Structural and spectroscopic investigations on the quenching free luminescence of europium oxalate nanocrystals

The structural features leading to the intense quenching free luminescence exhibited by europium oxalate nanocrystals, poly[[hexa­aqua­tri-μ₂-oxalato-dieuropium] 4.34-hydrate], {[Eu₂(C₂O₄)₃(H₂O)₆]·4.34H₂O}_n, is the focal point of this report. Europium oxalate nanocrystals were synthesized by a simple microwave-assisted co-precipitation method. Powder X-ray diffraction analysis revealed the monoclinic structure of the nanocrystals and the phase purity. The morphology and particle size were examined by transmission electron microscopy (TEM) analysis. Luminescence measurements on a series of samples of La_2–xEu_x(C₂O₄)₃·10H₂O, with x varying in the range 0.1 to 2, established the quenching free nature exhibited by the europium oxalate nanocrystals. A single-crystal structure analysis was carried out and the quenching free luminescence is explained on the basis of the crystal structure. A detailed photoluminescence characterization was carried out using excitation and emission studies, decay analysis, and CIE coordinate and colour purity evaluation. The various spectroscopic parameters were evaluated by Judd–Ofelt theoretical analysis and the results are discussed on the basis of the crystal structure analysis.

Computing details

Data collection: APEX3 (Bruker, 2012); cell refinement: SAINT (Bruker, 2012); data reduction: SAINT (Bruker, 2012); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2017 (Sheldrick, 2015b); molecular graphics: SHELXTL (Bruker, 2012) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXTL (Bruker, 2012) and VESTA (Momma & Izumi, 2011).

Poly[[hexaaquatri-µ₂-oxalato-dieuropium] 4.34-hydrate]

Crystal data

[Eu2(C2O4)3(H2O)6]·4.34H2O	F(000) = 711
Mr = 375.52	Dx = 2.538 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 11.0764 (5) Å	Cell parameters from 9988 reflections
b = 9.6272 (4) Å	θ = 2.9–45.8°
c = 10.1113 (5) Å	µ = 6.43 mm^−1
β = 114.281 (1)°	T = 296 K
V = 982.84 (8) Å^3	Block, colourless
Z = 4	0.25 × 0.20 × 0.15 mm

Data collection

Bruker Kappa APEXII CMOS diffractometer	2672 reflections with I > 2σ(I)
Radiation source: Sealed tube	Rint = 0.031
ω and φ scan	θmax = 30.0°, θmin = 2.9°
Absorption correction: multi-scan (SADABS; Bruker, 2012)	h = −15→15
Tmin = 0.25, Tmax = 0.41	k = −13→13
24930 measured reflections	l = −14→14
2875 independent reflections

Refinement

Refinement on F^2	Hydrogen site location: difference Fourier map
Least-squares matrix: full	All H-atom parameters refined
R[F^2 > 2σ(F^2)] = 0.013	w = 1/[σ^2(Fo^2) + (0.0131P)^2 + 0.507P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.030	(Δ/σ)max = 0.002
S = 1.09	Δρmax = 0.43 e Å^−3
2875 reflections	Δρmin = −0.39 e Å^−3
209 parameters	Extinction correction: SHELXL2017 (Sheldrick, 2015b), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
96 restraints	Extinction coefficient: 0.0152 (3)

