A New Cu, Al Fluoride Disilicate CuAl
2
F
2
(Si
2
O
7
) and its Relations to Topaz
Olga V. Yakubovich*
,a
, Ludmilla N. Dem'yanetz
b
, and Werner Massa*
,c
a,b
Moscow/Russia,
a
Department of Geology, Lomonosov State University and
b
Institute of Crystallography, Academy of
Sciences of Russia
c
Marburg, Fachbereich Chemie und Wissenschaftliches Zentrum fuÈr Materialwissenschaften der Philipps-UniversitaÈt
Received January 4th, 2000.
Professor Ulrich MuÈ ller zum 60. Geburtstag gewidmet
Abstract. CuAl
2
F
2
(Si
2
O
7
) has been prepared by hydrother-
mal synthesis and its crystal structure was determined by
single crystal X-ray diffraction: space group Pnma,
a = 8.8697(9), b = 14.084(2), c = 4.7553(5) A
Ê
,wR
2
= 0.056,
R = 0.022. Cu
2+
shows elongated square pyramidal coordina-
tion. Edge- and corner-sharing [AlO
4
F
2
] octahedra with
fluorine atoms in cis position form layers parallel to the ac
plane. Along b these layers are linked by Si
2
O
7
groups to
form a three-dimensional framework [Al
2
F
2
(Si
2
O
7
]
2±
. In ad-
dition, the [CuO
5
] pyramides connect two Al octahedra of
neighbouring layers. The crystal structure is discussed as a
derivative from topaz structure. The modular (or polyso-
matic) approach is used for this purpose, and for modelling
hypothetical related compounds.
Keywords: Copper aluminium fluoride disilicate; Topaz
variants; Crystal structure; Aluminium copper fluoride disili-
cate
Ein neues Cu, Al-Fluoriddisilicat CuAl
2
F
2
(Si
2
O
7
) und dessen Verwandtschaft
zum Topas
Inhaltsu
È
bersicht. CuAl
2
F
2
(Si
2
O
7
) wurde durch Hydrother-
malsynthese erhalten und seine Kristallstruktur durch Ein-
kristall-RoÈntgenbeugung bestimmt: Raumgruppe Pnma,
a = 8.8697(9), b = 14.084(2), c = 4.7553(5) A
Ê
,wR
2
= 0.056,
R = 0.022. Cu
2+
zeigt gestreckte quadratisch-pyramidale Ko-
ordination. Kanten- und eckenverknuÈpfte [AlO
4
F
2
]-Okta-
eder mit Fluoratomen in cis-Position bilden Schichten par-
allel zur ac-Ebene aus. In Richtung der b-Achse werden
diese Schichten durch [Si
2
O
7
]-Gruppen zu einem dreidimen-
sionalen Netzwerk [Al
2
F
2
(Si
2
O
7
)]
2±
verbunden. ZusaÈtzlich
verbruÈcken die [CuO
5
]-Pyramiden zwei Al-Oktaeder be-
nachbarter Schichten. Die Kristallstruktur wird als Ab-
koÈmmling von der Topas-Struktur diskutiert. Dazu wird das
modulare Aufbau-Konzept benutzt, mit dem auch weitere
hypothetische Strukturen modelliert wurden.
Introduction
The modular approach is well known in inorganic
crystal chemistry. It operates with characteristic struc-
tural units or fragments and interprets the crystal
structure as their association in different combina-
tions. According to [1] the modular analysis of crystal
structures is rather complex and combines several as-
pects of modular interpretation, such as symmetry, to-
pology, classification, hierarchy, model simulation etc.
This approach considers the individual structure from
the point of view of its relationship to others, having
some common modular features. It is not fruitful only
to describe structures in this way, the modular ap-
proach seems to be able to work as a very good in-
strument for structural design, too.
We present here the results of the X-ray crystal
structure investigation of a new fluoride disilicate
CuAl
2
F
2
(Si
2
O
7
). These results are interpreted in terms
of the modular concept.
Experimental
Single transparent crystals of aquamarine-like colour
were obtained by hydrothermal synthesis in the system
CuO±Al
2
O
3
±SiO
2
±NH
4
F±H
2
O (T = 450 °C, P = 1000 bar,
t = 100 h). The presence of Cu, Al, Si, O and F in the sam-
ples was confirmed by qualitative X-ray spectral analysis
(CamScan 4DV + EDX Noran Instruments. Voyager 4.0 with
Pioneer Detector with ultrathin window).
Structure determination. The unit cell parameters were
determined and refined on an IPDS area detector system
(Stoe) using MoK
a
radiation (graphite monochromator) and
a total of 5110 reflections were collected. The intensities
1514
Ó WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0044±2313/00/6261514±1518 $ 17.50+.50/0 Z. Anorg. Allg. Chem. 2000, 626, 1514±1518
* Dr. O. V. Yakubovich,
Department of Geology, Moscow Lomonosov State University,
Vorob'evy Gory,
119899 Moscow/Russia
E-mail: yakubol@geol.msu.ru
* Prof. Dr. W. Massa,
Fachbereich Chemie der Philipps-UniversitaÈt,
D-35032 Marburg/Germany,
Fax: +4 96 42 12 82 89 17
E-mail: massa@mail.chemie.uni-marburg.de
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