A High‐Pressure Praseodymium Fluoride Borate Linking Multiple Structural Features of Apatite‐Type Compounds

Abstract Pr5(BO4)3−x(BO3)x(F,OH)2.67O0.28 (x≈1.6), a boron‐containing fluoride‐oxoapatite‐like compound, was obtained by the application of high‐pressure/high‐temperature synthesis. It exhibits a superstructure of the apatite type with a tripled c lattice parameter (space group P63/m) and shows complex anion disorder along the 63 screw axis and occupation of distorted octahedra, as well as almost trigonal planar sites, by oxygen and fluorine atoms. Furthermore, a distinct BO4/(BO3+F) group disorder is found; 46 % of the sites being occupied by BO4 groups and 54 % by BO3 groups, with a fluoride ion located near the missing oxygen atom. The rare earth cations in the 4f sites exhibit a specific distorted tricapped trigonal prismatic coordination with a mean metaprism twist angle of 21.3°. The crystal structure of Pr5(BO4)3−x(BO3)x(F,OH)2.67O0.28 (x≈1.6) shows much “flexibility” resulting in split and off‐site positions of all other rare earth cations. The title compound therefore combines many structural features of apatite‐like compounds, for example biologically highly‐important carbonated apatites, shedding more light onto the complex structural chemistry of apatites.


Introduction
Due to their enormous biological significance as main mineral components of human and animal hard tissues such as bones and teeth, apatiteC a 5 (PO 4 ) 3 (F,Cl,OH) and compounds with similar structures have been objects of extensive studies. Calcium phosphate biomaterials have been especiallyi nvestigated as potentiali mplant materials, or as scaffolds for growth factors in bone regenerationp rocesses. [1] Recently,B arheine et al. have reported that disordered borate-containing apatite is responsible for the adhesion properties of organic molecules like proteins, andshows enhanced biodegradability in contrast to crystalline hydroxyapatite. [2] Besides their biomedical features, the catalytic and piezoelectric properties of numerous apatites are well known. [3] Apatite-type compounds containing rare earth elements show interestingo ptical behaviour,and are potentially applicable as fluorescent lamp phosphors and laser-material hosts. [4] Recently,r are earth containing apatite-type oxide ion conductors like La 8 Y 2 Ge 6 O 27 have come into focus as potential electrolytes for solid oxide fuel cells (SOFC). [5] The apatitep rototype Ca 5 (PO 4 ) 3 Fw as first reportedb yN avay et al. [6] in 1930 andw as confirmed to adopt P6 3 /m symmetry. [7] To date, al arge variety of compounds with the apatite structure type (M1) 2 (M2) 3 (AO 4 ) 3 X are known. [8] The metalc ations M can be alkaline earth metals,r are earth elements, Cd or Cr, whereasf or the anion X the substituents F, Cl, Br,O H, O, or S are found. [9] The tetrahedrally coordinated atom A can either be at ransitionm etal (V,M n, Cr) or ag roup 14 or 15 element (Si, Ge, P, or As). [10] Many apatitic compounds adopt structures in lower-symmetric trimetric space groups like P6 3 , P6 and P3 , as well as monoclinic ones, forexample, P2 1 /m and P2 1 .
The scope of the apatitef amily is broadened by the inclusion of less common M 5 (AO 5 ) 3 X and M 5 (AO 3 ) 3 X compounds, wheret he AO 4 tetrahedra are replaced by groups with square pyramidal or trigonalp lanarc oordinations. Onep erfect example is naturalh ydroxyapatite,b uilding up two thirds of human bone material,w here the phosphate groupsP O 4 are partially replacedb yc arbonate groups CO 3 .F innemanite Pb 5 (AsO 3 ) 4 Cl, [11] ar educed form of mimetite Pb 5 (AsO 4 ) 4 Cl, [12] adopts an apatite structure with P6 3 /m symmetry and ac omplete replacemento ft he AO 4 tetrahedra by AsO 3 groups, with the arsenic atoms lying above the triangular oxygen plane.U p to now,o nly af ew boron-containing apatitesh ave been described. In most of these structures, boron is present in trigonal planar BO 3 groups that partially substitute the tetrahedral AO 4 groups. [13] In Ca 5 (BO 3 ) 3 F, tetrahedral PO 4 groupsa re fully substituted by planar BO 3 groups, leading to am onoclinic structure related to fluoroapatite. [14] Ito et al. reported about the incorporation of boron as BO 3 groups in oxoapatite. [15] As far as we know,L a 10 (Si 3.96 B 1.98 O 4 ) 6 O 2 is the only compound in whose structure the position A is partially occupied by fourfold-coordinated boronatoms. [16] Furthermore,s omea patite superstructures have been reported in literature, mainly based on ad oubling of one lattice parameter. Ag ood overview of the crystal chemistry of apatites including al ist containing apatite superstructures is given in ref. [8].I odo-oxoapatite was reported by Henning et al. [17] as the first modulated apatitee xhibitings uperstructure ordering along [0 01], based on partial ordering of oxygen andi odine ions resulting in at ripling of the c lattice parameter in space group P6 3 /m. The final structure description, however, still displays disorder as related to the stacking sequence of anions within the iodine-oxygen columns, witht he anionsb eing split over two positions each.
