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

Dipotassium gadolinium(III) phosphate(V) molybdate(VI), synthesized from a high-temperature melt starting from GdF3 as a source of gadolinium, has a structure that is isotypic with other M I 2 M 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 of these layers, and the interstitial space is occupied by K cations having eightfold oxygen coordination.

In the case of Rb 2 Bi(PO 4 )(MoO 4 ):Eu 3+ powders, the quantum efficiency has been shown to reach ca 100% for the Rb 2 Bi 0.5 Eu 0.5 (PO 4 )(MoO 4 ) phosphor (Grigorjevaite & Katelnikovas, 2016).High color purity and emission spectra peculiarities make these compounds attractive for redcomponent 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.

Structural commentary
The three-dimensional framework of the title compound is organized by linking together slightly distorted GdO 8 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).
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) A ˚.Among the Gd-O bond lengths, those corresponding to the bidentately coordinated phosphate groups are the longest [2.427 (2) and 2.453 (2) A ˚].The chains built up from GdO 8 polyhedra are interlinked by phosphate moieties into [GdPO 4 ] layers propagating in the ac plane.The nearest Gd� � �Gd distance within a zigzag chain is 3.9332 (2) A ˚. [Gd(PO 4 )(MoO 4 )] nets are formed by adhesion of [GdPO 4 ] layers and MoO 4 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 GdO 8 , MoO 4 and PO 4 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 Na 2 Y(PO 4 )(WO 4 ), the NaO 6 sodium environment is described as an effective 3 + 3 coordination indicating a relatively large void between two successive [Y(PO 4 )(WO 4 )] layers (Daub et al., 2012).

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, Representation of the unit-cell content of K 2 Gd(PO 4 )(MoO 4 ).

Figure 2
Representation of the coordination environment of gadolinium atoms in K 2 Gd(PO 4 )(MoO 4 ).Displacement ellipsoids are drawn at the 50% probability level.[Symmetry codes: (i) 2 À x, 1 2 À y, z; namely, MoO 4 and PO 4 and two are eightfold coordinated, KO 8 and GdO 8 .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 MoO 4 tetrahedron has minor distortions, as indicated by the value of S of 0.053.In contrast, the PO 4 tetrahedron reveals more severe deviations, having C 2 site symmetry with a calculated value of S = 0.238.In case of GdO 8 , the lowest value of S of 2.725 was obtained for a triangular dodecahedron (TDD-8) (Casanova et al. 2005) and KO 8 is best described as as biaugmented trigonal prism, as indicated by the value of S of 3.999.Thus, the GdO 8 polyhedron in K 2 Bi(PO 4 )(MoO 4 ) is found to be a triangular dodecahedron (TDD-8), as has also been observed for K 2 Eu(PO 4 )(WO 4 ) (Terebilenko et al., 2022).

Synthesis and crystallization
Single crystals of the title compound were grown from molten salts 7K 2 Mo 2 O 7 -3K 4 P 2 O 7 containing 5% mol of GdF 3 .A mixture of K 2 Mo 2 O 7 and K 4 P 2 O 7 was heated in a platinum crucible up to 1273 K.After melting, 5% mol of GdF 3 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 À 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.

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

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.

Figure 3
Figure 3Zigzag chains build up from (a) GdO 8 and (b) KO 8 polyhedra

Table 1
Experimental details.