New data on rubidium vanadium phosphate

International Journal of Inorganic Materials 3 (2001) 9–15

New data on rubidium vanadium phosphate Structure determination of the disordered (V V –P V ) compound Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) with x|0.6 E. Le Fur, B. de Villars, J. Tortelier, J.Y. Pivan* ´ ´ ´ de Chimie de Rennes, Campus de Beaulieu, Avenue du General Leclerc, 35700 Rennes, Laboratoire de Physicochimie, Ecole Nationale Superieure France Received 3 July 2000; accepted 3 August 2000

Abstract Brownish crystals of Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) were prepared hydrothermally. The structure was solved from single ˚ b57.1407(2) A, ˚ crystal X-ray diffraction data in the non centrosymmetric orthorhombic space group Pmn2 1 (No. 31) a513.5505(2) A, ˚ (R 1 (Fo)50.048, wR 2 (Fo 2 )50.111). The structure of Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) is closely related to the c514.7040(2) A. previously reported M 3 (V2 O 3 )(VO)(PO 4 ) 2 (HPO 4 ) (M5K 1 , Tl 1 ) with a disordered occupation of the tetrahedral voids by V V and P V atoms.  2001 Published by Elsevier Science Ltd. Keywords: Hydrothermal synthesis; D. Crystal structure; Rubidium vanadium phosphates

1. Introduction The vanadium phosphate systems have been extensively revisited since the vanadyl pyrophosphate (VO) 2 P2 O 7 system was referred to as the most efficient catalyst for the ¨ anhydride from light hydrocarbons [1]. synthesis of maleıc As a result, the VPOs crystal chemistry has grown richer in recent years presenting more or less complex frameworks, open or dense, depending on the synthetic conditions and the nature of the precursors introduced into the reaction medium. Nowadays, most research teams which are working on this system agree that the mechanisms involved during hydrothermal syntheses are not well understood [2]. In relation to the large number of reaction parameters into the heterogeneous reaction medium, the attainable vanadium oxidation states (V III , V IV and V V ) and the particular properties of phosphate species which can be present at the 22 same time as PO 32 and H 2 PO 42 in the solid state, 4 , HPO 4 an exact control of the reaction pathway remains illusive in many cases. As already pointed out by Amoros et al. [2], ‘‘the black box nature inherent in the hydrothermal *Corresponding author. E-mail address: [email protected] (J.Y. Pivan).

reactor’’ prevents in situ analyses being performed that are required to get relevant dynamic data for a better understanding of the processes involved during the hydrothermal syntheses. So, while the crystal chemistry of VPOs grows richer (a few single crystals are sufficient to extract a structural model), the physical properties have been investigated far less because of the large amounts of clean products that are required (|100 mg) which are difficult to obtain in a reproducible way. Special efforts have been devoted to rationalise the synthesis of VPOs in order to attain reproducibility with good yields [3,4]; nevertheless, a great number of unanswered questions still remains. In the course of our investigations on the Rb–V–P–O system, we were able to isolate as a pure phase a new mixed (V IV –V V ) compound Rb |5 (VO) |10 (PO 4 ) 4 (HPO 4 ) 8 [5,6]. Subsequent syntheses to obtain single crystals of a quality suitable for structure determination have resulted in producing a- and b-RbV(HPO 4 ) 2 , as minor components of supplementary RbVPO phases, as previously reported by Haushalter et al. [7], and the new (V IV –V V ) Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ). The present paper deals with the synthesis and the crystal structure determination of Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ). Our results are dis-

1466-6049 / 01 / $ – see front matter  2001 Published by Elsevier Science Ltd. PII: S1466-6049( 00 )00100-8

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E. Le Fur et al. / International Journal of Inorganic Materials 3 (2001) 9 – 15

cussed and compared to the related phases M 3 (V2 O 3 ) 2 (VO)(PO 4 )(HPO 4 ) (M5K 1 , Tl 1 ) that were previously reported by Huan [8] and Vaughey [9].

