Synthesis and structural, spectroscopic and magnetic studies of two new polymorphs of Mn(SeO3)·H2O

Article abstract of DOI:10.1016/j.jssc.2005.09.034

Two new manganese(II) selenite polymorphs with formula Mn(SeO ₃)·H₂O have been synthesized by slow evaporation from an aqueous solution. The crystal structure of both compounds (1) and (2) have been solved from X-ray diffraction data. The structure of (1) was determined from single-crystal X-ray diffraction techniques. The compound crystallizes in the Ama2 space group, with a=5.817(1), b=13.449(3), c=4.8765(9)A and Z=4. The structure of (2) has been solved from X-ray powder diffraction data. This phase crystallizes in the P2₁/n space group with unit-cell parameters of a=4.921(3), b=13.121(7), c=5.816(1)A, β=90.03(2)° and Z=4. Both polymorphs exhibit a layered structure formed by isolated sheets of MnO₆ octahedra and (SeO₃) ^2- trigonal pyramids in the (010) plane. These layers, which contain one manganese and selenium atom crystallographically independent, are formed by octahedra linked between them through the selenite oxoanions. The difference of both compounds consists in the stacking of the layers along the b-axis. The IR spectra show the characteristic bands of the selenite anion. Studies of luminescence performed at 6 K and diffuse reflectance spectroscopy have been carried out for both phases. The Dq and Racah (B and C) parameters, from luminescence and diffuse reflectance spectroscopy, are Dq=705, B=750, C=3325cm-1 for (1) and Dq=720, B=745, C=3350cm-1 for (2). The ESR spectra of both compounds are isotropic with g-values of 1.99(1). Magnetic measurements indicate the presence of antiferromagnetic couplings in both phases. The J-exchange parameters have been estimated by fitting the experimental magnetic data to a model for square-planar lattice. The values obtained are J/k=-0.83, -0.91 K and J′/k=-0.97, -1.20 K, for polymorphs (1) and (2), respectively.

Full text of DOI:10.1016/j.jssc.2005.09.034

ARTICLE IN PRESS
Journal of Solid State Chemistry 178 (2005) 3686–3697
www.elsevier.com/locate/jssc
Synthesis and structural, spectroscopic and magnetic studies of two new
polymorphs of Mn(SeO ) ꢀ H O
3
2
a
b,
Ã
a
b
c
˜ ˜
Aitor Larranaga , Jose L. Mesa , Jose L. Pizarro , A. Pena , Roger Olazcuaga ,
´ ´
a
Marıa I. Arriortua , Teofilo Rojo
´ ´
b,
Ã
a
Departamento de Mineralog ı´ a y Petrolog ı´ a, Facultad de Ciencia y Tecnolog ı´ a, Universidad del Pa ı´ s Vasco, Apdo. 644, E-48080 Bilbao, Spain
b
Departamento de Qu ı´ mica Inorg a´ nica, Facultad de Ciencia y Tecnolog ı´ a, Universidad del Pa ı´ s Vasco, Apdo. 644, E-48080 Bilbao, Spain
c
ICMCB-CNRS[UPR9048], Bordeaux 1, 87 Av. A. Schweitzer, 33608 Pessac Cedex, France
Received 4 July 2005; received in revised form 14 September 2005; accepted 15 September 2005
Available online 25 October 2005
Abstract
Two new manganese(II) selenite polymorphs with formula Mn(SeO ) ꢀ H O have been synthesized by slow evaporation from an
3
2
aqueous solution. The crystal structure of both compounds (1) and (2) have been solved from X-ray diffraction data. The structure of (1)
was determined from single-crystal X-ray diffraction techniques. The compound crystallizes in the Ama2 space group, with a ¼ 5:817ð1Þ,
˚
b ¼ 13:449ð3Þ, c ¼ 4:8765ð9Þ A and Z ¼ 4. The structure of (2) has been solved from X-ray powder diffraction data. This phase
˚
crystallizes in the P2 /n space group with unit-cell parameters of a ¼ 4:921ð3Þ, b ¼ 13:121ð7Þ, c ¼ 5:816ð1Þ A, b ¼ 90:03ð2Þ1 and Z ¼ 4.
1
2
)
ꢁ
Both polymorphs exhibit a layered structure formed by isolated sheets of MnO
6
octahedra and (SeO
3
trigonal pyramids in the (010)
plane. These layers, which contain one manganese and selenium atom crystallographically independent, are formed by octahedra linked
between them through the selenite oxoanions. The difference of both compounds consists in the stacking of the layers along the b-axis.
The IR spectra show the characteristic bands of the selenite anion. Studies of luminescence performed at 6 K and diffuse reflectance
spectroscopy have been carried out for both phases. The Dq and Racah (B and C) parameters, from luminescence and diffuse reflectance
ꢁ
1
ꢁ1
spectroscopy, are Dq ¼ 705, B ¼ 750, C ¼ 3325 cm for (1) and Dq ¼ 720, B ¼ 745, C ¼ 3350 cm for (2). The ESR spectra of both
compounds are isotropic with g-values of 1.99(1). Magnetic measurements indicate the presence of antiferromagnetic couplings in both
phases. The J-exchange parameters have been estimated by fitting the experimental magnetic data to a model for square-planar lattice.
0
The values obtained are J=k ¼ ꢁ0:83, ꢁ0.91 K and J =k ¼ ꢁ0:97, ꢁ1.20 K, for polymorphs (1) and (2), respectively.
r 2005 Elsevier Inc. All rights reserved.
Keywords: X-ray diffraction; Single-crystal and powder; IR; Visible–UV; Luminiscence; ESR; Magnetic susceptibility
1
. Introduction
number of different compounds [3]. The transition
metal–selenium–oxygen system has been the subject of
many previous investigations. Several phases have been
reported in which selenium is found in an oxidation state
IV. Depending on the conditions in solution [4], transition
An important area in materials science is the design of
compounds with an open framework, which can give rise to
original physical properties with potential practical appli-
cations such as ion exchange, surface absorption chemistry,
etc., due to the diversity of available cations and the
arrangements that they can exhibit [1,2]. In this way, the
crystal chemistry of selenium(IV) compounds shows
abundant structural versatility expressed by the significant
ꢁ
2ꢁ
2ꢁ
metal compounds with (HSeO ) , (SeO ) or (Se O )
2
3
3
5
3
oxoanions are known [5–7]. Ions with an sp hybrid lone
4
+
pair, such as, Se play a peculiar role in crystal chemistry
because of their high polarizability and strong preference
for unusual stereochemistry [3,8]. Thus, it can be predicted
2
ꢁ
that the combination of asymmetric (SeO3) group with
various transition metals will result in a rich structural
chemistry in transition metal selenites. In this way, in the
Mn (H O) (SeO ) and Mn (H O)(SeO ) phases [9] which
Ã
Corresponding authors. Fax: +34 946013500.
E-mail addresses: qipmeruj@lg.ehu.es (J.L. Mesa), qiproapt@lg.ehu.es
(T. Rojo).
4
2
3
4 3
3
2
3
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.09.034

