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The structure of dilutetium silicon pentaoxide, Lu2SiO5, has isolated ionic SiO4 tetrahedral units and non-Si-bonded O atoms in distorted OLu4 tetrahedra. The OLu4 tetrahedra form edge-sharing infinite chains and double O2Lu6 tetrahedra along the c axis. The edge-sharing chains are connected to the O2Lu6 double tetrahedra by isolated SiO4 units. The structure has been determined by neutron diffraction.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270101005352/br1287sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270101005352/br1287Isup2.hkl
Contains datablock I

Comment top

Of all known scintillators, Lu2SiO5, LSO, has the best combination of density, atomic number, light output and speed for the detection of 511 keV annihilation photons in positron emission tomography (PET). PET is a medical imaging technique capable of measuring the concentration of labelled compounds in the human body as a function of time; it is an efficient method for measuring regional biochemical and physiological functions. A large number of these dynamic tracer studies have been conducted throughout the world, resulting in important discoveries in heart disease, brain disorders and cancer.

Since 1980, the scintillation crystal Bi4Ge3O12 (BGO) has been used for the detection of the 511 keV annihilation photons in PET, primarily because its density and atomic number give it a higher photoelectric efficiency than more conventional scintillators, such as NaI:Tl. More recently, the scintillation crystal Lu2SiO5:Ce (LSO) (Melcher & Schweitzer, 1992) was discovered to have a similar detection efficiency to BGO whilst being seven times faster and having a four times higher light output. This crystal is now being grown in large quantities by CTI Inc., Knoxville, Tennessee, USA, one of the largest producers of positron tomographs worldwide. This corporation has now constructed the first LSO positron tomograph, which contains over 120,000 2.1 × 2.1 × 7.5 mm LSO crystals. It is anticipated that the use of LSO in medical imaging will increase steadily in the foreseeable future.

It is necessary to understand the underlying scintillation mechanisms in order to develop even better scintillator materials for use in medical imaging, nuclear physics, high-energy physics and astrophysics. This is being done mainly through electronic structure calculatins, i.e. embedded cluster calculations in a Hartree-Fock approach or periodic band structure calculations using a full-potential linear muffin tin orbital approach. Structural information on the title material is therefore essential.

A single-crystal X-ray data set was collected on a Siemens CCD diffractometer and the structure was solved using direct methods. Because of the poor accuracy achieved for the positional parameters of the O atoms, it was also decided to collect a single-crystal neutron diffraction data set. This was done at the medium-flux R2 steady-state reactor at Studsvik, Sweden.

The structure of Lu2SiO5 is shown in Fig. 1. Infinite chains of edge-sharing OLu4 tetrahedra run along the c axis. These are connected by SiO4 tetrahedra. The structure can thus be described as SiO4 tetrahedra and non-Si-bonded O atoms surrounded by four Lu atoms in a distorted tetrahedron. This is the well known structure for the smaller rare-earth (Dy—Lu) orthosilicates (Felsche, 1971, 1973; Anan'eva, et al., 1981).

Related literature top

For related literature, see: Anan'eva, Korovkin, Merkulyaeva, Morozova, Petrov, Savinova, Startsev & Feofilov (1981); Felsche (1971, 1973); Lehman & Larsen (1974); McCandlish et al. (1975); Melcher & Schweitzer (1992).

Experimental top

LSO is commercially available from CTI Positron Systems Inc., Knoxville, Tennessee, USA, and crystals were grown by the Czochralski method (ref?).

Refinement top

The lattice parameters, space-group symmetry and initial coordinates were determined from a single-crystal X-ray study. Because of high X-ray absorption, refinement was carried out with intensities measured by neutron diffraction. A crystal of Lu2SiO5 (obtained from CTI Inc.) was mounted on an aluminium pin for the data collection. Net intensities were extracted from step-scanned profiles using the algorithm of Lehman & Larsen (1974). Standard reflection intensities were analyzed (McCandlish et al., 1975) and all intensities and their standard uncertainties were modified according to the variation of the standard reflections, which could be caused by crystal decomposition or instrumental instability. These modified standard uncertainties were later used, unaltered, as weights in the least-squares refinement. After absorption and Lorenz corrections, the atomic parameters from the X-ray structure determination were used as starting parameters for a refinement of the structure based on the neutron diffraction data. An anisotropic extinction correction was applied [extinction coefficients 2.06 (9)E+9, 1.20 (9)E+9, 1.71 (10)E+9, 8.3 (8)E+8, -1.34 (7)E+9 and -5.3 (7)E+8 Query.], but despite this 50 of the most extinction-affected reflections had to be omitted from the refinement. The largest peak in the final residual scattering power density was found close to atom O3.

