Crystal structure of a layered phosphate molybdate K₂Gd(PO₄)(MoO₄)

The title compound dipotassium gadolinium(III) phosphate(V) molybdate(VI), K₂Gd(PO₄)(MoO₄), was synthesized from a high-temperature melt starting from GdF₃ as a source of gadolinium. Its structure is isotypic with other M^I₂M^III(M^VIO₄)(PO₄) compounds, where M^I = Na, K or Cs, and M^III = rare-earth cation, M^VI = Mo or W. The three-dimensional framework is built up from [Gd(PO₄)(MoO₄)] anionic sheets, which are organized by adhesion of [GdPO₄] layers and [MoO₄] tetra­hedra stacked above and below these layers. The inter­stitial space is occupied by K cations having eightfold oxygen coordination. The polyhedron of GdO₈ was estimated to be a triangular dodeca­hedron by the continuous shape measurement method.

1. Chemical context

Layered phosphate(V) molybdates(VI) M^I₂M^III(M^VIO₄)(PO₄) comprising an alkali metal and a rare-earth metal M^III such as Sm (Zhao et al., 2009), Eu (Terebilenko et al., 2022), Y (Zhang et al., 2016) or Bi (Grigorjevaite et al., 2020) are considered to be promising luminescent materials (Guo et al., 2019). The initial structural models of this group of compounds, Na₂Y(PO₄)(MoO₄), were monoclinic, space group C2/c, as described by Ben Amara & Dabbabi (1987). Subsequent work determined that the material crystallizes in an ortho­rhom­bic system, space group Ibca (Marsh, 1987). The discovery of K₂Bi(PO₄)(MoO₄) by Zatovsky et al. (2006) opened a new group of luminescent materials that are isostructural to Na₂Y(PO₄)(MoO₄) and have high color purity and quantum yield (Grigorjevaite & Katelnikovas, 2016).

In the case of Rb₂Bi(PO₄)(MoO₄):Eu³⁺ powders, the quantum efficiency has been shown to reach ca 100% for the Rb₂Bi_0.5Eu_0.5(PO₄)(MoO₄) phosphor (Grigorjevaite & Katelnikovas, 2016). High color purity and emission spectra peculiarities make these compounds attractive for red-component design in near-UV LED-driven solid-state light sources (Zozulia et al., 2023). One of the main disadvantages of these luminescence hosts is the relatively high activator content needed (from 50 to 75%) to reach a high quantum efficiency (Grigorjevaite & Katelnikovas, 2016). Different strategies have been applied to improve the luminescence performance and lower the luminescent dopant content, including rare-earth co-doping (Naidu et al., 2012) and anion modifications (Guo et al., 2019). To tune the luminescence properties of these phosphors, the quest for new representatives of this group of compounds can shed light on the development of new phosphors based on them.

2. Structural commentary

The three-dimensional framework of the title compound is organized by linking together slightly distorted GdO₈ dodeca­hedra with non-condensed phosphate and molybdate tetra­hedra (Fig. 1). These moieties are arranged into layers perpendicular to the [010] direction with each phosphate layer being followed by two molybdate layers. In this packing, the gadolinium and potassium cations are eightfold coordinated by oxygen (Fig. 2) and ordered into zigzag chains (Fig. 3).

Figure 1 Representation of the unit-cell content of K2Gd(PO4)(MoO4).

Figure 2 Representation of the coordination environment of gadolinium atoms in K2Gd(PO4)(MoO4). Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) 2 − x,  − y, z; (ii)  − x,  − y,  − z; (iii)  + x, y,  − z; (iv)  − x, y, 1 − z; (v)  + x,  − x, 1 − z; (vii) 1 + x, y, z; (viii)  − x, y,1 − z; (ix) 1 − x,  − y, z.]