2. Experimental section

2.1. Synthesis Mixtures of V2 O 5 (0.1535 g), Rb 2 CO 3 (0.500 g), 85% H 3 PO 4 (1 ml), tetraethylammonium chloride (1 g) and water (|3.6 ml) were introduced together with reducing Zn 0 (0.10 g) in a Teflon acid digestion bomb (23 ml) then heated at 2008C for 48 h. The final brownish product was filtered off and dried in air. Prior to subsequent analyses, the reaction product was checked by visual examination under the microscope and appeared as intergrown brownish hexagonal shaped single crystals. These crystals were crushed to powder and analysed by X-ray diffraction using ˚ The resulting product was pure and l CuK a 51.54178 A). identified as Rb |2 (VO) |3 (HPO 4 ) |4 [5,6] and the X-ray powder pattern could be accounted on the basis of an hexagonal unit-cell by using the autoindexing program ˚ c|58.75 A. ˚ As Dicvol [10] with parameters a|12.24 A, the few single crystals tested by means of X-ray diffraction using a Kappa CCD diffractometer were not of sufficient quality to undertake structural determination, subsequent syntheses were tested to improve the quality of the crystals. The only parameters to be changed were the reaction time (24 h,t,7 days) and the temperature (180, T ,2458C) that led to supplementary Rb–VPO phases. So, V 31 containing phases such as a-RbV(HPO 4 ) 2 and bRbV(HPO 4 ) 2 , as previously reported by Haushalter [7], were obtained as roughly 50:50 mixtures after hydrothermal treatments at 2458C for 7 days. Other experiments (24 h,reaction time,48 h) led to mixtures of less reduced Rb–VPO solids namely the hexagonal phase Rb |2 (VO) |3 (HPO 4 ) |4 [5,6] and Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) in variable relative amounts depending on the conditions of the syntheses. The latter compound was obtained as few platelet single crystals with its colour varying from brownish to deep greenish. Subsequent single crystal X-ray investigation showed that these crystals corresponded to a new phase and on the basis of structural determination (vide infra), the subtle differences in colour could be ascribed to a variable protonation rate of the pyrophosphate group.

3. Single-crystal X-ray diffraction and structure determination The crystal structure of Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) was solved from single-crystal X-ray diffraction data. A well-shaped brownish platelet crystal with approximate dimensions of 0.253

0.2030.02 mm 3 was mounted on a glass fibre for intensity data collection on an Enraf-Nonius Kappa CCD diffrac´ tometer (Centre de Diffractometrie, Universite´ de Rennes). The experiment was carried out at room temperature using a graphite monochromated Mo Ka radiation ( l50.71073 ˚ The Collect program [11] was used to optimise the A). goniometer and detector angular settings during the intensity data collection which was performed in the v–f scanning mode. The unit cell and the orientation matrix were refined using the entire data set of 52 666 reflections collected within the range 1–358. A total of 463 frames were collected for a total exposure of 13.8 h. Reflection indexing, Lorentz-polarization and peak integration were performed using the Denzo program [11]. The data set was corrected using the Scalepack program [11]. The crystallographic data, conditions for intensity data collection, structure solution and refinement are given in Table 1. The systematic absences on the h0l reflections (h1l52n11) gave the Pmn2 1 or non-conventional Pmnm as possible space groups. The crystal structure of Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) was solved in the non-centrosymmetric space group using the direct-methods program SHELXL-86 [12] and the refinements were performed against F 2 using the program SHELXL-97 [13]. The entire structure model was easily obtained from direct methods and subsequent different Fourier syntheses. After the positional coordinates and isotropic then anisotropic displacement parameters of the non-hydrogen atoms were refined, the refinement converged at R 1 (Fo)|0.051 (wR 2 (Fo 2 )|0.127). At this stage of refinement, the overall topology of the structure was fully identified but the atomic displacement parameters of one phosphorus atom ˚ 2 ) which indicated were abnormally low (Ueq|0.007(5) A that the electronic contribution on the position was underestimated. On the other hand, the corresponding bond distances towards oxygen atoms were too long for ideal P–O bonds. To account for these anomalies, an occupancy factor of the position was added as additional variable parameter which converged on t 5145.8%(1). The residuals were slightly improved (R 1 (Fo)|0.048; wR 2 (Fo 2 )| 0.112) and the thermal factor was then correct [Ueq|0.018 ˚ 2 ]. In relation to the significant lengthening of the (6) A bondings and the obtained occupancy factor, (V V –P V ) substitution was considered on the position which was tested in subsequent refinements and resulted in a V/ P5 0.58(1) / 0.42(1) ratio. The final reliability factors R 1 (Fo) and wR 2 (Fo 2 ) defined in [13] were 0.048 and 0.111, respectively, for 3284 unique reflections and 244 variable parameters. The positional coordinates and atomic displacement parameters are listed in Table 2 and the bond distances and angles are collected in Table 3.

