Materials Research Bulletin 37 (2002) 1189±1195
Solvothermal synthesis of the complex ¯uorides
KMgF3 and KZnF3 with the Perovskite structures
Ruinian Huaa, Zhihong Jiaa, Demin Xieb, Chunshan Shia,*
a
Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, PR China
b
Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR China
(Refereed)
Received 14 November 2001; accepted 12 February 2002
Abstract
The complex ¯uorides KMgF3 and KZnF3 with Perovskite structures were solvothermally
synthesised at 150±1808C and characterised by means of X-ray powder diffraction, scanning
electron microscopy, thermogravimetric analysis and infrared spectroscopy. # 2002 Elsevier
Science Ltd. All rights reserved.
Keywords: A. Fluorides; B. Chemical synthesis; C. X-ray diffraction; C. Electron diffraction
1. Introduction
In recent years, complex ¯uorides which show various interesting structures have
been extensively studied due to their particular physical properties such as piezoelectric
characteristics [1], ferromagnetic [2], nonmagnetic insulator behaviour [3] and photoluminescence host materials [4,5]. Conventional synthesis routes to complex ¯uorides
include solid state reactions [6,7] at high-temperature (>4008C), high-pressure
(>100 MPa) hydrothermal technique [8±10]. The solid state synthetic apparatus,
however, requires a complicated set-up because the corrosive nature of ¯uorides has
limited the studies of ¯uorides in the materials chemistry as well as the fact that hightemperature, high-pressure hydrothermal techniques require special devices. Recently,
a mild hydrothermal synthesis of the complex ¯uorides at 120±2408C has been reported
[11±14]. The oxygen content in the complex ¯uorides synthesised by solid-state
*
Corresponding author. Tel.: ‡86-431-5262-041; fax: ‡86-431-5685-653.
E-mail address: [email protected] (C. Shi).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 7 3 2 - 8
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R. Hua et al. / Materials Research Bulletin 37 (2002) 1189±1195
reaction is higher than that of the corresponding complex ¯uorides synthesised by
hydrothermal techniques [15]. KMgF3 and KZnF3 are the important complex ¯uorides
because they show lasing action [16,17] when they are doped with a proper dopant.
Various isomorphous replacements in the framework of complex ¯uorides lead to many
controllable properties [18]. In order to explore a new method for the preparation of
complex ¯uorides, herein we report a convenient method for the synthesis of KMgF3 and
KZnF3 with Perovskite structures using a solvothermal process.
2. Experimental
2.1. Synthesis
Solvothermal synthesis of KMgF3 and KZnF3 was carried out in a Te¯on-lined
stainless steel autoclave under autogenous pressure. The starting reactants were KF
(A.R.), MgF2 (A.R.) and ZnF2 (A.R.). The mole ratios of initial mixtures for the
synthesis of KMgF3 and KZnF3 were 1.0KF:1.0MgF2 and 2.0KF:1.0ZnF2, respectively. The typical synthetic procedure for crystalline KMgF3 and KZnF3 were as
follows: 0.291 g KF and 0.312 g MgF2, 0.581 g KF and 0.518 g ZnF2 were mixed
respectively, and added into a Te¯on-lined autoclave of 20 ml capacity. Then the
autoclave was ®lled with ethylene glycol (for the synthesis of KMgF3) and absolute
ethanol(for the synthesis of KZnF3) up to 80% of the total volume. The autoclave was
sealed into a stainless steel tank and heated in an oven at 1808C for 7 days. After being
cooled to room temperature naturally, the ®nal powder product was ®ltered off, washed
with absolute ethanol and distilled water, then dried in air at ambient temperature.
2.2. Characterisation
All products were characterised by X-ray powder diffraction (XRD), using a Japan
Ê ). The XRD
Rigaku D/max-IIB diffractometer with Cu Ka1 radiation (l ˆ 1:5405 A
data for index and cell-parameter calculations were collected by a scanning mode
with a step of 0.028 in the 2y range from 10 to 1008 and a scanning rate of 4.08 min 1.