Figure 3 Zigzag chains build up from (a) GdO8 and (b) KO8 polyhedra

Each Gd cation is surrounded by two molybdate tetra­hedra and four phosphate tetra­hedra; two of the phosphate groups are coordinated in a bidentate manner (Fig. 2). The Gd—O bond lengths lie in the range 2.314 (3)–2.453 (3) Å. Among the Gd—O bond lengths, those corresponding to the bidentately coordinated phosphate groups are the longest [2.427 (2) and 2.453 (2) Å]. The chains built up from GdO₈ polyhedra are inter­linked by phosphate moieties into [GdPO₄] layers propagating in the ac plane. The nearest Gd⋯Gd distance within a zigzag chain is 3.9332 (2) Å. [Gd(PO₄)(MoO₄)] nets are formed by adhesion of [GdPO₄] layers and MoO₄ tetra­hedra above and below these layers (Fig. 1).

Both the phosphate and molybdate tetra­hedra have an almost regular geometry with typical bond lengths. The central atoms of the GdO₈, MoO₄ and PO₄ polyhedra are located on a twofold axis. The potassium cation resides inside the inter­layer space having eightfold coordination, as has been found for other potassium-based representatives of this family (Zatovsky et al., 2006). Importantly, there is a difference in the nearest oxygen coordination of sodium- and potassium-based frameworks. In case of Na₂Y(PO₄)(WO₄), the NaO₆ sodium environment is described as an effective 3 + 3 coordination indicating a relatively large void between two successive [Y(PO₄)(WO₄)] layers (Daub et al., 2012).

3. Coordination environment calculations

The distortions of the coordination environment of gadolin­ium, potassium, phospho­rus and molybdenum have been calculated by the continuous shape measurement method with the Shape 2.1 program (Llunell et al., 2013). The shape measurements in this work are taken from normalized coord­ination polyhedra (Alvarez, 2021). There are two types of polyhedra within the structure studied: two are tetra­hedral, namely, MoO₄ and PO₄ and two are eightfold coordinated, KO₈ and GdO₈. The shape measurements of a set of atoms with respect to a reference shape (e.g., the tetra­hedron, abbreviated T-4 by IUPAC) calibrates the overall distance of the atoms to the vertices of the tetra­hedral shape in the same position. Thus, a zero-shape measurement for a set of atoms indicates that the polyhedron has exactly the reference shape, expressed as S(T-4) = 0.00 for an ideal tetra­hedron. Increasing values of the shape measurement will be found for more distorted polyhedra, in other words, these values are essentially spatial distance minima of the central atom from a minimization polyhedral fitting procedure. For the title compound, the MoO₄ tetra­hedron has minor distortions, as indicated by the value of S of 0.053. In contrast, the PO₄ tetra­hedron reveals more severe deviations, having C₂ site symmetry with a calculated value of S = 0.238.

In case of GdO₈, the lowest value of S of 2.725 was obtained for a triangular dodeca­hedron (TDD-8) (Casanova et al. 2005) and KO₈ is best described as as biaugmented trigonal prism, as indicated by the value of S of 3.999. Thus, the GdO₈ polyhedron in K₂Bi(PO₄)(MoO₄) is found to be a triangular dodeca­hedron (TDD-8), as has also been observed for K₂Eu(PO₄)(WO₄) (Terebilenko et al., 2022).

4. Synthesis and crystallization

Single crystals of the title compound were grown from molten salts 7K₂Mo₂O₇–3K₄P₂O₇ containing 5% mol of GdF₃. A mixture of K₂Mo₂O₇ and K₄P₂O₇ was heated in a platinum crucible up to 1273 K. After melting, 5% mol of GdF₃ was added to the initial molten salts under stirring. The mixture was then held at this temperature for 2 h and cooled down to room temperature at a rate of 50 K h⁻¹. The solidified melt was leached out with warm water to dissolve the superfluous flux. The final product consisted of colourless plates. The yield was 64% by Gd.

5. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1.

