About the Rare-Earth Metal(III) Bromide Oxoarsenates(III) RE₅Br₃[AsO₃]₄ with A- (RE = La and Ce) or B-Type Structure (RE = Pr, Nd, Sm–Tb) and RE₃Br₂[AsO₃][As₂O₅] (RE = Y, Dy–Yb)

Abstract

The monoclinic rare-earth metal(III) bromide oxoarsenates(III) RE₅Br₃[AsO₃]₄ of the A-type (RE = La and Ce) crystallize in the space group C2/c with the lattice parameters a = 1834.67(9) pm, b = 553.41(3) pm, c = 1732.16(9) pm and β = 107.380(3)° for La₅Br₃[AsO₃]₄ and a = 1827.82(9) pm, b = 550.67(3) pm, c = 1714.23(9) pm and β = 107.372(3)° for Ce₅Br₃[AsO₃]₄ with Z = 4, while, for the B-type (RE = Pr, Nd and Sm–Tb), they prefer the space group P2/c with lattice parameters from a = 881.23(5) pm, b = 547.32(3) pm, c = 1701.14(9) pm and β = 90.231(3)° for Pr₅Br₃[AsO₃]₄ to a = 875.71(5) pm, b = 535.90(3) pm, c = 1643.04(9) pm and β = 90.052(3)° for Tb₅Br₃[AsO₃]₄ with Z = 2. The closely related rare-earth metal(III) bromide oxoarsenates(III) RE₃Br₂[AsO₃][As₂O₅] crystallize in the triclinic space group P$\overline{1}$ with lattice parameters from a = 539.15(4) pm, b = 870.68(6) pm, c = 1092.34(8), α = 90.661(2)°, β = 94. 792(2)° and γ = 90.223(2)° for Dy₃Br₂[AsO₃][As₂O₅] to a = 533.56(4) pm, b = 869.61(6) pm, c = 1076.70(8), α = 90.698(2)°, β = 94.785(2)° and γ = 90.053(2)° for Yb₃Br₂[AsO₃][As₂O₅] with Z = 2. All three structures have the same building units with [REO₈]¹³⁻ and [REO₄Br₄]⁹⁻ polyhedra as well as isolated ψ¹-tetrahedral [AsO₃]³⁻ anions in common, with the exception that, in the latter two, ψ¹-[AsO₃]³⁻ tetrahedra linked by a corner form a pyroanionic [As₂O₅]⁴⁻ entity. A- and B-type differ in the stacking sequence of their $\begin{array}{c}2\\ \infty \end{array}${[(RE3)O$\begin{array}{c}t\\ 4/1\end{array}$(Br1)$\begin{array}{c}v\\ 1/2\end{array}$(Br2)$\begin{array}{c}e\\ 3/3\end{array}$]^6.5−} layers. While the former have an ABC sequence, the latter exhibit an AAA variant. In the triclinic structures, the (RE3)³⁺ sites are thinned out, while the As³⁺ sites are simultaneously enriched, resulting in the mentioned condensed units.

1. Introduction

The rare-earth metal(III) bromide oxopnictogenates(III) have an impressive number of different structures to date, and the best-studied are the bismutates and antimonates. Only a single composition is realized with bismuth, where the tetragonal REBrBi₂O₄ representatives [1] with RE = Y, Pr, Nd and Sm–Lu crystallize in the space group P4/mmm. They contain [REO₈]¹³⁻ and corner-linked ψ¹-pyramidal [BiO₄]⁵⁻ units to form a checkerboard pattern. The bromide anions are isolated from these layers and only linked to them via secondary contacts. A very similar picture emerges for the antimonates, where there is an isostructural example with LaBrSb₂O₄ [2] and the cation-mixed Sm_1.5BrSb_1.5O₄ [3] examples. The same composition REBrSb₂O₄ is also known with two more different structure types. The rare-earth metal environment does not differ apart from a slight distortion. Here, the trivalent antimony surrounds itself with three oxygen atoms to form a ψ¹-tetrahedron [SbO₃]³⁻, which is linked to others via corners either to form chains $\begin{array}{c}1\\ \infty \end{array}${[SbO$\begin{array}{c}v\\ 2/1\end{array}$O$\begin{array}{c}t\\ 1/1\end{array}$]⁻} (v = vertex and t = terminal) for the monoclinic representatives crystallizing in the space group P2₁/c (RE = Y, Nd and Eu–Dy) [4,5,6,7] or isolated rings $\begin{array}{c}0\\ \infty \end{array}${[(SbO$\begin{array}{c}v\\ 2/1\end{array}$O$\begin{array}{c}t\\ 1/1\end{array}$)₄]⁴⁻} for the tetragonal representatives (RE = Er and Tm) [5,8] adopting the space group P42₁2. Furthermore, there is the orthorhombically crystallizing (space group: Pnma) La₂Br₄Sb₁₂O₁₉ [9], which features edge-linked, capped square prisms [LaO₉]¹⁵⁻, located within double-halfpipes of linked ψ¹-pyramidal [SbO₄]⁵⁻ and ψ¹-tetrahedral [SbO₃]³⁻ groups. For charge neutrality, only bromide anions that are only bound via secondary contacts again are located between these $\begin{array}{c}1\\ \infty \end{array}${[Sb₁₂O₁₉]²⁻} strands. In the case of arsenates, only two different examples are known to date, firstly tetragonal La₃OBr[AsO₃]₂ (space group: P4₂/mnm) [10], which displays trigonal prisms of oxygen around lanthanum that are doubly capped by bromide anions [LaO₆Br₂]¹¹⁻ and an additional trigonal prism [LaO₈Br]¹²⁻, which is tricapped by two oxygen atoms and a bromide anion. The ψ¹-tetrahedral [AsO₃]³⁻ anions are isolated here, but arranged in such a way that lone-pair channels form. Furthermore, Tm₃Br₂[AsO₃][As₂O₅] (space group: P$\overline{1}$) [11] is another individual case whose significantly extended series is presented for the first time in this publication, supplemented and compared with the previously completely unknown series of RE₅Br₃[AsO₃]₄ representatives with RE = La–Nd and Sm–Tb, moreover.

2. Materials and Methods

The new rare-earth metal(III) bromide oxoarsenates(III) with the composition RE₅Br₃[AsO₃]₄ were produced for RE = La–Nd and Sm–Tb via a metallothermic reaction of rare-earth metal (RE: ChemPur, Karlsruhe, Germany, 99.9%) with rare-earth metal tribromide (REBr₃: ChemPur, 99.99%) and arsenic sesquioxide (As₂O₃: Aldrich, Darmstadt, Germany, 99.99%) using cesium bromide (CsBr: Merck, Darmstadt, Germany, 99.9%) as flux according to Equation (1) at temperatures of 825 °C, as no single crystals of sufficient size for single crystal X-ray analysis were obtained at lower temperatures:

4 RE + REBr₃ + 4 As₂O₃ → RE₅Br₃[AsO₃]₄ + 4 As (RE = La–Nd, Sm–Tb)

(1)

The also new rare-earth metal(III) bromide oxoarsenates(III) with the composition RE₃Br₂[AsO₃][As₂O₅] with RE = Y and Dy–Yb were obtained from the rare-earth metal sesquioxides (RE₂O₃: ChemPur, 99.9%) with rare-earth metal bromide (REBr₃: ChemPur, 99.99%) and arsenic sesquioxide (As₂O₃: Aldrich, 99.99%) again with cesium bromide (CsBr: Merck, 99.9%) as a fluxing agent according to Equation (2) at temperatures of 780 °C, as no single crystals of sufficient size for single crystal X-ray analysis were obtained at lower temperatures:

7 RE₂O₃ + 4 REBr₃ + 9 As₂O₃ → 6 RE₃Br₂[AsO₃][As₂O₅] (RE = Y and Dy–Yb)

(2)

After removal of the cesium–bromide flux by rinsing off with water and subsequent drying of the crude batches, suitable crystals for single-crystal X-ray structure analysis were detected and selected in all cases. The always platelet-shaped crystals were transferred into glass capillaries (Hilgenberg, Malsfeld, Germany; outer diameter: 0.1 mm, wall thickness: 0.01 mm) and fixed there with grease. For all RE₅Br₃[AsO₃]₄ representatives (RE = La–Nd and Sm–Tb), the measurements were carried out on a STADI-VARI single-crystal X-ray diffractometer (Stoe & Cie, Darmstadt, Germany), while, for the RE₃Br₂[AsO₃][As₂O₅] representatives (RE = Y and Dy–Yb), they took place with a κ-CCD single-crystal X-ray diffractometer (Bruker-Nonius, Karlsruhe, Germany), using Mo-K_α radiation (λ = 71.07 pm) in all cases.

