New rubidium zinc hydrogen phosphate, Rb2Zn2(HPO4)3: Synthesis, crystal structure, and 31P single-crystal NMR

Article abstract of DOI:10.1021/ic9906757

A new rubidium zinc hydrogen phosphate, Rb₂Zn₂(HPO₄)₃, is prepared by an unusual method utilizing long nucleation times. This material is crystallized from a gel with an initial composition of 1.0 ZnO/0.94 P₂O₅/0.96 Rb₂O/0.04 Li₂O/41 H₂O, while the phosphate concentration equals 1.6 M and pH = 3.5. The gel is placed in a sealed Pyrex flask at 52 °C, and after 4.5 months crystallization of Rb₂Zn₂(HPO₄)₃ is noticed. This new crystalline compound has a three-dimensional framework structure built from spiral chains of alternating PO₄ and ZnO₄ tetrahedra connected pairwise and assembled by other PO₄ tetrahedra, rubidium ions, and hydrogen bonds. The two rubidium ions, Rb(1) and Rb(2), have an exceptionally low number of oxygen contacts in the first coordination sphere, five and seven, respectively. Crystal data: Monoclinic, P2₁/c (no. 14), a = 12.5880(4), b = 12.7170(8), c = 7.5827(8) A, β = 96.100(1)°, Z = 4. A single-crystal ³¹P NMR investigation of Rb₂Zn₂(HPO₄)₃ was performed employing a two-axis goniometer probe and reveals the presence of three chemically and six magnetically nonequivalent phosphorus sites, in accordance with the crystal structure. ³¹P chemical shielding anisotropies and isotropic chemical shifts (-3.3(3), -2.6(3), and 2.0(3) ppm) have been determined for the three phosphorus sites.

Full text of DOI:10.1021/ic9906757

2026
Inorg. Chem. 2000, 39, 2026-2032
New Rubidium Zinc Hydrogen Phosphate, Rb2Zn2(HPO4)3: Synthesis, Crystal Structure,
31
and P Single-Crystal NMR
Torben R. Jensen,*,† Rita G. Hazell,‡ Thomas Vosegaard,§,| and Hans J. Jakobsen§
Department of Chemistry, University of Southern Denmark, Odense, DK-5230 Odense M, Denmark,
Department of Chemistry, University of Aarhus, DK-8000 Aarhus C, Denmark, and Instrument Centre
for Solid-State NMR Spectroscopy, Department of Chemistry, University of Aarhus,
DK-8000 Aarhus C, Denmark
ReceiVed June 11, 1999
A new rubidium zinc hydrogen phosphate, Rb2Zn2(HPO4)3, is prepared by an unusual method utilizing long
nucleation times. This material is crystallized from a gel with an initial composition of 1.0 ZnO/0.94 P2O5/0.96
Rb2O/0.04 Li2O/41 H2O, while the phosphate concentration equals 1.6 M and pH ) 3.5. The gel is placed in a
sealed Pyrex flask at 52 °C, and after 4.5 months crystallization of Rb2Zn2(HPO4)3 is noticed. This new crystalline
compound has a three-dimensional framework structure built from spiral chains of alternating PO4 and ZnO4
tetrahedra connected pairwise and assembled by other PO4 tetrahedra, rubidium ions, and hydrogen bonds. The
two rubidium ions, Rb(1) and Rb(2), have an exceptionally low number of oxygen contacts in the first coordination
sphere, five and seven, respectively. Crystal data: monoclinic, P21/c (no. 14), a ) 12.5880(4), b ) 12.7170(8),
c ) 7.5827(8) Å, â ) 96.100(1)°, Z ) 4. A single-crystal 31P NMR investigation of Rb2Zn2(HPO4)3 was performed
employing a two-axis goniometer probe and reveals the presence of three chemically and six magnetically
nonequivalent phosphorus sites, in accordance with the crystal structure. 31P chemical shielding anisotropies and
isotropic chemical shifts (-3.3(3), -2.6(3), and 2.0(3) ppm) have been determined for the three phosphorus
sites.
Introduction
of materials.7 It is well established that the nucleation time
increases upon lowering the temperature of the synthesis.8,9 In
some cases the nucleation time can be several months.10,11
The chemistry of synthesized zinc orthophosphates shows a
variety of compounds, and recent research has expanded the
ZnO-P2O5-H2O-amine family of synthetic materials consid-
erably. The first members were discovered in 1992 and today
ca. 20 have been reported.12 Apparently, only one templated
alkali metal zinc orthophosphate is known, LiZn(HPO4)(PO4)‚
enH2‚H2O.10 An analogue to zeolite X, reported as Na67-
((CH4)4N)12Zn8(ZnPO4)96‚192H2O, is partly templated by hy-
drated sodium ions and partly by the ammonium ions.13
A number of new members in the system, M2O-ZnO-
P2O5-H2O where M ) alkali metal, have been synthesized
recently.3,11,14-20 We present here the synthesis of a new
An extraordinarily large number of orthophosphates have been
characterized in the past 2 decades, e.g., the discovery of open
framework aluminum phosphates (1982),1 beryllium phosphates
(1989),2 and zinc phosphates (1991),3 which in many cases have
interesting framework crystal structures. Transition metal sub-
stituted aluminum phosphates with open framework structures
are of considerable interest for a vide variety of purposes, e.g.,
catalysis.4 Poor thermal stability of these materials often limits
their utilization. This has prompted investigation of fully
substituted aluminum phosphate materials, i.e., Al3+ f Zn2+
+ M+ where M ) alkali metal, and exploration of new strategies
for their synthesis in our laboratory.
