Room temperature syntheses and thermal behaviors of hydrogarnets Sr3M2(OH)12 (M = Al, Cr and Fe)

Article abstract of DOI:10.1016/j.jallcom.2015.02.215

Hydrogarnets Sr₃M₂(OH)₁₂ (M = Al, Cr and Fe) were synthesized through a solid state reaction at room temperature with Al(NO₃)₃·9H₂O, CrCl₃·6H₂O, FeCl₃·6H₂O, Sr(NO₃)₂ and NaOH as the starting materials. All X-ray diffraction (XRD) data of Sr₃M₂(OH)₁₂ were indexed by the cubic system with the space group of Ia^-3d. XRD, differential thermal analysis and thermogravimetric (DTA-TG), and scanning electron microscopy (SEM) were employed to investigate the structural stability, thermal properties and morphologies of Sr based hydrogarnets. The cause of room temperature synthesis and the thermal decomposition behaviors of Sr-based hydrogarnets Sr₃M₂(OH)₁₂ (M = Al, Cr and Fe) are discussed.

Full text of DOI:10.1016/j.jallcom.2015.02.215

Journal of Alloys and Compounds 637 (2015) 248–252
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Room temperature syntheses and thermal behaviors of hydrogarnets
Sr3M2(OH)12 (M = Al, Cr and Fe)
a
a
a,
b
b
Yun-Feng Chen a, Zhi Yang a, , Yong-Ming Li , Jing Guo , Wen-Fu Fu , Ying-Xia Wang , Fu-Hui Liao ,
⇑
⇑
Jianye Li c
a College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650500, China
b College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
c University of Wisconsin-Madison, Madison, WI 53706, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrogarnets Sr3M2(OH)12 (M = Al, Cr and Fe) were synthesized through a solid state reaction at room
temperature with Al(NO3)3ꢀ9H2O, CrCl3ꢀ6H2O, FeCl3ꢀ6H2O, Sr(NO3)2 and NaOH as the starting materials.
All X-ray diffraction (XRD) data of Sr3M2(OH)12 were indexed by the cubic system with the space group
of Iaꢁ3d. XRD, differential thermal analysis and thermogravimetric (DTA–TG), and scanning electron
microscopy (SEM) were employed to investigate the structural stability, thermal properties and
morphologies of Sr based hydrogarnets. The cause of room temperature synthesis and the thermal
decomposition behaviors of Sr-based hydrogarnets Sr3M2(OH)12 (M = Al, Cr and Fe) are discussed.
Ó 2015 Elsevier B.V. All rights reserved.
Received 27 October 2014
Received in revised form 22 February 2015
Accepted 28 February 2015
Available online 5 March 2015
Keywords:
Hydrogarnets Sr3M2(OH)12
Synthesis
XRD
SEM
1. Introduction
reduces the reaction equipment requirements and the reaction
cost. Unfortunately, up to now, there is no any report on the pre-
The structures of hydrogarnets (A3M2(OH)12) are closely related
to those of garnets (A3M2M03O12) [1]. They are important inorganic
functional materials [2–4]. For examples, Ca3Al2Si3O12 and
Ca3Al2(OH)12 are representative compounds of garnets and
hydrogarnets, respectively. When each silicon ion in Ca3Al2Si3
O12(garnet) is replaced by four protons, Ca3Al2(OH)12 (hydrogar-
net) can be formed [5].
paration of hydrogarnets by solid state reaction at room tempera-
ture/low-heating temperature. In this paper, we first report the
syntheses of the hydrogarnets, Sr3M2(OH)12 (M = Al, Cr and Fe),
by solid state reaction at room temperature, and their thermal
decomposition behaviors are investigated by XRD and DTA–TG.
2. Experimental section
Hydrogarnets have been synthesized by aging of precipitated
gel process [6], sol–gel process [7], hydrolysis of complex oxides
[8], high temperature–pressure hydrothermal method [9] and mild
hydrothermal synthesis [10,11].
