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A new gadolinium ytterbium trifluoride has been grown for the first time by the Czochralski technique. Although GdF3 and YbF3 both present a high-temperature phase transition, the mixed compound Gd0.81Yb0.19F3 maintains its crystallographic structure upon cooling to room temperature. Taking into account that both Gd3+ and Yb3+ ions are distributed randomly on a single site (Wyckoff position 4c), this is attributed to a mean cationic radius coincident with that of the Tb3+ ion, so that the stability of the crystal structure resembles that of TbF3. The grown crystal melts noncongruently at ∼1413 K, it is transparent and colourless, and it has a high density.
Supporting information
A single crystal of the title compound was grown by the Czochralski (Cz)
technique under a high-purity CF4 (99.999%) atmosphere. Commercial GdF3
(99.99%) and YbF3 (99.99%) powders were mixed with a nominal composition of
Gd0.7Yb0.3F3, and loaded into a Pt crucible. This was surrounded by
refractory carbons and heated inductively using a 30 kW generator. Using a Pt
wire as a seed, a single crystal was pulled up at a speed of 2 mm h-1 and a
rotation of 10 r min-1.
The chemical composition of the grown crystal was determined by inductively
coupled plasma (ICP) atomic emission spectroscopy using an IRIS Advantage from
Nippon Jarrell–AshCo. Thermal analysis of the crystal was performed by
differential scanning calorimetry (DSC) using a DSC Rigaku Thermoplus 8270.
The heating rate was set to 10 K min-1 and the measurements were carried out
under a 200 ml min-1 flow of CF4 in order to prevent sample oxidation.
Data collection: SMART (Bruker, 2002); cell refinement: SAINT-Plus (Bruker, 2003); data reduction: SAINT-Plus (Bruker, 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2000); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).
Gadolinium ytterbium trifluoride
top
Crystal data top
Gd0.81Yb0.19F3 | Dx = 7.258 Mg m−3 |
Mr = 217.24 | Mo Kα radiation, λ = 0.71073 Å |
Orthorhombic, Pnma | Cell parameters from 866 reflections |
a = 6.518 (1) Å | θ = 4.6–40.3° |
b = 6.950 (1) Å | µ = 35.83 mm−1 |
c = 4.389 (1) Å | T = 293 K |
V = 198.82 (6) Å3 | Plate, colourless |
Z = 4 | 0.16 × 0.16 × 0.1 mm |
F(000) = 368.560 | |
Data collection top
Bruker SMART APEX CCD area-detector diffractometer | 656 independent reflections |
Radiation source: fine-focus sealed tube | 646 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.035 |
ω scans | θmax = 40.3°, θmin = 5.5° |
Absorption correction: empirical (using intensity measurements) (SADABS; Sheldrick, 1996) | h = −11→11 |
Tmin = 0.023, Tmax = 0.086 | k = −11→12 |
4098 measured reflections | l = −7→7 |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.020 | w = 1/[σ2(Fo2) + (0.0235P)2 + 0.5681P] where P = (Fo2 + 2Fc2)/3 |
wR(F2) = 0.052 | (Δ/σ)max < 0.001 |
S = 1.23 | Δρmax = 2.34 e Å−3 |
656 reflections | Δρmin = −1.69 e Å−3 |
23 parameters | Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
0 restraints | Extinction coefficient: 0.058 (2) |
Crystal data top
Gd0.81Yb0.19F3 | V = 198.82 (6) Å3 |
Mr = 217.24 | Z = 4 |
Orthorhombic, Pnma | Mo Kα radiation |
a = 6.518 (1) Å | µ = 35.83 mm−1 |
b = 6.950 (1) Å | T = 293 K |
c = 4.389 (1) Å | 0.16 × 0.16 × 0.1 mm |
Data collection top
Bruker SMART APEX CCD area-detector diffractometer | 656 independent reflections |
Absorption correction: empirical (using intensity measurements) (SADABS; Sheldrick, 1996) | 646 reflections with I > 2σ(I) |
Tmin = 0.023, Tmax = 0.086 | Rint = 0.035 |
4098 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.020 | 23 parameters |
wR(F2) = 0.052 | 0 restraints |
S = 1.23 | Δρmax = 2.34 e Å−3 |
656 reflections | Δρmin = −1.69 e Å−3 |
Special details top
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes)
are estimated using the full covariance matrix. The cell e.s.d.'s are taken
into account individually in the estimation of e.s.d.'s in distances, angles
and torsion angles; correlations between e.s.d.'s in cell parameters are only
used when they are defined by crystal symmetry. An approximate (isotropic)
treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s.
planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor
wR and goodness of fit S are based on F2, conventional
R-factors R are based on F, with F set to zero for
negative F2. The threshold expression of F2 >
σ(F2) is used only for calculating R-factors(gt) etc.
and is not relevant to the choice of reflections for refinement.
R-factors based on F2 are statistically about twice as large
as those based on F, and R-factors based on ALL data will be
even larger. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | Occ. (<1) |
F1 | 0.1649 (3) | 0.0642 (3) | 0.1159 (5) | 0.0131 (3) | |
F2 | 0.0208 (4) | 0.2500 | 0.5841 (6) | 0.0142 (4) | |
Gd1 | 0.36752 (2) | 0.2500 | 0.43768 (4) | 0.00909 (8) | 0.81 |
Yb1 | 0.36752 (2) | 0.2500 | 0.43768 (4) | 0.00909 (8) | 0.19 |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
F1 | 0.0141 (6) | 0.0108 (7) | 0.0145 (7) | 0.0008 (6) | −0.0031 (6) | −0.0020 (6) |
F2 | 0.0135 (10) | 0.0151 (12) | 0.0140 (11) | 0.000 | 0.0016 (7) | 0.000 |
Gd1 | 0.00965 (11) | 0.00723 (11) | 0.01039 (11) | 0.000 | −0.00085 (4) | 0.000 |
Yb1 | 0.00965 (11) | 0.00723 (11) | 0.01039 (11) | 0.000 | −0.00085 (4) | 0.000 |
Geometric parameters (Å, º) top
F1—Gd1 | 2.325 (2) | Gd1—F2iv | 2.325 (3) |
F1—Gd1i | 2.329 (2) | Gd1—F1v | 2.325 (2) |
F1—Gd1ii | 2.341 (2) | Gd1—F1vi | 2.329 (2) |
F2—Gd1iii | 2.325 (3) | Gd1—F1vii | 2.341 (2) |
F2—Gd1 | 2.349 (3) | Gd1—F2viii | 2.498 (3) |
F2—Gd1ii | 2.498 (3) | | |
| | | |
Gd1—F1—Yb1i | 140.93 (9) | F1ix—Gd1—F2 | 79.68 (5) |
Gd1—F1—Gd1i | 140.93 (9) | F1viii—Gd1—F2 | 145.48 (5) |
Gd1—F1—Yb1ii | 103.00 (8) | F1vii—Gd1—F2 | 145.48 (5) |
Yb1i—F1—Yb1ii | 114.09 (8) | F2iv—Gd1—F2viii | 130.97 (12) |
Gd1i—F1—Yb1ii | 114.09 (8) | F1v—Gd1—F2viii | 70.76 (7) |
Gd1—F1—Gd1ii | 103.00 (8) | F1—Gd1—F2viii | 70.76 (7) |
Yb1i—F1—Gd1ii | 114.09 (8) | F1vi—Gd1—F2viii | 110.13 (5) |
Gd1i—F1—Gd1ii | 114.09 (8) | F1ix—Gd1—F2viii | 110.13 (5) |
Yb1iii—F2—Gd1 | 131.33 (12) | F1viii—Gd1—F2viii | 64.96 (7) |
Gd1iii—F2—Gd1 | 131.33 (12) | F1vii—Gd1—F2viii | 64.96 (7) |
Yb1iii—F2—Gd1ii | 130.97 (12) | F2—Gd1—F2viii | 129.45 (9) |
Gd1iii—F2—Gd1ii | 130.97 (12) | F2iv—Gd1—Yb1ii | 142.27 (7) |
Gd1—F2—Gd1ii | 97.71 (9) | F1v—Gd1—Yb1ii | 38.66 (5) |
Yb1iii—F2—Yb1ii | 130.97 (12) | F1—Gd1—Yb1ii | 38.66 (5) |