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3687
exhibit a three-dimensional crystal structure constructed by
chains, there exist cavities delimited by the MnO6
octahedra and SeO trigonal pyramids, with the lone pair,
troscopy (ICP-AES). Observed (%): Mn, 27.2 and 27.3; Se,
39.3 and 39.2. Mn(SeO ) ꢀ H O (1) and (2). Calculated: (%)
3
2
Mn, 27.5; Se, 39.5. The density, measured by picnometry in
ꢁ3
3
3
in an sp hybrid orbital, pointing towards the center of
cavities. Similarly, the two Mn(SeO ) polymorphs [10] also
kerosene, was 3.5(1) g cm for both phases.
Thermogravimetric analysis was carried out under an
oxygen atmosphere in a SDC 2960 Simultaneous DSC-
3
exhibit a three-dimensional crystal structure formed by
chains that give rise to hexagonal channels with the lone
electronic pair directed towards the center of these
channels.
ꢁ
1
TGA TA Instrument, at 5 1C min
in the temperature
range 30–800 1C. A crucible containing ca. 20 mg of sample
was used. The decomposition curves for both phases reveal
two different steps. The first, in the 125–300 and
200–295 1C ranges for (1) and (2), respectively, corresponds
to the elimination of water. The experimental loss is
approximately, 11% and 9%, for (1) and (2), respectively,
the calculated loss being 9%. The second step corresponds
The crystal structures of the above-indicated three-
dimensional phases are more compact than the layered
crystal structures of the title compounds, probably due to
the different synthetic conditions. Mn (H O) (SeO ) and
4
2
3
4 3
Mn (H O)(SeO ) [9] were prepared by mild hydrothermal
3
3
2
2
ꢁ
synthesis at, approximately, 170 1C and 10 atm., the two
to the decomposition of the (SeO3) anion in the form of
SeO gas, and takes place in the 450–550 and 460–560 1C
polymorphs of Mn(SeO ) [10] were obtained using super-
critical hydrothermal conditions (500 or 600 1C and 500 or
3
2
ranges, for compounds (1) and (2). The loss experimental is
51.2%, 51.4%, for (1) and (2), respectively, the calculated
loss being 51.5%. The X-ray powder diffraction patterns of
the residues obtained at 800 1C show the presence of
1
100 atm., for every polymorph), whereas the two poly-
morphs of Mn(SeO ) ꢀ H O were synthesized at room
3
2
temperature and pressure by slow evaporation from an
aqueous solution.
˚
Mn O [space group Ia3, a ¼ 9:43ð1Þ A] [14].
2
3
Recently, Harrison et al. [11] and Choudhury et al. [12]
have prepared organically templated zinc and iron selenites
by using mild hydrothermal conditions. Later, Udayaku-
mar and Rao [13] were able to synthesize, using the
piperazinium cation as structure directing agent, a family
of organically templated compounds in which both the
hydrogenselenite and diselenite oxoanions exist simulta-
neously in the inorganic skeleton. On the other hand,
hydrothermal synthesis performed at moderate or high
temperature and pressure can be used to obtain new
condensed selenites [9,10]. The synthetic procedure carried
out at room temperature and pressure is also an adequate
method to obtain purely inorganic compounds with low
structural dimensionality. In this work we report the
synthesis in aqueous solution at room temperature, the
crystal structure and the thermal, spectroscopic and
magnetic properties of two new layered polymorphs in
The thermal behavior of these polymorphs was also
studied by time-resolved X-ray thermodiffractometry. The
X-ray powder patterns were collected in air on a PHILIPS
X’PERT automatic difractometer (Cu-Ka radiation)
equipped with a variable-temperature stage (Paar Physica
TCU2000) with a Pt sample holder. The patterns were
recorded in 2y steps of 0.021 in the range 5–401, counting
for 2 s per step and increasing the temperature at
ꢁ
1
5 1C min
from room temperature up to 800 1C. The
results are given in Figs. 1a and b. Polymorph (1) is stable
up to approximately 225 1C. A strong decrease of the peak
intensities is observed in the 225–350 1C range, indicating
the decomposition of the compound to give an amorphous
phase. At temperatures higher than 350 1C a crystalline
intermediate phase is formed, which cannot be identified in
the Powder Diffraction File. Finally, at temperatures
higher than 415 1C the inorganic residue is the Mn O
2
3
2
ꢁ
the system Mn(II)–(SeO3) with formula Mn(SeO ) ꢀ H O.
oxide [14], which is in good agreement with that obtained
from the thermogravimetric study (see Fig. 1a). Polymorph
(2) is stable up to approximately 250 1C. In the 250–475 1C
range the same crystalline intermediate compound is
formed. In the case of compound (1), the residue obtained
at 800 1C is Mn O , in accordance with the thermogravi-
3
2
2
. Experimental
2
.1. Synthesis and characterization
2
3
The new polymorphs of Mn(SeO ) ꢀ H O, compounds (1)
metric analysis (see Fig. 1b).
3
2
and (2), have been synthesized by slow evaporation from
an aqueous solution containing MnCl ꢀ 4H O (5.05 mmol)
2.2. Crystal structure determination of the polymorphs (1)
and (2)
2
2
and SeO (9.0 mmol). The pH of the reaction mixtures was
2
increased to, approximately, 7 using RbOH and 6.5 using
NH OH for compounds (1) and (2), respectively. After 2
2.2.1. Polymorph (1)
A prismatic single crystal of (1) with dimensions
4
days at room temperature, the evaporation of water yielded
light-pink single crystals in the case of phase (1) and a
microcrystalline powder for compound (2) of the same
color. The resulting products were filtered, washed with
water and acetone and dried over P O for 6 h.
3
0.02 ꢂ 0.02 ꢂ 0.01 mm was carefully selected under a
polarizing microscope. This crystal was used for the data
collection performed on an XCALIBUR2 automated
diffractometer equipped with a CCD detector, using
graphite-monochromated Mo-Ka radiation. The total
number of measured reflections was 1653, with 479
2
5
The percentage of the elements in the products was
determined by inductively coupled atomic emission spec-

