Attempts to synthesize Pr4Mo7O27 using Pr, Pr6O11 and MoO3 in a molar ratio of 8:6:77 led to a main product of scheelite-type Pr0.667[MoO4] and few single crystals of the triclinic A-type Pr6Mo10O39. The latter crystallizes in space group P1̅ (a=945.25(1), b=1058.49(2), c=1815.16(3) pm; α=104.149(1), β=95.220(1), γ=102.617(1)°, Z=2). Its crystal structure comprises six crystallographically independent Pr3+ cations, eight tetrahedral [MoO4]2− units, and one [Mo2O7]2− entity. The cations display coordination numbers of seven (1×) and eight (5×), while the [MoO4]2− tetrahedra are surrounded by five Pr3+ cations each. The [Mo2O7]2− anions exhibit a coordination environment of seven Pr3+ cations. The attempt to synthesize PrF[MoO4] using PrOF (from in situ thermal decomposition of PrF[CO3]) as reagent did not lead to the desired product but to monoclinic B-type Pr6Mo10O39. This slightly less dense modification compared to its triclinic analogue crystallizes in space group C2/c (a=1247.93(3), b=1989.68(6), c=1392.52 (4) pm, β=100.505(2)°, Z=4) with three crystallographically independent Pr3+ cations, four [MoO4]2− tetrahedra, and again one [Mo2O7]2− unit in the crystal structure. Thus, both Pr6Mo10O39 modifications are better described with the structured formula Pr6[MoO4]8[Mo2O7]. The coordination numbers around the Pr3+ cations are seven (1×) and eight (2×) while all four [MoO4]2− anions are again surrounded by five Pr3+ cations each. Six of the latter represent the coordination environment around the [Mo2O7]2− entities. Besides the thorough comparison of the crystal structures single crystal Raman spectra were recorded for both Pr6Mo10O39 phases.
Oxidomolybdates(VI) of the trivalent lanthanides are best classified by the molar ratio of the lanthanide sesquioxide Ln2O3 (Ln=La–Nd, Sm–Lu) to molybdenum trioxide MoO3. The combination of one equivalent Ln2O3 with three equivalents MoO3 results in the most common composition Ln2Mo3O12, which can be attributed to the structured formula Ln2[MoO4]3 and regarded as “sesquimolybdates” of the lanthanides. Depending on the size of the Ln3+ cations several crystal structures are known in the literature, that are either derivatives of the scheelite structure , , , , , , ,  or defect variants of the scheelite type Ca[WO4]  itself according to Ln0.667[MoO4] , , , . While any composition containing less MoO3 than three times the molar amount of Ln2O3 consists of non-molybdenum bonded oxide anions, compositions with a higher content contain both isolated and condensed oxidomolybdate anions. Besides several constitutions with integer molar Ln2O3: MoO3 ratios, such as 1:4 and 1:5 with formulae according to Ln2Mo4O15 (≡Ln2[MoO4]2[Mo2O7]) , , , , , , , ,  and Ln2Mo5O18 (≡Ln2[MoO4][Mo2O7]2) , respectively, two compositions have been identified that can be described with fractured ratios. Two equivalents Ln2O3 and seven equivalents MoO3 (molar ratio 1:3.5=2:7) result in the composition Ln4Mo7O27, which is known in the literature in three different structure types , , ,  and not all of them consist of isolated ortho- and dimolybdate entities, but also of more highly condensed units with coordination numbers of five around the Mo6+ cations. For the molar ratio Ln2O3:MoO3 of 3:10 (=1:3⅓) two structure types have so far been reported, a triclinic one for the representatives bearing large Ln3+ cations (Ce6Mo10O39 ) and a monoclinic C-centered one for the derivatives with the smaller Ln3+ cations (Ln6Mo10O39, Ln=Nd , Eu , Gd ). Thus, the title compounds are on the segue between the two structure types and two of the rare examples of a dimorphic composition within the family of lanthanide oxidomolybdates.
2 Experimental section
In an attempt to synthesize Pr4Mo7O27 (isotypic to La4Mo7O27  or Ce4Mo7O27 ), elemental praseodymium (Pr: 99.9%; ChemPur, Karlsruhe Germany), hexapraseodymium undecaoxide (Pr6O11: 99.9%; ChemPur, Karlsruhe, Germany) and molybdenum trioxide (MoO3: p. a.; Merck, Darmstadt, Germany) were mixed in 8:6:77 molar ratio according to eq. (1):
After tempering the reagents in evacuated silica ampoules for 6 days at 800°C the desired products were not obtained, but a mixture of scheelite-type Pr0.667[MoO4]  and A-type Pr6Mo10O39 was found, according to eqs. (2) and (3):
The few, coarse single crystals of the triclinic Pr6Mo10O39 modification are stable in air and water and show the typical green color of Pr3+ compounds .