4. Structure results and discussion The structure of Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) is shown in Fig. 1. It consists of vanadium polyhedra

E. Le Fur et al. / International Journal of Inorganic Materials 3 (2001) 9 – 15

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Table 1 Crystal data for Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) Crystal data Empirical formula Color; habit Crystal system Space group ˚ Unit cell dimensions (A)

˚ ) Volume (A Z Formula weight (g mol 21 ) Density (calc.) (g cm 23 )

Rb 12 (V2 O 3 ) 4 (VO) 4 (PO 4 ) 8 (H 2 P3 VO 14 ) Brownish; plate-like Orthorhombic Pmn2 1 a513.5505(2) b57.1404(2) c514.4840(4) V51401.2(2) 1 3000.6 3.56

Data collection Crystal size (mm 3 ) Absorption coefficient (Mo Ka) (mm 21 )) T min /T max Absorption correction Radiation Maximum 2u Data collected

0.2530.230.02 12.74 0.588 / 0.856 Gaussian method ˚ MoKa l50.71073 A 2u # 708 h: 0,121; k: 0,111; l: 223,0

Structure solution and refinement Total data collected Unique data after merging Unique data Free parameters Data / parameters ratio ˚ 3) Min., max. (e / A a Residuals GooF

52666 3284 2671 (F 2 .2.0s (F 2 )) 244 10.95 21.39, 12.99 R 1 50.048, wR 2 50.11 1.043

3

a

R 1 5 oiF0 u 2 uFc i / ouF0 u; wR 2 5 [ow(uF0 u 2 2 uFc u 2 ) / owuF0 u 2 ] 1 / 2 with w 5 1 / [s 2 (F 02 1 (0.0443P)2 1 7.86P]; P 5 (max(F 20 , 0) 1 2F 2c ) / 3.

commonly found in the vanadium phosphates chemistry. The first polyhedron is a distorted octahedron with the vanadium atom slightly off-centred towards one apex (V2 and V3 atoms) leading to a short O=V distance (d O=V |1.60 ˚ a longer trans O- - -V bond (d O - - -V |2.25 A) ˚ and four A), ˚ The oxygen atoms involved basal distances around 2.0 A. in the O=V and O- - -V bondings (namely O1 and O8) are shared through [ . . . O=V- - -O=V- - -O=V . . . ] chains that run along the [010] direction. The V=O- - -V bond angle is almost constant along the chain [V2–O8–V3, a 5 134.38(2) and V2–O1–V3, a 5136.25(2)]. The basal oxygen atoms around V2 and V3 (O2, O3, O4 and O10) are linked with adjacent orthophosphate groups which are alternated up and down with respect to the [ . . . O=V- - O=V- - -O=V . . . ] chain as can be seen in Fig. 2. These orthophosphate groups are quite regular with P–O bond ˚ and O–P–O distances in the range of 1.525(7)–1.556(6) A bond angles varying between 105.48 and 111.68. Each phosphorus atom shares two oxygen atoms with two adjacent [O=VO 4 - - -O] octahedra within the preceding chain and two additional [O=VO 4 ] square pyramids so that the phosphate units connect together four vanadium polyhedra resulting in an overall 3D VPO framework. The links between the PO 4 and the VO 5 are achieved via O5, O6, O7 and O9 bridges. The V1 and V4 atoms occupy these quite regular pyramidal sites with four basal V–O