Computing details top

Data collection: CRYO (ARACOR, 1981); cell refinement: please provide; data reduction: ARACOR (Lundgren, 1983); program(s) used to solve structure: please provide; program(s) used to refine structure: DUPALS (Lundgren, 1983); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: DISTAN (Lundgren, 1983).

Figures top
[Figure 1] Fig. 1. The structure of LSO, with displacement ellipsoids at the 50% probability level. Si—O bonds are filled, bonds in the OLu4 tetrahedra are open and other Lu—O bonds are thin lines.
Dilutetium silicon pentaoxide top
Crystal data top
Lu2SiO5Cell parameters taken from X-ray data
Mr = 458.02Dx = 7.36 Mg m3
Monoclinic, C2/cNeutron radiation, λ = 1.20700 Å
a = 14.2774 (7) ÅCell parameters from 1391 reflections
b = 6.6398 (4) Åθ = 1.5–22.5°
c = 10.2465 (6) ŵ = 0.16 mm1
β = 122.224 (1)°T = 293 K
V = 821.74 (8) Å3Prismatic, colourless
Z = 85.6 × 2.8 × 2.8 mm
F(000) = 37.07
Data collection top
ARACOR
diffractometer
949 reflections with Inet > 10σ(Inet)
Radiation source: reactorRint = 0.047
Cu monochromatorθmax = 52.0°, θmin = 0.0°
θ/2θ scan b/P/bh = 180
Absorption correction: integration
ABSSTOE (Lundgren 1983)
k = 08
Tmin = 0.607, Tmax = 0.684l = 1113
999 measured reflections3 standard reflections every 33 reflections
961 independent reflections intensity decay: 0.4%
Refinement top
Refinement on F2 1/σ2(F) modified for experimental instability
Least-squares matrix: full(Δ/σ)max = 0.050
R[F2 > 2σ(F2)] = 0.035Δρmax = 0.09 e Å3
wR(F2) = 0.078Δρmin = 0.06 e Å3
S = 2.91Extinction correction: B-C type 1 Gaussian anisotropic
949 reflectionsExtinction coefficient: see below
79 parameters
Crystal data top
Lu2SiO5V = 821.74 (8) Å3
Mr = 458.02Z = 8
Monoclinic, C2/cNeutron radiation, λ = 1.20700 Å
a = 14.2774 (7) ŵ = 0.16 mm1
b = 6.6398 (4) ÅT = 293 K
c = 10.2465 (6) Å5.6 × 2.8 × 2.8 mm
β = 122.224 (1)°
Data collection top
ARACOR
diffractometer
949 reflections with Inet > 10σ(Inet)
Absorption correction: integration
ABSSTOE (Lundgren 1983)
Rint = 0.047
Tmin = 0.607, Tmax = 0.6843 standard reflections every 33 reflections
999 measured reflections intensity decay: 0.4%
961 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.03579 parameters
wR(F2) = 0.078Δρmax = 0.09 e Å3
S = 2.91Δρmin = 0.06 e Å3
949 reflections
Special details top

Refinement. Anisotropic extinction coefficients. Type 1.

Value. 2055E+10. 1197E+10. 1715E+10. 8293E+09 -.1337E+10 -.5308E+09 s.u.. 9021E+08. 9376E+08. 1001E+09. 7746E+08. 7421E+08. 7477E+08

Translated to uniform units by tech ed: 2.06 (9)E+9 1.20 (9)E+9 1.71 (10)E+9 8.3 (8)E+8 - 1.34 (7)E+9 - 5.3 (7)E+8

