Mercury(II) tungstate powder, HgWO4, was prepared by boiling a mixture of HgO and H2WO4 in water. Rietveld refinements on neutron powder data showed that the monoclinic structure (C2/c) consists of zigzag chains of edge-sharing HgO6 and WO6 octahedra. The Hg atom lies on an inversion centre and the W atom lies on a twofold axis. The Hg atom forms two characteristic short collinear Hg-O bonds.
Supporting information
The title compound was prepared by mixing equal amounts of HgO and H2WO4 in
water. The mixture was boiled for a few minutes until the orange colour of HgO
disappeared. The product was filtered and dried at room temperature. The
resulting powder was pale yellow.
As HgWO4 is isostructural with HgMoO4 and the ionic radius of W is very
close to that of Mo, the structural parameters of HgMoO4 were taken as
starting parameters for the structural refinement of HgWO4. The program
FULLPROF (Rodriguez-Carvajal, 1997) was used for refining the cell and
structure. The profile shape was represented by a pseudo-Voigt function.
Profile, lattice, structure parameters, zero-point shift, six background
parameters and the scale factor were refined without correction for preferred
orientation. Atomic displacements were assumed to be isotropic. WINPLOTR
(Roisnel & Rodriguez-Carvajal, 1999) was used for plotting the powder
diffractogram and ATOMS (Dowty, 1998) was used for the polyhedral
representation. The weight function used in the refinements was 1/u2 where u
is the s.u. for the observed intensities of each data point.
Data collection: Fullprof (Rodriguez-Carvajal, 1997); cell refinement: Fullprof; data reduction: Fullprof; program(s) used to solve structure: Fullprof; program(s) used to refine structure: Fullprof; molecular graphics: ATOMS (Dowty, 1998); software used to prepare material for publication: WinPLOTR (Roisnel and Rodriguez-Carvajal, 1999).
mercury(II) tungstate
top
Crystal data top
HgWO4 | V = 323.32 (4) Å3 |
Mr = 448.44 | Z = 4 |
Monoclinic, C2/c | Dx = 9.212 Mg m−3 |
Hall symbol: -c_2yc | Neutron radiation, λ = 1.470 Å |
a = 11.3606 (8) Å | T = 295 K |
b = 6.0125 (4) Å | pale yellow |
c = 5.1482 (4) Å | cylinder, 10 × 10 mm |
β = 113.159 (4)° | Specimen preparation: Prepared at 373 K |
Data collection top
Neutron powder diffractometer | Data collection mode: transmission |
Radiation source: neutron reactor | Scan method: step |
Cu(220) monochromator | 2θmin = 12.0°, 2θmax = 139.92°, 2θstep = 0.08° |
Specimen mounting: vanadium can | |
Refinement top
Refinement on Inet | Excluded region(s): 4 to 12 degrees, no Bragg peaks |
Least-squares matrix: full with fixed elements per cycle | Profile function: Pseudo-Voigt |
Rp = 0.028 | 27 parameters |
Rwp = 0.035 | Weighting scheme based on measured s.u.'s |
Rexp = 0.029 | (Δ/σ)max < 0.01 |
RBragg = 0.047 | Background function: polynomial function adjusted iteratively in each cycle |
χ2 = 1.513 | Preferred orientation correction: none |
1700 data points | |
Crystal data top
HgWO4 | β = 113.159 (4)° |
Mr = 448.44 | V = 323.32 (4) Å3 |
Monoclinic, C2/c | Z = 4 |
a = 11.3606 (8) Å | Neutron radiation, λ = 1.470 Å |
b = 6.0125 (4) Å | T = 295 K |
c = 5.1482 (4) Å | cylinder, 10 × 10 mm |
Data collection top
Neutron powder diffractometer | Scan method: step |