˚ ˚ bonds [1.873(4) A#d basalV– O #2.015(7) A] and a shorter ˚ ˚ The oxyO=V distance [1.600(7) A#d #1.610(7) A]. O5V gen O16 and O18 around V1 and V4 are those involved in the terminal O=V bonds. Of the remaining basal oxygen, two are bonded to the orthophosphate groups described above, one is part of homonuclear V–O–V bridges (O13 for V1 [V1–O13–V1 bridge]; O17 for V4 [V4–O17–V4 bridge]) leading to [V2 O 3 ] dimeric units while the last one is linked to a somewhat unusual pyrophosphate-like unit as it is shown in Fig. 3. As already discussed in the preceding section, a partial (V V –P V ) substitution was considered for one of the tetrahedral sites which are part of the pyrophosphate block to account for significant lengthening of the bond distances within the site. The resulting pseudo pyrophosphate described as [P1.42 V0.58 O 7 ] is, for the sake of simplicity, hereafter named as a pyrophosphate group. Such a partial (V V –P V ) substitution in a pyrophosphate unit was already recently reported by Ninclaus et al. [14] for (NH 4 ) 2 VO(V22x Px O 7 ) which was hydrothermally prepared under reaction conditions very close to those given in the experimental section. The two sites are very different in size depending on the (V V –P V ) substitution. The first site, which contains the V V species, consists of ˚ to 1.769(9) A ˚ with an distances ranging from 1.571(11) A ˚ These bond distances average value of d av. 51.657(9) A. agree fairly well with similar distances reported for the

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Table 2 Atomic coordinates and equivalent isotropic displacement coefficients ˚ 2 3100) with their standard deviation in brackets Ueq. (A Atoms

x

y

z

Ueq.

Rb1 Rb2 Rb3 Rb4 V1 V2 V3 V4 P1 P2 (P,V )a P4 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14 O15 O16 O17 O18 O19

0.0 0.25379(8) 0.50 0.26051(9) 0.1296(1) 0.0 0.0 0.3720(1) 0.1492(2)) 0.0 0.50 0.1482(2) 0.0 0.1050(4) 0.1063(5) 0.1013(5) 0.1332(5) 0.2602(5) 0.3705(4) 0.0 0.2395(5) 0.1022(5) 0.0 0.5 0.0 0.3988(5) 0.4038(5) 0.3152(5) 0.5 0.1861(5) 0.5

0.8701(2) 0.6597(1) 0.6391(2) 0.1485(1) 0.4050(2) 0.4935(2) 0.9933(3) 0.0993(2) 0.2110(3) 0.1075(4) 0.4796(3) 0.7085(3) 0.2053(10) 0.0329(8) 0.5312(8) 0.3869(8) 0.1992(9) 0.2228(9) 0.3023(9) 0.7071(12) 0.2766(8) 0.8861(8) 0.1447(18) 0.3415(12) 0.4595(15) 0.4723(10) 0.9946(10) 0.9290(9) 0.0367(12) 0.5750(9) 0.6969(12)

0.2632(1) 0.10078(7) 0.2733(1) 0.92722(7) 0.3395(1) 0.0246(1) 0.0053(2) 0.1867(1) 0.1452(1) 0.5289(2) 0.0243(1) 0.8834(1) 0.9651(7) 0.1004(5) 0.9283(5) 0.1047(6) 0.2492(5) 0.1240(4) 0.2782(4) 0.0671(7) 0.4047(5) 0.9240(5) 0.4280(8) 0.1087(8) 0.3033(8) 0.3033(8) 0.0628(5) 0.2350(5) 0.2266(7) 0.2891(6) 0.0831(6)