ARTICLE IN PRESS
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3688
Fig. 1. Thermodifractograms of (a) polymorph (1) and (b) polymorph (2).
reflections being independent (Rint: ¼ 0:1046) and 476
observed, applying the criterion I42sðIÞ. The Lorentz
polarization and absorption corrections were made with
the diffractometer software [15], taking into account the
shape and size of the single crystal. The structure was
solved by direct methods with the program SHELXS97 [16]
belonging to the WINGX system [17]. The manganese and
selenium atoms were located directly and the oxygen atoms
from Fourier difference maps. The hydrogen atoms of the
water molecule were not determined. The refinement
procedure of the crystal structure was carried out by the
full-matrix least-squares method based on F , using the
SHELXL97 computer program [18]. A restraint was
imposed in the refinement, due to the absence of inversion
2

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A. Larra n˜ aga et al. / Journal of Solid State Chemistry 178 (2005) 3686–3697
3689
Table 1
Crystal data, intensity collection and some features of the structure
refinement for (1) from single crystal data
between the calculated and observed data. The analysis of
the systematic absences (0k0; k ¼ 2n and h0l; h þ l ¼ 2n)
indicated that the P2 /n is the more probable space group.
1
The preferred orientation (010) observed during the
characterization of compound (2) together with the
similarity of the powder diffractograms and the cell
parameters of both compounds, allowed us to consider a
similar layered structure for both polymorphs.
Taking into account geometric features, structural
models that fulfill the three following conditions were
considered: (i) the compound has (010) layers, (ii) the
sheets have the same topology than the layers observed
in compound (1), (iii) the structural model must be in
Empirical formula
Formula weight (g/mol)
Temperature (K)
MnSeO
199.91
293(2)
3
ꢀ H
2
O
˚
Wavelength (A)
0.71073
Orthorhombic, Ama2
Crystal system, space group
˚
a (A)
5.817(1)
13.449(3)
˚
b (A)
˚
c (A)
4.8765(9)
381.5(1)
3
Volume (A )
˚
3
Z, calculated density (g/cm )
4, 3.445
12.854
364
ꢁ1
Absorption coefficient (mm
F(000)
3
Crystal size (mm )
)
accord with the P2 /n monoclinic symmetry. This
1
0.02 ꢂ 0.02 ꢂ 0.01
Xcalibur2
4.45–31.69
Diffractometer
y range for data collection (deg.)
Limiting indices
process was monitored comparing the theoretical
diagrams for each model using the ATOMS program [23]
and the experimental X-ray diffractogram from poly-
morph (2).
ꢁ8oho8, ꢁ19oko19, ꢁ7olo4
1653/479 [RðintÞ ¼ 0:1046]
96.1
Reflections collected/unique
Completeness to y (%)
Refinement method
2
The Rietveld refinement (FULLPROF98 program) [24]
obtained using the best structural model gives rise to a
good agreement between the observed and calculated
diffractograms. Soft constrains for the Se–O and Mn–O
distances were used in the last refinement taking into
account the (010) preferred orientation (Fig. 2a). Fig. 2b
shows the comparison of a region of the powder patterns
for polymorphs (1) and (2). Additional maxima are
observed in (1) or (2) near to 2y ꢃ 231, due to its different
symmetry.
Full-matrix least-squares on F
476/1/34
1.299
Data/restraints/parameters
2
Goodness-of-fit on F
Final R indices [I42sðIÞ]
R indices (all data)
Largest diff. peak and hole (e A
R1 ¼ 0:0679, wR2 ¼ 0:1648
R1 ¼ 0:0682, wR2 ¼ 0:1649
1.402 and ꢁ1.329
ꢁ
3
)
˚
R1 ¼ ½SðjFoj ꢁ jFcjÞꢄ=SjFoj; wR2 ¼ ½S½wðjFoj ꢁ jFcj Þ ꢄ=S½wðjFoj Þ2ꢄ1=2;
2
2
2
2
2
2
2
2
2
w ¼ 1=½s jFoj þ ðxpÞ þ ðypÞꢄ; where p ¼ ½jFoj þ 2jFcj ꢄ=3 ; x ¼ 0:0644,
y ¼ 9:1281.
The final structural parameters and R-factors are
shown in Table 2. Atomic coordinates of both
compounds are given in Table 3. The structure parameters
have been deposited at the FIZ-Karlsruhe Centre (CSD
415550).