Gd1iii—F2—Yb1ii | 130.97 (12) | F1vi—Gd1—Yb1ii | 94.04 (5) |
Gd1—F2—Yb1ii | 97.71 (9) | F1ix—Gd1—Yb1ii | 94.04 (5) |
F2iv—Gd1—F1v | 142.42 (6) | F1viii—Gd1—Yb1ii | 133.92 (5) |
F2iv—Gd1—F1 | 142.42 (6) | F1vii—Gd1—Yb1ii | 133.93 (5) |
F1v—Gd1—F1 | 67.48 (10) | F2—Gd1—Yb1ii | 42.68 (6) |
F2iv—Gd1—F1vi | 74.68 (6) | F2viii—Gd1—Yb1ii | 86.76 (6) |
F1v—Gd1—F1vi | 132.31 (6) | F2iv—Gd1—Gd1ii | 142.27 (7) |
F1—Gd1—F1vi | 68.38 (5) | F1v—Gd1—Gd1ii | 38.66 (5) |
F2iv—Gd1—F1ix | 74.68 (6) | F1—Gd1—Gd1ii | 38.66 (5) |
F1v—Gd1—F1ix | 68.38 (5) | F1vi—Gd1—Gd1ii | 94.04 (5) |
F1—Gd1—F1ix | 132.31 (6) | F1ix—Gd1—Gd1ii | 94.04 (5) |
F1vi—Gd1—F1ix | 139.28 (10) | F1viii—Gd1—Gd1ii | 133.92 (5) |
F2iv—Gd1—F1viii | 74.62 (8) | F1vii—Gd1—Gd1ii | 133.93 (5) |
F1v—Gd1—F1viii | 135.70 (4) | F2—Gd1—Gd1ii | 42.68 (6) |
F1—Gd1—F1viii | 95.91 (7) | F2viii—Gd1—Gd1ii | 86.76 (6) |
F1vi—Gd1—F1viii | 65.91 (8) | F2iv—Gd1—Yb1viii | 91.36 (7) |
F1ix—Gd1—F1viii | 128.76 (5) | F1v—Gd1—Yb1viii | 103.47 (5) |
F2iv—Gd1—F1vii | 74.62 (8) | F1—Gd1—Yb1viii | 103.47 (5) |
F1v—Gd1—F1vii | 95.91 (7) | F1vi—Gd1—Yb1viii | 103.45 (5) |
F1—Gd1—F1vii | 135.70 (4) | F1ix—Gd1—Yb1viii | 103.45 (5) |
F1vi—Gd1—F1vii | 128.76 (5) | F1viii—Gd1—Yb1viii | 38.34 (5) |
F1ix—Gd1—F1vii | 65.91 (8) | F1vii—Gd1—Yb1viii | 38.34 (5) |
F1viii—Gd1—F1vii | 66.95 (10) | F2—Gd1—Yb1viii | 169.05 (7) |
F2iv—Gd1—F2 | 99.59 (5) | F2viii—Gd1—Yb1viii | 39.61 (6) |
F1v—Gd1—F2 | 67.66 (7) | Yb1ii—Gd1—Yb1viii | 126.372 (10) |
F1—Gd1—F2 | 67.65 (7) | Gd1ii—Gd1—Yb1viii | 126.372 (10) |
F1vi—Gd1—F2 | 79.68 (5) | | |
Symmetry codes: (i) −x+1/2, −y, z−1/2; (ii) x−1/2, y, −z+1/2; (iii) x−1/2, y, −z+3/2; (iv) x+1/2, y, −z+3/2; (v) x, −y+1/2, z; (vi) −x+1/2, −y, z+1/2; (vii) x+1/2, −y+1/2, −z+1/2; (viii) x+1/2, y, −z+1/2; (ix) −x+1/2, y+1/2, z+1/2. |
Experimental details
Crystal data |
Chemical formula | Gd0.81Yb0.19F3 |
Mr | 217.24 |
Crystal system, space group | Orthorhombic, Pnma |
Temperature (K) | 293 |
a, b, c (Å) | 6.518 (1), 6.950 (1), 4.389 (1) |
V (Å3) | 198.82 (6) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 35.83 |
Crystal size (mm) | 0.16 × 0.16 × 0.1 |
|
Data collection |
Diffractometer | Bruker SMART APEX CCD area-detector diffractometer |
