Hydrothermal oxidation - Reduction methods for the preparation of pure and single crystalline alunites: Synthesis and characterization of a new series of vanadium jarosites

Article abstract of DOI:10.1021/ja016832u

Three new redox-based, hydrothermal, synthetic methods have been developed for the preparation of a new series of jarosites, AV₃(OH)₆(SO₄)₂ (A = Na⁺, K⁺, Rb⁺, TI⁺, and NH₄⁺), in high purity and in single crystalline form. The V³⁺ jarosites have been characterized by single-crystal X-ray and elemental analysis, and by infrared and electronic absorption spectroscopy. The synthetic methods employed here represent a new approach for the preparation of the jarosite class of compounds, which for the past several decades, have been notoriously difficult to prepare in pure form. To demonstrate the impact of our new synthetic techniques on the magnetic properties of jarosites, the V³⁺ jarosites were also prepared according to the nonredox techniques used over the past 30 years. A comparative study of these samples and those prepared by our new synthetic methods reveals widely divergent magnetic properties, thus pointing to the importance of the new redox synthetic methods to future magnetism studies of jarosite compounds.

Full text of DOI:10.1021/ja016832u

Published on Web 02/21/2002
Hydrothermal Oxidation-Reduction Methods for the
Preparation of Pure and Single Crystalline Alunites:
Synthesis and Characterization of a New Series of Vanadium
Jarosites
Daniel Grohol and Daniel G. Nocera*
Contribution from the Department of Chemistry, 6-335, Massachusetts Institute of Technology,
77 Massachusetts AVenue, Cambridge, Massachusetts 02139
Received August 13, 2001
Abstract: Three new redox-based, hydrothermal, synthetic methods have been developed for the
preparation of a new series of jarosites, AV3(OH)6(SO4)2 (A ) Na+, K+, Rb+, Tl+, and NH4+), in high purity
and in single crystalline form. The V3+ jarosites have been characterized by single-crystal X-ray and
elemental analysis, and by infrared and electronic absorption spectroscopy. The synthetic methods employed
here represent a new approach for the preparation of the jarosite class of compounds, which for the past
several decades, have been notoriously difficult to prepare in pure form. To demonstrate the impact of our
new synthetic techniques on the magnetic properties of jarosites, the V3+ jarosites were also prepared
according to the nonredox techniques used over the past 30 years. A comparative study of these samples
and those prepared by our new synthetic methods reveals widely divergent magnetic properties, thus pointing
to the importance of the new redox synthetic methods to future magnetism studies of jarosite compounds.
about the triangular lattice.9-22 When the spins are ferromag-
netically coupled, as has been recently found for the Na+ form
Introduction
The alunite supergroup of minerals consists of more than 40
isostructural compounds with the general formula AM3(OH,-
OH2)6(TO4)2.1 The A site is usually occupied by a monovalent
of V3+ jarosite,23,24 metamagnetism of the pseudo-XY type is
observed.25
Despite nearly three decades of intense research, the mag-
netism of the jarosite compounds continues to elude complete
characterization. In such highly correlated systems, very pure
and stoichiometric samples are required to obtain consistent and
(Na+, K+, Rb+, Tl+, NH4+, H3O+), divalent (Ca2+, Ba2+, Pb2+
,
Hg2+), or a trivalent (Bi3+, rare earths) ion with a coordination
number greater or equal to nine, M is occupied by an Al3+
,
Cr3+, or Fe3+ in an octahedral oxygen coordination site, and T
is occupied by a S6+, P5+, or As5+ in a tetrahedral oxygen site.1
The three-dimensional structure of the alunites consists of A+
ions sandwiched between Kagome´ layers, which are hexagonal
sheets composed of corner-shared triangles of interconnected
MO6 octahedra. These elementary triangles are further capped
by TO4 tetrahedra positioned alternately above and below the
two-dimensional layers.2-7 The placement of spins at the vertices
of the triangular array can confer unusual magnetic properties
on the Kagome´ lattice of exchange-coupled moments.8 Indeed,
the jarosite subgroup of compounds, AM3(OH)6(SO4)2 (M )
Fe3+ and Cr3+) have long fascinated the solid-state physics
community as models for studying the phenomenon called spin-
frustration arising from antiferromagnetic coupling of spins
(9) Wills, A. S.; Harrison, A. J. Chem. Soc., Faraday Trans. 1996, 92, 2161-
2166.
(10) Wills, A. S.; Harrison, A.; Ritter, C.; Smith, R. I. Phys. ReV. B 2000, 61,
6156-6169.
(11) Inami, T.; Nishiyama, M.; Maegawa, S.; Oka, Y. Phys. ReV. B 2000, 61,
12181-12186.
(12) Frunzke, J.; Hansen, T.; Harrison, A.; Lord, J. S.; Oakley, G. S.; Visser,
D.; Wills, A. S. J. Mater. Chem. 2001, 11, 179-185.
(13) Earle, S. A.; Ramirez, A. P.; Cava, R. J. Physica B 1999, 262, 199-204.
(14) Harrison, A.; Wills, A. S.; Ritter, C. Physica B 1998, 241-243, 722-723.
(15) Oakley, G. S.; Pouget, S.; Harrison, A.; Frunzke, J.; Visser, D. Physica B
1999, 267-268, 145-148.
(16) Oakley, G. S.; Visser, D.; Frunzke, J.; Andersen, K. H.; Wills, A. S.;
Harrison, A. Physica B 1999, 267-268, 142-144.
(17) Wills, A. S.; Harrison, A.; Mentink, S. A. M.; Mason, T. E.; Tun, Z.
Europhys. Lett. 1998, 42, 325-330.
(18) Keren, A.; Kojima, K.; Le, L. P.; Luke, G. M.; Wu, W. D.; Uemura, Y. J.;
Takano, M.; Dabkowska, H.; Gingras, M. J. P. Phys. ReV. B 1996, 53,
6451-6454.
