Cadmium phosphates Cd2(PO4)OH and Cd5(PO4)2(OH)4 crystallizing in mineral structures

Cd2(PO4)OH and Cd5(PO4)2(OH)4 crystallize in the mineral structures of triplite and arsenoclasite, respectively.


Chemical context
In the quest for new oxidotellurates(IV) modified by incorporation of tetrahedral phosphate anions (Eder & Weil, 2020;Ok & Halasyamani, 2006;Yao et al., 2021;Zhao et al., 2021a,b;Zimmermann et al., 2011), crystals of Cd 2 (PO 4 )OH were serendipitously obtained under hydrothermal conditions when working in the system Cd/Te IV /P V /O/(H).During a targeted synthesis of Cd 2 (PO 4 )OH under Te-free conditions, another phosphate with composition Cd 5 (PO 4 ) 2 (OH) 4 had crystallized.We report here the synthesis conditions and crystal structure refinements of these two basic cadmium phosphates and their relationships with known mineral structures.
Cd 2 (PO 4 )OH is the first reported M 2 (XO 4 )Y compound with exclusively OH À ions at the Y site to crystallize in the triplite structure in space-group type I2/a.Such M 2 (XO 4 )OH compounds usually adopt the triploidite structure in spacegroup type P2 1 /a, like the arsenate analogue Cd 2 (AsO 4 )(OH) (Ðordevic ´& Kolitsch, 2013).Triploidite-like structures have twice the unit-cell volume of triplite-like structures and show no centering of the monoclinic unit-cell.However, for Cd 2 (PO 4 )OH, reflections hinting at a doubled unit-cell volume or violating the reflection conditions for an I-centered unit-cell were not found in the diffraction data.
Cd 2 (PO 4 )OH.The asymmetric unit of Cd 2 (PO 4 )OH comprises two Cd, one P and five O sites (O1 and O2 being positionally disordered over two sites each).All atomic sites are situated at the general Wyckoff position 8 f of space group I2/a.The resulting coordination polyhedra around Cd1, Cd2 and P are depicted in Fig. 1.For the sake of simplicity, the crystal structure of Cd 2 (PO 4 )OH will be described in the following without the disorder of atoms O1 and O2.Consid-ering Cd-O distances < 3.0 A ˚as relevant, both Cd sites are coordinated by six oxygen atoms forming significantly distorted [CdO 4 (OH) 2 ] octahedra (Table 1,