Open framework materials are generally prepared under mild
conditions because they are topologically metastable.5 Synthesis
at moderate temperatures is of increasing importance, and in
some cases crystallization of new materials is facilitated by slow
nucleation in silica gel6 or utilization of successive conversion
(6) Leech, M. A.; Cowley, A. R.; Prout, K.; Chippindale, A. M. Chem.
Mater. 1998, 10, 451.
(7) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press:
London, 1982.
* To whom correspondence should be addressed. Present address:
Condensed Matter Physics and Chemistry Department, Risø National
Laboratory, Frederiksborgvej 399, DK-4000 Roskilde, Denmark. Fax: +45
4677 4790. E-mail: torben.rene.jensen@risoe.dk.
(8) Francis, R. J.; Price, S. J.; O’Brien, S.; Fogg, A. M.; O’Hare, D.;
Loiseau, T.; Fe´rey, G. Chem. Commun. 1997, 521.
(9) Christensen, A. N.; Jensen, T. R.; Norby, P.; Hanson, J. C. Chem.
Mater. 1998, 10, 1688.
(10) Jensen, T. R.; Hazell, R. G. Chem. Commun. 1999, 371.
(11) Jensen, T. R. J. Chem. Soc., Dalton Trans. 1998, 13, 2261.
(12) Harrison, W. T. A.; Martin, T. E.; Gier, T. E.; Stucky, G. D. J. Mater.
Chem. 1992, 2, 175.
(13) Harrison, W. T. A.; Gier, T. E.; Moran, K. L.; Nicol, J. M.; Eckert,
H.; Stucky, G. D. Chem. Mater. 1991, 3, 27.
(14) Harrison, W. T. A.; Gier, T. E.; Nicol, J. M.; Stucky, G. D. J. Solid
State Chem. 1995, 114, 249.
† Department of Chemistry, University of Southern Denmark.
‡ University of Aarhus.
§ Instrument Centre for Solid-State NMR Spectroscopy, University of
Aarhus.
| Present address: Centre de Recherches sur les Mate´riaux a` haute
Tempe´rature, CNRS, 45071 Orle´ans Cedex 2, France.
(1) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen,
E. M. J. Am. Chem. Soc. 1982, 104, 1146.
(2) Harvey, G.; Meier, W. M. Stud. Surf. Sci. Catal. 1989, 49A, 411.
(3) Gier, T. E.; Stucky, G. D. Nature (London) 1991, 349, 508.
(4) Chen, J.; Thomas, J. M. J. Chem. Soc., Chem. Commun. 1994, 603.
(5) Francis, R. J.; O’Hare, D. J. Chem. Soc., Dalton Trans. 1998,
3133.
(15) Jensen, T. R.; Norby, P.; Stein, P. C.; Bell, A. M. T. J. Solid State
Chem. 1995, 117, 39.
(16) Gier, T. E.; Harrison, W. T. A.; Nenoff, T. M.; Stucky, G. D. Synthesis
of Microporous Materials; Occelli, M. L., Robson, H. E., Eds.; Van
Nostrand Reinhold: New York, 1992; p 407.
10.1021/ic9906757 CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/20/2000

Rb2Zn2(HPO4)3: Synthesis, X-ray, and 31P NMR
Inorganic Chemistry, Vol. 39, No. 10, 2000 2027
Table 1. Indexed Powder Diffraction Pattern of Rb2Zn2(HPO4)3
Based on the Unit Cell Parameteres a ) 12.576(5), b ) 12.729(4),
c ) 7.592(3) Å, â ) 96.13(4)°, V ) 1208.4(8) Å3
rubidium zinc hydrogen orthophosphate, Rb2Zn2(HPO4)3, which
has an unusually long nucleation time. A detailed structural
characterization combining X-ray diffraction and NMR spec-
troscopic measurements on single-crystal samples reveals an
exceptional coordination for the rubidium atoms.
h
k
l
dcalc
dobs
I/Io
1
1
0
0
-1
2
0
1
1
2
1
1
1
1
2
1
2
1
3
3
2
1
1
4
0
0
2
1
1
0
3
4
3
5
0
0
5
5
2
3
5
0
3
0
0
1
0
1
0
1
1
1
1
1
2
1
0
0
1
2
0
0
2
1
0
2
2
1
1
1
0
3
0
1
0
2
3
2
2
0
12.5039
8.9200
6.4927
6.3644
5.9916
5.6118
5.5574
4.7255
4.6438
4.3103
3.7180
3.6185
3.5983
3.5108
3.4869
3.3698
3.2807
3.1822
3.1261
3.0886
3.0631
3.0359
3.0015
2.9592
2.8413
2.8289
2.6974
2.5458
2.5193
2.5009
2.3838
2.3578
2.1551
2.1058
2.0610
1.9893
1.8706
12.4341
8.9232
6.4984
6.3598
5.9923
5.6045
5.5469
4.7237
4.6470
4.3114
3.7095
3.6223
3.5986
3.5125
3.4876
3.3689
3.2833
3.1878
3.1290
3.0907
3.0618
3.0348
2.9995
2.9606
2.8452
2.8240
2.6979
2.5460
2.5182
2.4983
2.3857
2.3573
2.1570
2.1044
2.0594
1.9908
1.8700
2
2
2
8
2
2
7
1
3
3
4
100
12
9
3
7
11
45
14
53
4
6
3
4
5
32
9
9
4
3
10
4
3
3
3
3
6
Experimental Section
Synthesis and Initial Phase Identification. A white gel was
prepared in the following way. The powdered solids LiOH‚H2O (0.158
g), RbCl (5.260 g), and Zn(CH3COO)2‚2H2O (10.072 g) were mixed
together. H2O (26.0 g) was added, and then 50% RbOH (9.181 g)
followed by 85% H3PO4 (9.961 g) were added slowly under stirring.
