A new hydrogen-containing whitlockite- type phosphate Ca ...DOI 10.1515/zkri-2014-1774 Received May...

Z. Kristallogr. 2014; 229(12): 823–830

Dina V. Deyneko*, Sergey M. Aksenov, Vladimir A. Morozov, Sergey Yu. Stefanovich, Olga V. Dimitrova, Oksana V. Barishnikova and Bogdan I. Lazoryak

A new hydrogen-containing whitlockite-type phosphate Ca9(Fe0.63Mg0.37)H0.37(PO4)7: hydrothermal synthesis and structure

Abstract: A new hydrogen-containing Ca9(Fe0.63Mg0.37)H0.37(PO4)7 phosphate with the whitlockite-type structure has been synthesized by a hydrothermal method and its structure has been studied by the single-crystal X-ray dif-fraction. The compound crystallizes in the trigonal space group R3c (traditional for compounds with the whitlockite-type structure) with unit-cell parameters: a = 10.3533(1) Å, c = 37.1097(4) Å. The structure has been determined using the “charge flipping” method. Ca9(Fe0.63Mg0.37)H0.37(PO4)7 structure is similar to that of other members of the whit-lockite-type family. The presence of hydrogen in the struc-ture leads to the formation of OH-group with one of the oxygen of PO4-tetrahedra. Based on an analysis of the bond valence sums (BVS) a conclusion has been made about localization of H atoms in the structure. Smaller values of BVS for O1 and O10 atoms than ones for other oxygen atoms indicate localization of H atoms between them in a position with site symmetry 18b.

Keywords: calcium phosphates; hydrothermal synthesis; single-crystal X-ray diffraction; whitlockite-type compounds.

DOI 10.1515/zkri-2014-1774Received May 28, 2014; accepted October 6, 2014; published online November 12, 2014

IntroductionCalcium phosphates are the most important inorganic constituents of biological hard tissues [1]. In biological

systems, calcium orthophosphates in the form of carbon-ated hydroxyapatite (HA) or three calcium orthophos-phate (Ca3(PO4)2 or TCP) are present in bones, teeth, tendons and some species of shells. Calcium phosphate ceramics based on HA and TCP are used extensively as bioactive implants in human bone surgery due to their similarity to the mineral component of bone [2]. Such bio-materials are slowly resorbed in the body to be replaced by natural bone. On this reason TCP and its variations were extensively studied as bioceramic [3] and lumines-cent [4–6] materials. In biomedicine, the mixture of TCP and hydroxyapatite is used as a biphasic calcium phos-phate (BCP) being a bone-substitution ceramic due to dif-ferent solubility of these calcium phosphates in aqueous systems [1]. The substitution of calcium cations by ions with the similar size and charge noticeably changes sta-bility and solubility of TCP [7–10].

β-Ca3(PO4)2 (β-TCP) (sp. gr. R3c, Z = 21 or β-Ca10.5(PO4)7, Z = 6) crystallizes in trigonal symmetry [11, 12] and belongs to whitlockite-group minerals (whitlockite – Ca18.19(Mg1.17Fe0.83)H1.62(PO4)14 [13], synthetic Mg- and Mn- whit-lockite – Ca9Mg(PO4)6(PO3OH) [13], Ca9Mn(PO4)6(PO3OH) [14], strontium whitlockite Sr9Mg(PO4)6(PO3OH) [15]; mer-rilite Ca9NaMg(PO4)7 [16]; ferromerrilite Ca9NaFe2+(PO4)7 [17] and bobdownsite Ca9Mg(PO4)6(PO3F) [18]) as well as cerite-group minerals [19] (cerite-(Ce) (REE,Ca)9(Mg,Fe3+)(SiO4)3[SiO3(OH)]4(OH)3 [20]; cerite-(La) (REE,Ca)9(Fe3+,Ca,Mg)(SiO4)3 [SiO3(OH)]4(OH)3 [21] and aluminocerite (Ce,Ca)9Al(SiO4)3[SiO3 (OH)]4(OH)3 [22]).