Figure 2
The idealized crystal structure of Cd 2 (PO 4 )OH without disorder of O1 and O2 in a projection along [010].The O atom of the OH group is given as a yellow sphere, the other O atoms as white spheres, the Cd atoms as green spheres; the [PO 4 ] unit is displayed as a red polyhedron.
unit shares four of its O atoms with [PO 4 ] tetrahedra, leading to a tri-periodic structure (Fig. 2).
Of the five oxygen sites, four (O1-O4) are bound to two Cd and one P atom each.The site associated with the OH group is bound to four Cd sites.This assignment is supported by bondvalence calculations (Brown, 2002), using the parameters of Brese & O'Keeffe (1991).The bond-valence sum (BVS) of the OH site amounts to 1.67 valence units (1.92-2.08 valence units for the other O sites).The OH site is the one occupied by the F À anion in the isotypic triplite-type structures.The [(OH)Cd 4 ] polyhedron has a distorted tetrahedral shape with bond lengths in the range 2.101 (6)-2.484(7) A ˚.The [(OH)Cd 4 ] tetrahedra are linked to each other by sharing two edges with neighbouring tetrahedra forming 1 1 [(OH)Cd 4/2 ] chains extending parallel to [001] (Fig. 2).
The environment of the OH site suggests multiple acceptor atoms for possible O-H� � �O hydrogen-bonding interactions and is the putative reason why the hydrogen atom could not be localized and also for the disorder of O1 and O2.Taking into account hydrogen-bonding interactions with O� � �O distances < 3.0 A ˚as significant, there are six O atoms in the vicinity of each OH site (Fig. 3).The shortest contact amounts to 2.635 (12) A ˚towards a symmetry-related OH site, the longest to 2.94 (8) A ˚to O2A.In the isotypic crystal structure of Cd 2 (PO 4 )F (Rea & Kostiner, 1974), the F site (corresponding to the OH site in the title structure) is not split and has four contacts < 3.0 A ˚to two F sites [2.756 (6) and 2.800 (7) A ˚] and to two O sites [2.828 (5) and 2.832 (5) A ˚].
Owing to the disorder present in Cd 2 (PO 4 )OH, a quantitative comparison with the ordered isotypic M 2 (XO 4 )Y crystal Comparison of bond lengths (A ˚) in the isotypic M 5 (XO 4 ) 2 (OH) 4 structures (M = Cd, Mn, Co; X = P, As, V) after standardization, and parameters of structural comparison with Cd 5 (PO 4 ) 2 (OH) 4 as the reference structure.
structures adopting the triplite-structure type was not undertaken.
Cd 5 (PO 4 ) 2 (OH) 4 .The asymmetric unit of Cd 5 (PO 4 ) 2 (OH) 4 comprises five Cd, two P and twelve O sites, all located on the general Wyckoff position 4 a of space group P2 1 2 1 2 1 ; the Hatom sites could not be localized.All five Cd sites are surrounded by six O atoms, resulting in a distorted octahedral environment for each metal atom.The Cd-O bond lengths are in a broad range between 2.184 (6) and 2.599 (6) A (Table 2).The mean bond lengths are 2.341 A ˚(Cd1), 2.283 A (Cd2), 2.222 A ˚(Cd3), 2.331 A ˚(Cd4) and 2.336 A ˚(Cd5), again in good agreement with the literature value specified above.The BVS values of the Cd atoms amount to 1.91, 2.16, 1.98, 2.00 and 1.90 valence units and thus show good agreement with the expected value of 2.
The The two types of ribbons are linked into a tri-periodic arrangement by sharing corners, as well as by sharing the two PO 4 tetrahedra (Fig. 4).The latter show deviations from an ideal tetrahedral arrangement, as revealed by slightly different bond lengths (Table 2) and by angular distortions, with O-P-O angles ranging from 106.2 (3) to 112.2 (3) � for P1 and 104.6 (4) to 114.0 (4) � for P2.
Cd 5 (PO 4 ) 2 (OH) 4 and the four isotypic M 5 (XO 4 ) 2 (OH) 4 crystal structures (M = Cd, Mn, Co; X = P, As, V) were quantitatively compared using the compstru software (de la Flor et al., 2016) available at the Bilbao Crystallographic server (Aroyo et al., 2006).For this purpose and for direct comparison of bond lengths (Table 2), the hydrogen atoms (if part of the model) were removed, and all crystal structures were standardized with STRUCTURE-TIDY (Gelato & Parthe ´, 1987).With Cd 5 (PO 4 ) 2 (OH) 4 as the reference structure, numerical values of parameter of comparison (degree of lattice distortion S, the arithmetic mean of the distance between paired atoms d av , the maximum difference between the atomic positions of the matching atoms d max , and the measure of similarity �) are collated in Table 2.As expected for isotypic structures, the low values for � indicate high similarities of Cd 5 (PO 4 ) 2 (OH) 4 with the four M 5 (XO 4 ) 2 (OH) 4 crystal structures.The differences in bond lengths of the individual structural units ([MO 6 ]; [XO 4 ]) are due to the different sizes of M II and X V , viz.0.745 A ˚for Co (high spin), 0.83 A ˚for Mn (high spin), 0.95 A ˚for Cd, and 0.17 A ˚for P, 0.335 A ˚for As, 0.355 A ˚for V; all values were taken from Shannon (1976).As a simple measure, the quotient X:M can be used for correlation.The closer the quotient is to that of P: Cd = 0.178, the higher is the similarity.

