Flux Synthesis, Crystal Structures, and Magnetism of the Series La2n+2MnSen+2O2n+2 (n = 0–2)

Abstract

Three members of the homologous series of manganese oxyselenides with the general formula La_2n+2MnSe_n+2O_2n+2 (n = 0–2) have been synthesized in a NaI/KI flux and characterized by single-crystal X-ray diffraction, powder X-ray diffraction and magnetic measurements. The structures consist of chains of edge-sharing MnSe₄O₂-octahedra along the b-axis which are linked together along the a-axis by edge-sharing OLa₄- and/or OLa₃Mn-tetrahedra forming infinite ribbons of increasing width. $mC$-La₂MnSe₂O₂ (Pb₂HgCl₂O₂-type, $C2/m$, a = 11.6621(5) Å, b = 3.9719(1) Å, c = 7.2049(3) Å, β = 121.655(2)${}^{\circ }$) represents a new polymorph of this compound. La₄MnSe₃O₄ ($P2/m$, a = 9.0055(4) Å, b = 4.0186(1) Å, c = 7.1946(3) Å, β = 109.715(2)${}^{\circ }$) and La₆MnSe₄O₆ ($C2/m$, a = 24.760(2) Å, b = 4.0359(3) Å, c = 7.1850(6) Å, β = 104.162(3)${}^{\circ }$) exhibit new structure types. Magnetic measurements suggest antiferromagnetic order of the moments below about 15 K with effective magnetic moments of 5.53(1), 5.99(1) and 6.01(1) ${\mu }_{\mathrm{B}}$ per formula unit for n = 1, 2 and 3, respectively.

1. Introduction

The research into structural chemistry and physical properties of transition-metal oxyselenides has been very fruitful during the last years. Numerous new compounds with the general formula RE₂TSe₂O₂ (RE = La, Ce; T = Cd, Fe, Mn, Zn) [1,2,3,4,5,6,7,8,9] have been discovered and their structures and magnetism have been studied. Among the particular families, different polymorphs may exist which makes the diversity even bigger. In order to distinguish the different polymorphs mentioned in this paper, we extend the formulae by the Pearson letters. Recently, we found new polymorphs in the RE₂FeSe₂O₂-family (RE = La, Ce) [2]. $mC$-La₂FeSe₂O₂ and $mC$-Ce₂FeSe₂O₂ crystallize in the Pb₂HgCl₂O₂-structure type with distorted edge-sharing FeSe₄O₂-octahedra linked together by edge-sharing OLa₃Fe-tetrahedra forming infinite ribbons which are two tetrahedra in width. Similar building blocks with an analogous connectivity between the TSe₄O₂-octahedra and the ORE₃T-tetrahedra have already been observed in the compounds RE_3.67Ti₂Se₆O₃ (RE = Ce, Nd, Sm) [10], La₄Ti₂Se₅O₄ [11], La₆Ti₃Se₉O₅ [11] and CeCrSe₂O [12]. Beside the iron compounds, there are two polymorphs known in the analogous manganese family. $oC$-La₂MnSe₂O₂ [1] has a defect ZrCuSiAs-type structure with slabs of edge-sharing OLa₄-tetrahedra stacked alternating with slabs of MnSe₄-tetrahedra. The structure of $oA$-La₂MnSe₂O₂ [8] consists of MnSe₄-tetrahedra and edge-sharing MnSe₄O₂-octahedra which are connected by infinite ribbons of edge-sharing OLa₄- and OLa₃Mn-tetrahedra four units wide.