Specimen mounting: vanadium can | 2θmin = 12.0°, 2θmax = 139.92°, 2θstep = 0.08° |
Data collection mode: transmission | |
Refinement top
Rp = 0.028 | χ2 = 1.513 |
Rwp = 0.035 | 1700 data points |
Rexp = 0.029 | 27 parameters |
RBragg = 0.047 | |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
Hg | 0.25 | 0.25 | 0 | 0.0096 (5)* | |
W | 0 | 0.1879 (8) | 0.25 | 0.0082 (14)* | |
O1 | 0.0966 (3) | 0.0918 (5) | 0.0279 (6) | 0.0065 (8)* | |
O2 | 0.1160 (3) | 0.3678 (5) | 0.4745 (7) | 0.0134 (9)* | |
Geometric parameters (Å, º) top
Hg—O1 | 2.039 (4) | Hg—Hg | 3.9577 (2) |
Hg—O2 | 2.627 (3) | Hg—Hg | 3.9577 (2) |
Hg—O2 | 2.731 (3) | Hg—Wiii | 3.5690 (5) |
W—O2 | 1.745 (4) | Hg—W | 3.8064 (5) |
W—O1 | 1.958 (4) | Hg—O2 | 2.627 (3) |
W—O1i | 2.201 (5) | Hg—W | 3.712 (3) |
Hg—W | 3.5690 (5) | Hg—O1 | 2.039 (4) |
Hg—Wi | 3.712 (3) | Hg—O2 | 2.731 (3) |
Hg—W | 3.8064 (5) | W—O2 | 1.745 (4) |
Hg—Hgii | 3.9577 (2) | W—W | 3.425 (4) |
W—Wi | 3.425 (4) | W—O1 | 1.958 (4) |
Hg—Hg | 3.9577 (2) | W—O1 | 2.201 (5) |
| | | |
O1—Hg—O1iv | 180 | W—Hg—Hg | 68.99 (7) |
O2—Hg—O2 | 180 | W—Hg—Hg | 111.01 (7) |
O2—Hg—O2 | 180 | W—Hg—Hg | 58.83 (6) |
W—Hg—W | 54.18 (7) | Hg—Hg—Hg | 81.144 (6) |
W—Hg—W | 56.08 (7) | Hg—Hg—Hg | 98.856 (6) |
W—Hg—Hg | 120.59 (3) | Hg—Hg—Hg | 180 |
W—Hg—Hg | 124.65 (3) | Hg—Hg—Hg | 81.144 (6) |
W—Hg—Hg | 55.35 (3) | Hg—W—Hg | 89.65 (10) |
W—Hg—Hg | 59.41 (4) | Hg—W—W | 64.32 (8) |
W—Hg—Wiii | 123.92 (7) | Hg—W—W | 59.85 (7) |
W—Hg—W | 125.82 (7) | Hg—W—Hg | 65.82 (3) |
W—Hg—W | 180 | Hg—W—Hg | 63.51 (3) |
O1v—Hg—O2 | 94.57 (12) | O2—W—O2 | 103.4 (3) |
O1v—Hg—O2 | 88.10 (11) | O2—W—O1 | 104.30 (17) |
O1v—Hg—O2 | 91.90 (11) | O2—W—O1 | 96.83 (17) |
O1v—Hg—O2 | 85.43 (12) | O2—W—O1 | 89.39 (13) |
O2—Hg—O2 | 68.73 (11) | O2—W—O1 | 163.3 (2) |
O2—Hg—O2 | 111.27 (11) | Hg—W—W | 61.50 (4) |
O2—Hg—O1 | 85.43 (12) | Hg—W—W | 127.34 (9) |
W—Hg—W | 88.476 (11) | Hg—W—Hg | 90.346 (16) |
W—Hg—Hg | 66.77 (6) | Hg—W—Hg | 168.74 (14) |
W—Hg—Hg | 122.92 (6) | W—W—Hg | 125.12 (9) |
W—Hg—Hg | 57.08 (6) | W—W—W | 97.46 (15) |
W—Hg—Hg | 113.23 (6) | W—W—Hg | 64.07 (4) |
W—Hg—Wiii | 91.524 (12) | Hg—W—Hg | 167.98 (15) |
W—Hg—W | 180 | O1—W—O1 | 145.7 (3) |
W—Hg—O2 | 156.54 (10) | O1—W—O1 | 69.25 (17) |
W—Hg—W | 125.82 (6) | O1—W—O1 | 84.45 (18) |
W—Hg—Hg | 121.17 (6) | O1—W—O1 | 80.37 (19) |
Symmetry codes: (i) −x, −y, −z; (ii) x, −y, z+1/2; (iii) x+1/2, y+1/2, z; (iv) −x+1/2, −y+1/2, −z; (v) −x+1/2, y+1/2, −z+1/2. |
Experimental details
Crystal data |
Chemical formula | HgWO4 |
Mr | 448.44 |
Crystal system, space group | Monoclinic, C2/c |
Temperature (K) | 295 |
a, b, c (Å) | 11.3606 (8), 6.0125 (4), 5.1482 (4) |
β (°) | 113.159 (4) |
V (Å3) | 323.32 (4) |
Z | 4 |
Radiation type | Neutron, λ = 1.470 Å |
µ (mm−1) | ? |
Specimen shape, size (mm) | Cylinder, 10 × 10 |
|
Data collection |
Diffractometer | Neutron powder diffractometer |
Specimen mounting | Vanadium can |
Data collection mode | Transmission |
Scan method | Step |
2θ values (°) | 2θmin = 12.0 2θmax = 139.92 2θstep = 0.08 |
|
Refinement |
R factors and goodness of fit | Rp = 0.028, Rwp = 0.035, Rexp = 0.029, RBragg = 0.047, χ2 = 1.513 |
No. of data points | 1700 |
No. of parameters | 27 |
No. of restraints | ? |