The molar composition of the thick milky gel was 1.0ZnO/0.94P2O5/
0.96Rb2O/0.04Li2O/41H2O. The concentration of phosphate was ca.
1.6 M, and the pH of the gel was 3.5. This batch was divided into
three portions denoted A, B, and C.
A portion of the gel, A, was heated in a Teflon lined steel autoclave
at 120 °C for 6 days, giving a needle-shaped crystalline product of
RbZn2(HPO4)(PO4).20 The clear solution (A) in equilibrium with the
crystalline material was decanted and placed in a beaker, allowing
evaporation of the solvent at room temperature. After 12 days a
crystalline material was isolated and identified as a recently described
new material, RbZn(H2PO4)(HPO4)‚H2O.19 This procedure was repeated
by again removing the solution from the crystalline material and
allowing evaporation of the liquid, leaving crystals of RbH2PO4 and a
mixture of RbH2PO4, RbCl, and possibly Rb2ZnCl4.21,22
Another portion, B, was placed in a sealed Pyrex flask at 52 °C for
4.5 months when transparent crystals were noticed, and after 6 months
three crystals with dimensions up to 7 mm and a total mass of 0.252
g were separated from the gel. Fragments of a crystal were used for
structural investigation, combining information from single-crystal
X-ray diffraction and 31P NMR, and a new material with composition
Rb2Zn2(HPO4)3 was identified as discussed in the following. The
remaining gel B was placed at 21 °C for 14 days in a sealed flask, and
then another rubidium zinc phosphate formed, RbZn(H2PO4)(HPO4)‚
H2O.19
A portion of the gel, C, was left to stand at room temperature for 6
months in a sealed Pyrex flask in order to prevent evaporation. The
polycrystalline product (white powder) was a mixture of RbZn(H2PO4)-
(HPO4)‚H2O, RbH2PO4, and possibly an unidentified impurity.19,21
The commercial chemicals were 85% H3PO4 (Fluka, extra pure
>98%), LiOH‚H2O (Fluka, puriss p.a. >99%), RbCl (Chempur, >99%),
50% RbOH aqueous solution (Aldrich, 99%), and Zn(CH3COO)2‚2H2O
(Fluka, purum p.a. >99.0%).
1
-2
-1
2
2
0
-1
2
3
3
-2
0
4
2
3
4
2
-3
-3
1
3
0
-1
5
-1
2
4
-2
1
5
6
Table 2. Crystallographic Data for Rb2Zn2(HPO4)3
chemical formula, Rb2Zn2(HPO4)3
a ) 12.5880(4) Å
b ) 12.7170(8) Å
c ) 7.5827(8) Å
â ) 96.100(1)°
fw ) 589.63 g mol-1
space group P21/c (No. 14)
T ) -153 °C
λ ) 0.710 73 Å
F
calcd ) 3.27 g cm-3
V ) 1207.0(1) Å3
Z ) 4
µ ) 123.2 cm-1
R(Fo)a ) 0.036
Rw(Fo)b ) 0.045
Powder X-ray diffraction data for phase identification and refine-
ment of unit cell parameters were obtained using a Siemens D5000
diffractometer equipped with a primary beam germanium monochro-
mator (Cu KR1 radiation, λ ) 1.5406 Å). The data were collected
from 5° to 90° in 2θ with a step length of 0.02° and counting time of
2-15 s per step, mounting ca. 20 mg of powdered sample on a glass
plate. The program CELLKANT23 was used to refine the unit cell
parameters from the observed d spacings. The indexed powder pattern
of Rb2Zn2(HPO4)3 (37 reflections) is provided in Table 1 using the
refined unit cell dimensions of Rb2Zn2(HPO4)3: monoclinic, a )
12.576(5), b ) 12.729(4), c ) 7.592(3) Å, â ) 96.13(4)°, and V )
1208.4(8) Å3.
a R(Fo) ) ∑(||Fo| - |Fc||)/(∑|Fo|). b Rw(Fo) ) [∑(w||Fo| - |Fc||)2/
2
(∑w|Fo|)2]1/2. Weighting scheme: w ) {[σcs(F 2) + F 2]1/2 - |F| }-2
.
A thermal investigation was performed by combining information
from thermogravimetric (TG) and differential scanning calorimetric
(DSC) measurements between 20 and 700 °C using a heating and
cooling rate of 5 °C min-1. TG was performed using a SETARAM
92-12 instrument, nitrogen atmosphere, and an open Al2O3 crucible
for the sample. DSC was performed using a SETARAM 92-16.18
instrument, argon atmosphere, and open platinum crucibles for the
sample and the reference, Al2O3. The temperature and enthalpy change
calibrations of the DSC measurements were performed using the low
to high quartz transition.