(19) Maegawa, S.; Nishiyama, M.; Tanaka, N.; Oyamada, A.; Takano, M. J.
Phys. Soc. Jpn. 1996, 65, 2776-2778.
(20) Townsend, M. G.; Longworth, G.; Roudaut, E. Phys. ReV. B 1986, 33,
4919-4926.
(1) Jambor, J. L. Can. Mineral. 1999, 37, 1323-1341.
(2) Szyman˜ski, J. T. Can. Mineral. 1985, 23, 659-668.
(3) Kato, T.; Miura, Y. Mineral. Mag. 1977, 8, 419-430.
(4) Menchetti, S.; Sabelli, C. Neues Jahrb. Mineral. Monatsh. 1976, 406-
417.
(21) Reimers, J. N.; Berlinsky, A. J. Phys. ReV. B 1993, 48, 9539-9554.
(22) Lee, S.-H.; Broholm, C.; Collins, M. F.; Heller, L.; Ramirez, A. P.; Kloc,
Ch.; Bucher, E.; Erwin, R. W.; Lacevic, N. Phys. ReV. B 1997, 56, 8091-
8097.
(5) Takano, M.; Shinjo, T.; Takada, T. J. Phys. Soc. Jpn. 1971, 30, 1049-
(23) Grohol, D.; Papoutsakis, D.; Nocera, D. G. Angew. Chem., Int. Ed. 2001,
40, 1519-1521.
1053.
(6) Wang, R.; Bradley, W. F.; Steinfink, H. Acta Crystallogr. 1965, 18, 249-
252.
(24) Papoutsakis, D.; Grohol, D.; Nocera, D. G. J. Am. Chem. Soc. 2002, 124,
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(7) Hendricks, S. B. Am. Mineral. 1937, 22, 773-784.
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(25) Grohol, D.; Huang, Q. Z.; Lee, Y. S.; Toby, B. H.; Lynn, J. W.; Nocera,
D. G. Manuscript in preparation.
9
2640 VOL. 124, NO. 11, 2002 J. AM. CHEM. SOC.
10.1021/ja016832u CCC: $22.00 © 2002 American Chemical Society

Synthesis of Vanadium Jarosites
A R T I C L E S
this solution, 0.86 mL (10 mmol) of dimethylsulfite was added. The
pressure vessel was closed and placed into an oven at 202 °C for 6 d.
The oven was then cooled at 0.1 °C min-1 to room temperature. Small,
dark-red crystals were filtered, washed, and dried in air. Yield: 1.22 g
(52%). Anal. Calcd for H6NaV3S2O14: H 1.29, Na 4.89, V 32.52, S
13.65. Found: H 1.33, Na 4.95, V 32.42, S 13.46.
reproducible results. Whereas synthetic approaches to the
jarosites have been studied extensively in conjunction with their
utility in the zinc refinery industry,26,27 methods have relied
exclusively on the simple precipitation at 100-160 °C of jarosite
from aqueous solutions containing the constituent ions A+, M3+
,
and SO4 ions,28-30
2-
Method 3. To 2.84 g of Na2SO4 (20.0 mmol) dissolved in 50 mL
of distilled water was added 1.94 mL (35 mmol) of H2SO4. The solution
was transferred into a Teflon liner of a 125-mL pressure vessel. A 0.51-g
pellet of metallic vanadium (10 mmol) was added to this solution. The
enclosed vessel was placed into an oven at 200 °C for 4 d. The oven
was then cooled at 0.1 °C min-1 to room temperature. After opening
the vessel, a small, unreacted piece of vanadium metal covered with
V3+ jarosite was carefully removed. The product that precipitated on
the walls of the Teflon liner was isolated, washed with distilled water
and dried in air. Yield: 0.91 g (58%). Anal. Calcd for H6NaV3S2O14:
H 1.29, Na 4.89, V 32.52, S 13.65. Found: H 1.32, Na 4.95, V 32.61,
S 13.74.
Preparation of KV3(OH)6(SO4)2 (KVS). Method 1. A 45-mL Parr
pressure vessel was charged with 4.0 mL of 2.0 M VOCl2 (8.0 mmol).
In a separate beaker, 0.316 g of K2SO3 (2.0 mmol), and 0.112 g of
KOH (2.0 mmol) was dissolved in 16 mL of distilled water and slowly
added to the Teflon liner containing the VOCl2 solution. The vessel
was sealed and placed into an oven at 205 °C for 4 d. The oven was
then cooled at 0.1 °C min-1 to room temperature. The product
precipitated on the walls of the Teflon liner; it was isolated, washed
with distilled water, and dried in air. Yield: 0.27 g (58%). Anal. Calcd
for H6KV3S2O14: H 1.24, K 8.04, V 31.44, S 13.19. Found: H 1.25,
K 8.11, V 31.38, S 13.32.
Method 2. A 2.0-mL amount of a 2.0 M VOCl2 solution (4.0 mmol)
was mixed with 10 mL of solution containing 0.22 g (4.0 mmol) of
KOH in a 23-mL Teflon liner. Into this solution, 0.18 mL (2.1 mmol)
of dimethylsulfite was added, and the Parr vessel was sealed. The
enclosed solution was heated at 202 °C for 7 d after which the oven
was cooled at 0.1 °C min-1 to room temperature. Small, dark-red
crystals that attached to the walls of the Teflon liner were collected by
filtration, washed with distilled water, and dried in air. Yield: 0.21 g
(41%). Anal. Calcd for H6KV3S2O14: H 1.24, K 8.04, V 31.44, S 13.19.
Found: H 1.26, K 8.04, V 31.36, S 13.25.
