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Jun 29, 2016 - ABSTRACT: Emerald-green single crystals of U(PO4)Cl were grown by chemical vapor transport in a temperature gradient (1000 → 900 °C)...
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Comprehensive Characterization of the Electronic Structure of U4+ in Uranium(IV) Phosphate Chloride Anna Bronova,⊥ Thomas Droß,†,⊥ Robert Glaum,*,⊥ Heiko Lueken,‡ Manfred Speldrich,‡ and Werner Urland§,∥ ⊥

Institute of Inorganic Chemistry, Rheinische Friedrich-Wilhelms-Universität, Gerhard-Domagk-Straße 1, D-53121 Bonn, Germany Institute of Inorganic Chemistry, RWTH Aachen, Professor-Pirlet-Str. 1, D-52074 Aachen, Germany § Institute of Inorganic Chemistry, Leibniz Universität Hannover, Callinstrasse 9, D-30167 Hannover, Germany ∥ Chemistry Department, University of Fribourg, Chemin du Musée 9, CH-1700 Fribourg, Switzerland ‡

S Supporting Information *

ABSTRACT: Emerald-green single crystals of U(PO4)Cl were grown by chemical vapor transport in a temperature gradient (1000 → 900 °C). The crystal structure of U(PO4)Cl (Cmcm, Z = 4, a = 5.2289(7) Å, b = 11.709(2) Å, c = 6.9991(8) Å) consists of a three-dimensional network of [PO4] tetrahedra and bicapped octahedral [UIVO6Cl2] groups. Polarized absorption spectra measured for two perpendicular polarization directions show a large number of well-resolved electronic transitions. These transitions can be fully assigned on the basis of a detailed ligand-field treatment within the framework of the angular overlap model. The magnetic behavior predicted on the basis of the spectroscopic data is in agreement with an f 2 system and perfectly matched by the results of temperaturedependent susceptibility measurements.

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INTRODUCTION Cations of d-block metals in comparatively low oxidation states can be stabilized as anhydrous phosphates (e.g., Ti3+ in TiPO4,1 Cr2+ in Cr3(PO4)2 and Cr2P2O7,2 Mo3+,4+,5+,6+ in Mo(PO3)3, Mo4P6Si2O25, MoP2O7, MoPO5, and Mo2P4O15,3 W4+,5+,6+ in WP2O7, WPO5, and W8P4O32,4 Re4+,5+,6+ in ReP2O7 and Re3P6Si2O25, RePO5, Re2P2O115). To extend our knowledge concerning the redox behavior of actinide metals in anhydrous phosphates we aimed at the synthesis of “reduced” uranium phosphates. These studies are also aiming to establish the chemistry of uranium compounds, which are of great interest in the context of rad waste disposal.48,49 Phosphates containing trivalent uranium were of particular interest to us, since this oxidation state is not well-characterized in oxo-compounds. In contrast, a whole series of phosphates containing tetravalent uranium has been characterized so far: UIV2O(PO4)2 (bright green color),6 UIVP2O7 (violet to light pink color, probably depending on conditions of synthesis),7 UIV(P4O12),8 UIV(PO3)4 (bright green),9 UIV(UVIO2)(PO4)2 (green powder),10 UIV(PO4)Cl·n H2O (n = 2, 4; emeraldgreen).11 Despite their bright color and the intriguing difference in color between UP2O7 and the other uranium(IV) phosphates so far only two systematic spectroscopic studies have been undertaken concerning U4+ ions doped in a ZrSiO4 host lattice12 and compounds with the formula M2USi6O15 (M: K, Rb).13 In experiments aiming at the crystallization of UP2O7 by chemical vapor transport14 using ZrCl4 as transporting agent © XXXX American Chemical Society

the new phosphate chloride was discovered as a result of a reaction between the phosphate solid and the halide.15 Crystals were heavily twinned and did not allow structure determination. Since more suitable crystals of U(PO4)Cl were obtained during equilibrium experiments in the ternary system uranium/ phosphorus/oxygen16 we performed a systematic study on its crystal structure and of the electronic structure of U4+ within. The results of this investigation are presented here and in the subsequent paper of this issue.