2.89(3) 2.94(2) 2.78(3) 3.09(2) 1.65(3) 1.23(4) 1.23(4) 1.62(3) 1.23(4) 1.99(5) 1.67(5) 1.18(4) 1.8(2) 1.7(1) 1.7(1) 2.0(1) 1.8(1) 1.8(1) 1.6(1) 1.5(1) 2.0(1) 1.7(1) 3.8(3) 3.1(2) 2.8(2) 2.8(2) 2.3(1) 2.3(1) 1.8(2) 2.5(1) 2.7(2)

a

V

along the [010] direction. Additionally, we were able to isolate different crystals with identical shape (platelet crystals), same unit cell parameters as determined from X-ray diffraction techniques but different colours – which varied from brownish (the title compound) to deep greenish. This indicated that variable protonation rates were attainable on the pyrophosphate groups which in turn allowed for coupled in situ redox mechanisms. The brownish phases would be mixed (V V –V IV ) compounds while the deep greenish crystals would correspond to the reduced phases containing only V IV atoms. As a result, the title compound appears as a member of a solid solution, the generic formula of which is Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (H x (P12yVy ) 2 O 7 ) with variable x and y values. The overall VPO topology is three-dimensional with more or less open voids in which the rubidium atoms are located. The coordination sphere of the Rb 1 cations consist of 9-(Rb1 and Rb4) and 10-oxygen atoms ˚ to (Rb2 and Rb3) at distances ranging from 2.787(8) A ˚ 3.442 A. At last, the obtained formula of Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) with x|0.6 was confirmed by EDX measurements performed on several crystals which gave ratios of Rb:V:P5 32.42(6):38.1(4):29.4(3) to be compared to the expected values from the structural study Rb:V:P533.3:36.2:30.6.

5. Discussion

V

V –P substitution with relative occupancy factors t P50.22(1), tV50.28(1).

M 2 O 7 species in the pyrovanadate / pyrophosphate series (NH 4 ) 2 VO(V22x Px O 7 ) (Table 4). The P–O bond lengths observed in the second site compare well with those reported above for the orthophosphates groups [1.494(12) ˚ ˚ ˚ A#d P – O #1.638(8) A]. The longer distances of 1.769(9) A ˚ within the pyrophosphate group involve and 1.638(8) A the oxygen bridging atom O19. On the other hand, one oxygen atom on each tetrahedron makes a terminal bond (namely O11 for the tetrahedron around P2 and O12 for V the tetrahedron containing V species). Electroneutrality requirements, coupled to bond valence sum calculations, showed that the pyrophosphate group was partially protonated as [HP1.42 V0.58 O 7 ]. The protons are likely disordered and attempts were not successful to locate them accurately. Nevertheless, on the basis of the available structural results, these protons were likely to be involved in strong ˚ H-bondings in that close O–O contacts (d O – O |2.8 A) actually occur in the structure. The O11, O12 from the pyrophosphate group and the bridging oxygen O19 within the dimeric [V2 O 3 ] unit participate to a large extent in these H-bondings which propagate in an alternated way

The structure of Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) is closely related to those of potassium and thallium vanadium phosphates M 3 (V2 O 3 )(VO)(PO 4 ) 2 (HPO 4 ) reported by Huan [9] and Vaughey [10]. While our structure crystallizes in the non-centrosymmetric space group Pmn2 1 , the compounds reported by Huan and Vaughey were described in the centrosymmetric space group Pnma. Though careful examination of the intensity data set did not reveal systematic absences indicative of an axial glide plane along the short stacking axis, attempts were made to refine the structure of Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) in the Pnma space group that resulted in higher residuals (R 1 50.070, wR 2 5 0.148). The overall VPO topology of M 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HPO 4 ) 2 resembles to that of Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) very much but the V/ P ratio and oxygen content are different. Indeed, V/ P51 and 32 oxygen are present in M 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HPO 4 ) 2 while V/ P|13 / 11 (.1) and there are only 31 oxygen atoms in the title compound. Looking after the structural units that differ between the structures (the phosphate groups) shows that similar problems as those we were confronted with are underlying in the structures of M 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HPO 4 ) 2 especially in the case of the thallium containing compound [9].