Synthesis and crystallization
Crystals of Cd 2 (PO 4 )OH and Cd 5 (PO 4 ) 2 (OH) 4 were both obtained from reactions under hydrothermal conditions.The starting materials were 0.1927 g (1.129 mmol) CdCO 3 , 0.1784 g (1.118 mmol) TeO 2 and 0.1289 g (1.118 mmol) of 85% wt H 3 PO 4 for the Cd 2 (PO 4 )OH batch, and 0.1874 g (0.607 mmol) Cd(NO 3 ) 2 •4H 2 O, 0.0296 g (0.257 mmol) 85% wt H 3 PO 4 , and 0.2197 g (3.916 mmol) KOH for the Cd 5 (PO 4 ) 2 -(OH) 4 batch.The reactants were weighed into small Teflon containers with a volume of ca 3 ml and mixed with deionized water so that the inner volume was filled to about two thirds with liquid.Then, the Teflon containers were placed into a steel autoclave and heated to 483 K for 7 d.Afterwards the autoclave was cooled down to room temperature within about 4 h.The formed solids were filtered off, washed with mother liquor, water and ethanol, and dried in air.
For the Cd 2 (PO 4 )OH batch, the reaction product was a mixture of a white and bright-yellow solid.An X-ray powder diffraction measurement revealed �-TeO 2 , which can be associated with the yellow solid, as a side product besides Cd 2 (PO 4 )OH.Small colourless block-shaped crystals of Cd 2 (PO 4 )OH could be isolated for single crystal X-ray diffraction.
For the Cd 5 (PO 4 ) 2 (OH) 4 batch, the reaction product was a white powder.Apart from Cd(OH) 2 and Cd 5 (PO 4 ) 2 (OH) 4 no other phases could be identified in the X-ray powder diffraction pattern.Colourless block-shaped crystals of Cd 5 (PO 4 ) 2 -(OH) 4 could be isolated for single crystal X-ray diffraction.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.
For structure refinement of triplite-type Cd 2 (PO 4 )OH, labelling and fractional coordinates of atoms were adapted from the crystal structure of triplite (Waldrop, 1969).For direct comparison with other triplite-like structures (Waldrop, 1969;Ðordevic ´& Kolitsch, 2013), the unconventional setting I2/a of space-group type No. 15 was chosen.The conventional setting in C2/c transforms with À a À c, À b, c to the chosen unconventional setting.Oxygen atoms O1 and O2 were found to be positionally disordered over two sites.The pairs O1A/ O1B and O2A/O2B were refined with common displacement parameters each.The site occupation factors were refined for the pair O1A/O2A and O1A/O2A to a ratio of 0.349 (18):0.651(18).Remaining positive and negative residual electron density close to the Cd1 position suggests possible positional disorder of this atom as well.However, using split positions for Cd1 led to a physically non-meaningful model and was not considered for the final refinement.H atoms could not be located for Cd 2 (PO 4 )OH.

Figure 1
Figure 1Coordination polyhedra in the crystal structure of Cd 2 (PO 4 )OH showing the disordered atoms O1 and O2.Displacement ellipsoids are drawn at the 50% probability level.Symmetry codes refer to Table1.

Figure 4
Figure 4The crystal structure of Cd 5 (PO 4 ) 2(OH)  4 in a projection along [100] in polyhedral representation.The [CdO 6 ] octahedra defining the first subunit are given in green, the [CdO 6 ] octahedra defining the second sub-unit are given in blue; [PO 4 ] tetrahedra are red.O atoms of OH groups are yellow, other O atoms are white.Displacement ellipsoids are displayed at the 74% probability level.

Table 3
Experimental details.
(Parsons et al., 2013)I À )](Parsons et al., 2013) 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.