Synthesis of inorganic functional materials by solid state reac-
tion at room temperature/low-heating temperature has attracted
much attention [12,13]. Solid state reaction at low-heating tem-
perature belongs to the scope of green chemistry and has many
advantages. For examples, Solid state reaction at low-heating tem-
perature can greatly decreases the synthesis temperature of the
compounds, and is very helpful to synthesize the metastable
phases and more new compounds. It not only decreases the tem-
perature and energy consumption of synthetic reaction, but also
2.1. Room temperature synthesis of Sr3M2(OH)12 (M = Al, Cr and Fe) samples
All starting materials used here, Al(NO3)3ꢀ9H2O, CrCl3ꢀ6H2O, FeCl3ꢀ6H2O,
Sr(NO3)2 and NaOH, are analytic pure (A.R.). Sr3M2(OH)12 (M = Al, Cr and Fe) sam-
ples were synthesized by solid state reaction at room temperature. The detailed
synthetic procedure for Sr3M2(OH)12 is described as follows. According to the stoi-
chiometric ratio, the amounts of M metal inorganic salts and Sr(NO3)2 were first
mixed completely, and then a certain amount of NaOH solid (10 mol% excess)
was added to the mixture of M metal inorganic salts and Sr(NO3)2, and then quickly
ground for a few minutes. The obtained products were washed with distilled water
until pH = 7, and dried at 80 °C for 30 min in air. The chemical reaction for
Sr3M2(OH)12 synthesis is presented as follows:
2MX3 ꢀ nH2O þ 3SrðNO3Þ2 þ 12NaOH ! Sr3M2ðOHÞ12 þ 6NaNO3 þ 6NaX þ nH2O
ð1Þ
ðM ¼ Al; Cr; Fe; X ¼ Clꢁ; NO3ꢁÞ
⇑
Corresponding authors. Tel.: +86 871 65941089; fax: +86 871 65941088.
E-mail addresses: kmyangz@hotmail.com (Z. Yang), fuwf@mail.ipc.ac.cn
The obtained Sr3M2(OH)12 samples were annealed at different temperatures for
(W.-F. Fu).
2 h in air, and their decomposition products were identified by X-ray diffraction.
http://dx.doi.org/10.1016/j.jallcom.2015.02.215
0925-8388/Ó 2015 Elsevier B.V. All rights reserved.

Y.-F. Chen et al. / Journal of Alloys and Compounds 637 (2015) 248–252
249
(M = Al, Cr and Fe; X = Clꢁ and NO3ꢁ) and Sr(NO3)2 solids, hydroxide
M(OH)3 (M = Al, Cr and Fe) generated, and the reaction heat
released from M(OH)3 formation is enough to quickly rise the
system temperature and provide the activation energy needed to
form Sr3M2(OH)12. According to our experimental result, the higher
the room temperature is, the easier the synthesis reaction is.
However, if NaOH solid is added into the aqueous solution of
MX3ꢀnH2O and Sr(NO3)2, because of the existence of a large amount
of water, the reaction heat released from M(OH)3 formation is
mostly absorbed by water in the system and cannot provide the
activation energy needed to form Sr3M2(OH)12. (2) An appropriate
amount of crystal water in the reactants is helpful for the ion diffu-
sion in solid state reaction at room temperature, which makes
react rapidly and completely.
2.2. Instruments
The phases of all powder products were characterized by means of X-ray
diffraction (XRD) on a DX-2700 X-ray powder diffractometer with a rotating target
and Cu K
a
radiation at room temperature. Simultaneous differential thermal analy-
Dupont 981
sis and thermogravimetric (DTA–TG) curves were recorded on
a
thermogravimetric analyzer (Dupont Company, USA) in Ar at a heating rate of
10 °C/min. The morphologies of the products were observed by SPM-S3400N scan-
ning electron microscopy (SEM).