Table 1
Experimental details

Crystal data
Chemical formula	K2Gd(PO4)(MoO4)
Mr	490.36
Crystal system, space group	Orthorhombic, Ibca
Temperature (K)	200
a, b, c (Å)	6.9527 (2), 19.7112 (6), 12.2466 (3)
V (Å^3)	1678.35 (8)
Z	8
Radiation type	Mo Kα
μ (mm^−1)	10.52
Crystal size (mm)	0.10 × 0.08 × 0.02
Data collection
Diffractometer	XtaLAB Synergy, Dualflex, HyPix
Absorption correction	Gaussian (CrysAlis PRO; Rigaku OD, 2020)
Tmin, Tmax	0.422, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	6547, 1079, 999
Rint	0.026
(sin θ/λ)max (Å^−1)	0.707
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.017, 0.045, 1.13
No. of reflections	1079
No. of parameters	61
Δρmax, Δρmin (e Å^−3)	1.53, −0.64
Computer programs: CrysAlis PRO (Rigaku OD, 2020), SHELXT2018/2 (Sheldrick, 2015a), SHELXL2018/3 (Sheldrick, 2015b) and OLEX2 (Dolomanov et al., 2009).

Supporting information

Computing details

Dipotassium gadolinium(III) phosphate(V) molybdate(VI)

Crystal data

K2Gd(PO4)(MoO4)	Dx = 3.881 Mg m^−3
Mr = 490.36	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Ibca	Cell parameters from 4624 reflections
a = 6.9527 (2) Å	θ = 3.3–30.0°
b = 19.7112 (6) Å	µ = 10.52 mm^−1
c = 12.2466 (3) Å	T = 200 K
V = 1678.35 (8) Å^3	Plate, clear light colourless
Z = 8	0.10 × 0.08 × 0.02 mm
F(000) = 1784

Data collection

XtaLAB Synergy, Dualflex, HyPix diffractometer	1079 independent reflections
Radiation source: micro-focus sealed X-ray tube, PhotonJet (Mo) X-ray Source	999 reflections with I > 2σ(I)
Mirror monochromator	Rint = 0.026
Detector resolution: 10 pixels mm^-1	θmax = 30.2°, θmin = 3.3°
ω scans	h = −8→8
Absorption correction: gaussian (CrysAlisPro; Rigaku OD, 2020)	k = −26→26
Tmin = 0.422, Tmax = 1.000	l = −16→16
6547 measured reflections

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: dual
R[F^2 > 2σ(F^2)] = 0.017	Secondary atom site location: difference Fourier map
wR(F^2) = 0.045	w = 1/[σ^2(Fo^2) + (0.0204P)^2 + 6.0211P] where P = (Fo^2 + 2Fc^2)/3
S = 1.13	(Δ/σ)max < 0.001
1079 reflections	Δρmax = 1.53 e Å^−3
61 parameters	Δρmin = −0.64 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
Gd1	1.000000	0.250000	0.42488 (2)	0.00554 (8)
Mo1	0.750000	0.41682 (2)	0.500000	0.00954 (10)
K1	0.71711 (11)	0.09429 (4)	0.32974 (5)	0.01672 (16)
P1	0.500000	0.250000	0.32047 (8)	0.0060 (2)
O1	0.6709 (3)	0.24105 (10)	0.40045 (17)	0.0095 (4)
O2	0.4787 (3)	0.18814 (11)	0.24608 (17)	0.0094 (4)
O3	0.9564 (3)	0.36581 (11)	0.47067 (18)	0.0139 (4)
O4	0.8056 (4)	0.46677 (12)	0.61376 (19)	0.0204 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Gd1	0.00341 (13)	0.00828 (12)	0.00493 (11)	−0.00007 (6)	0.000	0.000
Mo1	0.0102 (2)	0.00749 (16)	0.01089 (17)	0.000	0.00090 (13)	0.000
K1	0.0158 (4)	0.0130 (3)	0.0213 (3)	0.0014 (3)	−0.0007 (3)	0.0027 (2)
P1	0.0038 (6)	0.0093 (5)	0.0047 (5)	0.0000 (3)	0.000	0.000
O1	0.0037 (11)	0.0172 (10)	0.0076 (9)	0.0001 (8)	0.0006 (8)	0.0003 (8)
O2	0.0101 (11)	0.0110 (10)	0.0070 (9)	−0.0009 (8)	−0.0016 (7)	−0.0012 (8)
O3	0.0129 (11)	0.0111 (10)	0.0178 (11)	−0.0004 (9)	0.0034 (9)	−0.0019 (9)
O4	0.0209 (13)	0.0177 (11)	0.0226 (12)	0.0014 (10)	−0.0022 (10)	−0.0108 (10)