Method 3. To a solution of 4.36 g of K2SO4 (25 mmol) dissolved
in 50 mL of distilled water was added 1.94 mL (35 mmol) of H2SO4;
the resulting solution was transferred into a Teflon liner of a 125-mL
pressure vessel. A 0.538-g pellet of metallic vanadium (10.5 mmol)
was added to the solution, the vessel was enclosed and placed into an
oven at 205 °C for 7 d. The oven was then cooled at 0.1 °C min-1 to
room temperature. After opening the vessel, a small, unreacted piece
of vanadium metal was carefully removed. The product that precipitated
on the walls and on the bottom was isolated, washed with distilled
water, and dried in air. Yield: 0.58 g (34%). Anal. Calcd for
H6KV3S2O14: H 1.24, K 8.04, V 31.44, S 13.19. Found: H 1.20, K
7.96, V 31.19, S 13.11.
A+ + 3M3+ + 2SO42- + 6H2O f
AM3(OH)6(SO4)2 + 6H+ (1)
Precipitation of jarosites under these conditions occurs quickly
to yield small crystallites of 5-20 µm. Moreover, the maximum
occupancy of magnetic sites has been observed to be only 97%,
with most methods providing samples possessing magnetic site
occupancies of 83-94%.9 Accordingly, the low and varying
coverage of the M3+ sites in jarosite materials have led to
considerable discrepancies in their reported magnetic properties.
With the objective of using the jarosite structural framework
as a model system for investigating the magnetic properties of
the Kagome´ lattice, we sought to develop new synthetic methods
for the preparation of pure and highly crystalline materials. Our
synthetic approaches rely on employing redox-based hydro-
thermal methods to control the kinetics of product precipitation.
Here we report the development of such methods for the
synthesis and physical characterization of a new series of
magnetically pure and highly crystalline V3+ jarosites, AV3-
(OH)6(SO4)2 with A ) Na+, K+, Rb+, Tl+, and NH4+. As
discussed in the following paper,24 this class of compounds
allows for the systematic investigation of the magnetic properties
of a material composed of ferromagnetic Kagome´ layers at
varying distances of separation.
Experimental Section
General Procedures. All chemicals of reagent or analytical grade
were obtained from commercial companies, and they were used without
purification. Hydrothermal reactions were carried out in 23-mL, 45-
mL and 125-mL Teflon-lined, pressure vessels, which were purchased
from Parr Instruments. A Fisher Isotemp programmable oven with
forced-air circulation was used to obtain the desired temperature profiles
for hydrothermal reactions.
Synthesis by Hydrothermal Redox Methods: Preparation of
NaV3(OH)6(SO4)2 (NaVS). Method 1. VCl4 (5.3 mL) was added
dropwise to a stirring solution of ∼10 mL of ice-cold water; the
resulting reaction mixture was diluted to 25 mL to afford a solution
that was 2.0 M in VOCl2 and 4.0 M in HCl. A 2.0-mL amount of this
solution (4.0 mmol of VOCl2) was transferred into the Teflon liner of
a 23-mL pressure vessel. In a separate beaker, 0.227 g (1.80 mmol) of
Na2SO3, 0.20 g (5.0 mmol) of NaOH and 0.117 g (2.0 mmol) of NaCl
were dissolved in 8 mL of distilled water, and the solution was slowly
added to the VOCl2-charged Teflon liner. The vessel was sealed and
placed into an oven at 202 °C. After 6 d, the oven was cooled at 0.1
°C min-1 to room temperature. Small, dark-red to black crystals were
filtered, washed with distilled water, and dried in air. Yield: 0.31 g
(73%). Anal. Calcd for H6NaV3S2O14: H 1.29, Na 4.89, V 32.52, S
13.65. Found: H 1.32, Na 5.06, V 32.85, S 13.88.
Preparation of RbV3(OH)6(SO4)2 (RbVS). Method 1. A 2.0-mL
aliquot of a 2.0 M VOCl2 solution (4.0 mmol) was transferred into a
Teflon liner of a 23-mL Parr pressure vessel. In a separate beaker, 0.47
mL of a 50% solution of RbOH (4.0 mmol) and 2.1 mL of 6% H2SO3
(2.0 mmol) were dissolved in 6 mL of distilled water and slowly added
to the Teflon liner containing VOCl2. The bomb was sealed and placed
into an oven at 205 °C for 4 d. The oven was then cooled at 0.1 °C
min-1 to room temperature. The product that precipitated on the walls
of the Teflon liner was isolated, washed with distilled water, and dried
in air. Yield: 0.18 g (34%). Anal. Calcd for H6RbV3S2O14: H 1.14,
Rb 16.05, V 28.70, S 12.04. Found: H 1.12, Rb 15.57, V 31.12,
S 12.03.
Method 2. Ten millilliters of a 2.0 M VOCl2 solution (20 mmol),
prepared as described above, was mixed with a 50-mL solution
containing 1.00 g (25 mmol) of NaOH in a 125-mL Teflon liner. To
(26) Buban, K. R.; Collins, M. J.; Masters, I. M. JOM 1999, 51, 23-25.
(27) Dutrizac, J. E. JOM 1999, 51, 30-32.
(28) Dutrizac, J. E. Metall. Trans. B 1983, 14, 531-539.
(29) Dutrizac, J. E.; Kaiman, S. Can. Mineral. 1976, 14, 151-158.
(30) Kubisz, J. Mineral. Polonica 1970, 1, 47-57.
Method 2. A 2.0-mL amount of 2.0 M VOCl2 solution was mixed
with 8 mL of solution containing 0.59 mL (5.0 mmol) of 50% RbOH
in a 23-mL Teflon liner. Into this solution, 0.18 mL (2 mmol) of
9
J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002 2641

A R T I C L E S
Grohol
dimethylsulfite was added. The bomb was closed and placed into the
oven at 202 °C for 7 d. The oven was then cooled at 0.1 °C min-1 to
room temperature. Small dark-red crystals attached to the walls of the
Teflon liner were filtered, washed, and dried in air. Yield: 0.35 g (66%).
Anal. Calcd for H6RbV3S2O14: H 1.14, Rb 16.05, V 28.70, S 12.04.