Pr 5 (BO 4 ) 3Àx (BO 3 ) x (F,OH) 2.67 O 0.28 (x % 1.6) represents ah igh-pressure rare earth fluoride-oxoapatite-like compound exhibiting a superstructure featuring au nit cell tripled along [0 01], in combination with anion disordera long the 6 3 screw axis and BO 4 / (BO 3 + F) group disorder.T his compound therefore combines the two topics described above in aw ay that has not been reported so far,a nd could be ag ood basis for furtheru nderstanding of the crystal chemistry in natural hydroxyapatites.

Experimental Section
Synthesis:P r 5 (BO 4 ) 3Àx (BO 3 ) x (F,OH) 2.67 O 0.28 (x % 1.6) was obtained from a1 :1:2 mixture of Pr 6 O 11 ,P rF 3 ,a nd B 2 O 3 by applying highpressure/high-temperature conditions of 11.5 GPa and 1573 K, utilizing aW alker-type multianvil apparatus. The starting mixture was ground under argon atmosphere and filled into boron nitride crucibles, which were then positioned inside MgO octahedra and compressed by eight tungsten carbide cubes. Pr 5 (BO 4 ) 3Àx (BO 3 ) x (F,OH) 2.67 O 0.28 (x % 1.6) was obtained in form of greenish crystals that are stable in air at ambient conditions. Further experimental details on the synthesis are provided in the Supporting Information.
Single-crystal X-ray diffraction (SCXRD):S mall single crystals of Pr 5 (BO 4 ) 3Àx (BO 3 ) x (F,OH) 2.67 O 0.28 (x % 1.6) were isolated by mechanical fragmentation, and selected under an optical polarization microscope. In line with the known issue with crystal quality of mixed apatites, only one out of dozens of crystals proved to be suitable for detailed single-crystal structure determination. SCXRD data were collected at 173 Kw ith aB ruker D8 Quest diffractometer (Photon 100 detector) equipped with am icrofocus source generator (Incoatec GmbH, Geesthacht, Germany) combined with multi-layer optics (monochromatized Mo Ka radiation, l = 71.073 pm). Semiempirical absorption correction based on equivalent reflections was applied with SADABS-2014/5. [19] Systematic absences and Laue symmetry indicated the hexagonal space groups P6 3 /m or P6 3 .T he structure was solved with SHELXS [19b] (version 2013/1). Structure refinement (full-matrix least-squares against F 2 )w ith SHELXL [20] (version 2014/7) using Stoe X-Step32 [21] (Revision 1.05b) was successful in P6 3 /m;l ower symmetries including P6 did not result in less disorder or lower residuals. Due to large and highly anisotropic displacement parameters of the boron atoms B1A and B2, as well as the oxygen atoms O4 and O5, these positions were better refined as split positions B1/B1A, B2/B2A, O4/F4, and O5/F5. The ratio of occupation for these split positions was determined by free refinement of the occupancy factors, followed by small manual adaption to yield equal thermal displacement parameters for the boron positions B1:B1A and B2:B2A. The corresponding anion sites O4, O5, F4, and F5 were coupled with the boron occupancies to represent the chemically reasonable (BO 3 + F)/ BO 4 polyhedra. At this point, there was still significant electron density in the proximity of the cations Pr1, Pr2, and Pr4. Assuming that the strong anion disorder impacts the positions of the rare earth cations, Pr1 was offset by 21(1) pm from the position on the mirror plane, and Pr2 was also refined as as plit position (d = 22(1) pm). Pr4 was also refined as as plit position beneath its original position on the 6 axis. These modifications led to as table refinement and drastic improvement of the R values. In first approximation, the required F À /O 2À mixing ration was balanced by the disordered positions. In second approximation, the atomic positions on the 6 3 axis with high occupancy (F1, F2, and F3) were refined isotropically as fluorine atoms;t he remaining positions (O11, O21, O23, and O33) were refined isotropically as oxygen atoms. Ac omparison of the thermal displacement parameters showed higher values for the fluorine and lower values for the oxygen atoms. Due to the fact that we could