center in the spatial group Ama2. The scattering factors
were taken from Ref. [19]. All atoms were refined
anisotropicaly reaching final R factors of R1 ¼ 0:0682
[
wR2 ¼ 0:1649] (all data). Maximum and minimum peaks
ꢁ3
˚
in final difference map were 1.402, ꢁ1.329 e A . Details of
the crystal data, intensity collection and some features of
the structural refinement are reported in Table 1. The
structure parameters have been deposited at the FIZ-
Karlsruhe Centre (CSD 415549).
2.3. Physicochemical characterization techniques
The IR spectra (KBr pellets) were obtained with a
FT-IR
Nicolet
4
740
00–4000 cm range. Luminescence measurements were
spectrophotometer
in
the
ꢁ
1
2
.2.2. Polymorph (2)
Although polymorph (2) is isotypic with Co
carried out on a Spectrofluorimeter Fluorolog-2 SPEX
1680, model F212I at 6.0 K. The excitation source was a
high-pressure xenon lamp emitting between 200 and
1200 nm. Diffuse reflectance spectra were registered at
room temperature on a Cary 2415 spectrometer in the
210–2000 nm range. A Bruker ESP 300 spectrometer was
used to record the ESR polycrystalline spectra. The
temperature was stabilized with an Oxford Instrument
(ITC 4) regulator, the magnetic field was measured with a
Bruker BNM 200 gaussmeter and the frequency inside the
cavity was determined using a Hewlett-Packard 5352B
microwave frequency counter. Magnetic measurements on
powdered samples were performed in the temperature
range 5–300 K, using a Quantum Design MPMS-7 SQUID
magnetometer. The magnetic field was of 0.1 T, a value in
the range of linear dependence of magnetization vs.
magnetic field even at 5 K.
(
SeO ) ꢀ 2H O [20], attempts to resolve the crystal structure
3
2
starting from that of the cobalt(II) phase, did not yield
good reliability R-factors, due to the structural differences
in the stacking of the layers in the Mn(II) and Co(II)
isotypic phases.
The exact positions of the first 20 maxima of the diagram
of (2) were obtained with the WINPLOTR program [21].
The indexation of these peaks was carried out with the
TREOR90 program [22], giving as the best cell,
˚
a ¼ 5:81ð1Þ, b ¼ 13:11ð1Þ, c ¼ 4:91ð1Þ A and a ꢃ b ꢃ
g ꢃ 901. This unit cell is close to that of the orthorhombic
polymorph (1). The pattern matching refinement per-
formed with an orthorhombic symmetry did not allow a
good fit of the diffractogram, so, a monoclinic cell with b
near 901 was used to obtain a reasonable agreement

ARTICLE IN PRESS
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3690
Fig. 2. (a) Observed, calculated and difference X-ray powder diffraction patterns of polymorph (2). The observed data are shown by dots; the calculated
pattern by a solid line with the difference below. (b) A selected region of the powder patterns of polymorphs (1) and (2) showing the differences near at
2y ꢃ 231.
3
. Results and discussion
It is worth mentioning that the two polymorphs show
different orientation of the layers (Fig. 4). In polymorph
(1), two adjacent sheets are related by one ‘‘a’’ symmetry
plane and the binary axes parallel to the c-axis,
3
.1. Structural description of polymorphs (1) and (2)
Polymorphs (1) and (2) of Mn(SeO ) ꢀ H O exhibit a
˚
being separated by 3.37(1) A. In polymorph (2), the layers
are related by twofold screw axes parallel to the b-axis
and by inversion centers. These sheets are rotated 1801
3
2
layered crystal structure constructed from (010) isolated
2
sheets of MnO octahedra and (SeO ) trigonal pyramids.
These layers, which contain one crystallografically inde-
pendent manganese and selenium atom, are formed by
octahedra linked through corner sharing and selenite
oxoanions (Fig. 3).
ꢁ
6
3
˚
and separated by 3.28(1) A. As consequence of these
structural features compound (1) exhibits orthorhombic
symmetry, whereas that the symmetry for phase (2) is
monoclinic.