Found: H 1.10, Rb 16.12, V 29.11, S 12.07.
Method 3. To a solution of 3.74 g of Rb2SO4 (14 mmol) dissolved
in 50 mL of distilled water was added 1.94 mL (35 mmol) of H2SO4,
and the solution was transferred into a Teflon liner of a 125-mL pressure
vessel. A 0.616-g pellet of metallic vanadium (12.1 mmol) was added
to the solution. The vessel was enclosed and placed into an oven at
204 °C for 7 d. The oven was cooled at 0.1 °C min-1 to room
temperature, and after opening the vessel, a small, unreacted piece of
vanadium metal was carefully removed. The product that precipitated
on the walls and on the bottom of the liner was isolated, washed with
distilled water, and dried in air. Yield: 1.14 g (53%). Anal. Calcd for
H6RbV3S2O14: H 1.14, Rb 16.05, V 28.70, S 12.04. Found: H 1.06,
Rb 16.28, V 28.88, S 12.10.
and the resulting solution was slowly added to the bomb, which was
then closed and placed into an oven at 200 °C for 3 d. The bomb was
cooled to room temperature at 0.1 °C min-1. The product, which
precipitated on the walls of the liner, was isolated, washed with distilled
water, and dried in air. Yield: 0.22 g (47%). Anal. Calcd for
H10NV3S2O14: H 2.17, N 3.01, V 32.86, S 13.79. Found: H 2.14, N
2.93, V 32.74, S 13.65.
Preparation by Conventional Non-Redox Methods: Preparation
of NaVS. A 25-mL round-bottom flask was charged with a 10-mL
aqueous solution containing 1.420 g of Na2SO4 (10 mmol) and 0.040
g of NaOH (1.0 mmol). After the addition of 0.551 g of VCl3 (3.5
mmol), the flask was fitted with a condenser, and the solution was
refluxed for 24 h. The resulting suspension was cooled to room
temperature. A green precipitate was collected by filtration, washed
with distilled water, and air-dried. Yield: 0.49 g (84%). Analysis
revealed that the occupancy of the magnetic sites was 91.7% and that
25% of the Na+ sites were substituted by the H3O+ ions. Anal. Calcd
for H7.5Na0.75V2.75S2O14.25: H 1.64, Na 3.78, V 30.66, S 14.01. Found:
H 2.16, Na 3.85, V 30.48, S 13.43.
Preparation of TlV3(OH)6(SO4)2 (TlVS). Method 1. A solution
of 1.01 g of Tl2SO4 (2.0 mmol) dissolved in 5.5 mL of distilled water
and 3.1 mL (3.0 mmol) of 0.96 M H2SO3 was transferred into a Teflon
liner of a 23-mL Parr pressure vessel containing 1.5 mL of 2.0 M VOCl2
(3.0 mmol). The vessel was sealed and placed into an oven at 205 °C
for 3 d. The oven was then cooled at 0.1 °C min-1 to room temperature.
The product was isolated, washed with distilled water, and dried in
air. This method produced a heterogeneous mixture of dark maroon
crystals of TlVS and colorless crystals of TlCl. Crystals from this
synthetic procedure were of high quality and were characterized
structurally by single-crystal analysis. The TlVS jarosite could not be
obtained in the absence of TlCl due to this salt’s very low solubility in
water. To obtain TlCl-free TlVS, the following procedure was
developed to permit the separation of the two phases. A 2.0-mL amount
of 2.0 M VOCl2 was added into a Teflon liner of a 23-mL Parr pressure
vessel, and 2 mL of distilled water was added. In a separate beaker,
0.938 g of Tl2CO3 was added to 6.16 mL of 0.65 M solution of H2SO3
(4.0 mmol), and the mixture was added to the Teflon liner. The bomb
was enclosed and placed into an oven at 205 °C for 5 d. The oven was
then cooled at 0.1 °C min-1 to room temperature. White precipitate of
TlCl covered the bottom of the liner while dark maroon precipitate of
product formed on the walls. The product was collected. Yield: 0.13
g (20%). Anal. Calcd for H6TlV3S2O14: H 0.93, Tl 31.38, V 23.46, S
9.85. Found: H 1.04, Tl 32.37, V 23.62, S 9.87.
Method 2. A 23-mL Teflon-lined Parr vessel containing 0.938 g
(2.0 mmol) of Tl2CO3, 0.18 mL of dimethylsulfite (2.0 mmol), 2.0 mL
of 2.0 M VOCl2 (4.0 mmol), and 8 mL of distilled water was sealed
and placed in an oven at 202 °C for 3 d. The oven was then cooled at
0.1 °C min-1 to room temperature. A white precipitate of TlCl covered
the bottom of the liner, and maroon TlVS precipitated on the walls.
Yield: 0.093 g (14%). Anal. Calcd for H6TlV3S2O14: H 0.93, Tl 31.38,
V 23.46, S 9.85. Found: H 1.06, Tl 32.18, V 23.58, S 9.79.
Method 3. In 9.6 mL of distilled water were dissolved 0.151 g of
Tl2SO4 (0.30 mmol) and 0.44 mL of H2SO4, and the solution was
transferred into a Teflon liner of a 23-mL pressure bomb. A 0.163-g
pellet of metallic vanadium (3.2 mmol) was added to the solution, and
the bomb was enclosed and placed into an oven at 205 °C for 4 d. The
oven was then cooled at 0.6 °C min-1 to room temperature. After
opening the bomb, a small, unreacted piece of vanadium metal was
removed. The product that precipitated on the walls and on the bottom
was carefully isolated, washed with distilled water, and dried in air.
Yield: 0.130 g (33%). Anal. Calcd for H6TlV3S2O14: H 0.93, Tl 31.38,
V 23.46, S 9.85. Found: H 0.75, Tl 31.43, V 23.80, S 10.08.