Geometric parameters (Å, º)

Gd1—O1	2.314 (2)	K1—O1	3.037 (2)
Gd1—O1^i	2.314 (2)	K1—O2	2.687 (2)
Gd1—O1^ii	2.453 (2)	K1—O2^iv	2.755 (2)
Gd1—O1^iii	2.453 (2)	K1—O3^i	2.958 (2)
Gd1—O2^iv	2.427 (2)	K1—O3^vi	3.143 (2)
Gd1—O2^v	2.427 (2)	K1—O4^vii	2.970 (3)
Gd1—O3	2.370 (2)	K1—O4^viii	2.679 (2)
Gd1—O3^i	2.370 (2)	K1—O4^vi	3.180 (3)
Mo1—O3^ii	1.788 (2)	P1—O1	1.550 (2)
Mo1—O3	1.788 (2)	P1—O1^ix	1.550 (2)
Mo1—O4	1.749 (2)	P1—O2^ix	1.529 (2)
Mo1—O4^ii	1.749 (2)	P1—O2	1.529 (2)
O1—Gd1—O1^iii	126.66 (6)	O4^ii—Mo1—O4	111.50 (16)
O1^i—Gd1—O1^iii	68.18 (8)	O1—K1—O3^vi	58.59 (6)
O1—Gd1—O1^i	165.14 (10)	O1—K1—O4^vi	101.73 (6)
O1^iii—Gd1—O1^ii	58.64 (10)	O2—K1—O2^iv	79.43 (6)
O1^i—Gd1—O1^ii	126.66 (6)	O2^iv—K1—O3^i	60.76 (6)
O1—Gd1—O1^ii	68.18 (8)	O2^iv—K1—O3^vi	118.34 (6)
O1^i—Gd1—O2^v	77.86 (7)	O2—K1—O3^vi	76.61 (6)
O1—Gd1—O2^iv	77.86 (7)	O2—K1—O3^i	120.86 (7)
O1^i—Gd1—O2^iv	89.27 (7)	O2—K1—O4^vi	77.78 (6)
O1—Gd1—O2^v	89.27 (7)	O2^iv—K1—O4^vi	157.11 (7)
O1—Gd1—O3	88.71 (8)	O2^iv—K1—O4^vii	80.51 (7)
O1^i—Gd1—O3	94.81 (8)	O2—K1—O4^vii	93.86 (7)
O1^i—Gd1—O3^i	88.71 (8)	O3^i—K1—O1	70.21 (6)
O1—Gd1—O3^i	94.81 (8)	O3^i—K1—O3^vi	85.55 (7)
O2^v—Gd1—O1^ii	144.86 (7)	O3^vi—K1—O4^vi	53.60 (6)
O2^v—Gd1—O1^iii	133.34 (7)	O3^i—K1—O4^vii	117.92 (7)
O2^iv—Gd1—O1^iii	144.86 (7)	O3^i—K1—O4^vi	131.61 (7)
O2^iv—Gd1—O1^ii	133.34 (7)	O4^vii—K1—O1	131.43 (6)
O2^iv—Gd1—O2^v	60.80 (10)	O4^viii—K1—O1	147.62 (7)
O3^i—Gd1—O1^ii	77.67 (7)	O4^viii—K1—O2^iv	124.43 (7)
O3—Gd1—O1^iii	77.67 (7)	O4^viii—K1—O2	152.41 (7)
O3^i—Gd1—O1^iii	78.52 (7)	O4^viii—K1—O3^vi	99.58 (7)
O3—Gd1—O1^ii	78.52 (7)	O4^viii—K1—O3^i	85.55 (7)
O3—Gd1—O2^v	74.22 (7)	O4^vii—K1—O3^vi	155.97 (7)