Beside oxyselenides with the general chemical composition above, numerous other rare-earth oxyselenides are known, for example RE₄TiSe₄O₄ (RE = Sm, Gd, Tb, Dy, Ho, Er, Y) [13,14] and RE₄O₄Se(Se₂) (RE = La, Ce, Pr, Nd, Sm) [15]. RE₄TiSe₄O₄ can be regarded as a defect variant of $oA$-La₂MnSe₂O₂ with the same building blocks but without TiSe₄-tetrahedra. In contrast, RE₄O₄Se(Se₂) compounds contain Se²⁻ anions as well as (Se−Se)²⁻ dumbbells, which are separated by layers of distorted (RE₄O)¹⁰⁺ tetrahedra. Their magnetism origins from the $4f$-electrons of the particular rare-earth metal, and spin frustration is observed in the rare-earth-oxide tetrahedra of these compounds [15]. Formally, it should be possible to insert divalent transition-metals in this structure by breaking up the diselenide units leading to the formula RE₄TSe₃O₄.

In this paper, we present La₄MnSe₃O₄ as the first example of such compounds with only Se²⁻ anions in a new structure related to the RE₄TiSe₄O₄-type. La₄MnSe₃O₄ can formally be obtained by adding one La₂SeO₂-unit to La₂MnSe₂O₂. We were also successful to add a second La₂SeO₂-unit in order to obtain the compound La₆MnSe₄O₆, which represents the third member of the homologous series La_2n+2MnSe_n+2O_2n+2. Additional we have found $mC$-La₂MnSe₂O₂ as a new polymorph to the manganese-family and present magnetic susceptibility data of all compounds.

2. Results

Since conventional solid state synthesis did not lead to satisfying results, a flux synthesis was developed in order to get high purity samples (>95 wt %). $mC$-La₂MnSe₂O₂ was synthesized in a NaI/KI-flux at 1273 K starting from the low temperature polymorph $oC$-La₂MnSe₂O₂. The other two members of the homologous series were synthesized starting from the binary compounds La₂O₃, La₂Se₃ and MnSe, which were pre-reacted at 873 K, homogenized and then heated in a NaI/KI-flux at 1273–1373 K. After washing the reaction mixture to remove the flux, a large amount of bright brown, transparent rod-like crystals, as well as polycrystalline yellow-brown powders were obtained. The crystal structures were determined by single-crystal X-ray diffraction. A comparison of the obtained data is given in

Table 1. Crystal data and structure refinement of La_2n+2MnSe_n+2O_2n+2 (n = 0, 1, 2).

	$\mathit{mC}$-La2MnSe2O2	La4MnSe3O4	La6MnSe4O6
Space group	$C2/m$	$P2/m$	$C2/m$
a (Å)	11.6621(5)	9.0055(4)	24.760(2)
b (Å)	3.9719(1)	4.0186(1)	4.0359(3)
c (Å)	7.2049(3)	7.1946(3)	7.1850(6)
$\beta {(}^{\circ })$	121.655(2)	109.715(2)	104.162(3)
Volume (Å${}^3$)	284.08(2)	245.11(2)	696.16(10)
Z	2	1	2
$R_{int}$	0.024	0.028	0.056
$R_{\sigma }$	0.032	0.021	0.048
${\theta }_{max}{(}^{\circ })$	53.88	34.98	35.10
$R_1$ (obs)	0.024	0.023	0.026
$R_1$ (all)	0.036	0.032	0.049
$w{R}_2$ (obs)	0.048	0.050	0.041
$w{R}_2$ (all)	0.051	0.056	0.049
GooF	1.20	1.51	1.00
$\Delta e\left(e{\mathrm{\AA }}^3\right)$	+1.9/−2.5	+3.9/−1.7	+1.7/−1.7

, for details see

Table A1. Crystallographic data of mC-La₂MnSe₂O₂.