Preparation of NH4V3(OH)6(SO4)2 (NH4VS). Method 1. A 2.0-
mL amount of 2.0 M VOCl2 solution (4.0 mmol) was transferred into
a Teflon liner of a 23-mL Parr pressure bomb. In a separate beaker,
0.54 mL of a 14.8 M solution of NH4OH (8.0 mmol), and 2.1 mL of
6% H2SO3 (2.0 mmol) were dissolved in 6.0 mL of distilled water,
Crystallography. X-ray diffraction data were collected using a
Siemens three-circle single-crystal diffractometer equipped with a CCD
detector. All the data acquisitions were carried out at -90 °C in a
nitrogen stream using Mo KR radiation (λ ) 0.71073 Å), which was
wavelength-selected with a single-crystal graphite monochromator. For
each crystal, four data sets of 40-s frames were collected over a
hemisphere of reciprocal space using ω scans and a -0.3° scan width.
The data frames were integrated to hkl/intensity, and final unit cells
were calculated using the SAINT program. All structures were solved
by the Patterson methods and refined using the SHELXTL suite of
programs.
Methods. Electronic absorption spectra were measured on an OLIS-
modified Cary-17 spectrophotometer. Solid samples of V3+ jarosites
were finely ground, suspended in Nujol mull, and spread on Whatman
1 filter paper. All spectra were recorded over a spectral range of 360-
800 nm against a reference consisting of another filter paper moistened
with several drops of Nujol.
Infrared spectra of jarosites in KBr pellets were recorded on a Nicolet
Magna-IR 860 Spectrometer equipped with a KBr beam splitter and a
DTGS detector. For each spectrum, 32 scans were acquired with 4-cm-1
resolution over a wavelength range of 4000-400 cm-1
.
Magnetic susceptibilities of powdered samples prepared by conven-
tional nonredox method were measured in gelatin capsules using a
SQUID susceptometer (Quantum Design MPMSR2) over a 6-300 K
temperature range and at 0.5, 5.0, and 10.0 kOe. The Curie-Weiss
constants were calculated from linear fits of the inverse susceptibilities
vs temperature for T ) 150-300 K.
Results and Discussion
Syntheses. Five analogues of vanadium jarosite AV3(OH)6-
(SO4)2 (A ) Na+, K+, Rb+, Tl+ and NH4+) have been prepared
by three different hydrothermal, redox-based methods. With each
of the synthetic approaches, we have achieved control over the
precipitation of the jarosite by using redox reactions to slowly
generate V3+ throughout the course of the hydrothermal process.
Each of the approaches yields single-crystalline and highly pure
samples that analyze as 100.3 ( 1.1% in the M3+ ion, 100 (
1.5% in the A+ ion, 99.5 ( 1.1% in S, and 99.5 ( 3% in H.
Given the large standard deviation of the latter value, we note
that the elemental analysis does not reliably establish the
hydrogen content. In principle, missing protons from the lattice
could be charge-compensated by the presence of vanadium ions
in a +4 oxidation state. Our neutron diffractions studies,
however, have established that fully reduced V3+ materials are
obtained from the redox-based synthetic methods described
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2642 J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002

Synthesis of Vanadium Jarosites
A R T I C L E S
produced. Unlike methods 1 and 2, the acidity of the solution
decreases slowly during this reaction. The oxidation method
most often yields samples of the highest-quality single crystals.
We believe that the dissolution of metallic vanadium provides
a means of slowly generating V3+ ion, which is likely the
controlling step of the crystallization process. The preparative
method does, however, present one inconvenience. The reaction
does not proceed to completion due to passivation of the
vanadium pellet, which must be physically removed from the
vessel prior to collecting the jarosite product.
Structural Chemistry. The layered Kagome´ motif of the
alunite supergroup of compounds of formula AM3(OH)6(SO4)2
was first established by X-ray single-crystal analysis of a natural
potassium alunite sample.7 The correct space group for the
alunites was subsequently determined to be R3hm.6 The structures
of the new V3+ jarosites reported here conform to this latter
result, all crystallizing in the R3hm space group. Crystallographic
data of the Na+, K+, Rb+, and Tl+ derivatives are reported in
Table 1, and selected bond distances and angles, in Table 2.
Figure 1. Photograph of single crystals of KV3(OH)6(SO4)2 prepared by
method 3.
here.25 Figure 1 shows typical crystals, which are trigonal
antiprismatic plates of approximate dimensions of 0.2 mm ×
0.2 mm in width and 0.04 mm in thickness. The orange crystals
darken in intensity with increasing crystal size.
+
Crystals of the NH4 derivative were not of sufficient quality
to permit single-crystal data to be acquired. Consistent with the
initial structural reports of NaVS,23,31 the triangular subunit of
the Kagome´ lattice is composed of a sulfate anion, which caps
three vanadium octahedra that form a layered corner-sharing
triangular network (Figure 2a). This basic building block is not
perturbed by the A+ ion. A comparison of the derivatives reveals
that the a-dimension of the unit cell (Figure 2b) and all
equivalent bond distances and angles within the basic layer
framework are almost identical (Table 2). The only structural
value that appreciably changes within the series is the c-
dimension of the unit cell (Figure 2c), which is dictated by the
size of the monovalent cation residing between the Kagome´
layers. The monotonic increase in the interlayer spacing along
the series NaVS (d003 ) 5.617 Å), KVS (d003 ) 5.800 Å), RbVS
(d003 ) 5.945 Å), and TlVS (d003 ) 5.800 Å) follows directly
from the increasing ionic radius of the monocation.
The V3+ ion may be prepared by reducing V4+ in the form
of VOCl2 by sulfite anion SO3 in the presence of the A+
2-
cations under acidic conditions (pH ≈ 1.5),
A+ + 4 VO2+ + 2 SO32- + 4 H2O f
AV3(OH)6(SO4)2 + V3+ + 2 H+ (2)
The main constituent building blocks of the jarosite lattice, the
V
3+ and SO42- ions, are produced by a redox reaction. During
the course of the overall reaction, the pH of the reaction mixture
increases, attendant to the incorporation of hydroxides into the
V
3+ jarosite lattice; two moles of protons are generated per mole
of AV3(OH)6(SO4)2 produced. Small crystals are obtained from
reaction 2.
