High-pressure investigations of lanthanoid oxoarsenates: I. Single crystals of scheelite-type Ln[AsO₄] phases with Ln = La–Nd from monazite-type precursors

1 Introduction

For the lanthanoid(III) oxoarsenates(V) with the simple composition Ln[AsO₄], there are three structure types known so far. The first one is the monoclinic monazite type in the space group P2₁/n, which is limited to the largest lanthanoids (Ln = La–Nd) [1–5]. The monazite type has been named after the mineral monazite as a mixed lanthanoid(III) oxophosphate(V) Ln[PO₄] (mainly with Ln = La, Ce, Nd, Sm, and Gd). The second type is also related to the rare-earth metal(III) oxophosphates(V) by the xenotime minerals Ln[PO₄] (mainly with Ln = Yb) and Y[PO₄] crystallizing tetragonally in the space group I4₁/amd. The xenotime type of the oxoarsenates(V) is found for yttrium and all the smaller lanthanoids (Ln = Sm–Lu) [1–3, 6–12] exhibiting a decrease of the coordinating numbers for the involved Ln³⁺ cations from eight plus one in the monazite type to straight eight in the xenotime type, which represents the same structural arrangement as the mineral zircon (Zr[SiO₄]) [13]. After the thorough determination of these two crystal structure types, some high-pressure investigations have been carried out, which culminated in a third Ln[AsO₄] modification with the tetragonal space group I4₁/a. Because of its isotypism to the mineral scheelite (Ca[WO₄]) [14], this structure is called scheelite type in the following. In scheelite-type structures A[BO₄], the A metal cation shows also an eightfold coordination by oxygen atoms, as it has already been observed for the xenotime-type arrangement, while the B metal centers an oxygen tetrahedron, just like in all monazite- and xenotime-type phases. In 2006 and thus more than 50 years after the first member of the Nickel-Strunz mineral class VIII (phosphates, arsenates and vanadates), namely bismuth(III) oxoarsenate(V) Bi[AsO₄], which was found in the middle of the 20^th century [15], the first lanthanoid(III) oxoarsenate(V) in the scheelite-type structure, namely samarium(III) oxoarsenate(V) Sm[AsO₄] [16], could be synthesized in form of single crystals and characterized by X-ray diffraction. Further investigations in our research groups have shown that not only oxoarsenates(V) of the lanthanoids with the xenotime type (Ln = Sm, Tb, Dy, Er, Yb, and Lu) [15], but also those with the monazite-type structure (Ln = La, Ce, Pr, and Nd), can be transformed into their scheelite-type modifications at higher pressure [17].

2 Results and discussion

All crystal structures under consideration here, the scheelite, the xenotime, and the monazite type for the composition Ln[AsO₄] (Tables 1 and 2), contain just a single crystallographically independent position for the Ln³⁺ cations and also only one for the As⁵⁺ cations, but there are four crystallographic different O^2– anions in the monazite- in contrast to the xenotime- and scheelite-type structures, which also contain only one singular position for the oxygen atoms (Table 3). In the starting materials with monazite-type structure the Ln³⁺ cations (Ln = La–Nd) show a coordination number of eight plus one, which can be split into two structure elements (Fig. 1). On the one hand, there is a pentagonal plane (d(La–O) = 249–266 pm, d(Nd–O) = 242–259 pm) and on the other hand there is a bisphenoidally elongated tetrahedron (d(La–O) = 256–265 plus 291 pm, d(Nd–O) = 250–257 plus 292 pm), which becomes center-cut by the plane. In contrast the Ln³⁺ cations in the scheelite and xenotime type are only eightfold coordinated by oxygen atoms in the shape of a trigonal dodecahedron, comprising two bisphenoidally distorted tetrahedra interpenetrating each other, where the one built up by (O)^2– is oblate and the other one built up by (O′)^2– is elongated (Fig. 2). The shortest and longest Ln–O distances in the systems under consideration (d(La–O) = 256 and 291 pm, d(Nd–O) = 241 and 292 pm, see Fig. 3) with the lanthanum and the neodymium representatives as limiting members can mostly be found in the monazite-type structure, but the bond lengths in their scheelite-type analogs are in average shorter (d(La–O) = 251 and 253 pm, d(Nd–O) = 244 and 248 pm, see Tables 4 and 5). The oxygen environment around the arsenic(V) cations is more similar in both structure types. The As⁵⁺ cations are tetrahedrally coordinated by four O^2– anions and the distances diverge by only 1–2 pm for La[AsO₄] and Nd[AsO₄] (Fig. 4). The first noteable difference lies in the next coordination sphere, where two O^2– anions in the monazite type forms share one Ln³⁺ cation, which is not found in the scheelite-type modifications. The [LnO₈₊₁]^15– polyhedra are thus connected to five plus two [AsO₄]^3– anions via corners and via two edges in the monazite-type Ln[AsO₄] phases, whereas the [LnO₈]^13– polyhedra are connected to eight [AsO₄]^3– anions exclusively by corners in the scheelite-type Ln[AsO₄] phases (Fig. 5). Through the transformation from the monazite- into the scheelite-type lanthanoid(III) oxoarsenates(V) Ln[AsO₄], the densities increase by 4.3–4.8 % and the molar volumes are reduced by 4.1–4.9 % (Fig. 6).

