How Lewis Acidity of the Cationic Framework Affects KNaNbOF5
polymorphism
Kelvin B. Chang, Anastasiya Vinokur, Rachelle Ann F. Pinlac, Matthew R. Suchomel, Michael R.
Marvel and Kenneth R. Poeppelmeier*
Supporting Information
Synthetic Procedure
The synthesis of polymorphs of KNaNbOF5 was explored with a variety of potassium
sources, which included KNO3 (99.9%, Mallinckrodt), KCl (99.48%, Mallinckrodt), K2SO4
(99.0%, Aldrich), K2HPO4 (99%, Fluka), H2CO3 (99.9%, Mallinckrodt), and K3PO4 (98%,
Aldrich). Both the noncentrosymmetric (NCS) and centrosymmetric (CS) polymorphs of
KNaNbOF5 could be synthesized with KNO3, KCl, and K2SO4. If the ratio of K:Na in the
reaction mixture was greater than about 1:1, then the NCS polymorph would crystallize. A lower
K:Na ratio would result in the CS polymorph. The CS polymorph could be synthesized with
K2HPO4, H2CO3, and K3HPO4 with if a low K:Na ratio was used. Reactions with higher K:Na
ratios, however, did not yield the NCS polymorph. A number of different products were
observed, often times K2NaNbO2F4. Results are summarized in Table S1.
These results suggest the basicity of the anions plays an important role (pKb’s are given
in Table S1). Only the salts that do not hydrolize (or minimally hydrolize) can be used to form
either polymorph. The final pH was measured after heating to confirm the synthetic trend
observed with respect to anion basicity. For K2HPO4 and K2CO3, reactions with a final pH of 4
yielded the CS polymorph, and reactions with a final pH ranging from 5-7 resulted in
K2NaNbO2F4 (and/or other products). All reactions containing K3PO4 resulted in a final pH
within the 5-7 range, again resulting in K2NaNbO2F4 as the main product. These results explain
why the NCS cannot be formed using basic potassium sources. A higher K:Na ratio in the
reaction that is needed to form the NCS polymorph will result in a greater concentration of the
base, which will raise the pH and result in other products.
To counteract the hydrolysis and higher pH values, the pH of reaction solutions were
adjusted with acid to lower the pH to 0-1 in attempt to synthesize the NCS polymorph using the
basic K salts. Concentrated HNO3 was first used to adjust the pH because NO3- is shown to be a
spectator ion. Since K2HPO4 and K2CO3 gave similar results, K2HPO4 reactions were chosen
since the higher pKb value should have the best chance for synthesizing the NCS polymorph.
Some reactions, however, resulted in an unknown phase. No clear trend exists that would dictate
the formation of this unknown phase. Reactions with a potassium mole fraction of less than 0.81
in the K-Na-Nb phase space had solutions with a final pH ranging from 0-4. Most of these
reactions yielded the CS polymorph. Even reactions that contained high K:Na ratios that formed
the NCS phase in the KNO3, KCl, or K2SO4 systems resulted in forming the CS polymorph. The
reaction with the highest potassium mole fraction studied of 0.81 contained a solution with a
final pH of 5. Products consisted of a mixture of potassium and niobium oxides. Acidic solutions
are therefore needed to form KNaNbOF5. It is still unclear, however, why the NCS polymorph
did not form with K2HPO4 reactions with high K:Na ratios whose pH was lowered through the
addition of HNO3.
Table S1. Summary of products formed from varying
lower pH.
K source pKb of anion K:Na ratio Acid added
KNO3
Large
>~1:1
None
<~1:1
None
KCl
Large
>~1:1
None
<~1:1
None
K2SO4
12
>~1:1
None
<~1:1
None
K2HPO4 6.8
>~1:2
None
<~1:2
None
>~1:1
HNO3
<~1:1
HNO3
K2CO3
3.7
>~1:5
None
<~1:5
None
K3PO4
1.7
>~1:3
None
<~1:3
None
K sources, K:Na ratio, and addition of acid to
pH of final solution
2-3
2-3
2-3
2-3
3-4
3-4
5-6
4
0-4
0-4
5-7
4
5-7
3-5
Major Product formed
NCS
CS
NCS
CS
NCS
CS
K2NaNbO2F4 + other
CS
unknown
CS
K2NaNbO2F4 + other
CS
K2NaNbO2F4 + other
CS + K2NaNbO2F4
a)
b)
Figure S1. In situ powder X-ray diffraction patterns of a) heating and b) cooling of the
centrosymmetric KNaNbOF5 polymorph. Initial temperatures of each scan are shown to the
right.