Formula weight (g·mol−1)			522.7
Space group, Z			C2/m, 2
a, b, c (Å)			11.662(1), 3.972(1), 7.205(1)
β (${}^{\circ }$)			121.7(1)
Volume (Å3), ρX-ray (g·cm−3)			284.1(1), 6.11
Crystal size (mm3)			0.06 × 0.02 × 0.01
Diffractometer			Bruker D8 QUEST
Radiation λ (pm)			71.073
Absorption coeff. μ (mm−1)			29.6
2θ range (${}^{\circ }$)			6.64–107.76
Index range (hkl)			−19 ≤ h ≤ 25, k ± 8, −16 ≤ l ≤ 14
No. reflections collected			5770
No. unique data, Rint, Rσ			1731, 0.02, 0.03
No. data with I > 3σ(I)			1441
No. parameters			23
R1(obs/all)			0.024/0.036
wR2(obs/all)			0.048/0.051
$\Delta e(e/{\mathrm{\AA }}^3)$			1.92/−2.53
Atomic and Displacement Parameters
	Site	x, y, z	U11	U22	U33	Occ.
La	4i	0.6923(1), 0, 0.2430(1)	0.0073(1)	0.0058(1)	0.0059(1)	1
Mn	2d	0, $\frac{1}{2}$, $\frac{1}{2}$	0.0063(2)	0.0141(2)	0.0137(2)	1
Se	4i	0.9405(1), 0, 0.1797(1)	0.0087(1)	0.0088(1)	0.0080(1)	1
O	4i	0.8028(2), $\frac{1}{2}$, 0.4187(1)	0.0059(1)	0.0077(7)	0.0081(6)	1

,

Table A2. Crystallographic data of La₄MnSe₃O₄.

Formula weight (g·mol−1)			911.4
Space group, Z			P2/m, 1
a, b, c (Å)			9.006(1), 4.019(1), 7.195(1)
β (${}^{\circ }$)			109.7(1)
Volume (Å3), ρX-ray (g·cm−3)			245.1(1), 6.17
Crystal size (mm3)			0.04 × 0.02 × 0.01
Diffractometer			Bruker D8 QUEST
Radiation λ (pm)			71.073
Absorption coeff. μ (mm−1)			29.4
2θ range (${}^{\circ }$)			4.80–69.96
Index range (hkl)			h ± 14, −6 ≤ k ≤ 5, l ± 11
No. reflections collected			6887
No. unique data, Rint, Rσ			1210, 0.03, 0.02
No. data with I > 3σ(I)			1017
No. parameters			39
R1(obs/all)			0.023/0.032
wR2(obs/all)			0.050/0.056
$\Delta e\left(e{\mathrm{\AA }}^3\right)$			3.85/−1.71
Atomic and Displacement Parameters
	Site	x, y, z	U11	U22	U33	Occ.
La1	2n	0.2245(1), $\frac{1}{2}$, 0.1732(1)	0.0053(1)	0.0056(1)	0.0046(1)	1
La2	2m	0.6430(1), 0, 0.3496(1)	0.0053(1)	0.0044(1)	0.0048(1)	1
Mn1	1f	0, $\frac{1}{2}$, $\frac{1}{2}$	0.0120(6)	0.0113(5)	0.0115(5)	1
Se1	1e	$\frac{1}{2}$, $\frac{1}{2}$, 0	0.0072(3)	0.0062(3)	0.0048(3)	1
Se2	2m	0.0627(1), 0, 0.7971(1)	0.0078(2)	0.0095(2)	0.0079(2)	1
O1	2m	0.3651(4), 0, 0.3114(5)	0.0048(16)	0.0069(16)	0.0082(16)	1
O2	2n	0.2317(4), $\frac{1}{2}$, 0.5070(5)	0.0046(16)	0.0057(15)	0.0079(16)	1

and

Table A3. Crystallographic data of La₆MnSe₄O₆.