The monoclinic structures for A-type RE₅Br₃[AsO₃]₄ (RE = La and Ce) could be solved and refined in the space group C2/c and for B-type RE₅Br₃[AsO₃]₄ (RE = Pr, Nd and Sm–Tb) in the space group P2/c by direct methods using the SHELX-97 program package [12,13,14]. The triclinic structures of the RE₃Br₂[AsO₃][As₂O₅] representatives with RE = Y and Dy–Yb were solved and refined with the same methods in the space group P$\overline{1}$.

3. Results and Discussion

3.1. Crystal Structure of A-Type RE₅Br₃[AsO₃]₄ (RE = La and Ce)

The rare-earth metal(III) bromide oxoarsenates(III) RE₅Br₃[AsO₃]₄ (RE = La and Ce) of the A-type crystallize isotypically to their chloride analogs RE₅Cl₃[AsO₃]₄ with RE = La–Pr [15,16] in the monoclinic space group C2/c (no. 15) with lattice parameters of a = 1834.67(9) pm, b = 553.41(3) pm, c = 1732.16(9) pm and β = 107.380(3)° for La₅Br₃[AsO₃]₄ and a = 1827.82(9) pm, b = 550.67(3) pm, c = 1714.23(9) pm and β = 107.372(3)° for Ce₅Br₃[AsO₃]₄ with Z = 4.

The crystal structure contains three crystallographically different RE³⁺ cations. (RE1)³⁺ is surrounded by eight oxygen atoms (d(La1–O) = 245–260 pm and d(Ce1–O) = 241–255 pm) in the shape of a slightly distorted square antiprism (Figure 1, top left), and, similarly, the (RE2)³⁺ cation has eight O²⁻ anions arranged as a heavily distorted square hemiprism (d(La2–O) = 234–268 pm and d(Ce2–O) = 230–261 pm; Figure 1, top mid) as a first coordination sphere. The (RE3)³⁺ cation, on the other hand, is coordinated by four oxygen atoms (d(La3–O) = 237–258 pm and d(Ce3–O) = 233–259 pm) and four Br⁻ anions (d(La3–Br) = 318–327 pm and d(Ce3–Br) = 316–325 pm) in the form of a square antiprism (Figure 1, top right).

All six crystallographically independent oxygen atoms belong to two types of isolated, ψ¹-tetrahedral oxoarsenate(III) anions [AsO₃]³⁻ (Figure 2), which are formed from three oxygen atoms and a non-bonding electron pair around each As³⁺ center. The (As1)³⁺ cation is coordinated by three oxygen atoms (O1, O2 and O3) with distances of d(As1–O) = 173–181 pm for La₅Br₃[AsO₃]₄ and d(As1–O) = 175–180 pm for Ce₅Br₃[AsO₃]₄. For the (As2)³⁺ cation, the three oxygen ligands (O4, O5 and O6) occur with slightly longer distances (d(As2–O) = 174–182 pm for La₅Br₃[AsO₃]₄ and d(As1–O) = 177–184 pm for Ce₅Br₃[AsO₃]₄) than in the first case. Each oxygen atom of the two different discrete [AsO₃]³⁻ anions is coordinated by six RE³⁺ cations each, both via corners (4×) and via edges (2×).

In the crystal structure, the [(RE1)O₈]¹³⁻ and [(RE2)O₈]¹³⁻ polyhedra are connected to each other by oxygen-edge bonding to form fluorite-related layers $\begin{array}{c}2\\ \infty \end{array}${[(RE1/2)O$\begin{array}{c}e\\ 8/2\end{array}$]⁵⁻} parallel to the (100) plane (Figure 1). In turn, the polyhedral [(RE3)O₄Br₄]⁹⁻ around (RE3)³⁺ attach to these layers. Thus, bilayers result from the fused rare-earth metal-oxygen polyhedra, which, together with the isolated oxoarsenate(III) anions [AsO₃]³⁻, display the composition $\begin{array}{c}2\\ \mathrm{\infty }\end{array}${RE₅[AsO₃]₄}³⁺ and are stacked along the a-axis (Figure 3). Between these cationic bilayers reside the bromide anions (Br1)⁻ and (Br2)⁻. The two distinct Br⁻ anions are only surrounded by (RE3)³⁺ cations. The first anion (Br1)⁻ is linearly coordinated by two equidistant (RE3)³⁺ cations (d(Br1–La3) = 318 pm and d(Br1–Ce3) = 316 pm) at an angle of 180°, whereas the (Br2)⁻ anion shows a surrounding of three (RE3)³⁺ cations (d(Br2–La3) = 319–327 pm and d(Br2–Ce3) = 317–325 pm) in the shape of a pyramid. The [(RE3)O₄Br₄]⁹⁻ polyhedra, which contain the two crystallographically different Br⁻ anions, are linked via bromide corners and edges to form corrugated intermediate layers (Figure 1), in which all oxygen atoms of the [(RE3)O₄Br₄]⁹⁻ polyhedra work as components of the isolated [AsO₃]³⁻ units in such a way that the lone pairs on the As³⁺ cations point into the direction of the interlayers (Figure 3).

Table 1. Crystallographic data of A-type RE₅Br₃[AsO₃]₄ (RE = La and Ce).

RE5Br3[AsO3]4		La	Ce
Crystal system		monoclinic
Space group		C2/c (no. 15)
Lattice parameters,	a/pm	1834.67(9)	1827.82(9)
	b/pm	553.41(3)	550.67(3)
	c/pm	1732.16(9)	1714.23(9)
	β/°	107.380(3)	107.372(3)
Formula units, Z		4
Calculated density, Dx/g∙cm−3		5.643	5.776
Molar volume, Vm/cm3∙mol−1		252.69(9)	247.92(9)
Diffractometer		STADI-VARI (Stoe & Cie)
Wavelength		Mo-Kα (λ = 71.07 pm)
Electron sum, F(000)/e−		2472	2492
Measurement limit, θmax/°		33.00	32.85
Measurement range, ±h/±k/±lmin/max		27/8/26	27/8/25
Observed reflections		20793	18462
Unique reflections		3010	2917
Absorption coefficient, µ/mm−1		27.47	28.85
Rint/Rσ		0.096/0.053	0.045/0.039
R1/R1 with |Fo| ≥ 4σ(Fo)		0.084/0.056	0.066/0.036
wR2/Goof		0.089/0.873	0.084/0.990
Residual electron density, ρ/e−∙10−6∙pm−3		−2.54/2.73	−2.17/2.26
CSD number		2263604	2263605

contains the crystallographic data for A-type La₅Br₃[AsO₃]₄ and Ce₅Br₃[AsO₃]₄ with their determination,

Table 2. Fractional atomic coordinates and coefficients of the equivalent isotropic displacement parameters for the A-type RE₅Br₃[AsO₃]₄ representatives with RE = La and Ce.