The crystal structures of the whitlockite- and cerite-group minerals as well as structurally related compounds have a general formula (M(1–3))9□4M5(XO3Y)x(XO4)6-x (□,OH,F)3 (where M(1–3) = Ca2+, REE3+, Na+; M5 = Mg2+, Fe2+, Fe3+, Al3+; X = P5+ or Si4+ and Y = O2–, OH– or F–). The whitlockite-type (β-TCP) structure is built of isolated PO4 tetrahedra, which connect the MOn polyhedra into a 3D framework via common vertices with formation of two types of columns: A and B (Figure 1a). The whitlockite-type structure consists of two different layers: I and II. One of them is built only by B columns (Figure 1b), the second

*Corresponding author: Dr. Dina V. Deyneko, Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory, 1–3, Moscow 119991, Russia, E-mail: [email protected]; and Shubnikov Institute of Crystallography RAS, Moscow, RussiaSergey M. Aksenov: Lomonosov Moscow State University, Moscow, Russia; and Shubnikov Institute of Crystallography RAS, Moscow, RussiaVladimir A. Morozov, Sergey Yu. Stefanovich, Olga V. Dimitrova, Oksana V. Barishnikova and Bogdan I. Lazoryak: Lomonosov Moscow State University, Moscow, Russia

Authenticated | [email protected] author's copyDownload Date | 11/24/14 7:28 AM

824 D.V. Deyneko et al.: A new hydrogen-containing whitlockite-type phosphate Ca9(Fe0.63Mg0.37)H0.37(PO4)7

Fig. 1: (Color online). ab projection of the β-Ca3(PO4)7 structure (a). Columns A and B are indicated and the layer I the with B columns (b) and the layer II with the columns A and B (c) are shown. M4 site is half-occupied.

one – by columns A and B (Figure 1c). The B and A columns in the whitlockite-type structure can be presented as […–M1O8–M3O8–M2O8–PO4–PO4–…] and [….–PO4–M4O15–M5O6–M6O13–…], respectively. In the β-TCP structure the M4 site in the A columns is half-occupied while the M6 site is vacant so the A-type column in the β-TCP structure consists of a sequence of polyhedra and cavities.

The H-containing phosphates with the whitlockite-type structure are known for a long time. The structural data and properties of the H-containing whitlockite-type phases are important since the recovery of the bone tissue

realizes in aqueous systems. Ca9MH(PO4)7 (M = Mg2+ [23], Mn2+ [14]) were synthesized by the hydrothermal tech-nique while Ca9FeH0.9(PO4)7 [24] and Ca9FeD(PO4)7 [25] were obtained in the hydrogen atmosphere at 870 K. The positions of the deuterium atoms in the Ca9FeD(PO4)7 structure were located from neutron diffraction data [25].

In this paper we present results of the solution of the structure of a new hydrogen-containing Ca9(Fe0.63Mg0.37)H0.37(PO4)7 phosphate synthesized by a hydrothermal method.

ExperimentalMaterials and sample preparation

Single crystals of Ca9(Fe0.63Mg0.37)H0.37(PO4)7 phosphate were synthe-sized by a hydrothermal method. Standard Teflon-lined stainless steel autoclave of 5 mL capacity was used. The coefficient of the autoclave filling was selected so that pressure was constant. The syn-thesis was carried out at the general pressure of 70–80 atm in the temperature range from 543 K to 553 K. The bottom temperature point was limited by kinetics of the chemical reaction while the upper point was limited by equipment characteristics. The experiment duration is 20 days and it corresponds to full completion of the chemical reac-tion. The following analytical-grade compounds were used: CaCO3, NH4H2PO4, FeCl3·6H2O, MgCO3, H3PO4 and β-TCP. β-TCP was used as the main component of the charge. β-Ca3(PO4)2 was prepared from a stoichiometric mixture of CaCO3 and NH4H2PO4 by the solid-state method at 973 K for 50 h.

The synthesis was carried out in the region of phosphate excess with the CaO:P2O5 ratio 1:(2–3) and molar ratio Ca/Fe/Mg = 9:1:1, pH = 8. Final cooling after synthesis to room temperature was done in 24 h. The precipitate was separated by filtering a stock solution, washed several times with hot distilled water and finally dried at room tem-perature for 12 h. In according to optical microscopy study the pre-cipitate is a mixture of small colorless crystals and transparent white powder. Crystals were selected manually from the mixture for further studies. Powder X-ray diffraction (PXRD) study shows that white powder is a phase with the apatite-type structure. Element content of the selected single crystals was determined by inductively coupled plasma optical emission spectrometry and, independently, from the refinement of their crystal structure.