Selected geometric parameters (Å, º) topHg—O1 | 2.039 (4) | Hg—W | 3.5690 (5) |
Hg—O2 | 2.627 (3) | Hg—Wi | 3.712 (3) |
Hg—O2 | 2.731 (3) | Hg—W | 3.8064 (5) |
W—O2 | 1.745 (4) | Hg—Hgii | 3.9577 (2) |
W—O1 | 1.958 (4) | W—Wi | 3.425 (4) |
W—O1i | 2.201 (5) | | |
| | | |
O1—Hg—O1iii | 180 | | |
Symmetry codes: (i) −x, −y, −z; (ii) x, −y, z+1/2; (iii) −x+1/2, −y+1/2, −z. |
The structure determination of the title compound is part of a study of divalent metal ion tungstates (MWO4) (Åsberg Dahlborg & Svensson, 2000). These materials are of interest for their luminescent properties and find their applications as detector materials for high-energy radiation and particles (Blasse & Grabmaier, 1994). The high density (9.2 g cm-3) and strong absorption of high-energy radiation makes HgWO4 interesting for electromagnetic calorimetry applications.
HgWO4 has not been as thoroughly examined as the other tungstates but Swindells (1951) has previously reported the synthesis and emission spectra for HgWO4. Later Blasse & Heuvel (1974) investigated the luminescence properties further and compared them with other tungstates but no structure refinements have been carried out on HgWO4.
Most divalent metal ion tungstates (AWO4) belong to either the scheelite structure (Sillén & Nylander, 1943), if the radius of A is greater then 1 Å, or the wolframite structure (Keeling, 1957), if the radius of A is smaller then 1 Å. The radius of the Hg2+ ion is close to 1 Å and HgWO4 does not belong to either of the wolframite or the scheelite structure. The structure of HgMoO4 was published in 1973 (Jeitschko & Sleight, 1973) and is closely related to the wolframite structure. They also showed that HgWO4 belong to the same type structure but no structural data were published. Difficulties of growing single crystals and the combination of light and heavy atoms together with strong X-ray absorption promted us to use neutron powder diffraction and Rietveld refinements to get accurate structural parameters.
The structure of HgWO4 consists of zigzag chains of edge-sharing WO6 octahedra extending parallel to the c axis (Fig. 1). The O atoms form close-packed layers parallel to the yz plane. The stacking is close to cubic close-packing but adjacent ABC layers are slightly displaced relative to each other so that the fifth layer, B', corresponds to the A layer. Thus, the octahedral voids accommodating the Hg atoms are very distorted. As expected, mercury forms two short collinear Hg—O bonds with an Hg—O distance of 2.039 (4) Å. The other two pairs of Hg—O bonds are 2.627 (3) and 2.731 (3) Å, forming a very distorted octahedron. By edge-sharing, the HgO6 octahedra also form zigzag chains running along the c axis.
The structure of HgWO4 is closely related to the wolframite structure of the other d10 elements, ZnWO4 and CdWO4 (Åsberg Dahlborg & Svensson, 2000), since the polyhedra are interconnected in the same way. However, the coordination polyhedron around the Hg atom makes the HgWO4 structure different from Zn- and CdWO4. The O—Hg—O angles are all 180°, whereas the O—Zn—O and O—Cd—O angles are about 160° in ZnWO4 and CdWO4. The WO6 octahedra in the three structures are very similar. The WO6 octahedron in HgWO4, however, is more tetrahedral than in ZnWO4 and Cd.