Atom	Wyckoff Site	x/a	y/b	z/c	Ueq/pm2
La5Br3[AsO3]4
La1	4e	0	0.21052(19)	1/4	55(3)
La2	8f	0.49637(4)	0.23925(16)	0.08362(4)	44(2)
La3	8f	0.16655(5)	0.24725(16)	0.13643(4)	47(2)
Br1	4c	1/4	1/4	0	154(4)
Br2	8f	0.24296(7)	0.2473(3)	0.33316(7)	92(3)
As1	8f	0.36697(7)	0.2569(3)	0.22902(7)	43(3)
As2	8f	0.12415(7)	0.2579(3)	0.47883(7)	59(3)
O1	8f	0.4310(6)	0.3300(14)	0.1769(6)	155(24)
O2	8f	0.4083(7)	0.0106(14)	0.2901(7)	108(30)
O3	8f	0.4056(7)	0.4829(14)	0.3060(7)	88(27)
O4	8f	0.0444(6)	0.3002(14)	0.3949(6)	108(22)
O5	8f	0.0867(7)	0.0123(14)	0.5257(7)	99(31)
O6	8f	0.0919(7)	0.4932(14)	0.5341(7)	98(30)
Ce5Br3[AsO3]4
Ce1	4e	0	0.21009(9)	1/4	94(1)
Ce2	8f	0.49725(2)	0.23741(7)	0.08394(2)	93(1)
Ce3	8f	0.16519(2)	0.24647(7)	0.13604(2)	99(1)
Br1	4c	1/4	1/4	0	194(2)
Br2	8f	0.24408(4)	0.24738(11)	0.33314(4)	133(1)
As1	8f	0.36673(4)	0.25801(11)	0.22788(4)	89(1)
As2	8f	0.12440(4)	0.25794(11)	0.47806(4)	99(1)
O1	8f	0.4322(3)	0.3334(8)	0.1754(3)	144(10)
O2	8f	0.4117(3)	0.0093(7)	0.2908(3)	122(10)
O3	8f	0.4055(3)	0.4811(7)	0.3064(3)	102(10)
O4	8f	0.0426(3)	0.3086(8)	0.3934(3)	204(11)
O5	8f	0.0855(3)	0.0115(7)	0.5236(3)	109(10)
O6	8f	0.0913(3)	0.4954(7)	0.5354(3)	103(10)

the fractional atomic positions along with the coefficients of equivalent isotropic displacement parameters and

Table 3. Selected interatomic distances (d/pm) for the A-type RE₅Br₃[AsO₃]₄ representatives with RE = La and Ce.

RE		La	Ce
[(RE1)O8]13− polyhedron
RE1–O4	2×	244.7(8)	240.9(5)
RE1–O3	2×	255.5(12)	255.0(5)
RE1–O1	2×	258.6(8)	255.5(5)
RE1–O2	2×	260.2(10)	254.7(5)
[(RE2)O8]13− polyhedron
RE2–O1	1×	233.7(10)	229.5(5)
RE2–O6	1×	252.1(10)	247.9(5)
RE2–O5	1×	254.3(10)	251.0(5)
RE2–O5′	1×	257.6(12)	256.4(5)
RE2–O3	1×	258.0(11)	255.0(5)
RE2–O6′	1×	258.1(12)	257.0(5)
RE2–O4	1×	260.2(8)	253.5(5)
RE2–O2	1×	267.6(13)	261.2(5)
[(RE3)O4Br4]9− polyhedron
RE3–O6	1×	237.2(12)	233.0(4)
RE3–O3	1×	238.3(10)	235.4(4)
RE3–O5	1×	249.2(11)	248.5(4)
RE3–O2	1×	258.3(10)	258.7(5)
RE3–Br1	1×	317.65(7)	316.24(4)
RE3–Br2	1×	318.82(14)	317.17(7)
RE3–Br2′	1×	318.91(14)	318.04(7)
RE3–Br2″	1×	327.47(14)	325.35(7)
[(As1)O3]3− anion
As1–O1	1×	173.0(10)	174.8(5)
As1–O2	1×	175.3(10)	178.4(4)
As1–O3	1×	181.1(10)	180.3(4)
[(As2)O3]3− anion
As2–O4	1×	174.3(9)	176.7(5)
As2–O5	1×	181.7(10)	181.4(4)
As2–O6	1×	182.0(10)	184.3(4)

selected interatomic distances.

3.2. Crystal Structure of B-Type RE₅Br₃[AsO₃]₄ (RE = Pr, Nd and Sm–Tb)

The rare-earth metal(III) bromide oxoarsenates(III) RE₅Br₃[AsO₃]₄ (RE = Pr, Nd and Sm–Tb) of the B-type crystallize isotypically to La₅Cl₃[SbO₃]₄ [9] in the monoclinic space group P2/c (no. 13) with lattice parameters from a = 881.23(5) pm, b = 547.32(3) pm, c = 1701.14(9) pm and β = 90.231(3)° for Pr₅Br₃[AsO₃]₄ to a = 875.71(5) pm, b = 535.90(3) pm, c = 1643.04(9) pm and β = 90.052(3)° for Tb₅Br₃[AsO₃]₄ with Z = 2 nicely reflecting the consequences of the lanthanoid contraction. Three crystallographically different rare-earth metal(III) cations are present in the crystal structure, and all of them are surrounded by eight anions. In the case of the (RE1)³⁺ and (RE2)³⁺ cations, one finds exclusively O²⁻ anions, which form distorted square antiprisms [(RE1)O₈]¹³⁻ (d(Pr1–O) = 238–254 pm to d(Tb1–O) = 227–246 pm) or distorted cubes [(RE2)O₈]¹³⁻ (d(Pr2–O) = 227–259 pm to d(Tb2–O) = 219–253 pm) (Figure 4, top left and mid). Six As³⁺ cations (2× edge and 4× corner) graft at the [(RE1)O₈]¹³⁻ polyhedron, whereas there are only five of them (2× edge and 3× corner) in the case of the [(RE2)O₈]¹³⁻ polyhedron. These two types of polyhedra are linked via four of their edges to form layers $\begin{array}{c}2\\ \infty \end{array}${[(RE1/2)O$\begin{array}{c}e\\ 8/2\end{array}$]⁵⁻} within the (100) plane. It can be seen that each [(RE1)O₈]¹³⁻ polyhedron is linked to four [(RE2)O₈]¹³⁻ polyhedra, whilst each [(RE2)O₈]¹³⁻ polyhedron joins with two [(RE1)O₈]¹³⁻ and two [(RE2)O₈]¹³⁻ polyhedra (Figure 4, below left). The two crystallographically different Br⁻ anions occur in the crystal structure, resulting in a different bonding arrangement around the (RE3)³⁺ cations. While the (Br1)⁻ anions are linearly surrounded by two (RE3)³⁺ cations [(Br1)RE₂]⁵⁺, the (Br2)⁻ anions show a trigonal non-planar environment [(Br2)RE₃]⁸⁺. However, the square antiprism [(RE3)O₄Br₄]⁹⁻ formed by four O²⁻ and four Br⁻ anions around each (RE3)³⁺ cation is only slightly distorted (Figure 4, top right). The distances of contacts to the O²⁻ anions show a greater variance with d(Pr3–O) = 228–253 pm to d(Tb3–O) = 220–245 pm and d(Pr3–Br) = 316–320 pm to d(Tb3–Br) = 313–318 pm. Once again, the bromide anions are only connected to the rest of the structure via these (RE3)³⁺ cations. The [(RE3)O₄Br₄]⁹⁻ antiprisms share common edges of (Br2)⁻ anions to form bands that run along [010] and are finally corner-linked via (Br1)⁻ anions to create two-dimensional bilayers of composition $\begin{array}{c}2\\ \infty \end{array}${[(RE3)O$\begin{array}{c}t\\ 4/1\end{array}$(Br1)$\begin{array}{c}v\\ 1/2\end{array}$(Br2)$\begin{array}{c}e\\ 3/3\end{array}$]^6.5−}, also spreading out parallel to the (100) plane (Figure 4, bottom right).

As expected, both kinds of As³⁺ cations are surrounded by three O²⁻ anions to form tripodal ψ¹ tetrahedra (d(As–O) = 175–185 pm for Pr₅Br₃[AsO₃]₄ to d(As–O) = 176–184 pm for Tb₅Br₃[AsO₃]₄). In the next coordination sphere, one finds six RE³⁺ cations, which are twice edge-bridging and four times terminally bound to the oxide anions (Figure 5). A view from the oxygen atoms shows that one of the three O²⁻ ligands is only further coordinated by two RE³⁺ cations, while the remaining two have three contacts each. This is also reflected in the distances to the central As³⁺ cations, as the former are the shortest in both [AsO₃]³⁻ units.

Within the crystal structure, open channels remain between the bromide anion and rare-earth metal bilayers, into which the free electron pairs of the As³⁺ cations point (Figure 6).