O3^i—Gd1—O2^iv	74.22 (7)	O4^vii—K1—O4^vi	103.12 (7)
O3^i—Gd1—O2^v	132.85 (7)	O4^viii—K1—O4^vii	78.60 (7)
O3—Gd1—O2^iv	132.85 (7)	O4^viii—K1—O4^vi	78.20 (5)
O3^i—Gd1—O3	152.63 (11)	O1^ix—P1—O1	101.63 (17)
O3—Mo1—O3^ii	111.59 (14)	O2—P1—O1	111.11 (11)
O4—Mo1—O3	107.39 (11)	O2^ix—P1—O1^ix	111.11 (11)
O4—Mo1—O3^ii	109.51 (11)	O2—P1—O1^ix	113.12 (11)
O4^ii—Mo1—O3	109.50 (11)	O2^ix—P1—O1	113.12 (11)
O4^ii—Mo1—O3^ii	107.39 (11)	O2^ix—P1—O2	106.87 (17)
O1^ix—P1—O1—Gd1	−156.6 (3)	O2^ix—P1—O2—K1	133.88 (11)
O1^ix—P1—O1—Gd1^ii	−0.001 (1)	O2^ix—P1—O2—K1^x	−106.9 (2)
O1^ix—P1—O1—K1	112.05 (8)	O2^ix—P1—O2—P1^ix	0 (100)
O1^ix—P1—O1—P1^ix	0 (100)	O3^ii—Mo1—O3—Gd1	15.95 (10)
O1—P1—O2—Gd1^v	−123.85 (10)	O3^ii—Mo1—O3—K1^iii	−114.91 (9)
O1^ix—P1—O2—Gd1^v	122.59 (11)	O3^ii—Mo1—O3—K1^i	152.13 (15)
O1^ix—P1—O2—K1^x	15.7 (2)	O3^ii—Mo1—O4—K1^xi	32.18 (14)
O1—P1—O2—K1^x	129.30 (18)	O3—Mo1—O4—K1^xii	143.40 (15)
O1^ix—P1—O2—K1	−103.53 (11)	O3^ii—Mo1—O4—K1^xii	−95.27 (17)
O1—P1—O2—K1	10.03 (14)	O3—Mo1—O4—K1^xi	−89.15 (12)
O1—P1—O2—P1^ix	0 (78)	O3^ii—Mo1—O4—K1^iii	116.29 (10)
O1^ix—P1—O2—P1^ix	0 (100)	O3—Mo1—O4—K1^iii	−5.05 (11)
O2—P1—O1—Gd1^ii	−120.61 (11)	O4^ii—Mo1—O3—Gd1	−102.81 (16)
O2^ix—P1—O1—Gd1	−37.5 (3)	O4—Mo1—O3—Gd1	135.96 (16)
O2^ix—P1—O1—Gd1^ii	119.18 (11)	O4—Mo1—O3—K1^i	−87.85 (14)
O2—P1—O1—Gd1	82.7 (2)	O4^ii—Mo1—O3—K1^iii	126.33 (10)
O2^ix—P1—O1—K1	−128.76 (10)	O4^ii—Mo1—O3—K1^i	33.37 (16)
O2—P1—O1—K1	−8.56 (12)	O4—Mo1—O3—K1^iii	5.11 (11)
O2—P1—O1—P1^ix	0 (100)	O4^ii—Mo1—O4—K1^xi	150.89 (14)
O2^ix—P1—O1—P1^ix	0 (23)	O4^ii—Mo1—O4—K1^iii	−125.01 (10)
O2^ix—P1—O2—Gd1^v	0.000 (1)	O4^ii—Mo1—O4—K1^xii	23.44 (10)

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