Dimethylsulfite, (CH3O)2SO, is also a competent reductant
of VOCl2 under hydrothermal conditions at 200 °C,
Spectroscopic and Magnetic Characterization. Electronic
absorption spectra of the Na+, K+, and Rb+ derivatives are
almost identical. The visible absorption spectrum shows two
distinct shoulders at 410 and 570 nm. The energies of these
A+ + 4 VO2+ + 2 (CH3O)2SO + 8 H2O f
3
absorption bands are similar to those observed for the T1g(P)
AV3(OH)6(SO4)2 + V3+ + 6 H+ + 4 CH3OH (3)
3
3
3
r T1g (λmax ) 389 nm) and T2g r T1g (λmax ) 561 nm)
transitions of V(H2O)63+, respectively.32 An oxide ligand field
even more closely approximates that of the V3+ jarosites. The
observed transitions of V3+ doped into the pseudooctahedral
oxide environment provided by Al2O3 appear at 401 and 574
nm.32
This method leads to pure jarosites of high crystal quality, but
the size of the crystals is not significantly improved with respect
to the approach based on reaction 2.
Alternatively, V3+ may be produced by oxidation of V metal.
Under hydrothermal conditions, oxygen or protons present in
the reacting solution are the potential oxidants. To distinguish
between these possibilities, two reactions were performed under
identical hydrothermal conditions with the exception that one
was performed under an oxygen atmosphere and the other under
a nitrogen atmosphere. In both cases, V3+ jarosite was obtained
with similar yields. This result indicates that oxygen is not
essential for the formation of V3+ and consequently that protons
are the primary oxidant of V metal,
Infrared spectra of the Na+, K+, and Rb+ derivatives are very
similar to each other. The spectrum of the Na+ derivative shown
in Figure 3 is exemplary of the series. A very strong peak with
a maximum between 3420 and 3360 cm-1 and an equally strong
peak at 1030 cm-1 are attributed to the absorptions of the
O(3)-H stretching and V-O(3)-H bending modes, respec-
tively.33-35 The energy of the O-H stretching mode increases
along the series NaVS < KVS < RbVS by 25 and 27 cm-1
,
respectively (Table 3), suggesting the relative strength of the
A+-O interaction to follow the trend: Na+-O(3) > K+-O(3)
V + 3 H+ f V3+ + 3/2 H2
(4)
(31) Dobley, A.; Zavalij, P. Y.; Whittingham, M. S. Acta Crystallogr. C 2000,
56, 1294-1295.
(32) Lever, A. B. P. Inorganic Electronic Spectroscopy; Elsevier: Amsterdam,
1984; Chapter 6, pp 400-405.
With the coupling of reaction 4 to 1, this hydrothermal method
consumes three equiv of protons per equiv of V3+ jarosite
9
J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002 2643

A R T I C L E S
Grohol
Table 1. Crystallographic Data for the AV3(OH)6(SO4)2 Jarosites
NaVS
KVS
RbVS
TlVS
empirical formula
fw
H6NaV3S2O14
469.99
H6KV3S2O14
486.09
H6RbV3S2O14
532.46
H6TlV3S2O14
651.38
crystal system
space group
a (Å)
rhombohedral
R3hm
7.2857(6)
16.851(2)
90
120
3; 774.6(1)
3.022
rhombohedral
R3hm
7.2737(7)
17.399(2)
90
120
3; 797.2(2)
3.037
rhombohedral
R3hm
7.281(2)
17.835(7)
90
120
3; 818.8(5)
3.239
rhombohedral
R3hm
7.2834(6)
17.919(2)
90
120
3; 823.2(2)
3.941
c (Å)
R (deg)
γ (deg)
Z; V (Å3)
Fcalc
θ range (deg)
scan
3.45-23.25
3.44-23.09
3.43-23.27
3.41-23.26
ω
ω
ω
ω
temp (K)
183(2)
183(2)
183(2)
183(2)
no. of reflections collected
no. of unique reflections
no. of parameters
R1 (I > 2σ; all data)
wR2 (I > 2σ; all data)
GOF
1050
161
25
0.0295; 0.0306
0.0789; 0.0792
1.318
1048
160
25
0.0400; 0.0407
0.0973; 0.0975
1.288
1087
172
25
0.0365; 0.0412
0.0918; 0.0935
1.240
1137
173
25
0.0210; 0.0218
0.0474; 0.0479
1.263
for the interlayer A+ cation,
Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for
the AV3(OH)6(SO4)2 Jarosites
AM3(OH)6(SO4)2 + x H3O+ f A1-x(H3O)xM3(OH)6(SO4)2
NaVS
KVS
RbVS
TlVS
Bond Distances/Å
2.949(5)
(5)
A-O(2) 6×
A-O(3) 6×
S-O(1)
S-O(2) 3×
V-O(2) 2×
V-O(3) 4×
2.975(6)
2.851(5)
1.46(1)
1.471(6)
2.063(5)
1.991(2)
2.998(6)
2.928(6)
1.46(1)
1.478(7)
2.064(6)
1.988(3)
3.002(6)
2.943(5)
1.47(1)
1.486(6)
2.055(5)
1.983(2)
2.756(4)
1.461(8)
1.476(5)
2.054(4)
1.989 (2)
The other species is H2O resulting from the protonation of the
intralayer hydroxy group. For jarosites with M3+ ion deficien-
cies, proton transfer permits the charge neutrality of the com-
pound to be maintained.
