Hydrothermal growth of single crystals of the quantum magnets: Clinoatacamite, paratacamite, and herbertsmithite

Article abstract of DOI:10.1063/1.3562010

We present a hydrothermal method for growing millimeter-sized crystals of the quantum magnets with formula Cu_4-xZn_x(OH) ₆Cl₂: clinoatacamite (x=0), paratacamite (0.33

Full text of DOI:10.1063/1.3562010

Hydrothermal growth of single crystals of the quantum magnets:
Clinoatacamite, paratacamite, and herbertsmithite
Shaoyan Chu, Peter Müller, Daniel G. Nocera, and Young S. Lee
Citation: Appl. Phys. Lett. 98, 092508 (2011); doi: 10.1063/1.3562010
View online: http://dx.doi.org/10.1063/1.3562010
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Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS 98, 092508 ͑2011͒
Hydrothermal growth of single crystals of the quantum magnets:
Clinoatacamite, paratacamite, and herbertsmithite
Shaoyan Chu,1,a͒ Peter Müller,2 Daniel G. Nocera,2 and Young S. Lee3
1Center for Materials Science and Engineering, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139, USA
2Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
3Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
͑Received 24 January 2011; accepted 14 February 2011; published online 3 March 2011͒
We present a hydrothermal method for growing millimeter-sized crystals of the quantum magnets
with formula Cu4−xZnx͑OH͒6Cl2: clinoatacamite ͑x=0͒, paratacamite ͑0.33ϽxϽ1͒ and
herbertsmithite ͑x=1͒. These highly pure single crystals have been characterized by x-ray
diffraction, chemical analysis, Raman spectroscopy, and magnetic susceptibility measurements. This
synthesis success opens the door for detailed investigations of the magnetic ground-state properties
of these compounds. © 2011 American Institute of Physics. ͓doi:10.1063/1.3562010͔
Clinoatamite,1 paratacamite,2 and herbertsmithite3 are
related to the atacamite family of compounds of the general
formula Cu4−xZnx͑OH͒6Cl2. These materials have generated
much recent excitement in the condensed matter physics
community, as they are excellent candidates for research
on the effects of frustration in quantum magnets.4,5 The
presence of kagomé lattice planes ͑a network of corner-
sharing triangles͒ formed by the Cu ions in these compounds
provide a promising framework to explore exotic ground
states such as the quantum spin liquid. Recently, Nocera
and co-workers6 prepared microcrystalline polymorphs of
Cu4−xZnx͑OH͒6Cl2 by the treatment of malachite,
Cu2͑OH͒2CO3, with NaCl and HBF4 under hydrothermal
conditions. Since then, both synthetic and natural samples
have been used for investigating the structure and properties
of the materials.7–12 Though there exist natural crystals that
are about 1 mm in size, current research suffers from a lack
of larger synthetic crystals. Due to their limited purity and
size, natural mineral samples may not meet requirements for
crucial experiments such as magnetic anisotropy measure-
ments and neutron scattering. Growth of large crystals is
critical to further progress in investigating these fascinating
materials.
Synthesis of the atacamite family of compounds was un-
dertaken over a few decades ago.13 The challenge in growing
the atacamite family members arises from tiny differences in
the free energy between various products in the synthesis
process.14 In the naturally found minerals, the members of
the atacamite family always occur together. Depending on
the occupation number of zinc in the crystal lattice, there are
several different species in the Cu4−xZnx͑OH͒6Cl2 family.
For xϽ0.3, the crystal symmetry is orthorhombic ͑atacam-
ite͒ or monoclinic ͑botallackite and clinoatacamite͒. At
xϾ0.3, the crystal symmetry increases to rhombohedral.
This high symmetry phase with intermediate Zn occupancy
͑0.33ϽxϽ1͒ is zinc-containing paratacamite. The composi-
tional end members at x=1 are known as polymorphs of
herbertsmithite and kapellasite.15 In this report, we describe
crystal growth technology and characterizations of clinoata-
camite, paratacamite, and herbertsmithite. Hydrothermal
growth experiments were performed by using a three-zone
furnace. Typical starting materials and growth temperatures
for the three compounds that are the subject of this study
are listed in Table I. The formation of the compounds
likely proceeds according to the following reactions: ͑4
−x͒CuO + ZnCl2 + 3H2O→Cu4−xZnx͑OH͒6Cl2 + ͑1−x͒ZnO
or ͑4−x͒Cu͑OH͒2+ZnCl2→Cu4−xZnx͑OH͒6Cl2+͑1−x͒ZnO
+͑1−x͒H2O.