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
C1	0.44517 (18)	0.47391 (19)	0.4285 (2)	0.0169 (3)
C2	0.53338 (18)	0.42920 (18)	0.00267 (19)	0.0155 (3)
C3	0.99969 (17)	0.55174 (18)	0.55726 (19)	0.0142 (3)
Eu1	0.69025 (2)	0.54374 (2)	0.33136 (2)	0.01206 (4)
O8	0.33923 (14)	0.43628 (16)	0.43286 (16)	0.0253 (3)
O4	0.47209 (14)	0.47366 (15)	0.31954 (15)	0.0207 (3)
O7	0.48873 (13)	0.35775 (14)	−0.11128 (14)	0.0186 (3)
O6	0.62677 (14)	0.39709 (15)	0.11939 (15)	0.0239 (3)
O9	1.10866 (13)	0.57272 (14)	0.66274 (15)	0.0196 (3)
O5	0.89175 (13)	0.60721 (15)	0.53704 (15)	0.0210 (3)
O1	0.70997 (18)	0.30927 (17)	0.4269 (2)	0.0344 (4)
O3	0.79619 (17)	0.68432 (19)	0.2027 (2)	0.0346 (4)
O2	0.6667 (2)	0.78573 (17)	0.38334 (18)	0.0366 (4)
O10A	0.0767 (4)	0.8167 (6)	0.7903 (6)	0.089 (3)	0.668 (11)
O11C	0.0839 (12)	1.0230 (12)	0.5844 (15)	0.117 (3)	0.3552
O10B	0.0590 (5)	0.7008 (9)	0.8802 (8)	0.078 (3)	0.423 (9)
H2A	0.622 (3)	0.842 (3)	0.321 (3)	0.040 (8)*
H1A	0.647 (2)	0.259 (3)	0.427 (3)	0.052 (9)*
H3A	0.771 (4)	0.766 (2)	0.184 (4)	0.072 (12)*
H3B	0.8766 (19)	0.675 (5)	0.233 (4)	0.088 (14)*
H1B	0.772 (4)	0.283 (5)	0.503 (4)	0.13 (2)*
H2B	0.705 (5)	0.823 (5)	0.463 (3)	0.13 (2)*
O11D	0.1138 (19)	1.0379 (19)	0.469 (2)	0.107 (5)	0.1992
O11A	0.1578 (9)	0.8996 (10)	0.6823 (10)	0.107 (3)	0.4608
O11B	0.158 (3)	0.931 (3)	0.598 (3)	0.108 (6)	0.1669
O10C	0.096 (4)	0.737 (5)	0.757 (5)	0.090 (9)	0.069 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0140 (8)	0.0201 (9)	0.0173 (8)	−0.0038 (6)	0.0072 (7)	−0.0038 (6)
C2	0.0129 (8)	0.0188 (8)	0.0132 (8)	0.0020 (6)	0.0038 (6)	−0.0010 (6)
C3	0.0109 (7)	0.0170 (8)	0.0142 (8)	−0.0005 (6)	0.0046 (6)	0.0002 (6)
Eu1	0.00862 (5)	0.01613 (5)	0.01003 (5)	−0.00004 (3)	0.00242 (3)	0.00013 (3)
O8	0.0177 (7)	0.0404 (9)	0.0211 (7)	−0.0127 (6)	0.0114 (6)	−0.0090 (6)
O4	0.0164 (6)	0.0311 (8)	0.0159 (6)	−0.0068 (5)	0.0079 (5)	−0.0045 (5)
O7	0.0182 (6)	0.0194 (6)	0.0140 (6)	0.0036 (5)	0.0025 (5)	−0.0039 (5)
O6	0.0225 (7)	0.0250 (7)	0.0153 (6)	0.0100 (6)	−0.0012 (5)	−0.0031 (5)
O9	0.0100 (6)	0.0265 (7)	0.0175 (6)	0.0012 (5)	0.0007 (5)	−0.0067 (5)
O5	0.0105 (6)	0.0288 (7)	0.0201 (6)	0.0033 (5)	0.0026 (5)	−0.0078 (5)
O1	0.0270 (8)	0.0277 (8)	0.0479 (11)	−0.0020 (7)	0.0147 (8)	0.0157 (7)
O3	0.0247 (9)	0.0361 (9)	0.0434 (10)	0.0025 (7)	0.0144 (7)	0.0203 (8)
O2	0.0525 (11)	0.0243 (8)	0.0192 (7)	0.0145 (8)	0.0008 (7)	0.0009 (6)
O10A	0.047 (2)	0.097 (4)	0.100 (4)	0.014 (2)	0.006 (2)	−0.071 (3)
O11C	0.105 (7)	0.117 (7)	0.117 (7)	0.002 (6)	0.035 (6)	−0.016 (6)
O10B	0.037 (3)	0.111 (6)	0.079 (5)	−0.012 (3)	0.018 (3)	−0.062 (4)
O11D	0.084 (8)	0.111 (9)	0.113 (9)	−0.014 (7)	0.029 (8)	−0.013 (8)
O11A	0.082 (5)	0.097 (6)	0.109 (6)	0.005 (4)	0.007 (5)	−0.046 (5)
O11B	0.091 (9)	0.102 (10)	0.123 (11)	−0.007 (8)	0.034 (9)	−0.008 (10)
O10C	0.060 (12)	0.102 (12)	0.093 (12)	0.011 (11)	0.017 (10)	−0.049 (11)

Geometric parameters (Å, º)