ARTICLE IN PRESS
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3691
Table 2
Crystal data, intensity collection and some features of the structure
refinement for (2) from powder data
the oxygen atoms from the edge not shared follow the same
direction (see Fig. 3). The application of the symmetry
elements of every space group generates a displacement of
the layers along the [100] direction for polymorph (2). This
structural feature is not observed for polymorph (1) (see
Fig. 4).
Empirical formula
Formula weight (g/mol)
Temperature (K)
MnSeO
199.91
293(2)
1.5406
3
2
ꢀ H O
˚
Wavelength (A)
In polymorph (1) the MnO octahedra are linked to four
6
2
ꢁ
Crystal system, space group
Monoclinic, P2
1
/n
(SeO3) anions through four O(3) and one O(1) oxygen
atoms, the six-fold coordination is completed by the O(2)
from the water molecule. In polymorph (2) the hexacoo-
dination of the Mn(II) cations is achieved by the two O(4),
two O(2) and one O(3) belonging to the selenite groups and
one O(1) from the water molecule (Fig. 5). In polymorph
˚
a (A)
4.921(3)
13.121(7)
5.816(1)
90.03(2)
˚
b (A)
˚
c (A)
b (deg.)
3
Volume (A )
˚
375.5(4)
4, 3.499
38.684
364
3
Z, calculated density (g/cm )
(1) the links between the MnO octahedra is via four O(3).
6
ꢁ1
Absorption coefficient (mm
F(000)
)
For polymorph (2) the links between the MnO groups is
6
achieved by two O(2) and two O(4) common vertices
between the octahedra and the selenite anions (Fig. 6).
Selected bond distances and angles for (1) are listed in
Table 4. The metal–oxygen bond distances in the MnO6
Diffractometer
X’Pert
10.10–79.96
y range for data collection (deg.)
y step-scan increment (deg.)
Time-step (s/step)
2
0.020
12
˚
octahedra have a mean value of 2.20(6) A. The cis and trans
O–Mn–O bond angles range from 68.5(5)1 to 99.0(6)1 and
from 164.0(4)1 to 172.6(7)1, respectively. The distortion of
No. of points
3494
568/232
19
No. of reflections/independent
No. of structural parameters
No. of profile parameters
Scale factor
12
1
the MnO octahedra, from an octahedron (D ¼ 0) to a
6
trigonal prism (D ¼ 1) has been quantified by using the
Muetterties and Guggenberger method [25]. The value
obtained is 6.5%. These results indicate a distortion from
the ideal octahedral geometry. The mean value of the
No. atoms refined
6
R
R
Bragg, R
f
(%)
7.19, 4.95
5.34, 6.78
2.25
p
2
, wR (%)
p
w
˚
selenium–oxygen bond lengths is 1.70(5) A. The bond
valence analysis has been carried out by I.D. Brown
Rp ¼ Sjyiobs ꢁ ð1=cÞyicalcj=Syiobs;
wRp ¼ ½Swijjyiobs ꢁ ð1=cÞyicalcj ꢄ=Swi½yiobsꢄ2ꢄ1=2;
RB ¼ SjIobs ꢁ Icalcj=SIobs.
2
calculation method [26] using the S ¼ exp ½ðd=d Þ=Bꢄ
0
expression, included in the Rietveld program as the
subroutine BONDSTR [24], where d0 ¼ 1:811 and 1.790
4
+
2+
for Se and Mn cations, respectively, being B ¼ 0:370
Table 3
Atomic coordinates and equivalent isotropic displacement parameters
for both cations. The calculated oxidation states have
+
4
2+
values of 3.8(1) and 1.9(1) v.u., for Se
and Mn
2
˚
U
(eq) (A ) for (1) and (2)
cations, respectively, in good agreement with those
obtained from the structural resolution.
x
y
z
U
(eq)
Selected bond distances and angles for (2) are listed in
Table 4. The mean value for metal–oxygen bond distances
in the MnO octahedra is 2.3(1). The cis O–Mn–O angles
range from 65(2) to 108(2)1, and the trans O–Mn–O angles
from 154(2) to 175(2)1. The distortion of the MnO6
octahedra from the ideal octahedral geometry is 2.4%
(
1)
Se(1)
Mn(1)
O(1)
0.7500
0.7500
0.7500
0.7500
0.529(2)
0.1014(1)
0.2675(2)
0.138(1)
0.388(1)
0.1691(8)
ꢁ0.0054(2)
0.5670(7)
0.313(3)
0.0166(4)
0.0180(6)
0.024(3)
0.026(3)
0.022(2)
6
O(2)w
O(3)
0.876(4)
ꢁ0.157(2)
[
distances is 1.6(1) A. This value is smaller than that found
25]. The mean value of the selenium–oxygen bond
˚
(
2)
Se(1)
Mn(1)
O(1)w
O(2)
1.0253(5)
0.1029(7)
0.416(2)
0.858(3)
0.884(3)
1.348(2)
0.4026(1)
0.2684(2)
0.3908(8)
0.346(1)
0.329(1)
0.3661(7)
0.755(1)
0.248(2)
0.28(1)
0.028(8)
0.012(8)
0.031(19)
0.021(16)
0.014(15)
0.08(2)
˚
for polymorph (1) [1.70(6) A] and for the Co(SeO ) ꢀ H O
3
2
˚
isotypic phase [1.70(3) A], probably due to the use of
powder data in the Rietveld refinement of the crystal
structure of compound (2). The bond valence study [24,26]
0.982(2)
0.548(2)
0.716(4)
O(3)
O(4)
4
+
2+
for Se
values of 4.1(1) and 1.9(1) v.u., respectively.
and Mn
cations of the polymorph (2) gives
À Á
1
3
ꢅ
2
ꢅ
2
ꢅ
2
ꢅ
ꢅ
Ueq ¼
½ðU11ðaa Þ þ U22ðbb Þ þ U33ðcc Þ þ 2U13aca c cos bꢄ.
3.2. Infrared, UV–Vis and luminescence spectroscopies
The rotation of the layers in polymorph (2) determines
that the selenite groups are in alternated disposition in
adjacent sheets, with the vertex belonging to the edge not
shared in opposite directions. However, in polymorph (1)
The infrared spectra for compounds (1) and (2) show
bands corresponding to the water molecule and the
(SeO3) oxoanions. The n(OH) stretching modes of the
2
ꢁ