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Lu10.53734 (5)0.75593 (10)0.46705 (7)0.0053 (5)
Lu20.14093 (5)0.37735 (10)0.16362 (7)0.0052 (5)
Si0.31792 (8)0.59171 (15)0.19311 (12)0.0042 (7)
O10.41117 (6)0.50618 (12)0.36201 (9)0.0093 (6)
O20.38016 (7)0.78834 (12)0.17623 (10)0.0075 (6)
O30.20230 (6)0.64896 (13)0.17684 (9)0.0078 (6)
O40.29842 (7)0.42890 (13)0.06298 (9)0.0082 (6)
O50.01773 (6)0.40340 (12)0.10250 (9)0.0063 (6)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Lu10.0032 (5)0.0065 (4)0.0060 (5)0.0006 (2)0.0024 (4)0.0001 (3)
Lu20.0025 (5)0.0066 (4)0.0057 (5)0.0009 (2)0.0016 (4)0.0010 (3)
Si0.0012 (6)0.0054 (6)0.0048 (6)0.0008 (3)0.0008 (5)0.0004 (5)
O10.0048 (5)0.0102 (5)0.0074 (5)0.0016 (3)0.0005 (4)0.0039 (4)
O20.0060 (5)0.0065 (5)0.0108 (5)0.0010 (3)0.0050 (4)0.0006 (4)
O30.0029 (5)0.0122 (5)0.0085 (5)0.0010 (3)0.0031 (4)0.0018 (4)
O40.0063 (5)0.0078 (5)0.0067 (5)0.0009 (3)0.0010 (4)0.0024 (4)
O50.0039 (5)0.0084 (5)0.0068 (5)0.0006 (3)0.0030 (4)0.0001 (4)
Geometric parameters (Å, º) top
Lu1—O12.2561 (10)Lu2v—O4v2.2378 (10)
Lu1—O1i2.2949 (10)Lu2v—O4viii2.2356 (11)
Lu1—O22.6163 (11)Lu2v—O5v2.1652 (10)
Lu1—O2ii2.3301 (10)Lu2v—O5ix2.2642 (10)
Lu1—O3iii2.2756 (10)Si—O11.6242 (13)
Lu1—O5iv2.1598 (10)Si—O21.6395 (13)
Lu1—O5iii2.3432 (10)Si—O31.6138 (12)
Lu2v—O2vi2.2346 (10)Si—O41.6214 (13)
Lu2v—O3vii2.2350 (10)
O1—Lu1—O1i69.93 (4)
Symmetry codes: (i) x+1, y+1, z+1; (ii) x+1, y, z+1/2; (iii) x+1/2, y+3/2, z+1/2; (iv) x+1/2, y+1/2, z+1/2; (v) x, y, z+1; (vi) x+1/2, y+3/2, z+1; (vii) x, y+1, z+1/2; (viii) x+1/2, y+1/2, z+1; (ix) x, y, z+1/2.

Experimental details

Crystal data
Chemical formulaLu2SiO5
Mr458.02
Crystal system, space groupMonoclinic, C2/c
Temperature (K)293
a, b, c (Å)14.2774 (7), 6.6398 (4), 10.2465 (6)
β (°) 122.224 (1)
V3)821.74 (8)
Z8
Radiation typeNeutron, λ = 1.20700 Å
µ (mm1)0.16
Crystal size (mm)5.6 × 2.8 × 2.8
Data collection
DiffractometerARACOR
diffractometer
Absorption correctionIntegration
ABSSTOE (Lundgren 1983)
Tmin, Tmax0.607, 0.684
No. of measured, independent and
observed [Inet > 10σ(Inet)] reflections
999, 961, 949
Rint0.047
(sin θ/λ)max1)0.653
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.035, 0.078, 2.91
No. of reflections949
No. of parameters79
No. of restraints?
Δρmax, Δρmin (e Å3)0.09, 0.06

Computer programs: CRYO (ARACOR, 1981), please provide, ARACOR (Lundgren, 1983), DUPALS (Lundgren, 1983), ORTEPII (Johnson, 1976), DISTAN (Lundgren, 1983).

Selected bond lengths (Å) top
Lu1—O12.2561 (10)Lu2v—O4v2.2378 (10)
Lu1—O1i2.2949 (10)Lu2v—O4viii2.2356 (11)
Lu1—O22.6163 (11)Lu2v—O5v2.1652 (10)
Lu1—O2ii2.3301 (10)Lu2v—O5ix2.2642 (10)
Lu1—O3iii2.2756 (10)Si—O11.6242 (13)
Lu1—O5iv2.1598 (10)Si—O21.6395 (13)
Lu1—O5iii2.3432 (10)Si—O31.6138 (12)
Lu2v—O2vi2.2346 (10)Si—O41.6214 (13)
Lu2v—O3vii2.2350 (10)
Symmetry codes: (i) x+1, y+1, z+1; (ii) x+1, y, z+1/2; (iii) x+1/2, y+3/2, z+1/2; (iv) x+1/2, y+1/2, z+1/2; (v) x, y, z+1; (vi) x+1/2, y+3/2, z+1; (vii) x, y+1, z+1/2; (viii) x+1/2, y+1/2, z+1; (ix) x, y, z+1/2.
 

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