Table 4. Crystallographic data of the B-type RE₅Br₃[AsO₃]₄ series (RE = Pr, Nd and Sm–Tb) and their determination.

RE3Br3[AsO3]4		Pr	Nd	Sm	Eu	Gd	Tb
Crystal system		monoclinic
Space group		P2/c (no. 13)
Lattice parameters,	a/pm	881.23(5)	880.34(5)	878.46(5)	877.59(5)	876.64(5)	875.71(5)
	b/pm	547.32(3)	544.53(3)	541.19(3)	539.46(3)	537.71(3)	535.90(3)
	c/pm	1701.14(9)	1689.75(9)	1669.37(9)	1660.68(9)	1651.85(9)	1643.04(9)
	β/°	90.231(3)	90.196(3)	90.134(3)	90.107(3)	90.078(3)	90.052(3)
Formula units, Z		2
Calculated density, Dx/g∙cm−3		5.812	5.956	6.206	6.299	6.473	6.573
Molar volume, Vm/cm3∙mol−1		247.06(9)	243.89(9)	238.99(9)	236.74(9)	234.46(9)	232.16(9)
Diffractometer		STADI-VARI (Stoe & Cie)
Wavelength		Mo-Kα (λ = 71.07 pm)
Electron sum, F(000)/e−		1256	1266	1286	1296	1306	1316
Measurement limit, θmax/°		32.7	32.9	32.9	33.0	32.9	32.9
Measurement range, ±h/±k/±lmin/max		12/8/25	13/8/25	13/8/25	13/8/24	12/8/24	13/8/24
Observed reflections		15554	18023	15892	17671	15396	17361
Unique reflections		2839	2877	2785	2777	2711	2754
Absorption coefficient, µ/mm−1		29.93	31.30	34.09	35.68	37.19	38.98
Rint/Rσ		0.042/0.033	0.074/0.048	0.036/0.028	0.043/0.032	0.041/0.030	0.043/0.034
R1/R1 with |Fo| ≥ 4σ(Fo)		0.057/0.040	0.091/0.045	0.032/0.023	0.042/0.027	0.042/0.033	0.046/0.030
wR2/Goof		0.096/1.014	0.097/0.984	0.047/0.975	0.061/1.002	0.082/1.006	0.070/1.055
Residual electron density, ρ/e−∙10−6∙pm−3		−3.82/4.07	−4.31/4.46	−1.69/1.82	−2.16/2.27	−3.21/3.09	−3.29/3.45
CSD number		2394253	2394254	2394255	2394256	2394257	2394258

contains the crystallographic data for the B-type RE₅Br₃[AsO₃]₄ series with RE = Pr, Nd and Sm–Tb, and

Table 5. Fractional atomic coordinates and coefficients of the equivalent isotropic displacement parameters for the B-type RE₅Br₃[AsO₃]₄ representatives with RE = Pr, Nd and Sm–Tb.

Atom	Wyckoff Site	x/a	y/b	z/c	Ueq/pm2
Pr5Br3[AsO3]4
Pr1	2e	0	0.20745(9)	1/4	49(1)
Pr2	4g	−0.00328(5)	0.73670(7)	0.41419(3)	48(1)
Pr3	4g	0.32308(5)	0.24892(7)	0.41123(3)	67(1)
Br1	2f	1/2	0.24872(18)	1/4	160(3)
Br2	4g	0.50807(11)	0.75073(12)	0.42162(5)	91(2)
As1	4g	0.24419(11)	0.25781(12)	0.06087(5)	67(2)
As2	4g	0.26856(11)	0.75732(12)	0.26825(5)	59(2)
O1	4g	0.0847(7)	0.3092(9)	0.1213(3)	138(12)
O2	4g	0.1686(7)	0.0113(9)	0.0044(3)	84(11)
O3	4g	0.1813(7)	0.5041(9)	0.4929(3)	71(11)
O4	4g	0.1367(7)	0.8356(9)	0.1956(3)	92(11)
O5	4g	0.1741(7)	0.5076(9)	0.3180(3)	81(11)
O6	4g	0.1848(7)	0.9818(9)	0.3369(3)	67(11)
Nd5Br3[AsO3]4
Nd1	2e	0	0.20605(13)	1/4	89(2)
Nd2	4g	−0.00298(7)	0.73582(11)	0.41428(4)	84(1)
Nd3	4g	0.32186(7)	0.24851(11)	0.41117(4)	104(1)
Br1	2f	1/2	0.2490(4)	1/4	195(3)
Br2	4g	0.50710(11)	0.7511(3)	0.42205(7)	130(2)
As1	4g	0.24423(12)	0.2583(2)	0.06137(7)	104(2)
As2	4g	0.26815(12)	0.7580(2)	0.26804(7)	100(2)
O1	4g	0.0843(9)	0.3147(14)	0.1232(5)	188(18)
O2	4g	0.1687(9)	0.0114(14)	0.0038(5)	101(16)
O3	4g	0.1802(9)	0.5023(14)	0.4918(5)	145(18)
O4	4g	0.1359(9)	0.8406(14)	0.1957(5)	160(17)
O5	4g	0.1747(9)	0.5078(14)	0.3170(5)	141(18)
O6	4g	0.1847(9)	0.9822(14)	0.3373(5)	90(16)
Sm5Br3[AsO3]4
Sm1	2e	0	0.20307(7)	1/4	82(1)
Sm2	4g	−0.00144(4)	0.73417(6)	0.41437(2)	83(1)
Sm3	4g	0.31758(4)	0.24855(6)	0.41145(2)	89(1)
Br1	2f	1/2	0.24903(14)	1/4	186(2)
Br2	4g	0.50527(7)	0.75141(12)	0.42279(4)	130(1)
As1	4g	0.24327(8)	0.25875(11)	0.06269(4)	86(1)
As2	4g	0.26775(8)	0.75830(11)	0.26706(4)	81(1)
O1	4g	0.0843(6)	0.3210(7)	0.1234(2)	163(9)
O2	4g	0.1660(6)	0.0098(9)	0.0053(3)	110(9)
O3	4g	0.1779(6)	0.5016(9)	0.4923(3)	105(9)
O4	4g	0.1348(5)	0.8446(7)	0.1937(2)	123(9)
O5	4g	0.1729(7)	0.5024(8)	0.3168(3)	99(9)
O6	4g	0.1841(7)	0.9831(7)	0.3372(2)	94(9)
Eu5Br3[AsO3]4
Eu1	2e	0	0.20349(8)	1/4	56(1)
Eu2	4g	−0.00093(4)	0.73385(6)	0.41446(2)	57(1)
Eu3	4g	0.31635(4)	0.24839(6)	0.41165(2)	68(1)
Br1	2f	1/2	0.24905(16)	1/4	158(2)
Br2	4g	0.50475(8)	0.75134(13)	0.42294(4)	103(1)
As1	4g	0.24370(9)	0.25969(12)	0.06318(5)	65(1)
As2	4g	0.26775(9)	0.75885(12)	0.26672(5)	58(1)
O1	4g	0.0861(7)	0.3241(8)	0.1244(3)	157(12)
O2	4g	0.1654(6)	0.0109(9)	0.0065(3)	56(9)
O3	4g	0.1773(6)	0.5006(9)	0.4918(3)	60(9)
O4	4g	0.1355(6)	0.8458(8)	0.1934(3)	100(10)
O5	4g	0.1739(7)	0.5035(9)	0.3170(3)	82(11)
O6	4g	0.1830(7)	0.9822(8)	0.3385(3)	67(11)
Gd5Br3[AsO3]4
Gd1	2e	0	0.20140(9)	1/4	76(1)
Gd2	4g	−0.00089(4)	0.73331(6)	0.41445(2)	75(1)
Gd3	4g	0.31528(4)	0.24797(6)	0.41168(2)	80(1)
Br1	2f	1/2	0.24921(16)	1/4	172(3)
Br2	4g	0.50427(9)	0.75194(11)	0.42337(5)	122(2)
As1	4g	0.24290(9)	0.25997(11)	0.06338(5)	79(2)
As2	4g	0.26711(9)	0.75850(11)	0.26688(5)	75(2)
O1	4g	0.0850(7)	0.3268(9)	0.1250(3)	153(11)
O2	4g	0.1634(6)	0.0089(9)	0.0060(3)	93(10)
O3	4g	0.1775(7)	0.4993(9)	0.4926(3)	87(9)
O4	4g	0.1341(6)	0.8481(9)	0.1931(3)	102(10)
O5	4g	0.1724(7)	0.5024(9)	0.3172(3)	108(10)
O6	4g	0.1789(7)	0.9815(9)	0.3389(3)	96(10)
Tb5Br3[AsO3]4
Tb1	2e	0	0.20358(9)	1/4	88(1)
Tb2	4g	0.00008(5)	0.73453(7)	0.41434(3)	73(1)
Tb3	4g	0.31254(5)	0.24827(7)	0.41193(3)	68(1)
Br1	2f	1/2	0.2494(2)	1/4	162(3)
Br2	4g	0.50308(9)	0.75129(14)	0.42383(5)	109(2)
As1	4g	0.24279(9)	0.25921(13)	0.06434(5)	62(2)
As2	4g	0.26706(9)	0.75811(13)	0.26636(5)	65(2)
O1	4g	0.0841(8)	0.3303(12)	0.1258(4)	155(14)
O2	4g	0.1627(8)	0.0089(13)	0.0055(4)	90(12)
O3	4g	0.1751(9)	0.4989(14)	0.4937(4)	110(13)
O4	4g	0.1352(8)	0.8488(12)	0.1911(4)	116(12)
O5	4g	0.1734(9)	0.5009(13)	0.3171(4)	113(14)
O6	4g	0.1807(9)	0.9837(12)	0.3373(4)	75(13)