The monoclinic B-type modification of Pr6Mo10O39 emerged also as a by-product from solid-state reactions to form fluoride derivatives of praseodymium(III) oxidomolybdates(VI) with the formula PrF[MoO4] . For this reaction, active PrOF was generated in situ by thermal decomposition of bastnaesite-type PrF[CO3] according to Janka and Schleid . Tempering the aforementioned PrF[CO3] with equimolar amounts of molybdenum trioxide in open silica crucibles in two steps (2 h at 600°C for allowing the praseodymium fluoride carbonate to decompose according to eq. (4):
and another 6 days at 800°C expected to yield PrF[MoO4] did not result in the desired products, but in a mixture of Pr2Mo4O15 , monoclinic B-type Pr6Mo10O39 and several other unidentified by-products . As for the triclinic modification, its crystals are also stable to environmental influences, exhibit the typical Pr3+-green color and are coarse in appearance.
2.2 Crystal structure determination
Intensity data sets for both, the triclinic A- and the monoclinic B-type representatives of Pr6Mo10O39 were collected on a Nonius Kappa-CCD diffractometer (Delft, the Netherlands) using graphite-monochromatized MoKα radiation (wavelength λ=71.07 pm). Numerical absorption corrections were performed with the program Habitus  in each case. The crystal structure solutions and refinements were carried out utilizing the program package Shelx-2013 . Details of the data collections and structure refinements  are summarized in Table 1 and selected interatomic distances as well as bond angles are given in Tables 2–5 .
|Compound||A-type Pr6Mo10O39||B-type Pr6Mo10O39|
|Formula units, Z||2||4|
|Calculated density, Dx/g·cm−3||4.76||4.75|
|Molar volume, Vm/cm3·mol−1||510.32||511.83|
|Index range, ±h/±k/±l||12/14/24||16/26/18|
|Theta range, θmin−θmax/deg||2.05−28.24||1.95−28.23|
|Absorption coefficient, μ/mm−1||12.1||12.1|
|Data corrections||Background, polarization and Lorentz factors, numerical absorption correction with the program Habitus |
|Collected/unique reflections||59 943/8359||35 433/4209|
|Structure solution and refinement||Program package Shelx-2013 , scattering factors according to International Tables, Vol. C |
|R1 for (n) reflections with |Fo |>4σ(Fo)||0.029(6915)||0.025(3672)|
|R1/wR2 for all reflections||0.042/0.067||0.033/0.049|
|Goodness of fit, S||1.029||1.041|
|Residual electron density, ρ/e−·10−6 pm−3, min./max.||1.26/−1.74||0.99/−0.86|
|Distances||d, pm||Distances||d, pm||Distances||d, pm||Distances||d, pm|
|Angles||∢, deg||Angles||∢, deg||Angles||∢, deg||Angles||∢, deg|
|Distances||d, pm||Distances||d, pm||Distances||d, pm|
|Angles||∢, deg||Angles||∢, deg||Angles||∢, deg||Angles||∢, deg|
Further details of the crystal structure investigations may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: firstname.lastname@example.org) on quoting the deposition numbers CSD-432006 (triclinic A-type of Pr6Mo10O39) and CSD-432007 (monoclinic B-type).
2.3 Single crystal Raman spectroscopy
Raman spectroscopy for single crystals of both, the triclinic A- and the monoclinic B-type representatives of Pr6Mo10O39 was performed with the help of a Horiba XploRa spectrometer (Kyoto, Japan) using a LASER device with a wavelength of λ=638 nm for the irradiation of the crystals.
3 Results and discussion
3.1 Crystal structure
3.1.1 The triclinic structure of A-type Pr6Mo10O39
The triclinic modification of Pr6Mo10O39 crystallizes in space group P1̅ (a=945.25(1), b=1058.49(2), c=1815.16(3) pm; α=104.149(1), β=95.220(1), γ=102.617(1)°) with two formula units per unit cell and is thus isotypic to the cerium representative Ce6Mo10O39 . As this is both the denser phase as well as the one with the lighter lanthanides, it is henceforth named the A-type. All atoms in the unit cell are situated at Wyckoff positions 2i, therefore, the unit cell comprises as many crystallographic independent atoms as there are in the sum formula. The six Pr3+ cations are surrounded by seven (for Pr1) and eight (for Pr2–5) oxide anions, respectively, exhibiting rather irregular coordination polyhedra with shapes of mono- and bicapped trigonal prisms as well as square antiprisms (Fig. 1).