Single-Crystal X-ray Diffraction. A clear, colorless crystal fragment
of Rb2Zn2(HPO4)3 (0.20 × 0.10 × 0.04 mm3) was used for the data
collection performed on a Siemens SMART diffractometer equipped
with a CCD detector and graphite-monochromatized Mo KR radiation,
λ ) 0.71073 Å, using the SMART program.24 Approximately one
hemisphere of data was collected in frames covering 0.3° in ω in three
sets at different æ angles using a detector to crystal distance of 40.0
(17) Ng, H. Y.; Harrison, W. T. A. Microporous Mesoporous Mater. 1998,
23, 197.
(18) Broach, R. W.; Bedard, R. L.; Song, S. G.; Pluth, J. J.; Bram, A.;
Riekel, C.; Weber, H.-P. Chem. Mater. 1999, 11, 2076.
(19) Harrison, W. T. A.; Bircsak, Z.; Hannooman, L. J. Solid State Chem.
1997, 134, 148.
(20) Corcoran, E. W., Jr.; Savader, J. B.; Bhalla-Chawla, M. Molecular
sieVes; Occelli, M. L., Ed.; New York, 1992; p 400.
(21) Rusakov, A. A.; Kheiker, D. M. SoV. Phys. Crystallogr. 1978, 23,
228.
(22) Secco, A. S.; Trotter, J. Acta Crystallogr., Sect. C 1983, 39, 317.
(23) Ersson, N. O. CELLKANT (program for unit cell refinement from
powder X-ray diffraction data); Chemical Institute, Uppsala Univer-
sity: Uppsala, 1981.
(24) Siemens Analytical X-Ray Instruments, Inc., Madison, WI, 1995.

2028 Inorganic Chemistry, Vol. 39, No. 10, 2000
Jensen et al.
Figure 1. Unit cell content of Rb2Zn2(HPO4)3 shown as 50% thermal ellipsoids and giving the atom-labeling scheme. The crystal structure is
shown as an ab projection with the a axis horizontal.
Table 3. Atomic Coordinates and Isotropic Thermal Parameters of
Rb2Zn2(HPO4)3
mm, giving a total of 14 836 reflections (Rint ) 0.083). Data were
corrected for Lorentz polarization effects and for absorption using an
empirical method (program SADABS)24 [transmission (max/min) 0.69/
0.30], giving 3698 unique reflections of which 2691 were observed, I
> 3σ(I). The crystallographic details are given in Table 2.
A structural model was found using direct methods and the program
SIR97 in space group P21/c.25 Programs from the KRYSTAL software
package were used for structure refinement.26 Hydrogen atoms were
placed one-third of the distance between the oxygens: O(4)-O(3),
2.610 Å; O(8)-O(11), 2.566 Å; O(12)-O(11), 2.561 Å. The H atom
positions were not refined. All atoms were refined with anisotropic
displacement parameters except H atoms, which were kept isotropic.
Scattering factors of neutral atoms were applied throughout this work.
The refinement converged at R(Fo) ) 0.036 and Rw(Fo) ) 0.045, with
176 parameters refined using full matrix least squares based on all F
and a weighting scheme of w ) {[σCS(F2) + F2]1/2 - |F|}-2. Residual
electron density was located close to the heavy atoms, Rb [∆F (max/
min), 2.0(2)/(-1.6(2)) e Å-3]. Atomic coordinates and equivalent
thermal displacement parameters are shown in Table 3. Figure 1 shows
the atom-labeling scheme and 50% thermal ellipsoids of the unit cell
contents. Figure 1 was prepared using the program OrtepIII, and the
program Atoms, version 4.0, was used for preparation of the polyhedra
drawings (Figures 2 and 3).27,28
x
y
z
Uiso
Rb1
Rb2
Zn1
Zn2
P1
O1
O2
O3
O4
P2
O5
-0.36941(4)
0.14458(3)
-0.04815(4)
-0.33763(4)
-0.14946(9)
-0.2654(2)
-0.1312(3)
-0.1132(3)
-0.0767(3)
0.17346(9)
0.2594(2)
0.1011(2)
0.1050(3)
0.2293(3)
0.43721(9)
0.5321(3)
0.3536(2)
0.3861(2)
0.4862(3)
-0.0888
0.12050(4)
0.12898(4)
0.10354(4)
0.05181(4)
0.17971(9)
0.1773(2)
0.1112(3)
0.2926(2)
0.1329(3)
0.07887(8)
0.0049(2)
0.0310(2)
0.1234(2)
0.1723(2)
0.13548(9)
0.0996(2)
0.0485(2)
0.2336(3)
0.1617(3)
0.1575
0.62488(7)
0.10389(6)
0.65436(7)
0.13292(7)
0.01186(15)
0.0516(4)
0.8556(4)
0.9831(4)
0.1744(4)
0.5788(1)
0.6552(4)
0.4264(4)
0.7165(4)
0.4878(4)
0.0982(2)
0.2257(4)
0.0619(4)
0.1655(4)
-0.0804(5)
0.2768
0.0222(3)
0.0132(2)
0.0076(2)
0.0088(3)
0.0074(5)
0.013(2)
0.016(2)
0.015(2)
0.015(2)
0.0061(5)
0.015(2)
0.012(2)
0.013(2)
0.014(2)
0.0078(5)
0.013(2)
0.013(2)
0.016(2)
0.017(2)
0.028(17)
0.066(28)
0.142(47)
O6
O7
O8
P3
O9
O10
O11
O12
H4
H8
H12
0.2807
0.1909
0.1967
0.5232
-0.1654
Additional crystallographic results are given in Supporting Informa-
tion.