the fractional atomic positions and U_eq values, while

Table 6. Selected interatomic distances (d/pm) for the B-type RE₅Br₃[AsO₃]₄ representatives with RE = Pr, Nd and Sm–Tb.

RE		Pr	Nd	Sm	Eu	Gd	Tb
[(RE1)O8]13− polyhedron
RE1–O1	2×	238.1(6)	234.6(9)	233.0(4)	231.3(5)	229.8(5)	227.4(6)
RE1–O6	2×	251.8(6)	250.8(8)	247.8(5)	248.1(5)	245.1(5)	243.8(7)
RE1–O5	2×	252.5(6)	251.6(9)	248.4(5)	248.6(5)	247.6(5)	246.1(8)
RE1–O4	2×	254.2(6)	249.8(8)	246.1(4)	245.5(5)	242.4(5)	244.0(6)
[(RE2)O8]13− polyhedron
RE2–O4	1×	226.8(6)	226.6(8)	223.0(4)	222.8(5)	221.2(5)	218.5(6)
RE2–O3	1×	245.8(6)	243.3(8)	239.8(6)	238.2(6)	238.5(6)	237.4(7)
RE2–O6	1×	250.9(6)	249.7(8)	247.8(5)	245.0(6)	241.5(5)	242.7(7)
RE2–O2	1×	251.3(6)	251.2(8)	247.4(6)	245.9(6)	244.1(5)	243.5(8)
RE2–O1	1×	252.0(5)	248.3(8)	243.4(4)	242.0(5)	239.7(5)	238.1(6)
RE2–O2′	1×	255.6(6)	254.0(8)	252.6(6)	252.0(6)	250.5(5)	248.2(7)
RE2–O3′	1×	259.1(6)	257.9(9)	254.4(6)	253.6(6)	251.5(5)	249.1(8)
RE2–O5	1×	259.1(6)	259.0(9)	256.6(5)	255.4(6)	253.7(5)	253.6(7)
[(RE3)O4Br4]9− polyhedron
RE3–O6	1×	228.2(5)	226.0(8)	222.9(5)	221.4(5)	221.9(5)	220.2(7)
RE3–O3	1×	233.6(6)	231.0(9)	228.3(6)	226.2(5)	225.3(5)	225.0(7)
RE3–O5	1×	249.5(6)	248.8(8)	244.7(5)	243.3(5)	242.3(5)	239.7(7)
RE3–O2	1×	253.2(6)	250.7(8)	248.9(6)	249.0(5)	247.2(5)	244.7(7)
RE3–Br1	1×	316.0(1)	314.7(1)	313.9(4)	313.4(1)	312.5(1)	312.8(1)
RE3–Br2	1×	318.1(1)	316.7(2)	316.1(8)	315.6(1)	314.6(1)	314.6(1)
RE3–Br2′	1×	319.8(1)	319.1(1)	317.2(8)	316.1(1)	314.9(1)	314.9(1)
RE3–Br2″	1×	320.4(1)	319.1(2)	318.7(8)	318.3(1)	318.2(1)	317.6(1)
[(As1)O3]3− anion
As1–O1	1×	176.8(6)	178.3(8)	176.0(4)	175.3(5)	175.6(6)	176.0(7)
As1–O2	1×	178.4(6)	178.7(8)	178.6(6)	177.7(6)	179.0(5)	179.5(7)
As1–O3	1×	182.7(6)	184.2(8)	184.1(6)	184.8(5)	183.6(5)	183.8(8)
[(As2)O3]3− anion
As2–O4	1×	174.6(6)	174.4(8)	175.4(4)	174.4(5)	175.3(5)	176.0(6)
As2–O5	1×	181.2(5)	179.5(8)	181.8(5)	181.0(5)	181.1(5)	180.7(7)
As2–O6	1×	185.1(5)	184.6(8)	184.3(4)	185.1(5)	185.8(5)	184.2(6)

shows a compilation of selected distances.

3.3. Crystal Structure of RE₃Br₂[AsO₃][As₂O₅] (RE = Y and Dy–Yb)

The compounds with the composition RE₃Br₂[AsO₃][As₂O₅] (RE = Y and Dy–Yb) crystallize isotypically to RE₃Cl₂[AsO₃][As₂O₅] with RE = Sm–Gd [11,17] in the triclinic space group P$\overline{1}$ (no. 2) and lattice parameters from a = 539.15(4) pm, b = 870.68(6) pm, c = 1092.34(8), α = 90.661(2)°, β = 94.792(2)° and γ = 90.223(2)° for Dy₃Br₂[AsO₃][As₂O₅] to a = 533.56(4) pm, b = 869.61(6) pm, c = 1076.70(8), α = 90.698(2)°, β = 94.785(2)° and γ = 90.053(2)° for Yb₃Br₂[AsO₃][As₂O₅] with Z = 2. The crystal structure offers three different rare-earth metal(III) cation sites. (RE1)³⁺ and (RE2)³⁺ are both eightfold coordinated exclusively by oxygen atoms, forming square [(RE1,2)O₈]¹³⁻ prisms with d(Dy1–O) = 224–252 pm to d(Yb1–O) = 221–247 pm and d(Dy2–O) = 227–256 pm to d(Yb2–O) = 222–253 pm (Figure 7). These prisms join together via four common oxygen edges to form two-dimensional layers of composition $\begin{array}{c}2\\ \infty \end{array}${[(RE1/2)O$\begin{array}{c}e\\ 8/2\end{array}$]⁵⁻}, which propagate parallel to the ac-plane (Figure 7). The third crystallographically diverse rare-earth metal (III) cation, on the other hand, shows both a different coordination environment and a different linkage pattern. In contrast to the cations (RE1)³⁺ and (RE2)³⁺, which carry solely oxygen atoms, contacts to the Br⁻ anions occur for the first time for (RE3)³⁺, with four O²⁻ and four Br⁻ ligands erecting a square antiprism [(RE3)O₄Br₄]⁹⁻ (Figure 7). Bond lengths from d(Dy3–O) = 222–248 pm and d(Dy3–Br) = 287–321 pm to d(Yb3–O) = 219–243 pm and d(Yb3–Br) = 285–322 pm have to be stated. The (RE3)³⁺ cations are linked via common oxygen atoms to the previously described layers formed by the [REO₈]¹³⁻ polyhedra centered by the (RE1,2)³⁺ cations and graft above and below them (Figure 7). Amongst each other, the square [(RE3)O₄Br₄]⁹⁻ antiprisms are linked via common edges of bromide anions, but only (Br2)⁻ contributes to this linkage, whereas (Br1)⁻ maintains only a single contact to this (RE3)³⁺ cation. Thus, one-dimensional infinite double strands of the composition $\begin{array}{c}1\\ \infty \end{array}${[(RE3)O$\begin{array}{c}t\\ 4/1\end{array}$(Br1)$\begin{array}{c}t\\ 1/1\end{array}$(Br2)$\begin{array}{c}e\\ 3/3\end{array}$]⁷⁻} are formed, which propagate along the a-axis (Figure 7). Bonds are formed between these two structural motifs along the b-axis via contacts between the double strands and the two-dimensional layers containing the (RE1,2)³⁺ cations located above and below. Thus, two crystallographically different Br⁻ anions occur, which differ from each other by their different coordination spheres. While (Br1)⁻ only has only one contact in the area effective for bonding to (RE3)³⁺ cations with a bond length of around 286 pm, the (Br2)⁻ anion is triple-coordinated and deflected by around 90 pm from the triangular plane formed by its three RE³⁺ ligands.