Formula weight (g·mol−1)			1300.2
Space group, Z			C2/m, 2
a, b, c (Å)			24.760(1), 4.036(1), 7.185(1)
β (${}^{\circ }$)			104.2(1)
Volume (Å3), ρX-ray (g·cm−3)			696.2(1), 6.20
Crystal size (mm3)			0.03 × 0.02 × 0.01
Diffractometer			Bruker D8 QUEST
Radiation λ (pm)			71.073
Absorption coeff. μ (mm−1)			29.3
2θ range (${}^{\circ }$)			5.84–70.20
Index range (hkl)			−40 ≤ h ≤ 38, k ± 6, l ± 11
No. reflections collected			8892
No. unique data, Rint, Rσ			1375, 0.05, 0.05
No. data with I > 3σ(I)			957
No. parameters			53
R1(obs/all)			0.023/0.049
wR2(obs/all)			0.041/0.049
$\Delta e\left(e{\mathrm{\AA }}^3\right)$			1.65/−1.70
Atomic and Displacement Parameters
	Site	x, y, z	U11	U22	U33	Occ.
La1	4i	0.0788(1), $\frac{1}{2}$, 0.1453(1)	0.0050(2)	0.0060(2)	0.0049(2)	1
La2	4i	0.2263(1), 0, 0.2665(1)	0.0045(2)	0.0044(2)	0.0043(2)	1
La3	4i	0.1261(1), 0, 0.6051(61)	0.0056(2)	0.0045(2)	0.0052(2)	1
Mn1	2d	0, $\frac{1}{2}$, $\frac{1}{2}$	0.0110(9)	0.0110(7)	0.0100(8)	1
Se1	4i	0.8252(1), $\frac{1}{2}$, 0.0613(1)	0.0069(3)	0.0067(3)	0.0048(3)	1
Se2	4i	0.4781(1), $\frac{1}{2}$, 0.2116(1)	0.0076(4)	0.0095(3)	0.0075(4)	1
O1	4i	0.2752(2), $\frac{1}{2}$, 0.3975(6)	0.004(2)	0.009(2)	0.006(2)	1
O2	4i	0.1288(2), 0, 0.2662(7)	0.007(2)	0.007(2)	0.007(2)	1
O3	4i	0.0823(2), $\frac{1}{2}$, 0.4801(6)	0.003(2)	0.005(2)	0.010(2)	1

in the appendix. $mC$-La₂MnSe₂O₂ and La₆MnSe₄O₆ adopt monoclinic $C2/m$ (No. 12) symmetry, whereas La₄MnSe₃O₄ crystallizes in space group $P2/m$ (No. 10). It is noticeable, that the lattice parameter b increases (≈1.6%) and c decreases (≈0.3%) slightly with increasing n. In contrast, the monoclinic angle β decreases strongly (≈14%) with increasing n. Further relevant crystallographic data are compiled in the appendix.

Rietveld refinements of the X-ray powder patterns (Figure 1) of $mC$-La₂MnSe₂O₂ and La₆MnSe₄O₆ revealed small fractions of impurity phases of ≈1% MnSe or ≈4% La₂SeO₂, respectively. The La₄MnSe₃O₄ sample contained a minor impurity phase (peak at $2\theta \approx {31}^{\circ }$) which has not been identified.

2.1. $mC$-La₂MnSe₂O₂

$mC$-La₂MnSe₂O₂ crystallizes isotypic with $mC$-La₂FeSe₂O₂ and $mC$-Ce₂FeSe₂O₂ [2] in the Pb₂HgCl₂O₂-type structure. The iron compounds are low-temperature polymorphs in the RE₂FeSe₂O₂ family (dwelling temperature = 1073 K), while $mC$-La₂MnSe₂O₂ is observed at high temperatures (≥1273 K). The structure of $mC$-La₂MnSe₂O₂ consists of chains of distorted edge-sharing MnSe₄O₂-octahedra along the b-axis which are linked together along the a-axis by edge-sharing OLa₃Mn-tetrahedra forming infinite ribbons parallel to b (Figure 2). The La${}^{3+}$ ion has a distorted LaSe₅O₃ square antiprismatic coordination environment (Figure 3), which was already observed in RE₄TiSe₄O₄ compounds [13,14]. Similar to all RE₂FeSe₂O₂ polymorphs, the displacement ellipsoid of the manganese atom in $mC$-La₂MnSe₂O₂ exhibits an oblate spheroid shape in direction of the selenium atoms which is a consequence of the distorted MnSe₄O₂-octahedra. The distance between two manganese atoms in the MnSe₄O₂ chains (d${}_{intra}$) is 397.2(1) pm, whereas the distance between the chains (d${}_{inter}$) is 616.0(1) pm. Mn–O and Mn–Se bond lengths are 205.4(1) and 284.1(1) pm, respectively, which shows that MnSe₄O₂-octahedra are strongly compressed towards the oxygen atoms. The O–Mn–Se and Se–Mn–Se bond angles within the MnSe₄O₂-octahedra are nearly regular (88.7(1)${}^{\circ }$–91.3(1)${}^{\circ }$), while the La–O–La and La–O–Mn bond angles in the OLa₃Mn-tetrahedra are 105.1(1)${}^{\circ }$–115.8(1)${}^{\circ }$ and thus differ significantly from a perfect tetrahedral shape (109.5${}^{\circ }$).