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3692
2
)
ꢁ
anions.
Fig. 3. Layers of the polymorphs (a) (1) and (b) (2) showing the crystallographically independent MnO
6
octahedra and the (SeO
3
Fig. 4. Arrangement of the layers along the [100] direction for the (a) (1) and (b) (2).
water molecules appear in the 3420–2480 and
Luminescence measurements of the Mn(II) ions in
polymorphs (1) and (2) have been carried out at 6 K. The
emission spectra for both phases obtained under a 420 nm
excitation exhibit a unique red emission peaking at 670 and
665 nm for (1) and (2), respectively. This emission is
ꢁ
1
3
1
415–2860 cm ranges and the d(HOH) deformation at
625, 1615 cm for phases (1) and (2), respectively. The
ꢁ
1
symmetric and antisymmetric stretching vibrations,
n (SeO ), n (SeO ) and the deformation, d(SeO3),
s
3
as
3
5
appear at the following frequencies: n ¼ 1025, 995,
characteristic of octahedrally coordinated Mn(II) (d )
s
ꢁ
1
n
(
¼ 8502675, 850–680 and d ¼ 445 cm for compounds
(Fig. 7a). The excitation spectra obtained with emission of
630 nm reveal the spectral distribution of bands corre-
sponding to the excited levels of Mn(II) in octahedral
sites (Fig. 7b) [30]. The diffuse reflectance spectra,
recordered at room temperature, exhibit several very weak
as
1) and (2). The positions observed for these vibrational
modes are in good agreement with those expected for these
compounds and are similar to those found in the literature
for other selenite phases [27–29].

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3693
2
ꢁ
anions in polymorph (a) (1) and (b) (2).
Fig. 5. Coordination environment for the MnO
6
octahedra and the (SeO
3
)
Fig. 6. Link between coordination polyhedra for (a) (1) and (b) (2).
2
+
spin-forbidden d–d bands consistent with Mn high spin
ions in octahedral geometry. The values are similar to those
obtained from the excitation luminescent spectra. Table 5
shows the positions of the bands together with the
assignment of the electronic transitions obtained from the
luminescence and diffuse reflectance measurements for
polymorphs (1) and (2). Taking into account the results of
the luminescence and diffuse reflectance spectroscopies, the
Dq and Racah parameters have been calculated by fitting
the experimental distribution of spectral bands to an
indicating a significant degree of covalence in the Mn–O
bonds.
3.3. ESR and magnetic properties
The ESR spectra of phases (1) and (2) were performed at
the X-band on a powdered sample from room temperature
to 4.2 K. The spectra of both compounds exhibit isotropic
signals due to the presence of magnetically coupled high
spin Mn(II) cations (Fig. 8). The signals remain practically
unchanged down to 100 and 70 K for (1) and (2),
respectively; below these temperatures the signals broaden
and lose intensity. The ESR signals were fitted by a
Lorentzian curve and the ‘‘g’’ gyromagnetic factor was
calculated for both selenites. The result obtained for both
phases was 1.99(1). The thermal evolution of the intensity
of the signals shows a continuous increase with cooling the
5
energy level diagram for high spin octahedral d system.
The Dq and Racah parameters are Dq ¼ 705, B ¼ 750,
ꢁ
1
C ¼ 3325 cm
for (1) and Dq ¼ 720, B ¼ 745, C ¼
ꢁ1
3
in other octahedrically coordinated Mn
350 cm for (2). These results are similar to those found
+
2
compounds
9,10]. The B values for both selenites are approximately
[
7
2
+
ꢁ1
8% of the value for the free Mn
cation (960 cm ),

