The mild hydrothermal synthesis of complex fluorides of AZnF3 (A = Na, K)

Article abstract of DOI:10.1246/cl.2000.1106

The complex fluorides of AZnF₃ (A = Na, K), which are isostructural with perovskite phases were obtained by the method of hydrothermal synthesis at 160-220 °C. Compared with traditional high-temperature solid-state method, the products were pure and contained lower amount of oxygen.

Full text of DOI:10.1246/cl.2000.1106

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106
Chemistry Letters 2000
The Mild Hydrothermal Synthesis of Complex Fluorides of AZnF (A = Na , K)
3
Hong Li, Zhihong Jia, and Chunshan Shi*
The key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry,
Chinese Academy of Science, Changchun 130022, P.R.China
(Received June 16, 2000; CL-000596)
The complex fluorides of AZnF (A = Na, K), which are
3
isostructural with perovskite phases were obtained by the
method of hydrothermal synthesis at 160–220 °C. Compared
with traditional high-temperature solid-state method, the prod-
ucts were pure and contained lower amount of oxygen.
In recent years, there are many reports about the per-
ovskite-type complex fluorides due to the interesting magnetic
and optical properties. KZnF is one of the important com-
3
3
+
plex fluorides. In KZnF :Cr , the output of laser has been
3
achieved.1 Several methods have been employed to synthe-
size KZnF , which required very rigorous equipment in the
3
solid-state reaction at high temperature. Reports on the syn-
thesis of KZnF by high-temperature hydrothermal methods
about 600 °C, 98MPa, have been acquired and are valuable,
crystallization and purity of the products. The optimal ratio of
the reactant was K : Zn = 2 : 1. The pH value of the reaction
system, adjusted by hydrofluoric acid, was the key factor to the
crystallization of the product. From the Table 1, we found that
when the pH<7.0, the products formed gradually. However,
when the pH value of the reaction system was more than 7.0,
the degree of the crystallization was incomplete, and a mixture
of KZnF and ZnF was produced. Therefore the optimal pH
3
2
but the reduced reactive temperature is desired. Hydrothermal
synthesis under mild condition has been reported by Zhao et
3
,4
al. With an aim towards a mild condition synthesis route to
complex flourides, we studied the hydrothermal synthesis of
KZnF and NaZnF .
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3
The starting materials for the synthesis of AZnF were
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3
2
A CO (A = Na, K) (A.R), ZnF (A.R), HF (A.R), (NH )HF
value of the reaction was about 3–4.
2
3
2
4
2
(
A.R) (all of reagent grade from Beijing Chemical Industries
Crystallization time and temperature were the important
factors for a successful synthesis, too. High temperature was a
Co., Ltd., Beijing, China). Water was twice distilled. Under
typical synthesis the mole ratios of the starting materials were
advantage factor to form the product. Although KZnF could
3
1
.0 A CO : 1.0 ZnF : 1.0 (NH )HF . The typical synthesis
be crystallized at lower temperature, much reaction time was
needed. High yield and pure product can be achieved when the
reaction temperature is 220 °C and reaction time is about 4 days.
2
3
2
4
2
procedure for crystalline KZnF was as follows: 0.2073 g
K CO , 0.1552 g ZnF and 0.0856 g (NH )HF were added to
3
2
3
2
4
2
8
7
.0 mL water, the pH value of reaction mixture was about
–8. Then, hydrofluoric acid was used to adjust the acidity of
The optimal conditions of crystallization of NaZnF were
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also studied. The best conditions of synthesizing NaZnF were:
3
+
2+
the mixture to the pH value about 3–4. The reaction mixture
was sealed in a Teflon-lined stainless-steel autoclave and
heated at 220 °C under autogenous pressure for 4 days; at the
conclusion of the run, the autoclave was cooled in the air and
decompressed. The contents were washed with deionized
water and air-dried at room temperature.
Na :Zn = 2:1, pH = 2–3, reaction temperature is 220 °C and
reaction time is about 4 days.
The XRD pattern of products of the KZnF and NaZnF is
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3
shown in Figure 1.
All products were identified from X-ray powder diffrac-
tion on a Rigaku D/max-IIB diffractometer with a rotating tar-
get with Ni-filtered Cu Kα radiation at room temperature.
The XRD data for index and cell-parameter calculations were
collected by a scanning mode with a step of 0.02° and a scan-
–
1
ning rate of 0.2° min with silicon used as an internal stan-
dard. Observation of the crystallites by SEM was performed
on a Hitachi X-650 scanning electron microscope.
Thermogravimetric analysis was conducted using a TGS-2
thermogravimetric system.
The hydrothermal synthesis conditions were summarized
in Table 1.
The compositions of reactant and the pH value of the
reaction system were found to be important to the formation,
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
1107
From the Figure 1, we can find that the products which
were obtained from hydrothermal crystallization were pure and
no oxyfluoride peak, which usually occurs in solid-state reac-
Therefore XPS was employed to discuss whether the product
2
–
–
contains O ion or OH and the results of the measurements
were listed in Table 2. From Table 2, the values of F , Zn
2p3/2
1s
tions, is evident. The XRD patterns of KZnF can be indexed in
and Na1s are calculated, compared the values of standard sub-
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a cubic system, giving refined cell parameters a = 0.4058 nm.
The value of the a is in satisfactory agreement with the value on
the card of a = 0.4056 nm. While the XRD patterns of NaZnF3
can be indexed in an orthorhombic system, giving a refined cell
parameter a = 0.5570 nm, b = 0.7764 nm, c = 0.5402 nm, the
value of the a is in satisfactory agreement with the value on the
card of a = 0.5587 nm, b = 0.7775 nm, c = 0.5415 nm.
stance of ZnF , KF, NaF, typical bond of Zn–F, K–F, Na–F
were confirmed to be formed while the bond of O–F, Zn–O,
K–O, Na–O are not detected.
2
In summary, the perovskite-type complex fluorides KZnF3
and NaZnF were synthesized by hydrothermal method under
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mild conditions. The pH value of the initial mixture, the molar
ratio of the reactant, the reaction time and temperature domi-
nate the crystallization of the product. Compared with tradi-
tional high-temperature solid-state method, the hydrothermal
technique appears advantageous in terms of lower synthesis
temperature, high purity and crystallinity with no evidence for
insertion of oxygen.
At room temperature, the morphology of the KZnF and
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NaZnF is observed from SEM. The photograph of the shape
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was shown in Figure 2. The shape of the crystal was cubic and
This work was supported by the national key project of
China for fundamental research.
the diameter of grain was about 50 µm, whereas the configura-
tion of NaZnF was octahedral and the diameter of the grain
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was about 40 µm.
References
The thermal stability of the product is studied by TG-DTA
analyses in air. No phase transformation was observed up to
1
2
3
4
O. Pilla, P. T. C. Freire, and V. Lemos. Phys. Rev. B.,
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S. Somiya, S. Hirano, M. Yoshimura, and K. Yanagisawa,
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Commun., 1997, 945.
C. Zhao, S. Feng, Z. Chao, C. Shi, R. Xu, and J. Ni, Chem.
Commun., 1996, 1641.
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6
00 °C and no mass loss occurred for AZnF between 50 °C to
3
50 °C, which indicates that the products are not hydrated and
are stable in air.
Although inert gases were applied to protect products in
solid-state synthesis, the content of oxygen was still very high.
The oxygenous compounds may be formed in hydrothermal
–
–
synthesis due to the similarity of the radius of F and OH .