Fig. 1:

Coordination polyhedron [LaO₈₊₁]^15– in the monazite-type lanthanoid(III) oxoarsenates(V) Ln[AsO₄] (top) emphasizing a pentagonal plane (bottom left) and an elongated tetrahedron (bottom right).

Fig. 2:

Coordination polyhedron [LnO₈]^13– in the scheelite-type lanthanoid(III) oxoarsenates(V) Ln[AsO₄] (top) emphasizing an oblate tetrahedron (bottom left) and an elongated tetrahedron (bottom right).

Fig. 3:

Comparison of the distances in the coordination spheres of the [LnO₈₊₁]^15– polyhedra in the monazite type (dotted lines) and of the [LnO₈]^13– polyhedra in the scheelite type (solid lines) for the lanthanoid(III) oxoarsenates(V) Ln[AsO₄] (Ln = La–Nd).

Fig. 4:

Cationic Ln³⁺ surrounding of the tetrahedral [AsO₄]^3– anions in the monazite- (left) and the scheelite-type lanthanoid(III) oxoarsenates(V) Ln[AsO₄] (right).

Fig. 5:

Coordination polyhedra [LnO₈₊₁]^15– in the monazite- (top) and [LnO₈]^13– in the scheelite-type lanthanoid(III) oxoarsenates(V) Ln[AsO₄] (bottom) surrounded by seven (left) and eight (right) [AsO₄]^3– units, respectively.

Fig. 6:

Graph of calculated densities (D_cald in g cm⁻³) of the lanthanoid(III) oxoarsenates(V) Ln[AsO₄] in the scheelite- (top) and the monazite-type modification (bottom) versus the ionic radii (r_i in pm) [18] of the corresponding eightfold coordinated Ln³⁺ cations.

All samples were characterized by X-ray diffraction using a STOE Stadi P powder diffractometer (STOE & Cie, Darmstadt, Germany) with monochromatized CuK_α1 radiation (λ = 154.05 pm) and a NONIUS-κ-CCD single-crystal diffractometer (Bruker-Nonius, Karlsruhe, Germany) with graphite-monochromatized MoK_α1 radiation (λ = 71.07 pm).

3 Conclusion

Although the scheelite-type structure of the lanthanoid(III) oxoarsenates(V) Ln[AsO₄] shows a lower coordination number (CN = 8) for the Ln³⁺ cations, it is the denser modification as compared to the corresponding monazite-type phases with CN(Ln³⁺) = 8 + 1. This can be explained by the shorter average distances of the Ln–O contacts of 252 and 246 pm in scheelite-type La[AsO₄] and Nd[AsO₄], respectively, as compared to 256 and 250 pm (for the first eight O^2– anions) or even 260 and 255 pm (for all nine O^2– anions) in monazite-type La[AsO₄] and Nd[AsO₄]. Furthermore, there is one more [AsO₄]^3– anion arranged around the scheelite-type [LnO₈]^13– polyhedra and all eight are attached just by vertex connectivity. In addition, the [AsO₄]^3– anions are more symmetrically ordered around the scheelite-type [LnO₈]^3– polyhedra than it is the case for the monazite-type [LnO₈₊₁]^15– polyhedra. These results show that the scheelite-type structures of the lanthanoid(III) oxoarsenates(V) Ln[AsO₄] exist for all monazite-type compounds of the lanthanoids with Ln = La–Nd as high-pressure modifications.