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3694
Table 4
Selected bond lengths (A) and angles (deg.) for (1) and (2)
sample up to 25 and 30 K, for compounds (1) and (2),
respectively. Finally, a dramatic decrease of the intensity
occurs (Fig. 9). These results suggest the existence of
antiferromagnetic interactions in both polymorphs. The
temperature dependence of the line width shows a weak
increase from room temperature to 100 K. Below this
temperature the line width increases vigorously down to
˚
(
1)
Bond distances (A)
Se(1)O trigonal pyramid
(2)
˚
3
i
Se(1)–O(3)
Se(1)–O(3)
Se(1)–O(1)
1.74(1)
1.74(1)
1.63(1)
Se(1)–O(4)
Se(1)–O(3)
Se(1)–O(2)
1.55(8)
1.61(5)
1.64(5)
4.2 K, probably due to a strong spin correlation [31].
Magnetic measurements for polymorphs (1) and (2) were
6
Mn(1)O octahedron
ii
ii
Mn(1)–O(3)
2.28(1)
2.28(1)
2.14(1)
2.14(1)
2.21(2)
2.13(2)
Mn(1)–O(4)
Mn(1)–O(3)
Mn(1)–O(3)
Mn(1)–O(2)
Mn(1)–O(2)
2.29(8)
2.26(5)
2.11(5)
2.49(5)
2.28(5)
2.25(7)
performed on powdered samples at 1000 G from room
temperature to 5 K, in the modes Zero Field Cooling
(ZFC) and Field Cooling (FC) modes, being similar the
results obtained for both cases (Figs. 10a and b). The molar
v
i
Mn(1)–O(3)
iv
iii
iii
Mn(1)–O(3)
iii
Mn(1)–O(3)
Mn(1)–O(2)w
Mn(1)–O(1)
Mn(1)–O(1)w
magnetic susceptibility of both phases, w , increases with
m
decreasing temperature and reaches a maximum at 14 and
17 K for (1) and (2), respectively, indicating the establish-
ment of a magnetic ordering. Below these temperatures the
magnetic susceptibility decreases continuously up to 5 K.
The form of these maxima with an appreciable width
suggests the existence of magnetic couplings of low
dimensionality. The thermal evolution of the molar
magnetic susceptibility follows the Curie–Weiss law in the
40–300 K and 45–300 K ranges, for (1) and (2), respec-
tively. The values of the Curie and Curie–Weiss constant
Bond angles (deg.)
3
Se(1)O trigonal pyramid
i
O(1)–Se(1)–O(3)
O(3)–Se(1)–O(3)
O(3)–Se(1)–O(1)
104.2(5)
95.1(7)
104.2(5)
O(4)–Se(1)–O(2)
O(4)–Se(1)–O(3)
O(3)–Se(1)–O(2)
121(3)
92(3)
99(2)
i
6
Mn(1)O octahedron
ii
O(3) –Mn(1)–O(1)
ii
O(4) –Mn(1)–O(1)
82.5(5)
91.4(5)
164.0(4)
96.0(2)
68.5(5)
82.5(5)
91.8(3)
96.0(2)
164.1(3)
91.4(5)
93.3(4)
99.0(6)
91.5(4)
93.3(4)
172.6(7)
175(2)
89(2)
83(2)
ii
O(3) –Mn(1)–O(2)
ii
O(4) –Mn(1)–O(2)
ii
O(3) –Mn(1)–O(3)
iii
iv
v
ii
iii
iii
i
O(4) –Mn(1)–O(2)
ii
O(3) –Mn(1)–O(3)
ii
O(4) –Mn(1)–O(3)
86(2)
93(2)
91(2)
ii
O(3) –Mn(1)–O(3)
ii
O(4) –Mn(1)–O(3)
v
O(3) –Mn(1)–O(1)
i
O(3) –Mn(1)–O(1)
3
are 4.50 cm K/mol and ꢁ50.0 K for (1) and (2), in good
v
O(3) –Mn(1)–O(2)
i
O(3) –Mn(1)–O(2)
98.1(2)
108(2)
154(2)
92(2)
agreement with the existence of one Mn(II) cation in the
v
O(3) –Mn(1)–O(3)
iii
i
O(3) –Mn(1)–O(2)
iii
v
O(3) –Mn(1)–O(3)
iv
i
O(3) –Mn(1)–O(3)
iii
unit formula of these compounds. The w T vs. T curve
m
3
decreases continuously from 3.80 and 3.85 cm K/mol at
iv
O(3) –Mn(1)–O(1)
iii
O(3) –Mn(1)–O(1)
iv
O(3) –Mn(1)–O(2)
iii
O(3) –Mn(1)–O(2)
3
room temperature up to 0.25 and 0.18 cm K/mol at 5 K,
89(2)
65(2)
iv
O(3) –Mn(1)–O(3)
iii
iii
O(3) –Mn(1)–O(2)
iii
for the polymorphs (1) and (2), respectively.
Taking into account the structural features of these
polymorphs which show the existence of sheets separated
iii
O(3) –Mn(1)–O(1)
iii
O(2) –Mn(1)–O(1)
87(2)
170(2)
101(2)
iii
O(3) –Mn(1)–O(2)
iii
O(2) –Mn(1)–O(2)
O(2)w–Mn(1)–O(1)
O(2)–Mn(1)–O(1)
˚
by, approximately, 3.3 A, a three-dimensional magnetic
Symmetry codes used to generate the equivalent atoms for (1) and (2).
3
model can be disregarded for the analysis of the magnetic
behavior. At first glance, from the magnetic point of view
the layers inside could be considered, simultaneously, the
existence of triangular and square planar antiferromagnetic
lattices (see Fig. 11a and b).
1
1
1
(
1) i ¼ ꢁx þ , y, z; ii ¼ x; y; z ꢁ 1; iii ¼ x þ ; ꢁy þ , z+ , iv ¼ ꢁx þ 1,
2
2
2
2
1
2
1
2
3
ꢁ
y þ , z þ ; v ¼ ꢁx þ , y, z þ 1.
2
1 1 1
2
1
2
1
1
(
2) i ¼ x, y, 1+z, ii ¼ ꢁ þ x, ꢁy, +z; iii ¼ þ x, 2 ꢁ y, 2 þ z.
2
2
Fig. 7. (a) Luminescent emission spectra for phases (1) and (2) and (b) luminescent excitation spectra for phases (1) and (2).