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EXPERIMENTAL SECTION

Synthesis. Uranium(IV) diphosphate as starting material was obtained from UO2(NO3)2·5H2O and (NH4)2HPO4 following a procedure described in literature.17 Chemical vapor transport14 experiments were performed in silica tubes of typical dimensions l = 12 cm and d = 1.4 cm (V = 18.5 cm3). Using chlorine or hydrogen chloride as transporting agent no transport effect was observed for UP2O7. In contrast, application of gaseous tetrachlorides MCl4 (M: Ti, Zr, Hf, V) or of NbCl5 as transporting agent (TA) led to the migration of the diphosphate to the lower temperature zone of a temperature gradient (Table S1). Use of the tetrachlorides MCl4 (M: Ti, Zr, Hf) as transport agent always resulted in deposition of mixed crystals (U1−xMx)P2O7 at the lower temperature zone (“sink”). In addition, such experiments led to the first observation of U(PO4)Cl as neighboring phase besides the mixed crystal. Under similar experimental conditions (Table S1) the amount of Zr4+ or Hf 4+ Received: March 1, 2016

A

DOI: 10.1021/acs.inorgchem.6b00438 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ions in the mixed crystals was much higher than that of Ti4+. Mixed crystals (U1−xZrx)P2O7 and (U1−xHfx)P2O7 show a violet color, similar, however lighter than the one of pure crystals of UP2O7 that were obtained by chemical vapor transport using NbCl5 or VCl4 as TA (Table S1). Crystals of (U1−xTix)P2O7 show a distinctly different brownish-violet tint. U(PO4)Cl as main product besides small amounts of U2O(PO4)2 (occurring in the source only) were obtained by reaction of stoichiometric mixtures of UP2O7, UO2, and ZrCl4 (eq 1) followed by chemical vapor transport. Thus, crystals of the phosphate chloride with edge-lengths up to 0.5 mm have been grown. This procedure was reported by Kostencki.15

and interatomic distances are given in Tables 2 and 3. Additional crystallographic information is available in the Supporting Information.

Table 2. Atomic Coordinates and Isotropic Displacement Parameters for U(PO4)Cl

2UP2O7 (s) + 2UO2 (s) + ZrCl4(g) → 4U(PO4 )Cl(s) + ZrO2 (s) (1)

[σ2(Fo2) + (a·P)2 + b·P]; P =

d

Ueq (Å2)a

U P Cl O1 O2

4c 4c 4c 8g 8g

0 0 1/2 −0.2395(7) 0

0.090 80(1) −0.1885(1) 0.0042(2) −0.2609(3) −0.1038(3)

1/4 1/4 1/4 1/4 0.4227(7)

0.0060(2) 0.0053(3) 0.0192(4) 0.0133(6) 0.0107(8)

Pa

1 3

∑i ∑j Uij·a*j ·ai*·ai ·aj .

O1VI 2.207(4) O2X 2.579(4) O1 1.513(4)

O1VIII 2.207(4) O2 2.579(4) O1VII 1.513(4)

O2IV 2.296(5) Cl 2.8043(7) O2 1.563(4)

O2IX 2.296(5) ClV 2.8043(7) O2X 1.563(4)

a IV: −x, −y, −z + 1; V: x+ 1, y, z; VI: x + 1/2, y + 1/2, z; VII: −x, y, z; VIII: −x − 1/2, y + 1/2, z; IX: −x, −y, z − 1/2; X: x, y, −z + 1/2.

Magnetic Measurements. Magnetization measurements on polycrystalline samples (weighed portions 5−15 mg) encapsulated in quartz tubes were performed using a SQUID magnetometer (MPMS5S, Quantum Design, San Diego, CA; temperature range 1.7 ≤ T ≤ 800 K; applied field range of B0 ≤ 5 T). Details concerning sample arrangement and measurement technique are described elsewhere.22,23 Corrections for diamagnetic and conduction electron contributions as well as demagnetization were not applied. The results of the measurements are presented in Figure 1. Down to the low

Figure 1. Graphical representation of χmol vs T and μeff vs T for UPO4Cl and comparison to μcalc (solid line). For calculations, compare subsequent paper. temperatures, no cooperative effect is obtained. The deviation between observed and calculated susceptibilities at low temperatures is caused by the paramagnetic impurity. Polarized Single-Crystal Absorption Spectra. The polarized absorption spectra of U(PO4)Cl were measured using a modified CARY 17 spectro-photometer (Spectra Services, ANU, Canberra). Details are published.24 For a better signal-to-noise ratio the spectrophotometer is equipped with a chopper (optical chopper, model SR540, Stanford Research Systems, Inc.) for modulation of the incident beam and a lock-in amplifier (model SR510, Stanford