0.4526
Spectroscopic Investigation. Single-crystal 31P NMR experiments
were performed at 14.1 T (242.9 MHz) on a Varian Unity-INOVA
600 spectrometer employing a new goniometer design described
elsewhere.29 This probe is particularly designed for studies of small
crystals and has only two axes of rotation (a and b). Rotation of the
(25) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115.
(26) Hazell, A. KRYSTAL, An integrated system of crystallographic
programs; Aarhus University: Denmark, 1995.
(27) Burnett, M. N.; Johnson, C. K. ORTEP-III; Report ORNL-6895; Oak
Ridge National Laboratory: Oak Ridge, TN, 1996.
(28) Dowty, E. ATOMS, version 4.0; Shape Software: Kingsport, TN, 1997.
(29) Vosegaard, T.; Hald, E.; Daugaard, P.; Jakobsen, H. J. ReV. Sci.
Instrum. 1999, 70, 1771

Rb2Zn2(HPO4)3: Synthesis, X-ray, and 31P NMR
Inorganic Chemistry, Vol. 39, No. 10, 2000 2029
Figure 2. Crystal structure of Rb2Zn2(HPO4)3 shown as an ab
projection. Tetrahedra of PO4 are dark, and ZnO4 are light. Hydrogen
atoms are shown as small-size black circles, Rb atoms as larger
circles.
crystal is fully automated and computer controlled, which yields a high
degree of precision for the angle setting ((0.3°).30 For each axis of
rotation 25 spectra were recorded following an increment of 15° for
the rotation angle. Each spectrum is the sum of 16 transients measured
with a spectral width of 100 kHz, single-pulse excitation (τp ) 3.5
µs), and a repetition delay of 128 s. The crystal of Rb2Zn2(HPO4)3
used for the single-crystal NMR investigation was approximately 1 ×
1 × 2 mm3. The analysis of the single-crystal NMR spectra was
performed by the software package ASICS.30 The CSA parameters are
defined by δσ ) δzz - δiso and ησ ) (δxx - δyy)/δσ, where the principal
Figure 3. Crystal structure of Rb2Zn2(HPO4)3, viewed along the (110)
direction showing ladders of alternating four- and eight-rings running
along the c direction. Hydrogen atoms are shown as small-size dark
circles and Rb as larger circles. PO4 is represented as dark tetrahedra
and ZnO4 as light tetrahedra.
415 °C, with a significant mass loss of 3.2% in the range 280-
305 °C. This observation is in accordance with loss of water
(1.5H2O) from Rb2Zn2(HPO4)3, with a calculated mass loss of
4.58%. Differential scanning calorimetry of Rb2Zn2(HPO4)3
revealed three thermal events on the heating curve and one on
the cooling curve. An endothermic event in the temperature
range 298-329 °C (peak value at 320 °C) with an enthalpy
change of ∆H ) 82 ( 22 kJ/mol is equivalent to the mass loss
detected by TG. The obtained material is metastable and gives
an exothermic DSC effect in the range 452-473 °C (peak value
at 463 °C), ∆H ) -5.6 ( 1.5 kJ/mol. An endothermic signal
at 647-668 °C (peak value at 658 °C), ∆H ) 4.5 ( 1.2 kJ/
mol, corresponds to a reversible change in the sample be-
cause an exothermic event was visible on the cooling curve at
612-575 °C (peak value at 603 °C), ∆H ) -5.2 ( 1.4 kJ/
mol. The thermal decomposition product has not yet been
identified but might be a mixture containing RbZnPO4 and other
phases.
elements are defined in the order |δzz - δiso| > |δxx - δiso| > |δyy
-
δiso|. All principal elements and isotropic chemical shifts are in ppm
relative to 85% H3PO4.
Results and Discussion
Synthesis. The new material Rb2Zn2(HPO4)3 is prepared
using moderate temperature (52 °C) solvothermal synthesis, and
crystallization is noticed after ca. 4.5 months.31 Long nucleation
times have also been found in the initial preparation of
â-LiZnPO4‚H2O and LiZn(HPO4)(PO4)‚enH2‚H2O, ca. 3 months
at 21 °C.10,11 The nucleation time of the â-phase is found to
depend on the composition of the reaction mixture,11 which
suggests that a faster synthetic route might be developed for
the preparation of Rb2Zn2(HPO4)3. The synthesis presented here
reveals the diversity of solvothermal synthesis and phosphate
chemistry; one gel gives three different rubidium zinc ortho-
phosphates and at least three other phases.32
Description of the Crystal Structure. The crystal structure
of Rb2Zn2(HPO4)3 is illustrated in Figure 2 as an ab projection
and can be seen as a framework built from three crystallo-
graphically distinct tetrahedra of PO4 all having one longer P-O
bond with hydrogen on O(4), O(8), and O(12). The framework
is interrupted by the hydroxyl groups O(4), O(8), and O(12)
Thermal Investigation. Thermogravimetric measurements
showed a total mass loss of 4.4% in the temperature range 255-
(30) Vosegaard, T.; Hald, E.; Langer, V.; Skov, H. J.; Daugaard, P.; Bildsøe,
H.; Jakobsen, H. J. J. Magn. Reson. 1998, 135, 126.