Absorption correction | Empirical (using intensity measurements) (SADABS; Sheldrick, 1996) |
Tmin, Tmax | 0.023, 0.086 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 4098, 656, 646 |
Rint | 0.035 |
(sin θ/λ)max (Å−1) | 0.910 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.020, 0.052, 1.23 |
No. of reflections | 656 |
No. of parameters | 23 |
Δρmax, Δρmin (e Å−3) | 2.34, −1.69 |
Selected bond lengths (Å) topF1—Gd1 | 2.325 (2) | Gd1—F2iv | 2.325 (3) |
F1—Gd1i | 2.329 (2) | Gd1—F1v | 2.325 (2) |
F1—Gd1ii | 2.341 (2) | Gd1—F1vi | 2.329 (2) |
F2—Gd1iii | 2.325 (3) | Gd1—F1vii | 2.341 (2) |
F2—Gd1 | 2.349 (3) | Gd1—F2viii | 2.498 (3) |
F2—Gd1ii | 2.498 (3) | | |
Symmetry codes: (i) −x+1/2, −y, z−1/2; (ii) x−1/2, y, −z+1/2; (iii) x−1/2, y, −z+3/2; (iv) x+1/2, y, −z+3/2; (v) x, −y+1/2, z; (vi) −x+1/2, −y, z+1/2; (vii) x+1/2, −y+1/2, −z+1/2; (viii) x+1/2, y, −z+1/2. |
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Rare earth trifluorides (REF3) have long been studied as hosts for scintillation applications (Kobayashi et al., 2003). These crystals present a series of important characteristics, such as easy doping by isovalent Ce3+ substitution, high density, and high visible transparency. Among the near-UV–visible transparent REF3 (RE = La, Ce, Gd, Yb1 and Lu), only the well known LaF3 and CeF3 do not present a phase transition upon cooling.
Initial studies of the synthesis of REF3 identified a structural dimorphism along this series of compounds, which can crystallize in the tysonite hexagonal form and/or in the orthorhombic β-YF3 form (Zalkin & Templeton, 1953; Thoma & Brunton, 1966). Later authors have concluded that REF3 possess three different crystal structures. Depending on the occurrence or absence of a phase transition, REF3 are classified into four morphotropic series as a function of decreasing lanthanide ionic radius (Fedorov & Sobolev, 1995; Spedding et al., 1974; Sobolev et al., 1976; Greis & Cader, 1985; Petzel & Rathjen, 1994):
(1) LaF3 to NdF3 (trigonal LaF3-type, P3c1),
(2) SmF3 to GdF3 (hexagonal Schlyter type, P63/mmc; trigonal LaF3-type, P3c1; and orthorhombic β-YF3-type, Pnma),
(3) TbF3 to HoF3 (orthorhombic β-YF3-type, Pnma) and
(4) ErF3 to LuF3 (trigonal α-UO3-type, P3m1; and orthorhombic β-YF3-type, Pnma).