By varying the ratio of the starting materials and the
reaction temperature, the value of x can be adjusted from 0 to
1 in the end product. High purity starting materials were
combined with the desired composition ratios, and the mix-
ture was sealed in a reaction cell ͑quartz tube͒ with a fill
factor of ϳ85% after purging of air. The cell was then heated
to ϳ190 °C and dwelled for two days to form microcrystal-
line powder of almost single. After this pre-reaction, the cell
was held upright until the green-blue powder product depos-
ited at one end. Then, the cell was again laid horizontally
near the center of the three-zone furnace and slowly heated
͑2 °C/min͒ to the growth temperature. A temperature gradi-
ent at the cool end of the cell, where the crystals nucleated
and grew, was controlled to ϳ0.1–0.5 °C/cm. Growth of
the large crystals occurs as a re-crystallization process with
the microcrystalline powder at the hot end as the source.
Photographs of the as-grown crystals of clinoatacamite,
paratacamite, and herbertsmithite are presented in Fig. 1. The
crystals usually grow in aggregated clusters. Large flattened
crystals have a dark green-blue color with light green-blue
streaks. The color of crystals obtained from different batches
depends on the concentration of zinc in the products. Light
blue crystals normally have higher concentrations of zinc. In
agreement with natural minerals, the single crystals exhibit a
hexahedral shape. The dominant growth direction is normal
to the ͕101͖ plane for paratacamite and herbertsmithite and
the ͕011͖ plane for clinoatacamite. As-grown crystals are eas-
ily cleaned with water in an ultrasonic cleaner and disaggre-
gated by gentle crushing.
We used x-ray diffraction ͑XRD͒ ͑powder and single
crystal͒ to determine the structure of these three synthetic
species. The stoichiometric ratio of Cu/Zn was tested by in-
ductively coupled plasma with atomic emission spectroscopy
obtained by a Hololab 5000R Raman spectrometer. A super-
a͒
Electronic mail: sc79@mit.edu.
0003-6951/2011/98͑9͒/092508/3/$30.00
98, 092508-1
© 2011 American Institute of Physics
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092508-2
Chu et al.
Appl. Phys. Lett. 98, 092508 ͑2011͒
TABLE I. Structure and synthesis conditions of crystals of clinoatacamite, paratacamite, and herbertsmithite.
Clinoatacamite
Paratacamite
Herbertsmithite
Composition
x=0
x=0.34Ϯ0.01
Cu͑OH͒2 =0.20 g
CuCl2 =0.20 g
ZnCl2 =2.0 g
H2O=10.0 g
12, 8, 270
x=0.97Ϯ0.03
CuO=0.172 g
ZnCl2 =4.0 g
ZnO=0.50 g
H2O=4.0 g
12, 8, 191
190,187
Starting materials ͑g͒
CuO=2.0 g
CuCl2 ·2H2O=16.0 g
H2O=16.0 g
Reaction cell ͓outside diameter, inner diameter, length ͑mm͔͒
Growth temperature ͓hot-end, cool-end, ͑°C͔͒
Crystal system
16, 12, 260
180, 176
Monoclinic
P21 /n
175, 160
Trigonal
Trigonal
Space group
R-3
R-3m
a
b
a
a ͑Å͒
6.1575͑10͒
13.667͑32͒
6.8447͑6͒
b ͑Å͒
c ͑Å͒
␣͑°͒
6.8295͑11͒
9.1144͑14͒
13.667͑32͒
14.045͑12͒
90
6.8447͑6͒
14.116͑3͒
90
90
␤͑°͒
99.676͑2͒
90
90
␥͑°͒
Z
90
120
120
3
4
14.0, 6.5, 6.2, 5.8
Ϫ157
12
Magnetic transition temperatures ͑K͒
Curie–Weiss temperature, ␪ ͑K͒
Curie constant, C ͑cm3 K/mol Cu͒
4.7
None above 1.8
Ϫ300
Ϫ221
0.499
0.526
0.425
aIndicates the results obtained from single crystal XRD data at 100 K.
bIndicates the pulverized crystal’s XRD data at 273 K.
conducting quantum interference device ͑SQUID͒ magneto-
meter ͑Quantum Design, MPMS͒ was employed to measure
dc susceptibility at temperatures down to 1.8 K.
The chemical structures of these compounds can be fur-
ther characterized by Raman scattering as shown in Fig. 3.
When an unpolarized incident beam was perpendicular to the
͑011͒ plane of clinoatacamite and ͑101͒ plane of paratacam-
ite or herbertsmithite, we observed 12 phonon modes for
clinoatacamite and seven phonon modes for herbertsmithite
within a Raman shift region from 100 to 1000 cm−1. All of
these peaks are in good agreement with those found in pri-
mary reports on natural and synthetic minerals.14,16 However,
not much has been previously reported on the differences
between paratacamite ͑xϽ1͒ and herbertsmithite by Raman
spectroscopy. It is quite clear that as the concentration of
Clinoatacamite and paratacamite are classified in differ-
ent crystal systems and space groups, but their XRD patterns
are similar. A “fingerprint” on the powder XRD pattern for
distinguishing paratacamite from clinoatacamite is appear-
ance of diffraction peaks at a spacing of 28 nmϾd
Ͼ22 nm corresponding to 32° Ͻ2␪Ͻ41° for Cu K␣ radia-
tion. As shown in Fig. 2, for paratacamite, there are two-peak
and one-peak structures located at 2␪ϳ32.5° and ϳ40°, re-
spectively. In contrast, the diffraction pattern of clinoatacam-
ite poses four-peak and two-peak structures near the same
locations. This distinction is not removed by substituting
zinc for copper in zinc-containing clinoatacamite. A reason
for this is that the x-ray scattering factors for copper and zinc
are similar. With this in mind, it is easy to understand why
the powder XRD pattern of paratacamite is almost the same
as that of herbertsmithite. Attempts at refining the zinc/
copper ratio using XRD on both powder and single crystals
of these two compounds proved difficult, as expected. The
XRD refinement yields x with large uncertainty, where ICP-
AES is used as a consistency check. Experimentally, it is
impossible to distinguish paratacamite and herbertsmithite
with standard powder XRD alone.