Characterization

Element contents of the selected colorless single crystals was deter-mined by inductively coupled plasma optical emission spectrometry (ICP-OES) performed on an iCAP 6300 Thermo Scientific Spectrom-eter equipped with a high performance CID86 semiconductor detec-tor, RF plasma generator with power output of 1.35 kW at 27.12 MHz and the iTEVA software. For element content measurements the spec-trometer was thermostated at 301 ± 0.1 K. For ICP measurements the Ca9(Fe0.63Mg0.37)H0.37(PO4)7 sample (∼0.03 mg) was dissolved in a mix-ture of HCl and HNO3 acids (aqua regia) in the autoclave.


D.V. Deyneko et al.: A new hydrogen-containing whitlockite-type phosphate Ca9(Fe0.63Mg0.37)H0.37(PO4)7 825

PXRD patterns were collected on a Huber G670 Guinier diffrac-tometer (CuKα1 radiation, curved Ge(111) monochromator, transmis-sion mode, image plate). PXRD data were collected at TR over the 4°–100° 2θ range with a step of 0.01°.

The second harmonic generation (SHG) response of pow-der samples was measured with a Q-switched YAG:Nd laser at λω = 1064 nm in the reflection mode. The experimental set-up is described elsewhere [26]. The intensities of the SHG signal (I2ω) from the sample and that from a reference sample (polycrys-talline α-SiO2 with 3–5 μm particles size) were registered. The Ca9(Fe3+0.63Mg0.37)H0.37(PO4)7 single crystal shows an SHG response (I2ω/I2ω(SiO2)) ∼0.3. This insignificant nonzero SHG response is typical for other whitlockite-like phosphates with the noncen-trosymmetric crystal structures [27].

57Fe Mössbauer spectroscopy (MS) experiments were per-formed in transmission geometry at 300 K using a constant accel-eration Mössbauer spectrometer coupled with a 1024 multichannel analyzer. A 57Co/Rh γ-ray source was used. The velocity scale was calibrated relative to α-Fe. All isomer shift values (δ) given hereaf-ter are referred to α-Fe. The MS study of a crashed mixture of small colorless crystals and transparent white powder showed that MS

spectrum of the mixture did not differ from the similar spectrum of Ca9Fe(PO4)7 [24] and indicated the presence of only one type Fe cations with isomer shift (I.S.) δ = 0.33 mm/s. According to Menil [28] the isomer shift value is characteristic of high spin Fe3+ in an octa-hedral environment.

A colorless crystal (0.15 × 0.22 × 0.24 mm3) was used for single-crystal X-ray data collection. The single-crystal X-ray data were collected at room temperature on a Xcalibur Oxford Diffraction diffractometer with graphite monochromatized MoKα radiation (λ = 0.71073 Å) and a CCD detector using the ω–θ scanning mode. A total of 60868 reflections within the sphere limited by θ = 36.54° were measured. The experimental details of the data collection and refinement results are listed in Table 1. A semi-empirical absorption correction based on intensities of equivalent reflections was applied, and the data were corrected for the Lorentz, polari-zation and background effects. The refinement of the unit-cell parameters was performed using the CrysAlis software [29]. The structure determination and refinement were carried out using the JANA2006 program package [30]. Illustrations were produced with the JANA2006 program package in combination with the program DIAMOND [31].

Tab. 1: Crystal data, data collection and refinement of Ca9(Fe0.63Mg0.37)H0.37(PO4)7.