Lattice parameters were determined from 25 reflections (15.1° ≤ 2θ ≤ 62.0°) of a Guinier photograph (Cu Kα1, quartz monochromator, αSiO2 as internal standard; software: SOS1 and SOS221). bR1 = ∑∥Fo| 1 (F 2 3 o

z

Ua

a

∑ w(Fo2 − Fc 2)2 /∑ w(Fo2)2 .

y

Table 3. Selected Interatomic Distances (Å) Observed for U(PO4)Cl

chemical formula U(PO4)Cl formular weight (g·mol−1) 340.61 color emerald-green crystal system orthorhombic space group Cmcm (No. 63) Z 4 a, b, c (Å) 5.2289(7), 11.709(2), 6.9991(8)a 3 V (Å ) 428.54(9) D (g·cm−3) 5.71 crystal shape irregularly shaped fragment crystal size (mm3) 0.06·0.12·0.33 absorption coefficient (mm−1) 38.753 κ-CCD diffractometer (Enraf-Nonius), Mo Kα radiation, graphite monochromator (λ = 0.710 63 Å) absorption correction analytical (HABITUS)20 collected reflections 4934 independent reflections 365 (F0 > 4σ(F0)) theta range (deg) 3.5 ≤ θ ≤ 30.0 index ranges (whole sphere) −7 ≤ h ≤ 7, −16 ≤ k ≤ 16, −9 ≤ l≤9 internal residual Rint 0.049 software for structure determination and WinGX19 and SHELX9717 refinement extinction correction (SHELXL97) 0.0163(9) conventional residual R1b 0.018 (for all reflections) weighted residual wR2c 0.047 weighting scheme (SHELX97)d a = 0.0275; b = 0.3643

wR 2 =

x

Ueq =

Table 1. Crystallographic Data for U(PO4)Cl

c

site

a

The assignment of a Guinier photograph of U(PO4)Cl is provided (Table S2, Supporting Information). According to EDX analyses (scanning tunnel microscope, DMS 940, Zeiss) there is no detectable substitution of U4+ by Zr4+ in U(PO4)Cl. Evidence was found for the formation of U(PO4)Br from UP2O7 and ZrBr4 under similar conditions. Single-Crystal X-ray Study. Intensity data for U(PO4)Cl were collected using an area detector (κ-CCD, Enraf-Nonius). Structure determination and refinement were performed using the SHELX-9718 suite in the WinGX19 framework. Direct methods revealed the starting parameters for uranium, phosphorus, and chlorine. Subsequent ΔFourier syntheses allowed localization of the oxygen atoms. Eventually, an analytical absorption correction was applied to the measured data set using the program HABITUS.19 Information on crystal data, intensity measurement, and structure refinement is summarized in Table 1. Final atomic coordinates, isotropic displacement parameters,

− |F c ∥/∑|F o |.

atom

w = q/

2

+ 2Fc ). B

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Inorganic Chemistry Research Systems, Inc.). For measurements in the near IR (6600 to 16 000 cm−1) a liquid nitrogen-cooled Ge-photodiode detector (model 403, Applied Detector Corporation, Fresno, CA) was used. A photomultiplier (model PR-1400RF, Products for Research, Inc.) was used as detector in the UV−vis region (12 000 to 32 000 cm−1). The spectro-photometer is dedicated to the measurement of polarized absorption spectra of rather small single crystals having cross sections down to 0.1 mm. The electronic spectra (Figure 2) of weakly

2UP2O7 (s) + UCl4(g) ⇄ 4U(PO4 )Cl(s) + U(PO3)4 (s) (5)