4 Experimental section

The starting materials of the crystalline monazite-type lanthanoid(III) oxoarsenates(V) Ln[AsO₄] (Ln = La–Nd) were prepared as powder samples by dissolving the commercially available lanthanoid oxides (La₂O₃, CeO₂, Pr₆O₁₁, and Nd₂O₃: 99.99 %, ChemPur, Karlsruhe, Germany) in diluted nitric acid (HNO₃: 13 %, Scharr, Stuttgart, Germany) under heating. Subsequently, a stoichiometric amount of arsenic acid (H₃AsO₄: 99 %, Merck, Darmstadt, Germany) was added and stirred for an hour. In case of the cerium and praseodymium samples, an additional stoichiometric amount of hydrogen peroxide (H₂O₂: 30 %, Applichem, Darmstadt, Germany) was added in order to reduce any tetravalent cations. The solution was neutralized with 1 m caustic soda (NaOH solution), the precipitate was filtered off and dried in order to obtain monazite-type Ln[AsO₄] phases. The purity was proven by X-ray diffraction, which exclusively showed the reflections corresponding to the monazite-type lanthanoid(III) oxoarsenates(V) [1–5]. For the high-pressure/high-temperature experiments, these lanthanoid(III) oxoarsenates(V) Ln[AsO₄] were packed into cylindrical boron-nitride crucibles (Henze BNP AG, HeBoSint^® S10, Germany, volume: from 9 mm³ (14/8) up to 35 mm³ (18/11)) and sealed with fitting BN plates as lids. The scheelite-type single crystals for the unchanged composition Ln[AsO₄] were synthesized in a modified Walker-type module in combination with a 1000-t press (both devices from the company Voggenreiter, Mainleus, Germany). As pressure medium, precastable magnesium oxide octahedra (Ceramic Substrates & Components, Isle of Wight, UK) with edge lengths of 14 or 18 mm (14/8- or 18/11-assembly) were applied. Eight tungsten carbide cubes (TSM 10, Ceratizit, Austria) with the truncation edge lengths of 8 or 11 mm compressed the primary MgO octahedra. Further information on the construction of the assemblies are given in references (Walker [19] and [20], Huppertz [21], Rubie [22], Kawai and Endo [23]). The samples were compressed to 11 GPa within 3.5 h, followed by a heating period of 15 min, in which the samples reached a temperature of 1200 °C. The temperature was held for further 15 min, with a following segment of controlled cooling (90 min) to 1200 °C and a quenching of the sample to room temperature within 1 min. After decompression the WC cubes were removed, the recovered MgO octahedra were broken apart to isolate the samples for further analytical investigations. The surrounding hexagonal BN crucibles were carefully separated from the samples to obtain the air- and humidity-resistant compounds Ln[AsO₄] (Ln = La–Nd) as crystalline solids. To estimate the formation region of the compounds with respect to the parameters pressure and temperature, we performed several different high-pressure/high-temperature experiments with varying pressures and temperatures. Single crystals of La[AsO₄], Pr[AsO₄], and Nd[AsO₄] could be obtained with the characteristic colors of green for the praseodymium crystals and lavender for the neodymium compound. As expected for a lanthanum(III) oxoarsenate(V), these crystals are colorless. Despite of several attempts, no single crystals of Ce[AsO₄] could be found in the product which were large enough for single-crystal X-ray measurements. However, as the powder diffraction patterns showed phase pure Ce[AsO₄], a structure refinement with Rietveld methods was possible (Fig. 7). The result showed a quite good match with the data from the other three scheelite-type oxoarsenates(V).

Fig. 7:

Rietveld refinement from powder diffraction data (MoK_α1 radiation, λ = 71.07 pm) of scheelite-type cerium(III) oxoarsenate(V) Ce[AsO₄].

Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de; url: http://www.fiz-informationsdienste.de/en/DB/icsd/depot_anforderung.html) on quoting the deposition numbers CSD-430619 for La[AsO₄], CSD-430620 for Nd[AsO₄] and CSD-430621 for Pr[AsO₄].