3. Results and discussion
3.1. Phase formation of Sr3M2(OH)12
The XRD patterns for Sr3M2(OH)12 (M = Al, Cr and Fe) samples
synthesized by solid state reaction at room temperature are shown
in Fig. 1. All diffraction peaks of the patterns are consistent with
the standard JCPDS files No. 97-002-0529 [Sr3Al2(OH)12] and No.
97-002-0530 [Sr3Fe2(OH)12]. Their structures belong to the cubic
Table 1
ꢀ
Lattice parameters for Sr3M2(OH)12 (M = Al, Cr and Fe) samples.
symmetry with the Ia3d space group. Based on the structural mod-
V (Å3)
Rp (%)
Rwp (%)
els of Sr3Al2(OH)12 [14] and Sr3Fe2(OH)12 [15], their XRD data were
refined by the Rietveld method using the GSAS program [16]. The
lattice parameters for Sr3M2(OH)12 (M = Al, Cr and Fe) obtained
from the Rietveld refinement of XRD data are listed in Table 1.
The powder X-ray diffraction data for Sr3Cr2(OH)12 are also listed
in Table 2. The Rietveld profile fits of Sr3Cr2(OH)12 are shown in
Fig. 2.
As reported in Ref. [6], Sr3M2(OH)12 (M = Al, Cr and Fe) could be
synthesized by mild hydrothermal route at 240 °C for 3 days in a
strong alkaline aqueous solution. Herein we synthesized the
hydrogarnets, Sr3M2(OH)12, by solid state reaction at room tem-
perature (ꢂ25 °C). The synthetic reaction of hydrogarnet, Eq. (1),
could be decomposed into Eqs. (2) and (3):
Compound
a (Å)
Sr3Al2(OH)12
Sr3Cr2(OH)12
Sr3Fe2(OH)12
13.0434(1)
13.1504(4)
13.2188(7)
2219.08(7)
2274.1(2)
2309.8(4)
7.68
7.77
8.28
10.69
10.50
10.96
Table 2
Powder X-ray diffraction data for Sr3Cr2(OH)12
.
No.
2h
hkl
I (%)
84.5
11.2
76.4
46.6
100.0
43.8
8.8
63.6
0.9
63.6
1.4
4.5
9.4
28.9
21.3
43.4
12.3
No.
2h
hkl
800
I (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
16.481
19.048
25.300
27.092
30.355
33.347
34.759
37.436
38.695
42.339
43.499
46.806
47.882
49.986
50.994
51.997
54.942
211
220
321
400
420
422
431
521
440
532
620
631
444
640
552
642
651
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
55.894
58.703
62.333
63.200
64.950
65.804
66.683
69.207
72.549
75.834
78.241
79.049
79.844
82.232
83.003
85.401
13.6
2.7
1.4
10.5
9.4
6.0
6.8
5.4
1.9
6.3
8.6
2.2
7.2
5.1
2.9
3.3
653
752
840
842
655
664
763
772
765
864
961
MX3 ꢀ nH2O þ 3NaOH ! MðOHÞ3 þ 3NaX þ nH2O
ð2Þ
3SrðNO3Þ2 þ 2MðOHÞ3 þ 6NaOH ! Sr3M2ðOHÞ12 þ 6NaNO3
ð3Þ
Using the Eqs. (2) and (3), the room temperature formation of
Sr3M2(OH)12 could be explained as follows. (1) When a certain
amount of NaOH solid is added to the mixture of MX3ꢀnH2O
1042
963
880
776
Fig. 2. The experimental (crosses) and calculated (blue solid line) XRD patterns and
their difference (red solid line) for Sr3Cr2(OH)12 powders. The vertical bars (|) in the
Fig. 1. X-ray diffraction patterns of Sr3M2(OH)12 (M = Al, Cr and Fe) powders
synthesized by solid state reaction at room temperature for a few minutes. a:
middle belongs to Bragg positions of Sr3Cr2(OH)12. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
Sr3Fe2(OH)12; b: Sr3Cr2(OH)12; c: Sr3Al2(OH)12
.