Bond Angles/deg
O(1)-S-O(2) 3× 109.4(2)
109.7(2)
109.2(2)
180
92.4 (2)
87.6 (2)
180
110.0(3)
109.0(3)
180
92.0(2)
88.0(2)
180
91.0(4)
89.0(4)
110.2(2)
108.8(2)
180
91.6 (2)
88.4(2)
180
90.5(3)
89.5(3)
AV3(OH)6(SO4)2 + 3x H+ f AV3-x(OH2)3x(OH)6-3x(SO4)2
O(2)-S-O(2) 3× 109.6(2)
O(2)-V-O(2)
180
92.5 (1)
87.5(1)
(6)
O(2)-V-O(3) 4×
O(2)-V-O(3) 4×
Our contention of extrinsic impurities consisting of H2O and
H3O+ is confirmed by the elemental analysis of the NaVS
sample prepared by the conventional nonredox technique; an
analysis of Na0.75(H3O)0.25V2.75(OH2)0.75(OH)5.25(SO4)2 indicates
the presence of significant deficiencies in the contents of both
the Na+ and the V3+ ions. Therefore, both types of defects are
present in the lattice. The presence of both inter- and intralayer
defects have significant implications for the magnetic properties,
particularly for the nature of long-range ordering (vide infra).
Jarosite materials prepared by redox and nonredox hydro-
thermal methods exhibit profoundly different magnetic proper-
O(3)-V-O(3) 2× 180
O(3)-V-O(3) 2×
O(3)-V-O(3) 2×
90.9(3)
89.1(3)
90.5(3)
89.5(3)
> Rb+-O(3). Two very strong peaks at about 1190 and 1095
cm-1 arise from the antisymmetric S-O stretching modes of
the sulfate anion, and two medium peaks at 702 and 642 cm-1
are due to the S-O bending modes of the same moiety. A very
strong peak at 521 cm-1 is indicative of the V-O stretching
mode.
Two prominent IR absorption features distinguish V3+
jarosites prepared by the new redox-based methods from those
materials obtained with the conventional precipitation methods
employed previously to this study. As shown by the dashed
line of Figure 3, the IR spectrum of V3+ jarosite prepared by
nonredox methods exhibits a broad absorption near 3400 cm-1
for the O-H stretching vibration and an absorption at 1635
cm-1, which is characteristic of an H-O-H bending mode. In
stoichiometrically pure jarosites, the 3400 cm-1 absorption is
reduced in intensity and width; the 1635 cm-1 does not appear
because the H-O-H moiety is not intrinsic to the jarosite
structure. We believe that this result indicates a significant
presence of two extrinsic species in jarosites prepared by
conventional methods. One is the H3O+ ion, which can substitute
-1
ties. Plots of øM vs T for T ) 150-300 K are linear for all
samples. For the sodium member of the series, similar Curie-
Weiss constants of ΘCW ) 53(2) K and 50(2) K are obtained
for samples prepared by redox- and nonredox methods, respec-
tively. However, the magnetic properties of samples prepared
by the different methods are not commensurate in the low-
temperature region (T < 40 K). Figure 4 emphasizes the
magnetic susceptibility plots of NaVS over the low-temperature
range where three-dimensional antiferromagnetic ordering be-
tween neighboring Kagome´ layers becomes evident.23,24 The
critical ordering temperature, Tc ) 20.5 K, of the sample
prepared by the conventional precipitation technique is signifi-
cantly lower than the Tc ) 33.3 K observed for samples prepared
by the new redox technique. A similar result is observed for
the KVS samples (Tc (0.5 kOe) ) 15.5 K and Tc (0.5 kOe) )
31.0 K for nonredox and redox prepared samples, respectively).
These results indicate that the nature of the long-range interac-
tions is significantly influenced by lattice defects associated with
(33) Powers, D. A.; Rossman, G. R.; Schugar, H. J.; Gray, H. B. J. Solid State
Chem. 1975, 13, 1-13.
(34) Ripmeester, J. A.; Ratcliffe, C. I.; Dutrizac, J. E.; Jambor, J. L. Can. Mineral.
1986, 24, 435-447 and references therein.
(35) Sasaki, K.; Tanaike, O.; Konno, H. Can. Mineral. 1998, 36, 1225-1235.
9
2644 J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002

Synthesis of Vanadium Jarosites
A R T I C L E S
Figure 2. The X-ray crystal structure of (a) the magnetic subunit of a triangular µ-hydroxy vanadium trimer, (b) the in-plane segment of the Kagome´ layer
and (c) side-on view of stacked Kagome´ layers. The sphere coding of the atoms is presented in (a).
Table 3. Locations of Peaks in Infrared Spectra of the
AV3(OH)6(SO4)2 Jarositesa
mode
NaVS
KVS
RbVS
TlVS
NH VS
4
ν(O-H)
3361
-
3386
-
3413
-
3412
-
3412
3286
1425
1196
1088
1016
merged
671
ν(N-H)
δ(N-H)
νas(S-O)
νas(S-O)
δ(V-O-H)
δas(S-O)
δas(S-O)
ν(V-O)
-
-
-
-
1188
1105
1040
706
644
521
1190
1095
1030
702
642
521
1200
1090
1018
688
667
515
1194
1078
1011
merged
658
503
513
a Assignments taken from refs 33 and 35.
may also be of relevance to the magnetic properties of jarosites
possessing H3O+ ions as the A+ cation. For V3+ jarosite, (H3O)-
V3(OH)6(SO4)2, a Tc ) 21 K has recently been observed.37
A
possibility exists that the low Tc for this sample, as compared
to that observed for other A+ members of this series,24 is due
to defects arising from protonation of the hydroxides bridging
the vanadium centers. More generally, jarosites with wide ranges
of x values in eq 5 have been described in the literature for M
Figure 3. IR spectra of NaV3(OH)6(SO4)2 prepared by method 2 (solid
line) and by a conventional nonredox precipitation (dashed line).
the presence of the H-O-H moiety within the intralayer and
interlayer of the jarosite structure. The correlation between
depressed transition temperatures and the presence of site defects
in the Kagome´ lattice appears to be a general trend for the
jarosite compounds. Recent magnetic studies performed on
stoichiometrically pure A+ members of Fe3+ jarosites yielded
reproducible antiferromagnetic ordering temperatures that were
appreciably greater than those previously reported.36 Our results
) Fe3+ 9,10
(OH)6(SO4)2, has extensively been investigated, and it has long
been believed to be a spin-glass magnetic system.9,10,17
.