[There is a controversy about the existence of the high-temperature hexagonal Schlyter-type phase. On the one hand, its appearance is claimed (Greis & Cader, 1985; Schlyter, 1953), while on the other it has been argued that it corresponds to twinned crystals in space group P3c1 with balanced volume ratios (Maximov & Schulz, 1985).]
The transitions between these series are a consequence of their relative structural instabilities. According to previous studies regarding the stabilization of each structural type [see Sobolev et al. (1977), and references therein], the mean cationic radius is the geometric factor that plays a decisive role. In that particular paper, Sobolev dealt with the stability of the orthorhombic β-YF3-type in the phase diagrams of the GdF3–LnF3 systems (with Ln = Tb, Ho, Er or Yb). Taking into account the ionic radii (Shannon, 1976), one can calculate the stability range of this phase, which extends from peritectic to eutectic composition. In all four diagrams this range relates to a mean cationic radius which lies between the ionic radii of Tb and Ho, i.e. it matches the range (3) above where no phase transition is observed.
In the present work, we intended to synthesize a binary REF3 which was transparent and did not present any phase transitions. For this purpose, we chose Gd3+ and Yb3+ as cations and considered the GdF3–YbF3 phase diagram (Sobolev et al., 1977). Due to the noncongruent nature of mixed Gd1-xYbxF3 compounds, the initial composition of the melt was chosen to be x = 0.30, so that, according to the phase diagram, a crystal with the approximate composition Gd0.91Yb0.09F3 should crystallize at a temperature of about 1443 K with an orthorhombic structure.
A transparent crack-free single crystal was grown by the Czochralski technique. Chemical analysis carried out by inductively coupled plasma indicated that the crystal composition was Gd0.81Yb0.19F3. According to the phase diagram, this concentration of Yb1 should correspond to a starting melt value of x = 0.43, instead of the nominally used x = 0.3. X-ray diffraction measurements confirmed that the grown crystal has the expected β-YF3 structure, characterized by two anionic sites and a single cationic site. Consequently, both Gd3+ and Yb3+ occupy the same site randomly (Wyckoff position 4c), as shown in Fig. 1. Furthermore, as the two cations are very similar to each other, it is not possible to distinguish them by X-ray diffraction measurements. The lattice parameters obtained correspond precisely with those known for TbF3. Considering Vegard's law (Reference?) and the ionic radii for coordination number 9 for the cationic site, we found that the average radius coincides with that of Tb93+, namely 1.235 Å. Therefore, in agreement with previous observations about average ionic radius for the stabilization of the orthorhombic structure, our experiment reinforces the hypothesis that the Tb3+ ionic radius represents the maximum average limit.
The thermal behaviour of the as-grown crystal was investigated by differential scanning calorimetry and is shown in Fig. 2. Similar to the GdF3–YbF3 phase diagram, three transitions at high temperature can be clearly distinguished. The first one, at 1413 K, corresponds to the nucleation temperature, the second one, at 1527 K, with the peritectic temperature and the highest one, at 1540 K, with the completely liquid phase. These values are shifted from those indicated in the phase diagram for the measured crystal composition (x = 0.19), namely 1398, 1453 and 1466 K for the nucleation temperature, eutectic phase and total melting, respectively.
In summary, we have presented the first report of the growth of a single crystal of Gd0.81Yb0.19F3 by the Czochralski technique. Due to its noncongruent nature, the growth of large crystals will require non-standard techniques to counteract the continuous shift of the melt composition during growth. On the other hand, this growth behaviour could be an advantage for the synthesis of core–shell nanoparticles based on REF3 luminescent ions, which have specific applications in the fields of biology and optoelectronics (see e.g. Wang et al., 2006). Crystalline Gd0.81Yb0.19F3 is transparent and colourless, it crystallizes in the orthorhombic β-YF3 structure and it does not present any phase transition upon cooling. Furthermore, the density is as high as 7.25 Mg m-3. Therefore, Gd0.81Yb0.19F3 represents a new REF3 with attractive properties for scintillation and core–shell nanoparticle applications.