Composition Space Diagrams for Mixed Transition Metal Oxide Fluorides
Alexander J. Norquist, Kevin R. Heier, Charlotte L. Stern, and Kenneth R. Poeppelmeier*
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113
ReceiVed June 24, 1998
Composition space diagrams have been used to study phase stability of mixed transition metal oxide fluorides
synthesized by hydrothermal reaction of the metal oxides in (HF)
x
‚pyridine/H
2
O/pyridine solution (150 °C,
autogenous pressure). The combination of early (Ti, Zr, Hf, Nb, Ta, Mo, W) and late (Cu, Cd, Zn) transition
metals and the effects of varying the mole ratios of the metals, (HF)
x
‚pyridine, and water are examined in this
study. Single-crystal products containing [Cu(py)
4
]
2+
, [Cd(py)
4
]
2+
, or [Zn(py)
4
]
2+
cations and [TiF
6
]
2-
, [ZrF
6
]
2-
,
[HfF
6
]
2-
, [NbOF
5
]
2-
, [TaOF
5
]
2-
, [MoO
2
F
4
]
2-
, or [WO
2
F
4
]
2-
anions are recovered. Relative stability of the
crystalline products is governed by the negative charge distribution on the anions, as well as the concentrations
of each reactant in solution. Two new structures are reported: Cu(NC
5
H
5
)
4
TaOF
5
and Cu(NC
5
H
5
)
4
TiF
6
‚3H
2
O.
Crystal data: for Cu(NC
5
H
5
)
4
TaOF
5
, monoclinic, space group C2/c (No. 15), with a ) 10.541(3) Å, b ) 13.547(6)
Å, c ) 16.04(1) Å, ) 97.73(5)°, and Z ) 4; for Cu(NC
5
H
5
)
4
TiF
6
‚3H
2
O, monoclinic, space group C2/c (No.
15), with a ) 12.767(4) Å, b ) 12.048(2) Å, c ) 17.787(6) Å, ) 109.9(1)°, and Z ) 4.
Introduction
Hydrothermal reactions provide a convenient and efficient
method to study the crystallization fields of mixed transition
metal oxide flourides. Hydrogen fluoride is used to dissolve
metal oxides under hydrothermal conditions (150 °C, autogenous
pressure) in pyridinium poly(hydrogen fluoride)/H
2
O/pyridine
solutions. Slowly cooling these solutions results in single crystals
of new mixed metal oxide fluoride compounds. In each system
an early (Ti, Zr, Hf, Nb, Ta, Mo, W) and a late (Cu, Cd, Zn)
transition metal are used. The early transition metals are “hard”
acids and coordinate the “hard” bases in solution (F
-
and O
2-
),
while the late transition metals are “soft” acids and coordinate
the “soft” bases in solution (pyridine (py)). The “soft” metals
form [Cu(py)
4
]
2+
, [Cd(py)
4
]
2+
, and [Zn(py)
4
]
2+
cations, while
the “hard” metals form [TiF
6
]
2-
, [ZrF
6
]
2-
, [HfF
6
]
2-
, [NbOF
5
]
2-
,
[TaOF
5
]
2-
, [MoO
2
F
4
]
2-
, and [WO
2
F
4
]
2-
anions. Other important
species, such as hydronium (H
3
O
+
) and pyridinium (pyH
+
)
cations and F
-
anions, are also present in solution. Which
species combine to form the crystalline products depends on
the negative charge distribution on the anions, the fluoride
affinity of the cations, and the mole fraction in solution of each
reaction component (Figure 1).
To report the variables involved in these hydrothermal
reactions, “composition space” diagrams have been developed.
Composition space diagrams are similar to ternary phase
diagrams in that the products are directly related to initial
reactant mole fractions. The concentration of the three reactants,
namely, metal oxides, (HF)
x
‚pyridine, and H
2
O, are recorded
in each composition space diagram. Approximately 20 reactions
are run at varying mole fractions of each of these reactants,
while other variables such as temperature and amount of solvent
are held constant.
1
The crystalline products of the reactions are
analyzed and the results plotted on the diagram to determine
“crystallization fields”, that is, areas of selective crystallization,
for each product. Composition space diagrams are not phase
diagrams as only the crystalline products are analyzed, while
products which are amorphous or remain in solution are
neglected. In addition, the boundaries separating the crystal-
lization fields are not sharp. In the area adjacent to a boundary,
a mixture of crystalline products is often observed. Nevertheless,
the composition space diagram is a useful tool to help plan
reactions and rationalize the results in hydrothermal systems.
Composition space diagrams have been reported previously
for the (MO
x
,
1
/
2
Nb
2
O
5
)/(HF)
x
‚pyridine/H
2
O (MO
x
) CuO,
1
CdO
2
) systems. Composition space diagrams for the (CuO,
M′O
x
)/(HF)
x
‚pyridine/H
2
O (M′O
x
) TiO
2
, ZrO
2
, HfO
2
,
1
/
2
Ta
2
O
5
,
MoO
3
, WO
3
) and (ZnO,
1
/
2
Nb
2
O
5
)/(HF)
x
‚pyridine/H
2
O systems
are reported here. Composition space diagrams for the (CuO,
xWO
3
)/(HF)
x
‚pyridine/H
2
O(x ) 0.5, 1, 2) series are also
reported. The structures for two new mixed metal oxide fluoride
compounds, Cu(py)
4
TaOF
5
and Cu(py)
4
TiF
6
‚3H
2
O (py ) pyr-
idine), are reported in the Supporting Information.
Experimental Section
CAUTION! (HF)x‚pyridine is toxic and corrosive.
Materials. CuO (99%, Aldrich), MoO3 (99.5%, Aldrich), ZrO2 (99%,
Aldrich), TiO2 (99.9%, Aldrich), Nb2O5 (99.99%, Aldrich), WO3 (99%,
Aldrich), CdO (99.5%, Aldrich), Ta2O5 (99.99%, Aldrich), HfO2 (98%,
Aldrich), ZnO (99%, Aldrich), pyridine (99.8%, anhydrous, Aldrich),
and (HF)x‚pyridine (pyridinium poly(hydrogen fluoride), 70 wt % HF,
Aldrich) were used as received. Reagent amounts of deionized H2O
were used in the syntheses.
* Corresponding author.
(1) Halasyamani, P.; Willis, M. J.; Stern, C. L.; Lundquist, P. M.; Wong,
G. K.; Poeppelmeier, K. R. Inorg. Chem. 1996, 35, 1367.
(2) Halasyamani, P. S.; Heier, K. R.; Norquist, A. J.; Stern, C. L.;
Poeppelmeier, K. R. Inorg. Chem. 1998, 37, 369.
6495 Inorg. Chem. 1998, 37, 6495-6501
10.1021/ic9807238 CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/20/1998