The end member of this ion exchange, (H3O)Fe3-
A
possible explanation of this spin glass behavior may be due to
structural disorder arising from proton transfer from the inter-
layer hydronium ions to the bridging hydroxyl groups.
(37) Wills, A. S. Phys. ReV. B 2001, 63, 064430/1-064430/13.
(38) Gopalakrishnan, J. Chem. Mater. 1995, 7, 1265-1275.
(36) Grohol, D.; Papoutsakis, D.; Nocera, D. G. Submitted for publication.
9
J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002 2645

A R T I C L E S
Grohol
reactions under equilibrium. Under these hydrothermal condi-
tions, highly pure materials can be obtained.
The idea of introducing a step to control the production of a
metal ion prior to precipitation is not new, although previous
solvothermal protocols have not typically involved a redox step.
Rather, the metal ion was produced by its controlled dissolution
from a heterogeneous phase. Alberti and co-workers synthesized
large single crystals of R-zirconium phosphate by taking
advantage of the temperature-dependent equilibrium between
F- and Zr4+ ions.45 As the temperature was slowly raised, Zr4+
ions were de-complexed and made available for reaction with
the phosphate anion. This method was subsequently employed
for the preparation of organic derivatives of R-zirconium
phosphate and phosphonates and for the synthesis of a variety
of phosphonate compounds possessing transition metal cations
other than Zr4+. Zhang and Clearfield have used an alternative
approach to grow single crystals of divalent transition metal
phosphonates.46 In aqueous environment, urea slowly decom-
poses to generate ammonium hydroxide, thereby gradually
increasing the pH of the solution. In an environment with an
increasing pH, phosphonic acids can be slowly deprotonated
and thus made available for reaction with the transition metal
to form divalent phosphonates.
In summary, we have prepared and structurally characterized
a new series of vanadium jarosites. We have demonstrated that
representatives of the jarosite class of materials can be obtained
by deliberately inserting a kinetically controlling oxidation or
reduction step into the synthesis sequence in such a way as to
slow the formation of the product. With this approach, we have
developed three new methods for the synthesis of vanadium
jarosites in pure form and of high crystallinity. The described
synthetic methods are general and should be applicable to other
members of the alunite supergroup of minerals. For instance,
ongoing investigations exploiting redox-based hydrothermal
methods have yielded large and stoichiometrically pure single
crystals of the Fe3+ jarosites. Our studies of the V3+ and other
M3+ jarosites reveal that these new redox-based synthetic
approaches are keystones to obtaining reproducible magnetic
properties of the Kagome´ lattice of the jarosite class of
compounds.
Figure 4. Temperature dependence of M/H per mole of vanadium at applied
magnetic field strengths of 0.5 (O), 5.0 (+) and 10.0 (4) kOe for a sample
prepared by (bottom) conventional nonredox- and (top) redox-based
hydrothermal syntheses.
Concluding Remarks
Low-temperature synthetic techniques such as “chimie douce”,
flux, or solvothermal methods38-40 permit control over the
reaction pathway under mild conditions (T < 300 °C) as com-
pared to traditional high-temperature solid-state synthetic pro-
tocols.41-43 However, precipitation is often rapid when cations
and anions, exhibiting high thermodynamic affinities to each
other, are reacted under homogeneous conditions. If the nuclea-
tion rate of new crystals significantly exceeds the growth rate
of crystals already formed44 then microcrystalline powders will
result that may or may not be stoichiometrically pure. Under
extreme conditions, local deficiencies will be created when the
precipitation rate is faster than ion diffusion. As is the case for
jarosite materials, an available charged ion may substitute locally
depleted ions to yield poorly crystalline products that contain a
variety of structural and stoichiometric defects.
Acknowledgment. We thank Dr. Dimitris Papoutsakis for
acquiring the X-ray single crystal data sets. Financial support
for this work was provided by the National Science Foundation
(DMR-9311597) and by the Center for Materials Science and
Engineering Research at MIT (DMR-9808941).
Supporting Information Available: Tables of crystal data,
ORTEP representations with numbering schemes, atomic co-
ordinates, and isotropic and anisotropic thermal parameters for
AV3(OH)6(SO4)2 (A ) Na+, K+, Rb+ and Tl+) (PDF). An X-ray
crystallographic file in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
The newly discovered synthetic methods described herein are
designed to generate the magnetic M3+ ion by controlled redox
(39) Stein, A.; Keller, S. W.; Mallouk, T. E. Science 1993, 259, 1558-1564.
(40) Rouxel, J.; Tournoux, M.; Brec, R. Soft Chemistry Routes to New Materials;
Trans Tech Publications: Aedersmannsdorf, CH, 1994; Vol. 152-153.
(41) Corbett, J. D. Inorg. Chem. 2000, 39, 5178-5191.
(42) West, A. R. Solid State Chemistry and Its Applications; Wiley: New York,
1984.
JA016832U
(43) Hagenmuller, P. PreparatiVe Methods in Solid State Chemistry; Academic
(45) Alberti, G.; Costantino, U.; Giulietti, R. J. Inor. Nucl. Chem. 1980, 42,
1062-1063.
(46) Zhang, Y.; Clearfield, A. Inorg. Chem. 1992, 31, 2821-2826.
Press: New York, 1972.
(44) Leubner, I. H. J. Phys. Chem. 1987, 91, 6069-6073.
9
2646 J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002