(31) The term hydrothermal synthesis is restricted to temperatures higher
than 100 °C and water used as solvent (West, A. R. Solid State
Chemistry and its Applications; John Wiley and Sons Ltd.: Chichester,
1984; p 41). SolVothermal synthesis is a more general term used here
to indicate that the nucleation and crystal growth is solution-mediated
by dissolving the gel.
(32) Preparation of crystalline materials by evaporation of a solvent is a
method that has been known for a very long time. The residue from
a solvothermal synthesis in equilibrium with solid material has a unique
composition. Such solutions can in some cases be useful for preparation
of other materials.

2030 Inorganic Chemistry, Vol. 39, No. 10, 2000
Table 4. Selected Bond Lengths and Angles for the Crystal
Jensen et al.
a
Structure of Rb2Zn2(HPO4)3
P1-O2
P1-O1
P1-O3
P1-O4
1.508(3)
1.521(3)
1.529(3)
1.572(3)
Zn1-O6iii
Zn1-O2
Zn1-O7
Zn1-O3i
Zn2-O5iii
Zn2-O10Viii
Zn2-O9iV
Zn2-O1ix
1.913(3)
1.941(3)
1.952(3)
1.969(3)
P2-O5
P2-O6
P2-O7
P2-O8
1.503(3)
1.521(3)
1.531(3)
1.576(3)
1.932(3)
1.946(3)
1.949(3)
1.967(3)
Figure 4. Selected single-crystal 31P NMR spectrum of Rb2Zn2(HPO4)3
P3-O11
P3-O9
1.517(3)
1.525(3)
P3-O10
P3-O12
1.531(3)
1.582(3)
revealing six 31P resonances.
O4-H4
0.865
0.714
0.857
H4-O3
1.745
1.883
1.703
Table 5. Calculated Oxidation States, Vi, for the Structure of
Rb2Zn2(HPO4)3, Using Ref 33 and the Bond Lengths Given in Table
4 Excluding the H Atoms
O8-H8
H8-O11
H12-O11
O12-H12
Rb1-O1i
Rb1-O12ii
Rb1-O5iii
Rb1-O9iV
Rb1-O10
Rb2-O6V
2.965(3)
3.071(3)
3.098(3)
3.158(3)
3.194(3)
2.849(3)
Rb2-O10V
2.872(3)
2.892(3)
2.915(3)
2.928(3)
3.042(3)
3.076(3)
no.
Vi
CN
Rb2-O4
Rb2-O8Vi
Rb2-O7
cation
phosphorus
1
2
3
1
2
1
2
5.04
5.04
4.95
2.10
2.07
0.54
1.15
4
4
4
4
4
5
7
Rb2-O8V
Rb2-O2Vii
zinc
O1-P1-O2
O2-P1-O3
O2-P1-O4
O1-P1-O3
O1-P1-O4
O3-P1-O4
111.9(2)
111.0(2)
105.8(2)
110.8(2)
109.1(2)
108.0(2)
O2-Zn1-O6iii
O6iii-Zn1-O7
O3i-Zn1-O6iii
O2-Zn1-O7
O2-Zn1-O3i
O3i-Zn1-O7
O5iii-Zn2-O10Viii
O5iii-Zn2-O9iV
O1ix-Zn2-O5iii
O9iV-Zn2-O10Viii
O1ix-Zn2-O10Viii
O1ix-Zn2-O9iV
95.6(1)
119.6(1)
106.4(1)
113.8(1)
105.3(1)
113.8(1)
rubidium
oxygen
framework
1
2
5
6
7
9
10
4
1.94
1.98
2.01
2.07
1.94
1.89
2.05
1.31
1.41
1.21
1.31
1.76
3
3
3
3
3
3
4
2
2
2
1
2
O5-P2-O6
O5-P2-O7
O5-P2-O8
O6-P2-O7
O6-P2-O8
O7-P2-O8
113.0(2)
113.8(2)
107.7(2)
109.3(2)
103.2(2)
109.2(2)
112.9(1)
101.0(1)
110.6(1)
117.1(1)
107.9(1)
107.2(1)
H-bond donor
8
O9-P3-O11
O10-P3-O11
O11-P3-O12
111.3(2)
110.3(2)
109.4(2)
O9-P3-O10
O9-P3-O12
O10-P3-O12
112.0(2)
104.8(2)
108.8(2)
12
11
3
H-bond acceptor
P1-O4-H4
P2-O8-H8
P3-O12-H12
O1i-Rb1-O12ii
O1i-Rb1-O5iii
O1i-Rb1-O9iV
O1i-Rb1-O10iii
O5iii-Rb1-O12ii
O6V-Rb2-O10V
O4-Rb2-O6V
115.0(3)
120.9(3)
123.1(3)
P1-O3-H4i
123.6(2)
121.6(2)
119.3(2)
P3-O11-H8i
P3-O11-H12Vi
ladders, built from two spiral chains of alternating P(1,2)O4 and
ZnO4 tetrahedra, are assembled by P(3)O4, rubidium ions, and
hydrogen bonds to form a three-dimensional framework struc-
ture. Figure 3 shows that Rb(2) is placed in the openings of the
eight-rings.