250
Y.-F. Chen et al. / Journal of Alloys and Compounds 637 (2015) 248–252
3.2. SEM images of the hydrogarnets, Sr3M2(OH)12 (M = Al, Cr and Fe)
3.3. TG–DTA curves analysis of the hydrogarnets, Sr3M2(OH)12 (M = Al,
Cr and Fe)
Fig. 3 shows SEM images for Sr3M2(OH)12. Although three sam-
ples are indexed by cubic crystal system with same space group,
however their morphologies are obviously different. The similar
results were also observed and explained by using different crystal
growth habits in Ref. [10]. The SEM morphology of Sr3Fe2(OH)12
consists of irregular particles, the sizes of the small and large par-
Fig. 4 shows TG–DTA/DSC curves for Sr3M2(OH)12 in Ar. Fig. 4a–c
represents the TG–DTA/DSC curves for Sr3Fe2(OH)12, Sr3Cr2(OH)12
and Sr3Al2(OH)12, respectively. According to the analytical results
of Ref. [10], the weight losses for Sr3M2(OH)12 below 350 °C corre-
spond to a fast dehydration of 4H2O molecules. The last 2H2O
molecules will loss when the heating temperature is above 350 °C.
ticles are about 4 lm and about 15 lm, respectively (in Fig. 3a).
Fig. 3b presents SEM image of Sr3Cr2(OH)12, its morphology is
mainly irregular flaky crystal, and its size varies considerably.
Fig. 3c is SEM image of Sr3Al2(OH)12 and it mainly consists of short
rodlike crystals. The diameters and length of rodlike crystals are
3.4. The thermal decomposition behaviors of the hydrogarnets,
Sr3M2(OH)12 (M = Al, Cr and Fe)
2–3
lm and 6–10 lm, respectively. Relatively speaking, the
morphology of Sr3Al2(OH)12 (Fig. 3c) is relatively regular.
3.4.1. The thermal decomposition behavior of Sr3Al2(OH)12
Fig. 5 gives the XRD patterns for the decomposition products
after sintering Sr3Al2(OH)12 powders at different temperatures in
air for 2 h. When the annealing temperature is 400 °C, the main
decomposition product is Sr3Al2O6ꢀH2O (JCPDS No. 00-032-1222).
When the annealing temperature reaches to 700 °C or above, the
main decomposition product is Sr3Al2O6 (JCPDS No. 97-007-
1860). The main characteristic diffraction peaks of SrCO3 (JCPDS
No. 97-016-6088) are observed in all XRD patterns for the high-
temperature decomposition products (in Figs. 5–7), the possible
reasons will be discussed at the end of this paper.
3.4.2. The thermal decomposition behavior of Sr3Cr2(OH)12
Fig. 6 shows the XRD patterns for the decomposition products
after sintering Sr3Cr2(OH)12 powders at different temperatures in
air for 2 h. When the annealing temperature is 300 °C, the main
decomposition products are SrCrO4 (JCPDS No. 97-004-0922) and
a little substance with an unknown phase; the oxidation number
of Cr in SrCrO4 is +6, indicating Cr with +3 oxidation number is
easily oxidized to the Cr with +6 oxidation number by O2 in air.
When the annealing temperature reaches to 400 °C, 800 °C and
900 °C, the main decomposition products are SrCrO4, Sr2CrO4
(JCPDS No. 01-074-2324), as well as a weak unknown phase; the
oxidation number of Cr in Sr2CrO4 is +4. XRD results indicated +6
oxidation number of Cr in SrCrO4 is more stable at low temperature
than that of +4 oxidation number of Cr in Sr2CrO4; a part of +6
oxidation number of Cr is reduced into +4 oxidation number with
the increase of annealing temperature.