Crystal data Formula Ca9(Fe0.63Mg0.37)H0.37(PO4)7 Formula weight (g) 1069.7 Temperature (K) 293 Cell setting Trigonal Space group R3c Lattice Parameters a (Å) 10.3533(1) c(Å) 37.1097(4) V (Å3) 3444.89(6) Z 6 Calculated density, Dx (g cm–3) 3.093 Crystal size (mm) 0.15 × 0.22 × 0.24 Crystal form unshaped grain Crystal color Colorless

Data Collection Diffractometer Xcalibur Oxford Diffraction (CCD-detector) Radiation; λ MoK

α; 0.71073

Absorption coefficient, μ (mm–1) 3.073 F (000) 3179 Data range θ (°); h, k, l 4.27–36.45; –21


Results

Elemental composition

The elemental analysis performed by ICP gives 33.5 ± 0.4 wt% for Ca, 3.27 ± 0.03 wt% for Fe, 0.84 ± 0.01 wt% for Mg and 20.4 ± 0.1 wt% for P. This is close to the Ca8.92Fe0.62Mg0.37 (PO4)7 composition (calc. wt%: 33.54 (Ca), 3.25 (Fe), 0.84 (Mg) and 20.34 (P)).

The analysis of the whitlockite-type structure shows that three 18-fold sites M1–M3 and one 6-fold site M5 are always fully occupied [11–18, 24, 25]. Occupancy of three 18b sites by Ca2+ and one 6a site by R3+ leads to with the Ca9M(PO4)7 formula for the whitlockite-type phase. Ca8.92Fe0.62Mg0.37(PO4)7 composition determined from ICP data is close to the Ca9M(PO4)7 (M = x(Fe)+y(Mg), y = 1–x). So far as Fe3+ cations are contained in the crystal composi-tion to achieve electrical neutrality the chemical formula should contain both Fe3+ and H+ ions. This assumption corresponds to the chemical formula of Ca9FexMgyHy(PO4)7. The values x and y were defined as the result of the crystal structure refinement.

Crystal structure solution

The following unit-cell parameters have been obtained by the least-squares refinement: a = 10.3533(1) Å, c = 37.1097(4) Å. In accordance with the analysis of system-atic absence of reflections the R–lattice and space group R3c (the traditional space group for compounds with the whitlockite-type structure) were chosen. Atomic scattering factors for neutral atoms together with anomalous disper-sion corrections were taken from International Tables for X-Ray Crystallography [32]. A structure model was deter-mined by “charge flipping” method using the SUPERFLIP computer program [33].

The initial model for the structure refinement was based on coordinates of the β-TCP structure [11]. At the first stage of the model-parameter refinement cations were set using the f-curve of Ca2+ cations. Ca2+ cations were located in the M1–M5 sites of the whitlockite-type structure. The refinement of the occupancy factors (gf-Ca) for M1–M5 sites showed that M1–M3 sites are occupied by Ca2+ cations (gf-Ca(M(1–3)∼1 (0.98)). The occupancy of the M4 position by Ca2+ cations was close to zero. At this stage of the refinement the calculated Ca–O distances appeared to be 2.32–2.95 Å for Ca1O8–Ca3O8 polyhedra and did not differ noticeably from the similar Ca-O distances for β-Ca3(PO4)2 [11, 12] and Ca9MH(PO4)7 (M = Mg2+, Mn2+ [14, 23]) structures. The occupancy factor (gf-Ca) for M5 site

Tab. 2: Interatomic distances M5 – O in the β-Ca3(PO4)2 and H-containing whitlockite-related compounds.

Composition M5–O Reference

β-Ca3(PO4)2 2.238–2.287 2.263 [11]Ca18Mn2H2(PO4)14 2.147–2.148 2.147 [14]Ca9Fe0.92+Fe0.13+H0.9(PO4)7 2.16–2.08 2.12 [24]Ca9Fe2+D(PO4)7 2.088–2.124 2.106 [25]Ca18.36(Mg1.14Fe2+0.86)H1.38(PO4)14 2.075–2.104 2.090 [34]Ca9(Mg0.37Fe3+0.63)H0.37(PO4)7 2.079–2.101 2.090 this workCa18.88(Mg1.87Fe2+0.13)H0.23(PO4)14 2.078–2.097 2.088 [34]Ca18.07(Mg1.83Fe2+0.17)H1.86(PO4)14 2.069–2.107 2.088 [34]Ca18.12(Mg1.84Fe2+0.16)H2(PO4)14 2.066–2.102 2.084 [34]Ca18.19(Mg1.17Fe2+0.83)H1.62(PO4)14 2.076–2.088 2.082 [13]Ca18.12(Mg1.97Fe2+0.28)H2(PO4)14 2.052–2.087 2.070 [34]Ca18Mg2H2(PO4)14 2.057–2.077 2.067 [23]Ca9Fe3+(PO4)7 1.951–2.168 2.060 [35]

was slightly smaller 1 (gf-Ca(M5)


Tab. 3: Fractional atomic coordinates, site symmetry, equivalent atomic displacement parameters (Ueq) and site composition for Ca9(Fe0.63Mg0.37)H0.37(PO4)7.