ARTICLE IN PRESS
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3695
Table 5
Assignment of the electronic transitions obtained from the luminescence and diffuse reflectance measurements for (1) and (2)
ꢁ1
Assignment n (cm
)
Mn(SeO
3
) ꢀ H
2
O (1)
Mn(SeO
3
) ꢀ 2H
2
O (2)
Luminescence
Diffuse reflectance
Luminescence
Diffuse reflectance
6
6
6
6
6
6
4
4
A
A
A
A
A
1g( S)- T1g( G)
18345
22475
24100
27785
30770
18350
22470
24095
27780
30770
18350
22575
24220
28565
31540
18350
22570
24215
28570
31545
6
4
4
1g( S)- T2g( G)
6
4
4
4
4
g
1g( S)- A1g( G); E ( G)
6
4
4
1g( S)- T2g( D)
6
4
4
g
1g( S)- E ( D)
Fig. 8. Thermal variation of the ESR spectra for (a) (1) and (b) (2).
Fig. 9. Thermal variation of the intensity and line width for (a) (1) and (b) (2).
2
2
2
3
Considering this situation, the experimental magnetic
data were fitted to the expressions derived by Rushbrooke
and Wood for the triangular (Eq. (1)) and square-planar
w ¼ ð35Ng b =12kTÞ½1 þ 23:33x þ 147:78x ꢁ 405:48x
m
4
5
6
ꢁ1
þ 8171:3x þ 64961:8x ꢁ 158110x ꢄ
,
ð2Þ
(
Eq. (2)) lattices [32]:
where, x ¼ jJj=kT, k is the Boltzman constant, N the
Avogadro’s number and b the Bohr magneton. The solid
lines in Fig. 10 show the best fits obtained. These results
indicate a disaccord between the experimental and theore-
tical data when the triangular lattice model is considered.
However, the fit is reasonably good when the square-planar
2
2
w ¼ ð35Ng b =12kTÞ
m
2
3
4
ꢂ½1 þ 35x þ 221:667x ꢁ 1909:83x þ 6156:92x
5
6 ꢁ1
,
þ 84395:9x ꢁ 1522000x ꢄ
ð1Þ

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3696
model is used, with the magnetic exchange parameters,
0
It is necessary to indicate that in order to obtain a good
0
J=k ¼ ꢁ0:83, ꢁ0.91 K and J =k ¼ ꢁ0:97, ꢁ1.20 K, for the
fit, a high value of the J =k parameter, on the basis of the
polymorphs (1) and (2), respectively. The ‘‘g’’ value was
1
approximation of the molecular field [33], is necessary to
consider additionally to the J-exchange parameter asso-
ciated to the Rushbrooke and Wood equation. This
parameter could explain the contribution of the triangular
lattice to the total magnetic exchange in these phases.
Consequently, these results suggest that the theoretical
proposed model must be considered with caution.
.99, obtained from the ESR measurements.
4. Concluding remarks
Two new polymorphs with formula Mn(SeO ) ꢀ H O
3
2
have been synthesized and their crystal structures resolved
from single-crystal and powder diffraction data. The
phases have been characterized by chemical analysis,
thermal and IR measurements. The diffuse reflectance
and luminescence studies indicate slightly distorted octahe-
dral geometry for the Mn(II) cations. The magnetic
analysis shows similar antiferromagnetic couplings in both
phases. The J-exchange parameters have been calculated
by fitting the experimental data with a model for a square-
planar lattice.
Acknowledgments
This work has been financially supported by the
´
‘Ministerio de Educacion y Ciencia’’ (MAT-2004-02071)
‘
and the ‘‘Universidad del Paı
´
s Vasco’’ (UPV/EHU)
9/UPV00169.310-13494/
001). The authors thank the technicians of SGIker, Dr.
(
2
9/UPV00130.310-15967/2004;
J.P. Chapman and Dr. I. Orue, financed by the ‘‘National
Program for the Promotion of Human Resources within
the National Plan of Scientific Research, Development and
Innovation’’—‘‘Ministerio de Ciencia
and ‘‘Fondo Social Europeo (FSE)’’, for the X-ray
diffraction and magnetic measurements, respectively.
y Tecnologıa’’
´
Fig. 10. Thermal variation of the w
2). The solid lines shown the best fit for the models triangular-planar (TP)
and square-planar (SP) lattices.
m m
and w T curves for (a) (1) and (b)
(
Fig. 11. Schematic representation of (a) the triangular-planar and (b) the square-planar lattices.

ARTICLE IN PRESS
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3697
˜
Dr. A. Larranaga wishes to thank the Gobierno Vasco/
Eusko Jaurlaritza for funding.
[11] W.T.A. Harrison, M.L.F. Philips, J. Stanchfield, T.M. Nenoff,
Angew. Chem., Int. Ed. 39 (2000) 3808.
[12] A. Choudhury, D. Udayakumar, C.N.R. Rao, Angew. Chem., Int.
Ed. 41 (2002) 158.
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