100
c: Solid, Sr3Al2(OH)12
b: Dash, Sr3Cr2(OH)12
a: Dot, Sr3Fe2(OH)12
95
90
85
80
b
a
c
0
100
200
300
400
500
600
700
0
-1
-2
-3
b
a
c
0
100
200
300
400
500
600
700
Temperature (oC)
Fig. 3. SEM images of Sr3M2(OH)12
.
a: Sr3Fe2(OH)12
;
b: Sr3Cr2(OH)12
;
c:
Fig. 4. TG–DSC/DTA curves of Sr3M2(OH)12 (M = Al, Fe and Cr) in Ar. a: Dot,
Sr3Fe2(OH)12; b: Dash, Sr3Cr2(OH)12; c: Solid, Sr3Al2(OH)12
Sr3Al2(OH)12
.
.

Y.-F. Chen et al. / Journal of Alloys and Compounds 637 (2015) 248–252
251
3.4.3. The thermal decomposition behavior of Sr3Fe2(OH)12
Fig. 7 presents the XRD patterns for the decomposition products
after sintering Sr3Fe2(OH)12 powders at different temperatures in
air for 2 h. As can be seen in Fig. 7, When the annealing tempera-
ture are 300 °C and 750 °C, the main decomposition product is
Sr4Fe4O11 (JCPDS No. 97-009-1064); when the heating temperature
reaches to 850 °C and 950 °C, the main decomposition products are
Sr4Fe4O11, and small amount of Sr3Fe2O7 (JCPDS No. 97-000-2684)
as well as small amount of material with an unknown phase. The
oxidation numbers of Fe in Sr4Fe4O11 and Sr3Fe2O7 are +3.5 and
+4, respectively. When Sr3Fe2(OH)12 is heated at 750 °C or below
750 °C, iron with +3 oxidation number will be oxidized to iron with
+3.5 oxidation number by O2 in air, the oxidation number will be
partly raised to +4 from +3 when the annealing temperature
increases to 750 °C and above, so the weak characteristic diffrac-
tion peaks of Sr3Fe2O7 phase are obviously observed in Fig. 7c
and d.
Based on XRD analysis above, weak diffraction peaks of SrCO3
are obviously observed in Figs. 5–7, which is good consistent with
Refs. [10,11]. During the washing and drying process of hydrogar-
nets in air, CO2 in air may react with Sr3M2(OH)12, the correspond-
ing reaction may be expressed by following equation:
Fig. 5. X-ray diffraction (XRD) patterns for the samples obtained by annealing
Sr3Al2(OH)12 at different temperatures for 2 h in air. a: 400 °C; b: 700 °C; c: 900 °C.
Sr3M2ðOHÞ12 þ 3CO2 ! 3SrCO3 þ 2AlðOHÞ3 þ 3H2O
The white products, SrCO3, can be clearly observed on the surface of
Sr3M2(OH)12 after the obtained products are placed or dried at 80 °C
for long time in air. So in order to avoid the formation of SrCO3
impurities, the products, Sr3M2(OH)12, should be avoided to contact
with air for long time. Because the main diffraction peaks for
Sr3M2(OH)12 and SrCO3 are overlapping, so the weak diffraction
peaks of small amount of SrCO3 impurities are covered by the
strong diffraction peaks of Sr3M2(OH)12, and not observed in Fig. 1.
4. Conclusions
In conclusion, using aluminum, chromium and iron nitrates or
chlorides, Sr(NO3)2 and NaOH as the raw materials, the hydrogar-
nets Sr3M2(OH)12 (M = Al, Cr and Fe) were synthesized by solid
state reaction at room temperature, and their thermal behaviors
were investigated by X-ray powder diffraction. Compared with
the hydrothermal reaction, the room temperature synthesis of
hydrogarnet has the advantages of quick reaction and easy
operation.
Fig. 6. X-ray diffraction (XRD) patterns for the samples obtained by heating
Sr3Cr2(OH)12 at different temperatures for 2 h in air. a: 300 °C; b: 400 °C; c: 800 °C;
d: 900 °C.
Acknowledgement
This work was financially supported by the National Natural
Science Foundation of China (NSFC 21367026).
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