Atom x y z Site Ueq, Å2 Occupancy

M1 0.72107(4) 0.84659(4) 0.43734(1) 18b 0.0090(1) CaM2 0.61447(4) 0.81351(4) 0.23477(1) 18b 0.0085(1) CaM3 0.19636(5) 0.37192(6) 0.34177(2) 18b 0.0180(2) CaM5 0 0 0.00283(2) 6a 0.0073(2) 0.63(1) Fe3++0.37(1) Mg2+P1 0 0 0.25632(3) 6a 0.0120(2) PP2 0.68463(6) 0.85977(5) 0.13684(1) 18b 0.0070(2) PP3 0.65091(6) 0.84077(6) 0.03380(1) 18b 0.0071(2) PO1 0 0 0.2990(1) 6a 0.0310(1) OO2 –0.0211(2) 0.8497(1) 0.24424(4) 18b 0.0150(5) OO3 0.7375(2) 0.8737(2) 0.17652(4) 18b 0.0116(5) OO4 0.7483(2) 0.7774(2) 0.11597(4) 18b 0.0133(5) OO5 0.7334(2) 0.0176(2) 0.12331(4) 18b 0.0116(4) OO6 0.5129(2) 0.7671(2) 0.13323(4) 18b 0.0111(4) OO7 0.6061(1) 0.9376(2) 0.05609(4) 18b 0.0101(4) OO8 0.5800(2) 0.6801(2) 0.04710(4) 18b 0.0132(5) OO9 0.8224(2) 0.9134(1) 0.03882(4) 18b 0.0116(5) OO10 0.6110(2) 0.8511(2) 0.99475(4) 18b 0.0148(5) O

Note: Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.

Tab. 4: Anisotropic atomic displacement parameters for Ca9(Fe0.63Mg0.37)H0.37(PO4)7.

Site U11 U22 U33 U12 U13 U23

M1 0.0105(2) 0.0085(2) 0.0080(2) 0.0047(1) –0.0018(1) –0.0004(1)M2 0.0095(2) 0.0089(2) 0.0075(2) 0.0050(1) –0.0003(1) –0.0011(1)M3 0.0117(2) 0.0306(2) 0.0145(2) 0.0128(2) 0.0027(1) 0.0105(2)M5 0.0077(2) 0.0077(2) 0.0064(3) 0.0039(1) 0 0P1 0.0096(2) 0.0096(2) 0.0169(5) 0.0048(1) 0 0P2 0.0070(2) 0.0079(2) 0.0058(2) 0.0036(1) 0.0004(1) –0.0001(2)P3 0.0079(2) 0.0065(2) 0.0067(2) 0.0035(2) 0.0001(2) –0.0001(2)O1 0.037(1)a 0.037(1)a 0.018(2) 0.0186(6) 0 0O2 0.0147(6) 0.0078(5) 0.0239(7) 0.0066(4) 0.0067(5) 0.0030(4)O3 0.0173(6) 0.0139(6) 0.0046(6) 0.0085(5) –0.0036(5) –0.0026(4)O4 0.0150(6) 0.0178(6) 0.0115(5) 0.0114(5) 0.0021(4) –0.0020(5)O5 0.0136(6) 0.0091(6) 0.0099(5) 0.0040(5) –0.0020(4) 0.0008(4)O6 0.0087(6) 0.0097(5) 0.0135(6) 0.0037(5) –0.0008(4) –0.0001(4)O7 0.0122(5) 0.0127(5) 0.0079(5) 0.0082(4) 0.0011(4) –0.0008(4)O8 0.0175(6) 0.0072(5) 0.0117(5) 0.0038(5) 0.0019(4) 0.0014(4)O9 0.0092(6) 0.0102(5) 0.0157(7) 0.0050(5) 0.0019(5) 0.0007(4)O10 0.0215(7)a 0.0177(6) 0.0070(5) 0.0113(6) 0.0007(4) 0.0003(5)

aBold font is highlighting increased anisotropic atomic displacement parameters.