The Pr–O interatomic distances range between 233 and 275 pm (Table 2) and therefore correspond very well with those in A-type Pr2O3, which show values between 234 and 267 pm . According to the structure formula Pr6[MoO4]8[Mo2O7] eight ortho-oxidomolybdate units are found in the unit cell with a cationic surrounding of five Pr3+ atoms around each tetrahedron. The Mo–O bond lengths are in the range of 173–182 pm (Table 2), which are in good agreement to those in lanthanum(III) sesquimolybdate(VI) La2[MoO4]3 (d(Mo–O)=173–182 pm ). The O–Mo–O angles in these tetrahedra differ with values from 104 to 118° (Table 3) a maximum of 8% from the ideal tetrahedral angle of 109.47° (Table 3). The [Mo2O7]2− anions exhibit staggered conformation with a Mo9–O36–Mo10 angle of about 162° (Fig. 2, bottom) and a coordination environment of seven praseodymium cations (Fig. 2, top). The bond lengths within both tetrahedra lie between 171 and 174 pm while the ones to the bridging (O36)2− atoms show values of 185–187 pm (Table 2), which is typical for compounds bearing [Mo2O7]2− anions (e.g. Mg[Mo2O7]: d(Mo–Obr)=185–188 pm , br=bridging). The O–Mo–O angles within the dimolybdate anions (105–116°) are again close to the one found in ideal tetrahedral environments.
Both the ortho- and the dimolybdate entities are loosely distributed in a layer along the crystallographic ac plane in the lower half of the unit cell (centering around y/b≈¼), and in the upper half (centering around y/b≈¾) the same build-up is found inversely (Fig. 3). The Pr3+ cations are found between the two aforementioned sheets of oxidomolybdate anions at y/b≈0 and ½.
3.1.2 The monoclinic structure of B-type Pr6Mo10O39
The monoclinic modification of Pr6Mo10O39 crystallizes in space group C2/c (a=1247.93(3), b=1989.68(6), c=1392.52(4) pm, β=100.505(2)°, Z=4) isotypic to Nd6Mo10O39 , Eu6Mo10O39 , and Gd6Mo10O39 . Its crystal structure is slightly less dense than the previously described triclinic phase and henceforth referred to as B-type. Except for (O20)2−, all atoms are situated at the general Wyckoff positions 8f, therefore three crystallographically distinguishable praseodymium cations are found in the crystal structure, which show coordination numbers of seven (Pr1) and eight (Pr2 and Pr3) oxide anions, again building up irregular polyhedra. While those around (Pr1)3+ and (Pr3)3+ can be regarded as distorted mono- and bicapped trigonal prisms, respectively, the coordination environment around (Pr2)3+ is best described as distorted pentagonal bipyramidal with an additional, second vertex below the pentagonal plane (Fig. 4).
The interatomic distances within these polyhedra range between 228 and 263 pm (Table 4) which correspond very well with those in the A-type of the title compound, as well as in A-type Pr2O3 . Even a distance as short as 228 pm is found in several praseodymium compounds (e.g. G-type Pr2[Si2O7]: dmin(Pr–O)=227 pm ). Furthermore, the crystal structure comprises four crystallographically independent [MoO4]2− tetrahedra with a cationic surrounding of five Pr3+ around each of these units. The Mo–O bond lengths lie between 173 and 181 pm, which represents the same range as found in the triclinic phase of Pr6Mo10O39. The O–Mo–O angles differ by about ±6% from the ideal value of 109.47° in tetrahedral units, ranging between 103 and 115° (Table 5). The [Mo2O7]2− entities exhibit the bridging O20 atom at Wyckoff position 4e (symmetry 2), a skew conformation (Fig. 5, bottom) with a Mo5–O20–Mo5 bridging angle of 146°, and a cation coordination of six Pr3+ cations (Fig. 5, top). Typically for these units, the Mo–O bond lengths to the terminal oxide anions remain rather short (172–173 pm, Table 4), while those to the bridging ones are as large as 187 pm (Table 4) as already seen in A-type Pr6Mo10O39. The same applies for the angles within each tetrahedron of the dimolybdate unit with values ranging from 107 to 115° (Table 5).
The Pr3+ cations, the [MoO4]2−, and the [Mo2O7]2− anions are arranged layer-like parallel to the ab plane in the crystal structure of the monoclinic B-type modification of Pr6Mo10O39 (Fig. 6).