The crystal structure exhibits three different sites for the As³⁺ cations. The (As1)³⁺ cation is only coordinated by three oxygen atoms and centers isolated ψ¹-tetrahedral [AsO₃]³⁻ units, allowing a strong stereochemical lone-pair activity (Figure 8, left). A distinction must be made between different cases when analyzing the second coordination sphere. The oxygen atom O1 only forms bonds to two rare-earth metal(III) cations, while there are three for O2 and O3. This is the reason why the distance between the (As1)³⁺ cation and O1 represents the shortest with d(As1–O1) = 174 pm for Dy₃Br₂[AsO₃][As₂O₅] to d(As1–O1) = 175 pm for Yb₃Br₂[AsO₃][As₂O₅], while the remaining bond lengths fall into the range of d(As1–O2/3) = 180–185 pm for Dy₃Br₂[AsO₃][As₂O₅] to d(As1–O2/3) = 180–184 pm for Yb₃Br₂[AsO₃][As₂O₅]. The (As2)³⁺ and (As3)³⁺ cations are surrounded by a total of five oxygen atoms, forming a [As₂O₅]⁴⁻ pyro-anion, in which one oxygen atom bridges both arsenic(III) cations according to [O₂(As2)(O6)(As3)O₂]⁴⁻ (Figure 8, right), frequently addressed as “conversural ψ¹-bitetrahedron” [17]. The free electron pairs of both As³⁺ cations are oriented into the same direction. If the second coordination sphere of the As³⁺ cations is regarded, it becomes noticeable that the oxygen atoms O4 and O5 each form bonds to three RE³⁺ cations, while there are only two such contacts for O6 to O8. As a result, the lengths of the arsenic–oxygen bonds in the case of O4 and O5 with values of d(As2–O4/5) = 177–184 pm for Dy₃Br₂[AsO₃][As₂O₅] to d(As2–O4/5) = 179–186 pm for Yb₃Br₂[AsO₃][As₂O₅] are somewhat longer than those of O7 and O8, which range between d(As3–O7/8) = 174–177 pm for Dy₃Br₂[AsO₃][As₂O₅] and d(As3–O7/8) = 175–176 pm for Yb₃Br₂[AsO₃][As₂O₅]. Of all those occurring in the entire crystal structure, the bridging oxygen atom O6 always shows the largest value in relation to any As³⁺ cations in terms of bond lengths (d(As2/3–O6) = 186–196 pm for Dy₃Br₂[AsO₃][As₂O₅] to d(As2/3–O6) = 186–196 pm for Yb₃Br₂[AsO₃][As₂O₅]) with distances about 15 pm longer than those of the remaining arsenic–oxygen bonds. This fact gives the pyro-anion [As₂O₅]⁴⁻ a highly asymmetric shape with ∢(As2–O6–As3) bond angles from 116° for RE = Dy to 115° for RE = Yb.

Within the crystal structure, channels are left behind between the bromide anion and rare-earth metal strands, into which the free electron pairs of the As³⁺ cations point (Figure 9).

Table 7. Crystallographic data of the RE₃Br₂[AsO₃][As₂O₅] representatives with RE = Y and Dy–Yb and their determination.

RE3Br2[AsO3][As2O5]		Y	Dy	Ho	Er	Tm	Yb
Crystal system		triclinic
Space group		P$\overline{1}$
Lattice parameters,	a/pm	538.06(4)	539.15(4)	537.54(4)	536.08(4)	534.76(4)	533.56(4)
	b/pm	870.48(6)	870.68(6)	870.39(6)	870.12(6)	869.81(6)	869.61(6)
	c/pm	1089.37(8)	1092.34(8)	1087.91(8)	1083.75(8)	1080.05(8)	1076.70(8)
	α/°	90.665(2)	90.661(2)	90.672(2)	90.683(2)	90.691(2)	90.698(2)
	β/°	94.790(2)	94.792(2)	94.798(2)	94.786(2)	94.784(2)	94.785(2)
	γ/°	90.189(2)	90.223(2)	90.173(2)	90.127(2)	90.088(2)	90.053(2)
Formula units, Z		2
Calculated density, Dx/g∙cm−3		5.091	6.500	6.596	6.688	6.763	6.883
Molar volume, Vm/cm3∙mol−1		153.07(7)	153.86(7)	152.72(7)	151.67(7)	150.73(7)	149.89(7)
Diffractometer		κ-CCD (Bruker-Nonius)
Wavelength		Mo-Kα (λ = 71.07 pm)
Electron sum, F(000)/e−		700	862	868	874	880	886
Measurement limit, θmax/°		27.48	33.05	27.48	27.48	27.48	27.48
Measurement range, ±h/±k/±lmin/max		6/11/14	8/13/16	6/11/14	6/11/14	7/11/14	6/11/13
Observed reflections		16792	17816	18942	16738	21574	17988
Unique reflections		2331	3603	2323	2313	2286	2267
Absorption coefficient, µ/mm−1		34.55	39.19	40.78	42.49	44.20	45.89
Rint/Rσ		0.093/0.040	0.085/0.048	0.075/0.037	0.074/0.037	0.066/0.026	0.083/0.034
R1/R1 with |Fo| ≥ 4σ(Fo)		0.089/0.050	0.074/0.052	0.044/0.034	0.039/0.032	0.021/0.019	0.037/0.037
wR2/Goof		0.098/1.021	0.133/1.033	0.083/1.046	0.071/1.083	0.043/1.109	0.092/1.081
Residual electron density, ρ/e−∙10−6∙pm−3		−1.49/1.34	−3.15/3.24	−1.93/2.40	−2.07/2.16	−1.16/1.09	−2.36/2.43
CSD number		2330957	2330958	2330959	2330960	2330961	2330962

contains the crystallographic data for the RE₃Br₂[AsO₃][As₂O₅] representatives with RE = Y and Dy–Yb,

Table 8. Fractional atomic coordinates and coefficients of the equivalent isotropic displacement parameters for the RE₃Br₂[AsO₃][As₂O₅] representatives with RE = Y and Dy–Yb.