(024)
(113)
Paratacamite (x = 0.34)
(021)
(202)
(006)
(211)
(022)
(004)
(220)
Clinoatacamite (x = 0)
(013)
(211)
(022)
(210)
(202)
(213)
(121)
(103)
30
32
34
36
38
40
42
2 Theta
FIG. 1. ͑Color online͒ Morphology of as-grown crystals of clinoatacamite
͑left͒, paratacamite ͑middle͒, and herbertsmithite ͑right͒.
FIG. 2. ͑Color online͒ Partial powder XRD patterns for paratacamite
͑x=0.34͒ and clinoatacamite ͑x=0͒ with Cu K␣ radiation.
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092508-3
Chu et al.
Appl. Phys. Lett. 98, 092508 ͑2011͒
7000
0.012
0.011
0.01
12
10
8
x = 1.0
x = 0.34
x = 0.00
x = 0.34, ZFC
x = 0.34, FC
x = 0, ZFC
x = 0, FC
6000
5000
4000
3000
2000
1000
0.009
0.008
0.007
0.01
10
12
14
16
18
0.008
0.006
0.004
T
6
4
x = 1, ZFC
x = 1, FC
200
400
600
800
1000
2
-1
Raman wavenumber (cm )
0
FIG. 3. ͑Color online͒ Raman spectra for the crystals of Cu4−xZnx͑OH͒6Cl2,
clinoatacamite ͑x=0͒, paratacamite ͑x=0.34͒, and herbertsmithite ͑x=1͒,
measured with ␭=785 nm excitation line.
2
3
4
5
6
7
8
Temperature, T (K)
zinc increases, a three-band structure at wavenumbers of
891.7, 924.4, and 964.4 cm−1 for clinoatacamite gradually
merge into a single-band structure at 940.0 cm−1 for her-
bertsmithite. Meanwhile, the increase in zinc concentration
FIG. 4. ͑Color online͒ Temperature dependence of dc susceptibility for cli-
noatacamite ͑x=0͒, paratacamite ͑x=0.34͒ and herbertsmithite ͑x=1͒ in the
field of 20 Oe with random orientation. ZFC and FC notes zero-field-cool
and field-cool measurement modes. Inset: separation of ZFC and FC indi-
cates a magnetic phase transition of clinoatacamite ͑x=0͒ at Tϳ14 K.
−1
enhances ͑reduces͒ the intensity of a band at around ␭
ϳ700 cm−1 ͑ϳ800 cm−1͒. Five common phonon modes for
these compounds correspond to Raman bands at wave num-
bers of ϳ120, ϳ145, ϳ360, ϳ505, and ϳ935 cm−1.
The synthetic crystals have been used to examine a the-
oretical prediction that strong geometrical frustration in spin
systems may suppress any magnetic long-range ordering
͑LRO͒ down to very low temperatures. Significant differ-
ences in the magnetic susceptibilities ͑␹=M/H͒ for these
three compounds are shown in Fig. 4. A sharp change in the
temperature dependence of ␹ generally indicates the occur-
rence of a magnetic phase transition. On cooling, clinoata-
camite undergoes multiple transitions, indicated by kinks in
the susceptibility. In contrast, paratacamite ͑x=0.34͒ has
only one transition involving a frustrated antiferromagnetic
phase with weak ferromagnetism. However, there is no de-
finable magnetic LRO temperature for the herbertsmithite
crystals down to the lowest measured temperature of 1.8 K.
Since the magnetic susceptibility measurements using a
SQUID probe the bulk magnetization of the sample, a single
magnetic transition ͑or the lack thereof͒ for synthetic parata-
camite ͑herbertsmithite͒ indicates that our crystals are free of
inclusion of any other family members with lower lattice
symmetry.
these kagomé systems are topics of future work.
The authors acknowledge helpful discussions with
Tianheng Han and Patrick Lee. This work was supported
by the Department of Energy under Grant No. DE-FG02-
04ER46134. This work used facilities supported by the MR-
SEC Program of the NSF under Award No. DMR 0819762.
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