2.2. La₄MnSe₃O₄ and La₆MnSe₄O₆

La₄MnSe₃O₄ and La₆MnSe₄O₆ crystallize in new structure types which contain two or three crystallographically independent La atoms with two different coordination environments, respectively. The crystal structures are closely related to that of $mC$-La₂MnSe₂O₂ (Figure 2). They also consist of chains of edge-sharing MnSe₄O₂-octahedra along the b-axis. However, these octahedra are connected by two edge-sharing OLa₃Mn- as well as either two or four additional OLa₄-tetrahedra along the a-axis, respectively. The framework of four units wide, edge-sharing tetrahedra forming infinite ribbons was already observed in RE₄TiSe₄O₄ [13,14] and $oP$/$oA$-La₂TSe₂O₂ (T = Fe, Mn) [8]. Frameworks with six units wide tetrahedra are, to our best knowledge, unknown so far. La₄MnSe₃O₄ has two crystallographically independent La${}^{3+}$ ions which have either a La(1) Se₅O₃ square antiprismatic or a La(2) Se₃O₄ monocapped trigonal antiprismatic coordination environment. La₆MnSe₄O₆ contains a third independent La${}^{3+}$ ion which shows also a monocapped trigonal antiprismatic coordination. A comparison of the coordination evironments of the different lanthanum ions is depicted in Figure 3. The coordination polyhedra of La(1) and La(2) as well as La(2) and La(3) are connected via common Se–O edges. Compared to $mC$-La₂MnSe₂O₂, d${}_{intra}$ is slightly and d${}_{inter}$ is significantly longer in La₄MnSe₃O₄ and La₆MnSe₄O₆ as depicted in

Table 2. Distances between the manganese atoms in (d${}_{intra}$) and between (d${}_{inter}$) the MnSe₄O₂- octahedra chains and selected bond lengths (/pm) of $mC$-La₂MnSe₂O₂ (n = 0), La₄MnSe₃O₄ (n = 1) and La₆MnSe₄O₆ (n = 2).

n	d${}_{\mathit{intra}}$	d${}_{\mathit{inter}}$	d(Mn–O)	d(Mn–Se)	d(La–O)	d(La–Se)
0	397.2(1)	616.0(1)	205.4(1)	284.1(1)	234.4(1)–240.8(1)	315.9(1)–337.1(1)
1	401.9(1)	900.6(1)	207.0(1)	284.8(1)	236.4(1)–246.5(1)	313.0(1)–333.8(1)
2	403.6(1)	1247.8(2)	207.7(1)	284.8(1)	236.2(1)–245.2(1)	309.4(1)–333.3(1)

.

Selected bond lengths of the three members of the homologous series are depicted in Table 2. The Mn–O, Mn–Se and La–O bond lengths of La₄MnSe₃O₄ and La₆MnSe₄O₆ are very similar leading to strongly compressed MnSe₄O₂-octahedra analogue to those in $mC$-La₂MnSe₂O₂. As observed in $mC$-La₂MnSe₂O₂, the octahedra in La₄MnSe₃O₄ and La₆MnSe₄O₆ are more regular than the tetrahedra with respect to the bond angles listed in

Table 3. Selected bond angles (${}^{\circ }$) of $mC$-La₂MnSe₂O₂ (n = 0), La₄MnSe₃O₄ (n = 1) and La₆MnSe₄O₆ (n = 2).