not exclude partial occupation by oxygen atoms on the positions of F4 and F5 within the (BO 3 + F) groups, refinement of all seven possible positions on the 6 3 axis with statistical occupation by F/O would not allow for an exact analysis of distribution. To maintain overall charge neutrality,f inal refinement cycles were performed using fixed occupancy factors based on the results of their free refinement. The positional parameters of all atoms except those located either on split positions or on the 6 3 screw axis were refined with anisotropic displacement parameters. Final difference Fourier synthesis did not reveal any significant residual peaks. Due to the fact that oxygen and fluorine cannot be distinguished unequivocally by means of SCXRD data, the differentiation between Oa nd Fo nt he 6 3 screw axis, and also within the (BO 3 + F) groups, is ar easonable model based on interatomic distances (see Table S4 in Supporting Information). Occupation of the F4 and F5 positions by oxygen atoms can almost certainly be excluded, due to the fact that the system would therefore relax forming the BO 4 group. While not indicated by the single-crystal structure data or infrared (IR) spectroscopy,t he presence of OH groups or water molecules in the title compound cannot be excluded. Due to the preparation of the high-pressure assembly at ambient conditions under air,apartial hydrolysis of the starting materials could occur. Furthermore, the presence of H 2 Oa nd OH groups in oxoapatites and halogen-oxoapatites is quite common. [22] Therefore, we assume the composition of the title compound to be best-represented with the formula Pr 5 (BO 4 Relevant details of the data collection and evaluation are listed in Table S1. Positional parameters, anisotropic and equivalent isotropic displacement parameters, interatomic distances, and interatomic angles (within BO 3 /BO 4 groups only) are provided in Ta bles S2-S5.

Results and Discussion
Crystals tructure refinement shows that Pr 5 (BO 4  The superstructure involves tripling of the c lattice parameter with respect to the basic apatitestructure type. Ar epresentation of the crystal structure with displacemente llipsoids at 90 %p robability level (Figure 2) illustrates the nature of the superstructure within the unit cell, and depicts the rather strong anisotropic displacement of the rare earth cations as well as the oxygen atoms forming the BO 3 and BO 4 groups.
It is obvious that the superstructure arises from partial ordering of oxygen and fluorine atoms along [0 01], as well as the partial ordering of the BO 4 /(BO 3 + F) group disorder (Figure 3).
In fact, only the (B2)O 3 groups are found on two faces of the tetrahedron, whereas the (B1A)O 3 group is only located on one side of the tetrahedron. This is depicted in Figure 3, including all relevant interatomicdistances. In all cases,only the adjacent planes of BO 3 triangles oblique to [0 01]a re occupied. Carbonate hydroxyapatites are known to show as imilar AO 4 /AO 3 group disorder, where trigonal planar CO 3 groups partially substitute tetrahedral PO 4 groups, forming one oxygen vacancy per substituting group. However,f or these carbonate apatites, including human and animal tooth enamels, no concurring modelsa re reported regarding the locations of vacancies on the oxygen sites of the tetrahedra. Dependingo nt he positions of the vacancies (seen as reduced occupancy factorsi nt he positionalp arameters of the crystal structure solution), al ocation of the faces of the tetrahedra, where the CO 3 2À groups are partially located, is possible.  [23] However,p olarized IR spectroscopy showst he planeso ft he CO 3 triangles to be oblique to [0 01] at an angle of 37 AE 48. [24] The latter is consistentw ith the positions of the BO 3 groups in the crystal structure of Pr 5 (BO 4 ) 3Àx (BO 3 ) x (F,OH) 2.67 O 0.28 (x % 1.6), with part of these groups additionally found on two planesa nd partly on only one of the four tetrahedral planes.
