Artikel
Tb
2
Se
2
O
7
: Terbium(III) Oxide Oxoselenate(IV) according to Tb
2
O[SeO
3
]
2
with
a “LoneⴚPair” Channel Structure
Joseph Wontcheu and Thomas Schleid*
Stuttgart, Institut für Anorganische Chemie der Universität
Received March 4th, 2002.
Abstract. The colourless title compound Tb
2
Se
2
O
7
(⬅ Tb
2
O-
[SeO
3
]
2
, terbium(III) oxide oxoselenate(IV)) was obtained by re-
acting a mixture of Tb
4
O
7
, Tb and SeO
2
in a 3:2:14-molar ratio at
850 °C for seven days in an evacuated silica vessel as needle-shaped
single crystals. It crystallizes tetragonally (space group: P4
2
/ncm,
a ⫽ 1064.57(8), c ⫽ 521.56(4) pm, c/a ⫽ 0.490, Z ⫽ 4) and exhibits
Tb
3⫹
cations in square-antiprismatic coordination of eight oxygen
atoms each. Six of the latter belong to six different isolated ψ
1
-
tetrahedral [SeO
3
]
2⫺
anions whereas the remaining two are not
bonded to selenium at all. Their coordination sphere actually con-
sists of four Tb
3⫹
cations arranged as tetrahedron. These [OTb
4
]
10⫹
tetrahedra form infinite one-dimensional
1
⬁
{[OTb
4/2
]
4⫹
} chains
along [001] by sharing trans-oriented edges which are packed such
Tb
2
Se
2
O
7
: Terbium(III)-Oxid-Oxoselenat(IV) gemäß Tb
2
O[SeO
3
]
2
mit einer
Kanalstruktur aus nicht-bindenden Elektronenpaaren
Inhaltsübersicht. Die farblose Titelverbindung Tb
2
Se
2
O
7
(⬅ Tb
2
O-
[SeO
3
]
2
, Terbium(III)-Oxid-Oxoselenat(IV)) wurde durch die Um-
setzung eines Gemenges Tb
4
O
7
, Tb und SeO
2
im molaren Verhält-
nis 3:2:14 in evakuierten Quarzglasampullen innerhalb von sieben
Tagen bei 850 °C in Form von nadelförmigen Einkristallen erhal-
ten. Sie kristallisiert tetragonal (Raumgruppe: P4
2
/ncm; a ⫽
1064,57(8), c ⫽ 521,56(4) pm, c/a ⫽ 0,490; Z ⫽ 4) und enthält
Tb
3⫹
-Kationen in quadratisch-antiprismatischer Koordination von
acht Sauerstoffatomen. Sechs davon gehören zu sechs unterschied-
lichen, isolierten, ψ
1
-tetraedrischen [SeO
3
]
2⫺
-Anionen, während die
restlichen beiden nicht an Selen gebunden sind. Deren Koordina-
tionssphäre besteht vielmehr aus vier Tb
3⫹
-Kationen in tetraedri-
scher Anordnung. Diese [OTb
4
]
10⫹
-Tetraeder bilden durch Ver-
Introduction
Among different other formula types (e. g. M
2
SeO
5
,
M
2
Se
1.5
O
6
,M
2
Se
3
O
9
,M
2
Se
3.5
O
10
,M
2
Se
4
O
11
[1, 2]) the
existence of phases with the composition M
2
Se
2
O
7
in the
phase diagrams of the systems M
2
O
3
/SeO
2
(M ⫽ rare-earth
elements) has been pointed out by many authors [3, 4] re-
cently. In spite of some arguments over their thermo-
dynamic stability [5, 6], thermogravimetric studies, sup-
* Prof. Dr. Thomas Schleid
Institut für Anorganische Chemie, Universität Stuttgart
Pfaffenwaldring 55, D-70569 Stuttgart
Telefax: ⫹49(0)711/685-4241
E-Mail: schleid@iac.uni-stuttgart.de
Z. Anorg. Allg. Chem. 2002, 628, 1941⫺1945 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 0044⫺2313/02/628/1941⫺1945 $ 20.00⫹.50/0 1941
that a central strand is quadratically surrounded in (001) projection
by four chains at a distance of a/
冪
2 and another square of four
more of them a factor of
冪
2(⬅ the value of a) further apart and
rotated by 45°. Charge balance and three-dimensional intercon-
nection of this particular rod-packing occurs via discrete trigonal
[SeO
3
]
2⫺
pyramids so that the whole structure gets strongly influ-
enced by the stereochemical lone-pair activity of these groups. Re-
markably enough, the 4
¯
symmetry of the
1
⬁
{[OTb
4/2
]
4⫹
} chains is
perfectly repeated by the “lone-pair” channels of the non-binding
electrons at the Se
4⫹
cations.
Keywords: Terbium; Oxides; Selenates(IV); Crystal structure; Ste-
reochemical “Lone-Pair” activity
knüpfung über trans-ständige Kanten eindimensional unendliche
Ketten
1
⬁
{[OTb
4/2
]
4⫹
} parallel [001], die so gepackt sind, daß ein
Zentralstrang in (001)-Projektion im Abstand a/
冪
2 quadratisch von
vier nahen und von noch vier um den Faktor
冪
2(⬅ a) weiter ent-
fernten Ketten, deren projiziertes Quadrat um 45° gedreht ist, um-
geben wird. Der Ladungsausgleich und die dreidimensionale Ver-
netzung dieser speziellen Stabpackung erfolgt durch diskrete, trigo-
nale [SeO
3
]
2⫺
-Pyramiden, so daß die Gesamtstruktur stark von der
stereochemischen Aktivitätdernicht-bindenden Elektronenpaare
dieser Baugruppen beeinflußt ist. Bemerkenswerterweise wieder-
holt sich die 4
¯
-Symmetrie der
1
⬁
{[OTb
4/2
]
4⫹
}-Ketten auch in den
Kanälen, die durch die nicht-bindenden Elektronenpaare („lone
pairs“) der Se
4⫹
-Kationen gebildet werden.
ported by IR and X-ray powder diffraction measurements,
verified this beyond doubt. Although these M
2
Se
2
O
7
-type
compounds have been often obtained at temperatures of
about 800 °C details concerning their crystal structure were
not available so far. The unambiguous presence of discrete
anionic selenite units ([SeO
3
]
2⫺
) suggested their formulation
as rare-earth(III) oxide oxoselenates(IV) with lonesome
O
2⫺
anions which are not bonded to any selenium atoms
according to M
2
O[SeO
3
]
2
. A phase with a similar situation
in the homologous M
2
O
3
/TeO
2
system has already been de-
scribed recently for the oxotellurate(IV) Pr
2
Te
2
O
7
[7] as ad-
opting the pyrochlore-type crystal structure (cubic, space
group: Fd3
¯
m) according to Pr
2
O(TeO
3
)
2
. In this case, the
non-tellurium bonded oxide anions form a three-dimen-
sional network
3
⬁
{[OPr
4/2
]
4⫹
} of vertex-shared [OPr
4
]
10⫹
tetrahedra with anti-cristobalite-type topology which inter-
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