E. Le Fur et al. / International Journal of Inorganic Materials 3 (2001) 9 – 15

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Table 3 ˚ bond angles (8) with their standard deviation in brackets for Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) Selected bond lengths (A),

V1

V2

V3

V4

P1

P2

(P, V)

P4

Distance

Angles

O18 O13 O5 O9 O14

1.610(7) 1.874(4) 1.968(7) 1.989(7) 2.016(7)

99.2(4) 104.5(3) 101.9(3) 99.3(3)

89.9(4) 158.8(4) 88.4(4)

87.3(3) 156.1(3)

85.7(3)

O8 O4 O4 O3 O3 O1

1.646(9) 1.953(7) 1.953(7) 2.021(6) 2.021(6) 2.233(8)

98.1(3) 98.1(3) 97.7(3) 97.7(3) 179.2(5)

89.4(4) 164.2(3) 87.8(3) 82.5(3)

87.8(3) 164.2(3) 82.5(2)

90.7(4) 81.7(2)

81.7(3)

O1 O10 O10 O2 O2 O8

1.621(8) 1.971(6) 1.971(6) 1.999(7) 1.999(7) 2.229(9)

98.5(3) 98.5(3) 96.6(3) 96.6(3) 177.4(5)

89.1(4) 164.9(3) 88.1(3) 83.4(3)

88.1(3) 164.9(3) 83.4(2)

90.6(4) 81.5(2)

81.6(3)

O16 O17 O7 O6 O15

1.600(7) 1.881(4) 1.964(6) 1.973(6) 1.990(7)

97.5(4) 105.1(3) 99.8(3) 102.3(3)

88.6(4) 162.6(3) 89.4(4)

88.5(3) 152.6(3)

85.3(3)

O5 O4 O6 O2

1.525(7) 1.530(6) 1.538(7) 1.548(7)

111.3(4) 109.8(4) 108.3(4)

107.1(4) 110.4(4)

109.9(3)

O11 O15 O15 O19

1.490(12) 1.554(7) 1.554(7) 1.636(8)

113.6(3) 113.6(3) 106.8(6)

114.1(5) 103.7(3)

103.7(3)

O12 O14 O14 O19

1.571(11) 1.644(7) 1.644(7) 1.773(9)

114.1(3) 114.1(3) 100.2(4)

113.2(5) 107.0(3)

107.0(3)

O10 O3 O7 O9

1.532(6) 1.535(6) 1.546(7) 1.555(6)

111.6(3) 110.6(4) 105.5(3)

108.4(4) 109.7(3)

111.1(4)

Indeed, the atomic displacement parameters of one oxygen atom which takes part in the near neighbouring of the ˚ 2 ). phosphorus species is abnormally high (Beq.| 5.7(5) A This oxygen is bonded to different P atoms from adjacent ˚ PO 4 units with very long P–O bond lengths of 1.64(2) A ˚ These bond distances compare well with the or 1.74(3) A. distances that we observe in the structure described herein ˚ respectively. The same 1.638(8) and 1.769(9) A, anomalies are underlying in the isostructural K 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HPO 4 ) 2 compound (high temperature factor of the same oxygen atom) but the corresponding tetrahedral sites are much less distorted.

6. Concluding remarks The structure of a new RbVPO solid, Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) with x|0.6 has been solved from X-ray crystal intensity diffraction data recorded using a CCD detector. It crystallizes in the non-centrosymmetric space group Pmn2 1 and is closely related to the structures of M 3 (V2 O 3 )(VO)(PO 4 ) 2 (HPO 4 ). The three dimensional overall topology results from the heterocondensation of [VO 6 ] distorted octahedra and [VO 5 ] pyramids with either orthophophate or pyrophosphate-like units. The rubidium atoms are located inside

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E. Le Fur et al. / International Journal of Inorganic Materials 3 (2001) 9 – 15

Fig. 1. View of Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) along the [010] direction.

tunnels. One of the most striking structural results described herein arises from (V V –P V ) substitution in a tetrahedral site. To the best of our knowledge, Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) is the second solid obtained from soft hydrothermal synthesis that exhibits such a substitution.