Silicon was used as an internal standard. Observation of crystallites by SEM was
performed on a JXA-840 scanning electron microscopy. Thermogravimetric analysis
(TGA) was conducted using a DT-30 thermogravimetric system. IR spectra were
obtained with a Magna 560 spectrometer in the range 400±4000 cm 1. The samples
were pressed KBr pellets for the spectral measurements.
3. Results and discussion
3.1. Characterisation
Table 1 shows the solvothermal synthesis conditions for KMgF3 and KZnF3. In the
synthesis of KMgF3, the K/Mg ratio and the solvents were found to be crucial to the
R. Hua et al. / Materials Research Bulletin 37 (2002) 1189±1195
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Table 1
Solvothermal synthesis conditions for KMgF3 and KZnF3
Starting
materials
a:b
Solvent
(mole ratio)
a
b
KF
KF
KF
KF
KF
KF
KF
KF
KF
KF
KF
KF
KF
KF
KF
KF
KF
MgF2
MgF2
MgF2
MgF2
MgF2
MgF2
MgF2
MgF2
ZnF2
ZnF2
ZnF2
ZnF2
ZnF2
ZnF2
ZnF2
ZnF2
ZnF2
1:1
2:1
1:2
1:1
1:1
1:1
1:1
1:1
1:1
2:1
3:1
1:2
2:1
2:1
2:1
2:1
2:1
Ethylene
Ethylene
Ethylene
Ethylene
Ethylene
Ethylene
Ethylene
Ethylene
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Reaction Reaction
Phases in product
time (day) temperature (8C)
glycol
glycol
glycol
glycol
glycol
glycol
glycol
glycol
7
7
7
4
3
7
6
7
7
7
7
7
1
2
2
3
7
180
180
180
180
180
150
150
120
180
180
180
180
180
180
150
150
120
KMgF3
KMgF3
KMgF3 ‡ MgF2
KMgF3
KMgF3 ‡ multiphase
KMgF3
KMgF3 ‡ multiphase
KMgF3 ‡ multiphase
KZnF3
KZnF3
KZnF3
KZnF3 ‡ ZnF2
KZnF3
KZnF3
KZnF3 ‡ ZnF2
KZnF3
KZnF3 ‡ ZnF2
formation, crystallisation and purity of the products. When the mole ratio K/Mg of
mixture was 1 or 2, and ethylene glycol or pyridine was used as solvent, the pure and
well-crystallised product was prepared. However, when the mole ratio K/Mg was 0.5,
impurity phases appeared. In the synthesis of KZnF3, the K/Zn ratio is a dominating
factor. Although when the mole ratio K/Zn was from 1 to 3, KZnF3 was formed. A
large K/Zn ratio was favourable for crystallisation and purity of the products, and
when K/Zn ratio <1 an impurity phase of ZnF2 was obtained.
Crystallisation temperature and reaction times were also important factors for an
effective synthesis. Although KMgF3 and KZnF3 can be crystallised at temperature
below 1808C, however, lower temperatures require longer reaction times. For
instance, in the KF±MgF2±ethylene glycol system, KMgF3 is obtained after 4 days
at 1808C, but at 1508C, 7 days are required. In the KF±ZnF2±ethanol system, KZnF3 is
obtained after 1 day at 1808C, but at 1508C, 3 days are needed. The possible reaction
mechanism can be formulated as follows:
H2 O
KF „ K‡ ‡ F ; a trace of water is coming from the solvents
MF2 ‡ nR„MF2 nR
MF2 nR ‡ K‡ ‡ F ˆ KMF3 ‡ nR
(M ˆ Mg or Zn, R ˆ ethylene glycol, ethanol, n ˆ 1 and 4)
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R. Hua et al. / Materials Research Bulletin 37 (2002) 1189±1195
Fig. 1. XRD patterns of KMgF3 (a) and KZnF3 (b).