3.1.3 Structural comparison of the triclinic A-type and the monoclinic B-type of Pr6Mo10O39
Both structure types exhibit several similarities, such as comparable coordination environments around the praseodymium cations and the isolated [MoO4]2− anions. However, the ratio of seven-fold to eight-fold coordinated Pr3+ cations is 1:2 in the monoclinic and 1:5 in the triclinic phase. The differences between the two structures increase as the [Mo2O7]2− entities are regarded. The cation coordination numbers decrease from seven in the triclinic A-type to six in the monoclinic B-type. Therefore, together with the higher ratio of eight-fold coordinated Pr3+ it is quite comprehensible that the triclinic phase has to be the denser one, if only by about 3%. Furthermore, the conformation of the aforementioned dimolybdate units differ from staggered with an almost linear Mo–O–Mo bridging angle of 163° to skew with a much smaller angle of 146°. All these distinctions substantially reduce the possibility to find a correlation between the two structures via a group-subgroup relationship. The best comparison is achieved by contemplating the crystal structure of the triclinic A-type along  (Fig. 7) and the monoclinic B-type along  (Fig. 8).
These views allow both structures to be described with a layer-like arrangement, however different to the ones in Figs. 3 and 6. In triclinic A-type Pr6Mo10O39 half of the crystallographically distinguishable [MoO4]2− tetrahedra are arranged together with the [Mo2O7]2− anions in sheets A around x/a=0, while the other four [MoO4]2− anions are also set up in sheets B, however, around x/a=½. The overall stacking of these layers can be considered as AB. The Pr3+ cations are distributed between these anions to fill voids and provide for electroneutrality (Fig. 7). A similar arrangement is found in the crystal structure of monoclinic B-type Pr6Mo10O39 if viewed along . The [Mo2O7]2− containing layers A are set up around y/b=¼ and ¾ while the others B are situated at y/b=0 and ½. Hence, a layer stacking according to B AB′ A′ is found with A′ being the respective inverted layer and B′ shifted by ½ along  (Fig. 8). Again, the praseodymium cations are distributed in the voids between the anions to ensure electroneutrality.
3.2 Raman spectroscopy
Single crystal Raman spectra were recorded for both modifications, the triclinic A-type (Fig. 9, bottom) and the monoclinic B-type of Pr6Mo10O39 (Fig. 9, top). As expected, the spectrum for the less symmetric triclinic modification shows more bands than the more symmetric monoclinic one. The area of symmetric and asymmetric stretching vibrations within both the [MoO4]2− and the [Mo2O7]2− units is located roughly between 800 and 950 cm−1, which is in good agreement with other compounds containing the respective complex entities . The asymmetric stretching vibration of the Mo–O–Mo bridge is usually found between 700 and 800 cm−1, while the symmetric one resides at around 560 cm−1 . The latter is rather weakly represented in the spectrum of the A-type and almost undetectable in the spectrum of the B-type modification. The area between 250 and 450 cm−1 can be attributed to both, bending vibrations of the complex oxidomolybdate(VI) entities and to stretching vibrations within the oxide polyhedra surrounding the Pr3+ cations, which renders a distinctive assignment of the bands seen in both spectra virtually impossible.
Crystals of Pr6Mo10O39 have proven to be dimorphic, exhibiting a triclinic A-type phase, which is slightly denser than the monoclinic B-type modification. Although both derivatives can be ascribed according to the same structure formula Pr6[MoO4]8[Mo2O7], a group-subgroup relationship was not found. This fact is mainly due to significant differences in the conformation of the [Mo2O7]2− units (staggered with a 163° Mo–O–Mo angle in the A-type and skew with a 146° bridging angle in the B-type phase) as well as to the different, however, always layer-like arrangement of the building blocks in the crystal structure of both modifications. The single crystal Raman spectra of both phases were collected and the bands of both spectra were assigned to the respective vibrations.
The authors are indebted to Prof. Dr. Thomas Schleid for the opportunity to conduct experiments as well as spectroscopic and diffraction measurements in his laboratory at the Institute for Inorganic Chemistry at the University of Stuttgart, Germany. We also thank Dipl.-Chem. Adrian Geyer for collecting the single crystal Raman spectra with the spectrometer of Prof. Dr. Hans-Joachim Massonne (Institute for Mineralogy and Crystal Chemistry, University of Stuttgart, Germany). Finally the financial support of the state of Baden-Württemberg (Stuttgart, Germany) is gratefully acknowledged.
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