The oxygen atoms O3 in carbonate hydroxyapatite (equaling O1, O3, respectively O6, O7 in Pr 5 (BO 4 ) 3Àx (BO 3 ) x (F,OH) 2.67 O 0.28 (x % 1.6)) are split in two positions (at ad istance of 37 pm), depending on coordination by phosphorus as PO 4 3À or by carbon as CO 3 2À . [23] While this is not the case in Pr 5 (BO 4 ) 3Àx (BO 3 ) x (F,OH) 2.67 O 0.28 (x % 1.6), we herein find sites near the positions of O4 and O5, which are partly occupied by fluorine atoms (F4 and F5). F4 is shifted 49(1) pm apart from O4, and F5 about 48(1) pm apart from O5, both in the direction off the center of the BO 4 tetrahedra. The resulting positions and interatomic distances lead to the conclusion that the positions of F4 and F5 are occupiedw herever the tetrahedral BO 4 group is replaced by aB O 3 group, as is shown in Figure 3. To the best of our knowledge,t his has not been reportedi na na patitic structure so far.
However,asimilars tructurals ituation can be found in solidsolution fluoride borates like, for example, Eu 3 (BO 3 ) 2 + x F 3À3x , [25] Ba 3.12 Sr 3.88 (BO 3 ) 3.65 F 3.05 , [26] and Ba 7 (BO 3 ) 4Àx F 2 + 3x (x = 0.49) [26b, 27] with [(BO 3 )F] 4À $[F 4 ] 4À anionic isomorphism. In these compounds, planarB O 3 groups directly neighbored to af luoride anion in trigonal pyramidal geometry,t ogetherw ith( 4F) 4À groups, are statistically distributed over the crystal structure. The BO 3 groups thereby can appear on each face except at the basal plane of the trigonal pyramid. In the crystal structure of  Very recently,L ie tal. reported the possible formation of tri-coordinated planar triangle( PO 3 + O) groups in a mixed borate-phosphate-fluoride with the composition Ca 5Ày (BO 3 ) 3Àx (PO 4 ) x F( CBP x F):yBi 3 + , [28] when BO 3 is gradually substitutedb yP O 4 .A tt he composition x = 2.0-2.5 and y = 0.5, 0.15, PO 4 groups may partially be described as ap lanar PO 3 group in combination with an isolated oxygen atom above the phosphorus atom. At x < 1.2, the crystal structure of Ca 5Ày (BO 3 ) 3Àx (PO 4 ) x F( CBP x F):yBi 3 + is reported to correspond to Ca 5 (BO 3 ) 3 F(space group Cm). [29] The presence of BO 4 as well as BO 3 groups in the crystal structureo fP r 5 (BO 4 ) 3Àx (BO 3 ) x (F,OH) 2.67 O 0.28 (x % 1.6) was confirmed by IR spectroscopy measurements of ab ulk sample, as is presented in detail in the SupportingI nformation. The ratio of occupation with BO 4 and (BO 3 + F) groups in  [31] ). Partial substitution of oxygen in the BO 3 /BO 4 groups by fluorine is therefore very unlikely,a st his would result in even smaller mean interatomic distances than the literature values given above (mean literature value d(BÀO/F) in tetrahedral BF 4 groups = 139 pm, [32] in BO 2 F 2 = 142.8 pm, [33] and in BO 3 F = 145.0 pm). [33b, 34] Furthermore, calculations of the bond valence sums within the BO 4 and BO 3 groups have been performed accordingt ot he bond-length/bond-strength concept. [35] These result in formal charges for the central boron atoms lower than + 3( B1 =+2.90, B1A =+2.86, B2 =+2.82, B2A =+2.83) and would decrease further by approximately 0.1 each for every oxygen atom being substituted by fluorine.