3.2. X-ray diffraction patterns
The XRD patterns of the KMgF3 and KZnF3 are shown in Fig. 1 (see Tables 2 and 3
for the XRD data of KMgF3 and KZnF3 ) and can be indexed in the primitive cubic
Ê and for
system. The unit-cell parameter [19] for KMgF3 is a ˆ 3:9919 0:0027 A
Ê
KZnF3 is a ˆ 4:0563 0:0007 A. The value for KMgF3 is slightly bigger than that of
Ê)
the corresponding KMgF3 synthesised by solid-state reaction (a ˆ 3:9889 A
[JCPDS Card 18-1033]. The value for KZnF3 is similar to that of the corresponding
Ê ) [JCPDS Card 6-0439]. The
KZnF3 synthesised by solid-state reaction (a ˆ 4:056 A
powder XRD patterns show that the samples are single phase.
3.3. SEM observation
The scanning electron micrographs of the complex ¯uorides KMgF3 and KZnF3
are shown in Fig. 2. As Fig. 2 clearly indicates, the crystallites have regular
R. Hua et al. / Materials Research Bulletin 37 (2002) 1189±1195
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Table 2
XRD data for KMgF3
h
k
l
Ê)
dobs (A
Ê)
dJCPDS (A
(I/I0)obs
1
1
2
2
2
3
3
2
3
1
1
0
1
2
1
1
2
2
0
1
0
1
0
0
1
2
1
2.8255
2.3064
1.9976
1.6295
1.4112
1.2638
1.2036
1.1516
1.0665
2.819
2.302
1.994
1.628
1.410
1.261
1.202
1.151
1.066
84
70
100
24
42
9
11
11
10
Table 3
XRD data for KZnF3
h
k
l
Ê)
dobs (A
Ê)
dJCPDS (A
(I/I0)obs
1
1
1
2
2
2
2
3
3
2
3
3
4
0
1
1
0
1
1
2
0
1
2
2
2
0
0
0
1
0
0
1
0
0
0
2
0
1
0
4.0551
2.8679
2.3422
2.0282
1.8138
1.6565
1.4340
1.3523
1.2823
1.1712
1.1250
1.0840
1.0141
4.055
2.869
2.343
2.029
1.814
1.656
1.434
1.352
1.283
1.171
1.125
1.084
1.014
30
100
5
70
15
39
29
5
14
8
2
14
4
morphology and implying that the samples are single phase. The two complex
¯uorides KMgF3 and KZnF3 have the same cubic of morphology, and the average
grain sizes are ca. 17 and 1 mm, respectively.
Fig. 2. SEM photographs of KMgF3 (left) and KZnF3 (right).
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R. Hua et al. / Materials Research Bulletin 37 (2002) 1189±1195
3.4. Thermal analysis
The thermal stability of the as-prepared KMgF3 and KZnF3 were studied by TGDTA analysis in air. Neither KMgF3 is decomposed up to 8308C nor KZnF3 up to
8708C. A small mass of ca. 3.5% surface water was evident for KMgF3 and ca. 1% for
KZnF3 between 50±1108C. The presence of water is con®rmed by IR at 3420.8 and
1643.4 cm 1, 3436.1 and 1642.7 cm 1, respectively. Upon increasing the temperature, the surface water is removed.
4. Conclusion
A new method for the synthesis of KMgF3 and KZnF3 by solvothermal crystallisation at 150±1808C is presented. Both KMgF3 and KZnF3 crystallise in cubic
systems with Perovskite structure. All the products have uniform grain shapes and
sizes. The mole ratios and solvents are effective for the synthesis. Compared with
traditional high temperature solid-state methods, high-temperature, high-pressure
hydrothermal synthesis method and mild hydrothermal synthesis method, the solvothermal synthesis method to complex ¯uorides appears advantageous in terms of
lower synthesis temperature, simple operation, single phase and well-crystallisation.
Acknowledgments
This work was supported by the State Key Project of Foundation Research
(G1998061306) and National Nature Science Foundation of China (50072031).
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