[36]. BVS results for Ca9(Fe0.63Mg0.37)H0.37(PO4)7 structure are presented in Table 6. BVS calculations were performed using the bond-length parameters for the Ca2+–O, Fe3+–O, Mg2+–O, and P5+–O. BVS for the M1, M2, M3, M5, P1, P2 and P3 sites are 2.13, 2.04, 1.90, 2.46, 4.98, 4.96 and 4.96 (valence units) (Table 6). The value of BVS for the M5-site confirms the mixed occupancy by the trivalent (Fe3+) and bivalent (Mg2+) cations. In accordance with Table 6 BVS data for two oxygen atoms O1 and O10 differ from others.

The calculated BVS value for O1 (1.14 v.u.) is considerably lower than for other oxygen atoms and corresponds to formation of O1H-bonds in the Ca9(Fe0.63Mg0.37)H0.37(PO4)7 structure. Thus [P1O23O1] tetrahedra usually observed in the whitlockite-type structure are transformed into [P1O23(O1H)] tetrahedra. Moreover the decreasing of the BVS value for O10 atoms (1.84 v.u.) in the comparison with O2–O9 atoms (1.93–2.14 v.u.) indicates formation of hydro-gen bonds H···O10 in the structure. The P1–O1 distance is



Tab. 5: Selected interatomic distances (Å) and angles in the [PO4] tetrahedra for Ca9(Fe0.63Mg0.37)H0.37(PO4)7.

Bond d, Å Bond d, Å

M1 -O7 2.371(2) M2 -O4 2.315(2)-O10 2.371(1) -O3 2.426(2)

-O2 2.381(1) -O9 2.446(2)-O6 2.437(2) -O5 2.456(1)-O8 2.464(2) -O2 2.466(1)-O6 2.468(1) -O9 2.478(1)-O4 2.625(1) -O7 2.509(2)-O5 2.652(1) -O8 2.763(2)

2.471 2.482M3 -O5 2.378(1) P3 -O8 1.526(2)

-O7 2.382(2) -O10 1.526(1)-O8 2.392(2) -O7 1.539(2)-O3 2.473(2) -O9 1.555(2)-O1 2.523(2) 1.537-O3 2.553(2)-O4 2.655(2) O1 -O10 3 × 2.754(4)-O2 2.951(2)

2.538 AngleM5 -O6 3 × 2.101(2) O1-P1-O2 107.07(7)

-O9 3 × 2.079(2) O2-P1-O2 111.76(8) 2.090 O4-P2-O5 115.17(8)P1 -O1 1.584(4) O4-P2-O6 107.07(8)

-O2 3 × 1.526(1) O5-P2-O6 107.25(9) 1.555 O7-P2-O8 113.50(8)P2 -O4 1.524(2) O7-P2-O9 105.30(8)

-O5 1.533(2) O7-P2-O10 106.5(1)-O6 1.547(2) O8-P2-O9 107.4(1)-O3 1.552(2) O8-P2-O10 112.60(8)

1.539 O9-P2-O10 111.33(9)

Tab. 6: Bond valence calculation for Ca9(Fe0.63Mg0.37)H0.37(PO4)7.

Atom M1 M2 M3 M5 P1 P2 P3 Vi

O1a 1.14 1.14O2 0.32 0.26 0.07 1.28 × 3↓ 1.93

1.28→O3 0.28 0.21+0.26 1.21 1.96O4 0.17 0.39 0.16 1.29 2.01O5 0.16 0.26 0.33 1.25 2.00O6 0.26+0.28 0.39 × 3↓ 1.21 2.14

0.39→O7 0.34 0.21 0.33 1.23 2.11O8 0.26 0.12 0.32 1.27 1.97O9 0.25+0.27 0.43 × 3↓ 1.18 2.13

0.43→O10b 0.34 0.22 1.28 1.84Vi 2.13 2.04 1.90 2.46 4.98 4.96 4.96

aOH-group.bO···H bond.The bold font is highlighting reduced values of BVS.Note: The value Vi was calculated for M5 site occupied by Fe3+, as a predominant element. The value of BVS of M5-site confirms the mix occupancy by trivalent (Fe3+) and bivalent (Mg2+) cations.