Clearly, the driving force for reaction 4 is the optimization of Lewis acid−base interactions in terms of the HSAB principle.27 The formation of uranium metaphosphate8,9 in (5) is somewhat speculative, since it was never observed as equilibrium solid at the high temperatures of the transport experiments (Table S1). Uranium(IV)-metaphosphate can be obtained at 800 °C in air from uranyl(VI) nitrate and phosphoric acid as single-phase microcrystalline powder.16 However, even in sealed ampules U(PO3)4 showed at ϑ > 800 °C almost complete decomposition to solid UP2O7 and gaseous P4O10. This limited thermal stability is the prerequisite for the chemical vapor transport of UP2O7 by reaction 2. Furthermore, the expected amount of the metaphosphate would be rather small compared to that of the phosphate chloride, and its reflections in the XRPD might well be hidden. Despite the chemical and crystallographic similarities in the pairs U2O(PO4)26/Zr2O(PO4)2,45 UP2O77/ZrP2O7,46 and U(PO3)48,9/Zr(PO3)447 no hints on formation of “Zr(PO4)Cl” were observed. Crystal Structure. The crystal structure of U(PO4)Cl consists of polyhedra [UIVO6Cl2] and [PO4] tetrahedra (Figures 3 and 4). The coordination polyhedron around U4+

Figure 2. Polarized electronic absorption spectra of an (010) crystal face of U(PO4)Cl. “hpol” and “vpol” denote perpendicular polarization directions of the incident light beam. Angular overlap modeling (a) with calculated intensities, (b) all transitions. dichroic (emerald-green/yellowish-green) crystals were recorded at room temperature using a rectangular crystal with well-developed faces (010) and (01̅0) (d = 0.1 mm). Polarization directions of the incident light correspond to the crystallographic axes as vpol∥c-axis and hpol∥aaxis.

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RESULTS AND DISCUSSION Synthesis and Chemical Vapor Transport. Our experiments (Table S1) show that uranium(IV) diphosphate can be crystallized by chemical vapor transport14,25 in a temperature gradient due to endothermic reactions using various metal chlorides as transport agent. We suggest that heterogeneous equilibria like eq 2 are responsible for the observed transport effect. 2UP2O7 (s) + 4ZrCl4(g) ⇄ 2UCl4(g) + 4ZrOCl 2(g) + P4 O10 (g)

(2)

As has already been pointed out in literature there might be an additional contribution of volatile uranium oxide chlorides (e.g., UOCl2, UOCl3, UOCl4, UO2Cl2) to the transport process.26 On the one hand, for chemical vapor transport of UTaO5 with chlorine the presence of gaseous UO2Cl2 as uranium carrier has been proven by mass spectrometric measurements.16 On the other hand, using nonoxidizing chlorides as transport agents the presence of uranium(VI) in the gas phase appears to be unlikely. Chemically, a contribution of the homogeneous equilibrium (3), in combination with the heterogeneous equilibrium (2), is reasonable. UCl4(g) + ZrOCl 2(g) ⇄ UOCl 2(g) + ZrCl4(g)

Figure 3. Projection of the U(PO4)Cl structure along [100] (a) and along [001] (b) with schematic representation of the coordination polyhedra [UIVO6Cl2] gray and tetrahedra [PO4] light-gray. (○) Oxygen and (●) chlorine atoms. Vertices of the unit cell are marked by asterisks (software: ATOMS V. 6.335).

might be described as bicapped octahedron, with two atoms O2 showing significantly longer distances d(U−O) (Table 3). These oxygen atoms are located above two cis-faces of the octahedron and originate from one phosphate group. Thus, an edge is shared between the [UIVO6Cl2] unit and one [PO4] tetrahedron (Figure 3). The uranophane structure motif31 is present here. The observed distances d(U−O) and d(U−Cl) compare well to those found for other compounds containing tetravalent uranium (e.g., U(PO3)4: 2.29 ≤ d(U−O) ≤ 2.42

(3)

Formation of uranium(IV) phosphate chloride from UP2O7 and zirconium tetrachloride can be understood in terms of metathesis reaction 4 followed by reaction 5. UP2O7 (s) + ZrCl4(g) ⇄ UCl4(g) + 4ZrP2O7 (s)

(4) C

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Figure 5. Splitting of the ground-state 3H4 with assignment of the irreducible representations to the split terms.

ions in UPO4Cl atoms exhibit eightfold coordination by six oxygen and two chlorine atoms. UV−vis absorption spectra and magnetic susceptibilities of UPO4Cl reveal significant ligandfield splitting for the parental terms of the U4+ ions. These ligand-field effects within the [UIVO6Cl2] chromophore can be rationalized by calculations within the AOM framework using the newly developed computer program BonnMag.40,41 The calculated splitting of the 3H4 ground state, which determines the temperature-dependent magnetic susceptibilities, as well as the energies of the excited states observed in the optical spectra chromophore, are in good agreement with the experimental data. Our study on the [UIVO6Cl2] chromophore in UPO4Cl may serve as example for further ligand-field analyses of 5f n systems within the AOM. The computer program BonnMag allows to perform these calculations rather easily and fast for all n (1 to 13). A detailed account of angular overlap modeling for UPO4Cl is given in the subsequent article (next paper in this journals issue).