n	∡(La–O–La)	∡(La–O–Mn)	∡(Se–Mn–Se)	∡(Se–Mn–O)
0	105.1(1)–115.8(1)	106.4(1)–111.7(1)	88.7(1)–91.3(1)	89.5(1)–90.5(1)
1	102.8(1)–116.4(1)	106.9(1)–109.8(1)	89.8(1)–90.2(1)	87.8(1)–92.2(1)
2	102.8(1)–117.3(1)	106.0(1)–109.9(1)	89.8(1)–90.2(1)	88.0(1)–92.0(1)

.

2.3. Magnetism

The magnetic susceptibilities of $mC$-La₂MnSe₂O₂, La₄MnSe₃O₄ and La₆MnSe₄O₆ (Figure 4) obey the Curie–Weiss rule and indicate antiferromagnetic ordering of the moments near 15 K, similar to the Néel temperature of $mC$-La₂FeSe₂O₂ ($T_{\mathrm{N}}$ = 20 K, [2]). In case of $mC$-La₂MnSe₂O₂ and La₄MnSe₃O₄, ${\chi }_{\mathrm{mol}}$ decreases clearly below this temperature, whereas the effect is more distinctive for $mC$-La₂MnSe₂O₂. In contrast, the susceptibility of La₆MnSe₄O₆ shows no decrease in ${\chi }_{\mathrm{mol}}$ but a clear turning point of the curve at this temperature.

Isothermal magnetization plots (see Figure A1 in the Appendix A) at 300 K are linear with field. $mC$-La₂MnSe₂O₂ exhibits a tiny hysteresis at 1.8 K, which can be due to small impurities, while those of La₄MnSe₃O₄ and La₆MnSe₄O₆ are nearly linear at 1.8 K. Plots of the inverse susceptibilities versus temperature (inserts in Figure 4) allow Curie–Weiss fits. The resulting effective magnetic moments (${\mu }_{\mathrm{eff}}$) are 5.53(1), 5.98(1) and 6.01(1) ${\mu }_{\mathrm{B}}$ per formula unit for n = 1, 2 and 3, respectively. Only the value for $mC$-La₂MnSe₂O₂ is slightly smaller than the theoretical moment of Mn${}^{2+}$ (5.92 ${\mu }_{\mathrm{B}}$). Negative values of the Weiss constant θ (

Table 4. Effective magnetic moments (${\mu }_{\mathrm{eff}}$), Weiss-constants (θ), and Curie-constants (C) of $mC$-La₂MnSe₂O₂, La₄MnSe₃O₄, and La₆MnSe₄O₆.

Compound	${\mathit{\mu }}_{\mathbf{eff}}$ (${\mathit{\mu }}_{\mathbf{B}}$)	θ (K)	C (cm${}^3$·K·mol${}^{-1}$)
$mC$-La2MnSe2O2	5.53(1)	$-46.5\left(1\right)$	3.82(1)
La4MnSe3O4	5.99(1)	$-62.1\left(1\right)$	4.48(1)
La6MnSe4O6	6.01(1)	$-57.4\left(1\right)$	4.51(1)

) support antiferromagnetic ordering in all compounds.

The detailed magnetic structure is not ascertainable from susceptibilty data. We have recently determined the spin structure of La₂CrSe₂O₂ which is isotypic to $mC$-La₂MnSe₂O₂. Neutron powder diffraction [16] experiments revealed a three-dimensional (G-type) ordering, tantamount to antiferromagnetic order within and between the chains of CrSe₄O₂-octahedra. $mC$-La₂MnSe₂O₂ has presumably the same magnetic structure, and the almost identical Néel temperatures suggest at least similar magnetic ordering patterns in La₄MnSe₃O₄ and La₆MnSe₄O₆.