∼ 1.584 Å and the longest P–O distance in the structure, the long P1–O1 distance resulting from the fact that the O1 oxygen is hydroxyl oxygen [34].

DiscussionIn according to the ICP measurement, MS data and the crystal structure refinement the composition of our

Fig. 2: (Color online). The environment of the octahedral M5 site in the Ca9(Fe0.63Mg0.37)H0.37(PO4)7 structure.



Fig. 3: (Color online). a) Part of the A column in the Ca9(Fe0.63Mg0.37)H0.37(PO4)7 structure. Dashed lines shows oxygen atoms connected with hydrogen. b) The environment of P1O4 tetrahedra with P3O4 tetrahedra and probable location of H+ between O1 and O10 with for-mation of O1H-bonds and hydrogen bonds H···O10 in the structure.

crystals synthesized by the hydrothermal method can be described as Ca9(Fe0.63Mg0.37)H0.37(PO4)7 . Main structural features of a new H-containing phosphate are reflected in its crystal chemical formula (Z = 6): M1–3[Ca]9M5[Fe3+0.63 Mg0.37][PO4]6[PO3.63(OH)0.37], where square brackets denote the composition of the M-sites and isolate phosphate-oxygen anions. The composition of the natural whit-lockite mineral is Ca9.095(Fe0.415Mg0.58)H0.81(PO4)7 [13] and the composition of the synthesized crystal is close to it. Ca9(Fe0.63Mg0.37)H0.37(PO4)7 structure is similar with struc-tures of other members of whitlockite-group compounds. Calcium ions fully occupy large M1, M2 and M3-sites of the whitlockite-type structure while the M4 and M6 sites are completely free. The octahedral M5 site of the

whitlockite-type Ca9(Fe0.63Mg0.37)H0.37(PO4)7 structure is occupied by Fe3+ and Mg2+ cations with two different M5–O distances of 2.079(1) and 2.101(1) Å, respectively (Figure 2, Table 5).

The refinement of the crystal structure using single-crystal X-ray data did not allow us to detect H+ ions in the structure and to clarify their position. However, the pres-ence of H+ ions in the structure of our crystals is necessary to preserve their electrical neutrality. Besides it is known [14, 23] that the whitlockite-type phases obtained by the hydrothermal synthesis usually contain H+ ions in the structure. Nevertheless the location of H+(D+) ions in the whitlockite-type structure in the 18-fold position was deter-mined during the refinement of Ca9FeD(PO4)7 structure using the neutron powder diffraction data [25]. For other phosphates a conclusion about the location of H+ ions in the structure was made based on the analysis of P–O dis-tances and ADPs for O atoms without using the BVS cal-culation method. Using of BVS allows us to locate the H+ ions position between O1 and O10 atoms (dO1–O10 = 2.754(4) Å (Table 5)), O1 and O10 forming O1H-bonds and H···O10 hydrogen bonds in the structure (Figure 3). H+ ions are located near the 3-fold axe in the position with 18b site symmetry and structurally disordered. 2.22 (6(Z) × 0.37) H+ ions statistically occupy 18b positions in the structure. This disordering with the statistically formation of O1H- and H···O10 hydrogen bonds leads to increasing anisotropic ADPs for O1 (U11 and U22) and O10 atoms (U11) in compari-son with other O atoms. Similar large anisotropic ADPs for O1 atoms were observed in other H-containing phosphates with the whitlockite-type structure [14, 23].

Acknowledgments: D.V.D. and S.M.A are grateful for financial support of the Foundation of the President of the Russian Federation (Grants MK-4990.2014.5). D.V.D., V.A.M. S.Yu.S and B.I.L. are grateful for financial support of the Russian Foundation for Basic Research (Grants 14-03-01100). The authors are grateful to Dr. K.V. Pokholok (Department of Chemistry, Moscow State University) for Mössbauer spectroscopy measurement and P.A. Volkov for ICP measurement.

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