107.20(9)
94.56(8)
91.69(9)
136.82(9)
158.25(9)
101.61(9)
64.66(9)
O9iV-Rb1-O12ii
O10iii-Rb1-O12iii
O5iii-Rb1-O9iV
O5iii-Rb1-O10iii
O9iV-Rb1-O10iii
O4-Rb2-O8Vi
120.63(9)
64.44(8)
57.17(8)
99.58(8)
130.04(8)
115.55(9)
96.99(9)
Oxidation states, Vi, were calculated as a sum of bond
valencies using the bond lengths shown in Table 4 not including
the H atoms, and the results are given in Table 5.33 Framework
oxygen atoms (nos. 1, 2, 5, 6, 7, and 9) coordinated to one Rb,
Zn, and P each have Vi in the range 1.89-2.07 and O(10)
coordinated to two Rb, one Zn, and P have Vi of 2.05, in
agreement with the expected (numerical) value of 2.0. The
hydroxyl oxygens O(4), O(8), and O(12) have Vi of 1.31, 1.41,
and 1.20, and O(3) and O(11) have calculated oxidation states
of 1.76 and 1.30, since they are all participating in hydrogen
bonding (the sum of Vi for these five oxygens is 7, in accordance
with the missing bond valence from three H atoms). The cations
P, Zn, and Rb(2) have calculated oxidation states in agreement
with the expected values. The low value of 0.54 for Rb(1) is
caused by an unusually low number of oxygen contacts, five,
in the first coordination sphere, but Rb(1) has some longer
oxygen contacts. The calculated oxidation state for Rb(1) is Vi
) 0.75, including oxygen contacts in the range 3.31-3.87 Å
to O(2), O(1)*, O(12)*, O(11), O(3), O(9)*, and O(9)**. A
similar calculation for Rb(2) gives Vi ) 1.30, including oxygen
contacts up to 3.88 Å to O(11), O(7)*, O(2)*, and O(3) (the
atoms marked with an asterisk are symmetry-related to atoms
shown in Table 4).
O4-Rb2-O7
O6V-Rb2-O8Vi
O6V-Rb2-O7
O6V-Rb2-O8V
O2Vii-Rb2-O6V
O4-Rb2-O10V
O8Vi-Rb2-O10V
O7-Rb2-O10V
O8V-Rb2-O10V
O2Vii-Rb2-O10V
a Symmetry codes: (i) x, 0.5 - y, z - 0.5; (ii) x - 1, y, 1 + z;
(iii) -x, -y, 1 - z; (iV) x - 1, y, z; (V) x, y, 1 + z; (Vi) x, 0.5 - y,
0.5 + z; (Vii) -x, -y, 2 - z; (Viii) -x, -y, -z; (ix) x, y, z - 1; (x) 1
+ x, y, z.
138.42(9)
145.40(9)
48.51(8)
57.50(9)
159.77(9)
84.64(9)
86.58(9)
86.47(9)
73.62(9)
O4-Rb2-O8V
O2Vii-Rb2-O4
O7-Rb2-O8Vi
O8Vi-Rb2-O8V
O2Vii-Rb2-O8Vi
O7-Rb2-O8V
O2Vii-Rb2-O7
O2Vii-Rb2-O8V
O2Vii-Rb2-O8V
93.81(9)
86.25(9)
75.31(9)
91.71(8)
156.44(9)
165.79(8)
94.07(9)
95.83(9)
95.83(9)
and by O(11), which is pendant and acceptor for two hydrogen
bonds [from H(8) and H(12)]; O(3) is acceptor for H(4). Selected
bond lengths and angles are shown in Table 4.
The apparent “triple four-ring units” visible in Figure 2
formed by the Zn, P(1), and P(2) tetrahedra are interconnected
by P(3) to form chains along the a axis. These parallel chains
are related by the c glide planes and the screw axes. Figure 3
shows apparent “triple four-ring units” viewed along the (110)
direction and reveals that they are in fact ladders of alternating
four- and eight-rings running along the c direction. These
(33) Brown, I. D.; Altermatt, D. Acta Crystallogr., Sect. B 1985, 41, 244.

Rb2Zn2(HPO4)3: Synthesis, X-ray, and 31P NMR
Inorganic Chemistry, Vol. 39, No. 10, 2000 2031
Figure 5. Rotation plots for the three crystallographically distinct 31P sites. The NMR probe rotation axes are not parallel to the crystallographic
axes giving rotation plots for each site, which are not exact mirrors of each other.
Table 6. 31P Chemical Shielding Anisotropy Parameters (δσ, ησ,
mirrors of each other because the NMR probe rotation axes are
not parallel to the crystallographic axes.
δiso) for Rb2Zn2(HPO4)3
site
δσ (ppm)a
ησ
δ
iso (ppm)a
Comparison with Related Materials. The crystal structures
of rubidium zinc (hydrogen) orthophosphates are usually built
from alternating phosphorus (the units H2PO4/HPO4/PO4) and
zinc tetrahedra connected by rubidium ions and hydrogen bonds.
Rubidium shows large coordination flexibility with coordination
numbers (CN) in the range 8-11 in these materials.19,20 An
exceptionally low CN of 5 and calculated oxidation state of
0.54 are found for one Rb ion in Rb2Zn2(HPO4)3 presented here.