All praseodymium cations in the crystal structure of Pr 5 (BO 4 ) 3Àx (BO 3 ) x (F,OH) 2.67 O 0.28 (x % 1.6) are in the trivalent state. While minor amounts of tetravalent praseodymium cations cannotb ee xcluded by any method, there is no experimental evidencef or significant amountso ft etravalent cations. The light-green color of the product sample, as wella st he relatively high meani nteratomic distances( see Ta ble S4) compared to literaturev alues of the crystal radii clearly account for Pr 3 + cations (sum of crystal radii [36] : r(Pr 3 + ,C .N. . For mixed valent rare earth oxides, for example Pr 9 O 16 and Pr 10 O 18 ,s uch comparison of mean interatomic distances to sums of crystal radii has been shownt ob ea pplicable for distinguishing between trivalent and tetravalent rare earth cations. [37] Moreover,t he title compoundh as been found to preferably form at the edge of the as that the metaprism twist angle f of the (M1)O 6 polyhedra in apatite-type compounds varies inversely with average ionic radii and unit cell volume. [8] Although no data are available for apatitic compounds with praseodymium as the cation M1, the nearly equal ionic radii of Pr 3 + (C.N. = 8: 127 pm;C .N. = 9: 132 pm) and Ca 2 + (C.N. = 8: 126 pm;C .N. = 9: 132 pm) enable good comparability of the mean metaprism angle obtained for Pr 5 (BO 4 ) 3Àx (BO 3 ) x (F,OH) 2.67 O 0.28 (x % 1.6) (V = 0.5266 nm 3 ,c orresponding to 1 = 3 of the supercell) of 21.38 with the values reported in literature for Ca 10 (PO 4 ) 6 F 2 (V = 0.5226 nm 3 )o f2 3.38 and Ca 10 (PO 4 ) 6 Cl 2 (V = 0.5376 nm 3 )o f19.18. [8] Indicatedb yl arge anisotropic displacementp arameters,t he rare earth cations Pr1 and Pr4 were positioned off their original positions on the mirrorp lane and the 6 axis, respectively,w ith equal partial occupancy each. Furthermore, the cation Pr2 was refined as split position (Pr2 and Pr2A) to comply with singlecrystal XRD data. Pr1 and Pr2 are each 7/8 coordinated, depending on the occupation of anions along [0 01]. This is similar to most common apatite-types tructures.
The anionsi nt he channelsa long [0 01]a re strongly disordered and their average positions in the crystal structure of Pr 5 (BO 4 ) 3Àx (BO 3 ) x (F,OH) 2.67 O 0.28 (x % 1.6) can only be estimated due to the positional disorder,w hich is aw ell-known phenomenon for apatite-types tructures.T he possible sites of the oxygen and fluorine atoms along the 6 3 screw axis, together with the resulting coordinations pheresa re shown in Figure 5. Whereast he positions of the fluorine atoms are almostf ully occupied, the oxygen sites are only occupiedt oaminor frac-tion (see Ta ble S2 in Supporting Information). Note that mainly every other octahedral and trigonal planar site is occupied( F3 and F1, respectively). In naturalf luoroapatite, the fluoridei ons occupy the sites 2a (z = 1 = 4 ,t riangular interstices between Ca atoms) along [0 01]o nly,w hereas the anionso fa patites containingl arger halideso rh ydroxides( hydroxyapatite) tend to occupy2 b (z = 0, octahedral coordination by Ca atoms) or to statistically occupy 4e sites.

Conclusion
By the application of high-pressure/high-temperature synthesis, Pr 5 (BO 4 ) 3Àx (BO 3 ) x (F,OH) 2.67 O 0.28 (x % 1.6), af luoride-oxoborate with apatite-like structure was obtained. The title compound exhibits complex disorder of anionsa long [0 01], as well as BO 4 /(BO 3 + F) group disorder. Partialordering resultsinanapatite superstructure with at ripled lattice parameter c. Pr 5 (BO 4 ) 3Àx (BO 3 ) x (F,OH) 2.67 O 0.28 (x % 1.6) therebyc ombines structural features that have only been reported in different apatitic compounds, for example, the AO 4 /AO 3 group disorder in carbonateh ydroxyapatite, the main component of human and Figure 5. Disordered anion sites in the crystal structure of Pr 5 (BO 4 ) 3Àx (BO 3 ) x (F,OH) 2.67 O 0.28 (x % 1.6) along the 6 3 screw axis occupiedb y fluorinea nd to aminor extent with oxygen atoms (partial occupancy), indicating the possible coordination sites.F urthermore, the distorted pentagonal bipyramidal coordination spheres of the rare earth cations Pr2/Pr2A and Pr1 are depicted. Note that the distinction between F, O, and OH is tentative. animal hard tissues, and the superstructure of iodo-oxoapatite Ca 15 (PO 4 ) 9 IO. The formation of BO 4 tetrahedra insteado ft rigonal planar BO 3 groups is preferred at high-pressure conditions during the synthesis-known as the pressure-coordination rule. Therefore, the synthesis of apatite structures with exclusively BO 4 groups replacing the PO 4 groups of the apatitep rototypes will be an interesting objective for future research. Owing to their high potential as scaffolds of growth factors and carriers for controlled protein releasef or bone regeneration and for biodegradable bone implants, further research in high-pressure synthesis of borate-containing hydroxyapatites and their characterization will be of great interest.