Atom	Wyckoff Site	x/a	y/b	z/c	Ueq/pm2
Y3Br2[AsO3][As2O5]
Y1	2i	0.24645(19)	0.00069(13)	0.37073(11)	166(3)
Y2	2i	0.26430(19)	0.00226(13)	0.87296(11)	171(3)
Y3	2i	0.26578(19)	0.30990(13)	0.63195(11)	184(3)
Br1	2i	0.30265(19)	0.50700(13)	0.84590(11)	241(3)
Br2	2i	0.23049(19)	0.49526(13)	0.38910(11)	217(3)
As1	2i	0.27732(19)	0.75960(13)	0.59460(11)	172(3)
As2	2i	0.18944(19)	0.72135(13)	0.15158(11)	165(3)
As3	2i	0.18310(19)	0.25610(13)	0.09726(11)	173(3)
O1	2i	0.3677(13)	0.9207(9)	0.6873(7)	226(18)
O2	2i	0.0108(13)	0.8407(8)	0.5079(7)	186(17)
O3	2i	0.4994(13)	0.8262(8)	0.4858(7)	187(17)
O4	2i	0.4436(12)	0.8275(8)	0.2209(7)	179(17)
O5	2i	0.0197(12)	0.1823(8)	0.7481(7)	146(16)
O6	2i	0.0643(12)	0.8423(8)	0.0213(7)	168(17)
O7	2i	0.4142(13)	0.1363(9)	0.0443(7)	241(18)
O8	2i	0.0815(13)	0.1367(8)	0.2119(7)	188(17)
Dy3Br2[AsO3][As2O5]
Dy1	2i	0.24538(11)	0.00096(7)	0.37052(6)	152(1)
Dy2	2i	0.26454(11)	0.00171(7)	0.87292(6)	153(1)
Dy3	2i	0.26669(11)	0.31302(7)	0.63249(6)	164(1)
Br1	2i	0.3030(3)	0.50816(18)	0.84651(14)	231(3)
Br2	2i	0.2309(3)	0.49457(17)	0.38864(13)	198(3)
As1	2i	0.2768(2)	0.75899(16)	0.59427(13)	158(3)
As2	2i	0.1889(2)	0.72071(16)	0.15190(13)	155(3)
As3	2i	0.1821(2)	0.25736(16)	0.09621(13)	155(3)
O1	2i	0.3660(18)	0.9183(12)	0.6857(9)	221(20)
O2	2i	0.0131(16)	0.8400(11)	0.5070(9)	147(17)
O3	2i	0.4965(17)	0.8255(11)	0.4844(9)	168(18)
O4	2i	0.4441(16)	0.8288(11)	0.2207(9)	158(17)
O5	2i	0.0200(18)	0.1822(12)	0.7475(9)	174(18)
O6	2i	0.0618(16)	0.8419(11)	0.0228(9)	158(17)
O7	2i	0.4109(18)	0.1386(11)	0.0451(9)	183(19)
O8	2i	0.0818(18)	0.1372(11)	0.2116(9)	177(18)
Ho3Br2[AsO3][As2O5]
Ho1	2i	0.24644(7)	0.00085(5)	0.37100(4)	123(1)
Ho2	2i	0.26396(7)	0.00192(5)	0.87279(4)	123(1)
Ho3	2i	0.26531(7)	0.31149(5)	0.63203(4)	151(1)
Br1	2i	0.30222(17)	0.50643(12)	0.84633(9)	208(2)
Br2	2i	0.23100(16)	0.49506(11)	0.38905(8)	180(2)
As1	2i	0.27705(17)	0.75895(11)	0.59450(9)	143(2)
As2	2i	0.19021(17)	0.72145(11)	0.15191(9)	146(2)
As3	2i	0.18328(17)	0.25670(11)	0.09680(9)	145(2)
O1	2i	0.3681(11)	0.9203(8)	0.6874(6)	192(15)
O2	2i	0.0111(11)	0.8403(7)	0.5084(6)	108(12)
O3	2i	0.4991(11)	0.8283(8)	0.4859(6)	151(14)
O4	2i	0.4438(11)	0.8261(8)	0.2207(6)	163(14)
O5	2i	0.0207(11)	0.1810(8)	0.7479(6)	150(14)
O6	2i	0.0598(11)	0.8430(7)	0.0216(6)	152(14)
O7	2i	0.4133(11)	0.1378(7)	0.0432(6)	155(14)
O8	2i	0.0824(11)	0.1368(8)	0.2122(6)	165(14)
Er3Br2[AsO3][As2O5]
Er1	2i	0.24639(7)	0.00057(5)	0.37118(4)	143(1)
Er2	2i	0.26408(7)	0.00229(5)	0.87282(4)	143(1)
Er3	2i	0.26480(7)	0.30989(5)	0.63183(4)	164(1)
Br1	2i	0.30195(18)	0.50585(12)	0.84580(9)	218(2)
Br2	2i	0.23051(17)	0.49557(11)	0.38950(9)	195(2)
As1	2i	0.27795(17)	0.75918(11)	0.59523(9)	151(2)
As2	2i	0.19065(17)	0.72116(11)	0.15179(9)	149(2)
As3	2i	0.18349(17)	0.25634(11)	0.09761(9)	151(2)
O1	2i	0.3713(12)	0.9190(8)	0.6891(6)	199(15)
O2	2i	0.0121(12)	0.8418(8)	0.5080(6)	152(14)
O3	2i	0.4982(12)	0.8284(8)	0.4859(6)	156(14)
O4	2i	0.4479(12)	0.8283(8)	0.2218(6)	174(14)
O5	2i	0.0193(12)	0.1804(8)	0.7474(6)	157(14)
O6	2i	0.0634(12)	0.8440(8)	0.0223(6)	164(14)
O7	2i	0.4136(12)	0.1373(8)	0.0444(6)	171(14)
O8	2i	0.0810(12)	0.1363(8)	0.2129(6)	168(14)
Tm3Br2[AsO3][As2O5]
Tm1	2i	0.24704(4)	0.00003(3)	0.37102(2)	93(1)
Tm2	2i	0.26454(4)	0.00269(3)	0.87266(2)	92(1)
Tm3	2i	0.26456(4)	0.30786(3)	0.63165(2)	108(1)
Br1	2i	0.30173(11)	0.50478(7)	0.84510(5)	163(1)
Br2	2i	0.23078(10)	0.49554(6)	0.38983(5)	147(1)
As1	2i	0.27809(10)	0.75946(6)	0.59570(5)	94(1)
As2	2i	0.19064(10)	0.72049(6)	0.15175(5)	90(1)
As3	2i	0.18288(10)	0.25530(6)	0.09807(5)	94(1)
O1	2i	0.3736(7)	0.9187(4)	0.6895(3)	140(8)
O2	2i	0.0108(7)	0.8427(4)	0.5087(3)	88(7)
O3	2i	0.4987(7)	0.8288(4)	0.4857(3)	94(7)
O4	2i	0.4485(7)	0.8276(4)	0.2222(3)	114(8)
O5	2i	0.0185(7)	0.1801(4)	0.7469(3)	96(7)
O6	2i	0.0630(7)	0.8439(4)	0.0220(3)	112(8)
O7	2i	0.4143(7)	0.1355(4)	0.0445(3)	108(8)
O8	2i	0.0795(7)	0.1355(4)	0.2134(3)	105(8)
Yb3Br2[AsO3][As2O5]
Yb1	2i	0.24714(11)	−0.00059(7)	0.37125(5)	144(2)
Yb2	2i	0.26475(11)	0.00341(7)	0.87265(5)	144(2)
Yb3	2i	0.26418(11)	0.30675(7)	0.63146(5)	165(2)
Br1	2i	0.3011(2)	0.50392(18)	0.84506(13)	214(3)
Br2	2i	0.2309(2)	0.49578(17)	0.38989(12)	202(3)
As1	2i	0.2788(2)	0.75968(17)	0.59596(12)	153(3)
As2	2i	0.1910(2)	0.72064(17)	0.15149(12)	143(3)
As3	2i	0.1830(2)	0.25445(17)	0.09896(12)	151(3)
O1	2i	0.3755(17)	0.9179(12)	0.6902(9)	229(22)
O2	2i	0.0125(16)	0.8431(11)	0.5085(8)	129(17)
O3	2i	0.4981(17)	0.8298(11)	0.4856(8)	145(18)
O4	2i	0.4479(16)	0.8311(12)	0.2227(8)	160(19)
O5	2i	0.0196(16)	0.1772(11)	0.7462(8)	134(17)
O6	2i	0.0630(16)	0.8450(11)	0.0217(9)	159(19)
O7	2i	0.4147(17)	0.1337(11)	0.0445(8)	145(18)
O8	2i	0.0787(17)	0.1338(12)	0.2128(9)	173(19)

summarizes the fractional atomic positions and U_eq values and

Table 9. Selected interatomic distances (d/pm) for the RE₃Br₂[AsO₃][As₂O₅] representatives with RE = Y and Dy–Yb.