The crystal structures of RbZn(H2PO4)(HPO4)‚H2O and RbZn2-
(HPO4)(PO4) can be described as one- and two-dimensional
chains and layers of alternating ZnO4 and PO4 tetrahedra con-
nected by rubidium ions and hydrogen bonds.19,20 The compound
RbZnPO4 has a three-dimensional framework structure built
from ZnO4 and PO4 tetrahedra with rubidium placed in the eight
ring channels of the zeolite ABW type structure.35
1
2
3
-47.4(7)
-48.9(6)
-48.9(7)
0.28(2)
0.09(3)
0.25(2)
-3.3(3)
2.0(3)
-2.6(3)
a All principal elements and isotropic chemical shifts are in ppm
relative to 85% H3PO4.
Single-Crystal 31P NMR. A typical single-crystal 31P NMR
spectrum of Rb2Zn2(HPO4)3, which displays six resonances of
equal intensity, is shown in Figure 4. Preliminary analyses of
the rotation plots shown in Figure 5 indicate the presence of
only three crystallographically distinct P sites because the
magnitudes of the six shielding tensors are pairwise equal. The
three phosphorus positions split up in 2 × 3 magnetically
inequivalent positions because of the mirror symmetry (the
NMR tensors are insensitive to translation and inversion). This
observation is in agreement with the space group P21/c and
three phosphorus atoms in the asymmetric unit. Magnetically
inequivalent but crystallographically equivalent nuclei have the
same magnitude but different orientations of their NMR tensors.
The rotation plots for the three phosphorus sites are shown
in Figure 5 where circles and squares indicate the two magneti-
cally nonequivalent P atoms for each site and the solid lines
correspond to the optimized parameters listed in Table 6. The
observed isotropic chemical shift values are in the range of
values found for other hydrogen phosphates.34 Site 2 has almost
axial symmetry (ησ ) 0.09(3)) and is therefore tentatively
assigned to P(2) because this phosphorus atom is coordinated
to three framework oxygens and one oxygen acting as a
hydrogen bond donor. The two other P sites, 1 and 3, have
similar NMR parameters corresponding to an asymmetric
chemical shift tensor, which possibly is P(1) and P(3) each
having one oxygen acting as hydrogen bond donor and another
as acceptor. The two rotation plots for each site are not exact
Apparently, only three alkali metal zinc monohydrogen phos-
phates are known: Na2Zn(HPO4)2‚4H2O, Rb2Zn2(HPO4)3, and
CsZn2.5(HPO4)3‚2H2O.10,36 The sodium- and cesium-containing
phases are built from ZnO4 and HPO4 layers intersected by open
12- and 8-rings where the alkali ions and water molecules are
placed. Hydrogen phosphates often form complex crystal
structures because of hydrogen bonds, e.g., the trigonal struc-
ture of RbFe(HPO4)2, which has a very long c axis of 52.75
Å.37 The structure of Rb2Zn2(HPO4)3 is no exception because
this new material has a complex three-dimensional crystal
structure built from spiral chains of phosphorus and zinc
tetrahedra.
(34) Turner, G. L.; Smith, K. A.; Kirkpatrick, R. J.; Oldfield, E. J. Magn.
Reson. 1986, 70, 408.
(35) Elammari, L.; Elouadi, B. J. Chim. Phys. Phys.-Chim. Biol. 1991, 88,
1969.
(36) Harrison, W. T. A.; Nenoff, T. M.; Gier, T. E.; Stucky, G. D. J. Solid
State Chem. 1994, 113, 168.
(37) Lii, K.-H.; Wu, L.-S. J. Chem. Soc., Dalton Trans. 1994, 1577.

2032 Inorganic Chemistry, Vol. 39, No. 10, 2000
Jensen et al.
Conclusion
Acknowledgment. A Danish Technical Research Council
Ph.D. grant is gratefully acknowledged. T.V. acknowledges
financial support from Carlsbergfondet. I. G. Krogh Andersen,
University of Southern Denmark, Odense, is thanked for DSC
measurements. The Siemens SMART diffractometer at Depart-
ment of Chemistry, Aarhus University was partly financed by
Carlsbergfondet. We gratefully acknowledge the use of the
facilities at the Instrument Centre for Solid-State NMR Spec-
troscopy, University of Aarhus, sponsored by the Danish
Research Councils (SNF and STVF), Teknologistyrelsen, Carls-
bergfondet, and Direktør Ib Henriksens Fond. We thank Aarhus
University Research Foundation for equipment grants.
A new rubidium zinc orthophosphate, Rb2Zn2(HPO4)3, has
been crystallized after a nucleation period of 4.5 months,
implying an unusual method of synthesis. This technique may
provide other new materials with a range of different composi-
tions. Rb2Zn2(HPO4)3 has a complex crystal structure built from
spiral chains of alternating ZnO4 and PO4 tetrahedra with
connecting rubidium ions. X-ray diffraction and 31P NMR on
single-crystal samples have proven to be exceptionally useful
in providing complementary structural information about long-
range crystalline order and the magnitude and orientation of
the 31P NMR tensors, i.e., the first and second coordination
sphere of the nuclei. It has been observed that phosphorus and
zinc coordinated to oxygen form an almost ideal net work, which
is in contrast to the coordination of the rubidium ions. This study
reveals the extraordinarily large coordination flexibility of
rubidium, which shows exceptionally low coordination numbers
and calculated oxidation states.
Supporting Information Available: One X-ray crystallographic
data file in CIF format is available. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC9906757