3. Materials and Methods

All starting materials (purity ≥ 99.9%) were handled in an argon-filled glove box (M. Braun, p(O₂) ≤ 1 ppm, p(H₂O) ≤ 1 ppm). Powder samples and single crystals of $mC$-La₂MnSe₂O₂ were prepared in a NaI/KI-flux starting from $oC$-La₂MnSe₂O₂, which was prepared as described in Reference [1]. $oC$-La₂MnSe₂O₂ (0.10 g) was sandwiched with 0.30 g of an eutectic mixture of NaI/KI (mass ratio 0.6:0.4, dried in dynamic vacuum at 673 K) in an alumina crucible. The crucible was sealed in an argon filled silica tube and heated to 1273 K at a rate of 100 K/h, kept at this temperature for 50 h and cooled to room temperature at a cooling rate of 100 K/h. Powder samples and single crystals of La₄MnSe₃O₄ and La₆MnSe₄O₆ were prepared in a two step reaction of appropriate amounts of La₂O₃, La₂Se₃ and MnSe. The rare-earth oxide was heated to 1273 K prior to use. La₂Se₃ was prepared by the reaction of freshly filed rare-earth metal and selenium powder at 573 K for 12 h. MnSe was prepared by the reaction of manganese and selenium powder at 1023 K for 24 h. The starting materials (0.20 g) were mixed intimately, filled in an alumina crucible, sealed in an argon filled silica tube and heated to 873 K at a rate of 25 K/h, kept at this temperature for 10 h and slowly cooled to room temperature at a cooling rate of 50 K/h. In a second step, the reaction mixture was ground in an agate mortar and sandwiched between 0.50 g of an eutectic mixture of NaI/KI. The crucibles were sealed in argon filled silica tubes and heated to 1073 K at a rate of 50 K/h, then to 1273–1373 K at a rate of 25 K/h, kept at this temperature for 40 h and cooled to room temperature at a cooling rate of 100 K/h. The reaction mixtures were then washed with deionized water and ethanol and dried in vacuum. The resulting samples were yellow-brown powders (purity > 95 wt %) and bright brown, transparent single crystals. The compounds are stable in air for months.

Single crystal X-ray diffraction data was collected with a Bruker D8 QUEST (fixed-χ goniometer, Mo-Kα, IμS with HE-LIOS multi-layer optics, PHOTON 100 detector, Bruker, Karlsruhe, Germany). Reflection intensity integration, data reductions, and multi-scan absorption corrections were done with APEX2 [17] and SADABS [18]. The structures were solved with Jana2006 [19]. Further details of the crystal structure investigations may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-Mail: [email protected], http://www.fiz-karlsruhe.de/request for deposited data.html) on quoting the depository numbers CSD-432365 ($mC$-La₂O₂MnSe₂), CSD-432366 (La₄MnSe₃O₄) and CSD-432367 (La₆MnSe₄O₆).

X-ray powder diffraction patterns were measured with a Huber G670 diffractometer (Cu-K${\alpha }_1$ radiation, Ge-monochromator, Huber Diffraktionstechnik GmbH & Co. KG, Rimstig, Germany). Rietveld refinements were done with TOPAS [20]. Magnetization isotherms and susceptibility measurements were performed with a MPMS-XL SQUID magnetometer (Quantum Design Inc., San Diego, CA, USA).

4. Conclusions

In this article we have reported the synthesis, structural characterization and magnetic properties of three new oxyselenides, which are members of the homologous series La_2n+2MnSe_n+2O_2n+2 (n = 0–2). The compounds are accessible using a flux synthesis at high temperatures (1273–1373 K). The crystal structures contain edge-sharing distorted MnSe₄O₂-octahedra along the b-axis as common building block. These are linked together along the a-axis by edge-sharing OLa₃Mn- and/or OLa₄-tetrahedra forming ribbons of increasing width. The lattice parameters b and c vary only slightly, whereas the monoclinic angle decreases strongly with increasing n. The magnetic susceptibilities obey the Curie–Weiss rule with effective magnetic moments compatible to Mn${}^{2+}$ and indicate antiferromagnetic ordering below 15 K.