)	371.357 (14)	Napper et al. (2004)

Note: (a) only the lengths of the unit-cell axes are reported.

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