RE		Y	Dy	Ho	Er	Tm	Yb
[(RE1)O8]13− polyhedron
RE1–O8	1×	223.2(7)	223.5(9)	222.7(6)	221.8(6)	221.0(4)	220.9(9)
RE1–O1	1×	232.4(7)	234.0(10)	232.1(6)	231.2(7)	229.6(4)	228.9(10)
RE1–O3	1×	234.6(7)	234.3(10)	233.1(6)	232.1(6)	231.0(4)	229.3(9)
RE1–O2	1×	241.8(7)	243.6(9)	241.3(6)	240.5(6)	239.2(4)	239.4(9)
RE1–O5	1×	242.8(7)	242.5(10)	242.3(6)	240.8(7)	239.6(4)	237.3(9)
RE1–O2′	1×	247.7(7)	247.0(9)	247.9(6)	245.7(6)	245.7(4)	244.0(9)
RE1–O3′	1×	248.2(8)	250.8(10)	246.7(6)	246.4(7)	245.9(4)	245.3(10)
RE1–O4	1×	251.1(7)	251.5(9)	252.1(6)	250.5(7)	249.6(4)	246.6(9)
[(RE2)O8]13− polyhedron
RE2–O7	1×	224.5(7)	226.6(9)	225.8(6)	223.1(7)	223.0(4)	221.7(9)
RE2–O1	1×	224.9(8)	227.3(11)	224.6(6)	224.6(7)	222.4(4)	221.8(10)
RE2–O7′	1×	227.8(9)	229.6(10)	227.5(6)	227.5(7)	226.2(4)	224.7(9)
RE2–O8	1×	233.5(7)	233.9(9)	233.5(6)	232.8(7)	231.9(4)	230.5(10)
RE2–O5	1×	240.9(7)	241.9(10)	239.8(6)	239.4(6)	239.1(4)	236.7(9)
RE2–O4	1×	245.0(7)	244.5(9)	245.6(6)	242.9(6)	242.3(4)	240.2(9)
RE2–O6	1×	245.8(7)	248.0(9)	246.6(6)	245.3(7)	245.2(4)	244.4(9)
RE2–O6′	1×	257.0(7)	256.2(10)	254.6(6)	254.4(6)	253.9(4)	252.8(10)
[(RE3)O4Br4]9− polyhedron
RE3–O5	1×	221.1(7)	222.3(9)	221.6(6)	220.8(6)	219.5(4)	219.1(9)
RE3–O3	1×	221.2(7)	222.8(9)	223.0(6)	222.0(6)	220.4(4)	220.0(9)
RE3–O2	1×	241.1(8)	243.4(10)	241.9(6)	241.0(7)	239.9(4)	239.3(9)
RE3–O4	1×	246.7(7)	248.4(10)	246.6(7)	244.5(7)	242.5(4)	243.1(9)
RE3–Br1	1×	287.1(2)	286.7(2)	286.0(1)	285.5(1)	284.9(6)	284.5(2)
RE3–Br2	1×	310.7(2)	310.6(2)	309.7(1)	309.2(1)	308.9(6)	308.7(2)
RE3–Br2′	1×	316.2(2)	316.2(2)	315.3(1)	314.6(1)	314.9(6)	314.6(2)
RE3–Br2″	1×	321.8(2)	320.6(2)	321.0(1)	321.4(1)	321.9(6)	321.9(2)
[(As1)O3]3− anion
As1–O1	1×	176.2(8)	174.2(11)	176.4(7)	176.0(7)	175.6(4)	175.1(11)
As1–O2	1×	180.1(7)	179.6(9)	179.4(6)	180.0(6)	180.2(4)	179.6(9)
As1–O3	1×	184.8(7)	185.2(9)	185.3(6)	184.6(6)	184.9(4)	184.4(9)
[(As2)(As3)O5]4− anion
As2–O4	1×	176.0(7)	177.3(9)	174.8(6)	177.5(7)	177.6(4)	178.7(9)
As2–O5	1×	183.1(7)	184.0(9)	184.0(6)	183.9(7)	183.8(4)	185.8(9)
As2–O6	1×	185.9(7)	185.8(9)	186.9(6)	185.9(6)	186.0(4)	186.2(9)
As3–O8	1×	175.4(7)	176.5(9)	176.1(6)	176.0(6)	175.7(4)	174.9(10)
As3–O7	1×	175.5(7)	173.6(10)	174.7(6)	174.2(7)	175.0(4)	175.6(9)
As3–O6	1×	196.3(7)	196.1(9)	195.0(6)	197.1(7)	195.9(4)	196.1(9)

offers selected interatomic distances.

3.4. Crystal-Structure Comparison

The rare-earth metal(III) bromide oxoarsenates(III) of formula type RE₅Br₃[AsO₃]₄ in the A- (RE = La and Ce) and B-type structure (RE = Pr, Nd and Sm–Tb) exhibit very similar structural motifs, if only all features are considered. The anionic environments of the RE³⁺ cations are almost identical, although the polyhedra of the (RE2)³⁺ cations in the A-type resemble more of a square antiprism, while, in the B-type, they are more similar to a cube. The distances between the rare-earth metal and oxygen develop according to the lanthanoid contraction with no particular exceptions. The same applies to the anionic environment of the As³⁺ cations, as here are no real differences too. By stacking several layers, however, the differences become clear, as both structures represent stacking variants. In the A-type, there is a one-third offset from layer to layer, so that two particles of the following layers come to rest below a gap in one layer before the next gap occurs (Figure 10, left). As a result, every fourth bromide layer is congruent. The B-type does not show this behavior, because, here, the gaps of each layer lie directly on top of each other and are therefore congruent, creating additional channels along [100] (Figure 10, right). To summarize, it can be said that the A-type follows an ABC layer sequence, and the B-type is just an AAA variation.

At first glance, it is not noticeable that the compositions RE₅Br₃[AsO₃]₄ and RE₃Br₂[AsO₃][As₂O₅] have a lot in common, but they share the completely same building blocks, except that, in the latter, there are two ψ¹-tetrahedral [AsO₃]³⁻ anions, which are conversurally connected to [As₂O₅]⁴⁻ units. If the bromide anions and oxygen atoms in both empirical formulae are brought to the same denominator, the following juxtaposition is obtained: RE₅Br₃As₄O₁₂ (for RE₅Br₃[AsO₃]₄) versus RE_4.5Br₃As_4.5O₁₂ (for RE₃Br₂[AsO₃][As₂O₅]). This shows that there has been a thinning of the rare-earth metal RE³⁺ and, at the same time, an enrichment of the arsenic As³⁺ in the sum formula by one-half each. This manifests structurally in a way that the (RE3)³⁺ cations are thinned out and, instead of the double layers $\begin{array}{c}2\\ \infty \end{array}${[(RE3)O$\begin{array}{c}t\\ 4/1\end{array}$(Br1)$\begin{array}{c}v\\ 1/2\end{array}$(Br2)$\begin{array}{c}e\\ 3/3\end{array}$]^6.5−}, only double strands $\begin{array}{c}1\\ \infty \end{array}${[(RE3)O$\begin{array}{c}t\\ 4/1\end{array}$(Br1)$\begin{array}{c}t\\ 1/1\end{array}$(Br2)$\begin{array}{c}e\\ 3/3\end{array}$]⁷⁻} are able to be formed, and, on the other hand, instead of isolated [AsO₃]³⁻ units, higher condensed oxoarsenates(III) with [As₂O₅]⁴⁻ anions occur.

4. Conclusions

With a total of fourteen different representatives in three different structures, the range of rare-earth metal(III) bromide oxoarsenates(III) has been considerably expanded and supplemented. Future research must show whether the composition RE₅Br₃[AsO₃]₄ and RE₃Br₂[AsO₃][As₂O₅] can also exist in parallel. Furthermore, the β angle in the B-type structure approaches closer and closer to 90°, which raises the question whether a structurally novel C-type for the heavier lanthanides might be formed, showing orthorhombic symmetry. Furthermore, under the synthetic conditions reported in this article, it was not possible to realize one of the two compositions for RE = Lu. Here, high-pressure experiments would be conceivable to realize an eightfold coordination for Lu³⁺, as this tends to prefer six- or sevenfold coordination in contrast to other rare-earth metals.