Figure 4. ORTEP representation of the coordination polyhedron [UIVO6Cl2] and adjacent phosphate groups; ellipsoids are given at the 95% probability level (software: ATOMS V. 6.335).

Å;28 U(P4O12): 2.20 ≤ d(U−O) ≤ 2.50 Å;29 UP2O7: d(U−O) = 2.24 Å;30 U2O(PO4)2: 2.08 ≤ d(U−O) ≤ 2.56 Å;32 UCl4: 2.61 ≤ d(U−Cl) ≤ 2.94 Å;32 UOCl2: 2.17 ≤ d(U−O) ≤ 2.40 Å and 2.66 ≤ d(U−Cl) ≤ 3.15 Å;33 U(PO4)Cl·2 H2O: 2.17 ≤ d(U−O) ≤ 2.55 Å and d(U−Cl) = 2.84 Å34). Electronic Structure of the U4+ Cation. For a better understanding of the electronic structure of U4+ in UPO4Cl angular overlap modeling36−39 (AOM) was performed. The calculated transition energies and estimated absorption coefficients (Judd−Ofelt theory40,41) comply very well with the observed absorption spectra (see Figure 2). Slater− Condon−Shortley parameters, the AOM parameters eσ and eπ for all ligands in the [UO6Cl2] chromophore as well as the Judd−Ofelt parameters for the calculation of the relative absorption coefficients are summarized in Table 4. A discussion

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Table 4. AOM Parameters for the [UO6Cl2] Chromophore in UPO4Cl Slater−Condon−Shortley parameters (cm−1) F2 = 190.9 F4 = 33.74 F6 = 3.996 75 spin−orbit coupling constant (cm−1) ζ = 1797.026 AOM parameters ek(U−O), ek(U−Cl) (k: σ,π) ligand distance (Å) eσ (cm−1) O1 2.207 2052 O2 2.296 1556 O2 2.579 690 Cl 2.804 1270 Judd−Ofelt parameters (1 × 10−24 m2)44 Ω2 = 1.078 Ω4 = 2.014 Ω6 = 0.4529

eπ (cm−1) 513 389 173 318

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00438. Crystallographic data were deposited with Fachinformationszentrum Karlsruhe, Abt. IDNT, D-76344 Eggenstein-Leopoldshafen (e-mail: crysdata@fiz-karlsruhe.de) and can be obtained by contacting FIZ (Reference No. CSD430866). X-ray crystallographic data (CIF) Tabulated data. Synthesis and chemical vapor transport of U(PO4)Cl and UP2O7. Assignment of the XPRD pattern of U(PO4)Cl. Anisotropic displacement parameters for U(PO4)Cl (PDF)

eπ/eσ 0.25 0.25 0.25 0.25

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. (R.G.)

of the modeling procedure, a brief description of the computer program BonnMag42,43 used for the calculations, and of the significance and chemical meaning of the derived AOM parameters is given in the immediately following article of this issue. The splitting of the ground-state 3H4 of the U4+ ion in UPO4Cl, as obtained by the AOM, is shown in Figure 5.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. † In memoriam T. D. who passed away too early.

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CONCLUSIONS Emerald-green single crystals of uranium(IV) phosphate chloride UPO4Cl have been obtained using chemical vapor transport reactions with the unusual transport agent ZrCl4. U4+

ACKNOWLEDGMENTS We gratefully acknowledge support with crystal growth and structure determination by Dr. A. Kostencki (Univ. of Gießen) D

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as well as preliminary AOM calculations by Dr. A. Rohde (Univ. of Hannover).

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DOI: 10.1021/acs.inorgchem.6b00438 Inorg. Chem. XXXX, XXX, XXX−XXX