Acknowledgements The authors are indebted to Dr T. Roisnel for the single crystal intensity data collection on the Kappa CCD diffractometer and J.C. Jegaden for microprobe analysis (Universite´ de Rennes, LCSIM, UMR 6511).

Fig. 2. View of the [ . . . O=V- - -O=V- - -O=V . . . ] chain along the [001] direction. Large white circles are V atoms, black circles are O atoms. The PO 4 tetrahedra are hatched.

E. Le Fur et al. / International Journal of Inorganic Materials 3 (2001) 9 – 15

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Fig. 3. View along [100] (a) and [001] (b) of the linkage between the [V2 O 3 ] and the [P22xVx O 7 ] units. Large white circles are V atoms, black circles are O atoms. The [P22xVx O 7 ] units are hatched.

Table 4 ˚ in (NH 4 ) 2 VO(V22x Px O 7 ) and Rb 6 (V2 O 3 ) 2 (VO) 2 (PO 4 ) 4 (HP22xVx O 7 ) (RbVPO) within the pyrophosphate unit [P,V]–O distances (A)

(P,V )–O apical (P,V )–O bridging (P,V )–O

(NH 4 ) 2 VO(V2 O 7 )

(NH 4 ) 2 VO(V22x Px O 7 )

(NH 4 ) 2 VO(P2 O 7 )

RbVPO

1.660 (5) 1.793(2) 1.709(3) (23)

1.570(6) 1.692(3) 1.619(4) 1.621(4)

1.491 (6) 1.611(3) 1.524(4) (23)

1.571(11) 1.769(9) 1.644(7)

References [1] Centi G, Trifiro F, Ebrer JR, Franchetti VM. Chem Rev 1988;88:55. ´ P, Dolores Marcos M, Beltran-Porter ´ ´ [2] Amoros A, Beltran-Porter D. Curr Opinion Solid State Mater Sci 1999;4:123, and references therein. [3] Schindler M, Joswig W, Baur WH. J Solid State Chem 1999;145:15. ´ P, Alamo J, Beltran-Porter ´ [4] Roca M, Marcos MD, Amoros A, ´ Beltran-Porter D. Inorg Chem 1997;36:3414. ` [5] Le Fur E, Pivan JY. In: VIIIeme Rencontres Marocaines de Chimie de l’Etat Solide, Tetouan (Morocco), 1999. [6] Le Fur E, Pivan JY. In: XIIIth International Conference on Solid Compounds of Transition Elements, Stresa (Italy), 2000. [7] Haushalter RC, Wang Z, Thompson ME, Zubieta J. Inorg Acta 1995;232:83.

1.494 (12) 1.638(8) 1.554(7) (23)

[8] Huan G, Johnson JW, Jacobson AJ, Cormoran EW, Goshorn DP. J Solid State Chem 1991;93:515. [9] Vaughey JT, Harrison WTA, Jacobson AJ. J Solid State Chem 1994;110:305. ¨ D, Boultif M. J Appl Cryst 1991;24:987. [10] Louer [11] Collect, Denzo, Scalepack, Sortav, Kappa CCD Program Package, Delft, The Netherlands: Nonius BV, 1998. [12] Sheldrick GM. SHELXS-86. Program for crystal structure refinement, ¨ Germany: University of Gottingen, 1986. [13] Sheldrick GM. SHELXL-97. Program for crystal structure refinement, ¨ Germany: University of Gottingen, 1997. [14] Ninclaus C, Retoux R, Riou D, Ferey G. J Solid State Chem 1996;122:139.