C1—O8	1.246 (2)	C3—Eu1	3.2453 (17)
C1—O4	1.254 (2)	Eu1—O6	2.4170 (14)
C1—C1^i	1.541 (4)	Eu1—O5	2.4213 (13)
C2—O6	1.247 (2)	Eu1—O2	2.4259 (16)
C2—O7	1.256 (2)	Eu1—O1	2.4294 (16)
C2—C2^ii	1.541 (3)	Eu1—O4	2.4639 (14)
C2—Eu1	3.2452 (18)	Eu1—O9^iii	2.4723 (13)
C3—O5	1.247 (2)	Eu1—O7^ii	2.4806 (13)
C3—O9	1.256 (2)	Eu1—O3	2.4816 (16)
C3—C3^iii	1.530 (3)	Eu1—O8^i	2.5431 (14)
O8—C1—O4	126.73 (17)	O2—Eu1—O3	72.91 (7)
O8—C1—C1^i	116.9 (2)	O1—Eu1—O3	136.94 (6)
O4—C1—C1^i	116.4 (2)	O4—Eu1—O3	142.16 (5)
O6—C2—O7	126.38 (17)	O9^iii—Eu1—O3	69.17 (5)
O6—C2—C2^ii	116.86 (19)	O7^ii—Eu1—O3	72.35 (5)
O7—C2—C2^ii	116.76 (19)	O6—Eu1—O8^i	140.61 (5)
O6—C2—Eu1	39.50 (9)	O5—Eu1—O8^i	66.67 (5)
O7—C2—Eu1	165.66 (12)	O2—Eu1—O8^i	69.97 (6)
C2^ii—C2—Eu1	77.44 (13)	O1—Eu1—O8^i	73.91 (6)
O5—C3—O9	126.32 (16)	O4—Eu1—O8^i	64.20 (4)
O5—C3—C3^iii	117.21 (19)	O9^iii—Eu1—O8^i	118.25 (5)
O9—C3—C3^iii	116.47 (19)	O7^ii—Eu1—O8^i	116.66 (5)
O5—C3—Eu1	39.73 (8)	O3—Eu1—O8^i	136.14 (6)
O9—C3—Eu1	165.89 (12)	O6—Eu1—C2	19.16 (4)
C3^iii—C3—Eu1	77.53 (13)	O5—Eu1—C2	151.84 (4)
O6—Eu1—O5	137.59 (5)	O2—Eu1—C2	119.22 (5)
O6—Eu1—O2	137.32 (5)	O1—Eu1—C2	90.81 (6)
O5—Eu1—O2	73.43 (5)	O4—Eu1—C2	75.87 (5)
O6—Eu1—O1	75.86 (6)	O9^iii—Eu1—C2	88.18 (4)
O5—Eu1—O1	89.57 (6)	O7^ii—Eu1—C2	46.95 (4)
O2—Eu1—O1	143.73 (7)	O3—Eu1—C2	79.50 (6)
O6—Eu1—O4	82.83 (5)	O8^i—Eu1—C2	139.88 (5)
O5—Eu1—O4	130.54 (5)	O6—Eu1—C3	118.42 (5)
O2—Eu1—O4	94.74 (6)	O5—Eu1—C3	19.22 (4)
O1—Eu1—O4	72.00 (5)	O2—Eu1—C3	90.08 (5)
O6—Eu1—O9^iii	71.77 (5)	O1—Eu1—C3	81.14 (5)
O5—Eu1—O9^iii	65.83 (4)	O4—Eu1—C3	140.49 (4)
O2—Eu1—O9^iii	127.34 (6)	O9^iii—Eu1—C3	46.65 (4)
O1—Eu1—O9^iii	68.67 (5)	O7^ii—Eu1—C3	147.72 (4)
O4—Eu1—O9^iii	137.12 (5)	O3—Eu1—C3	76.46 (5)
O6—Eu1—O7^ii	66.05 (4)	O8^i—Eu1—C3	81.02 (5)
O5—Eu1—O7^ii	142.04 (5)	C2—Eu1—C3	134.00 (4)
O2—Eu1—O7^ii	73.12 (5)	C1—O8—Eu1^i	119.79 (12)
O1—Eu1—O7^ii	128.31 (6)	C1—O4—Eu1	122.65 (12)
O4—Eu1—O7^ii	69.82 (5)	C2—O7—Eu1^ii	118.79 (11)
O9^iii—Eu1—O7^ii	125.08 (4)	C2—O6—Eu1	121.34 (12)
O6—Eu1—O3	83.19 (6)	C3—O9—Eu1^iii	119.31 (11)
O5—Eu1—O3	81.09 (6)	C3—O5—Eu1	121.04 (11)
O4—C1—O8—Eu1^i	−177.88 (15)	O7—C2—O6—Eu1	−176.53 (14)
C1^i—C1—O8—Eu1^i	2.2 (3)	C2^ii—C2—O6—Eu1	3.8 (3)
O8—C1—O4—Eu1	−177.70 (15)	O5—C3—O9—Eu1^iii	−177.02 (14)
C1^i—C1—O4—Eu1	2.2 (3)	C3^iii—C3—O9—Eu1^iii	2.8 (3)
O6—C2—O7—Eu1^ii	−176.47 (16)	Eu1—C3—O9—Eu1^iii	175.2 (4)
C2^ii—C2—O7—Eu1^ii	3.2 (3)	O9—C3—O5—Eu1	−177.03 (14)
Eu1—C2—O7—Eu1^ii	174.6 (5)	C3^iii—C3—O5—Eu1	3.2 (3)

Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) −x+1, −y+1, −z; (iii) −x+2, −y+1, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2A···O4^iv	0.82 (2)	1.87 (2)	2.693 (2)	178 (3)
O1—H1A···O7^v	0.85 (2)	1.98 (2)	2.822 (2)	170 (3)
O1—H1A···O6^v	0.85 (2)	2.54 (3)	3.172 (2)	132 (3)
O3—H3A···O8^iv	0.83 (2)	2.09 (2)	2.885 (2)	159 (3)
O3—H3B···O10A^vi	0.82 (2)	2.05 (2)	2.859 (5)	171 (4)
O3—H3B···O10B^vi	0.82 (2)	2.29 (3)	2.936 (7)	136 (4)
O3—H3B···O10C^vi	0.82 (2)	2.49 (5)	3.23 (4)	151 (4)
O1—H1B···O10A^vii	0.83 (2)	2.10 (3)	2.865 (5)	153 (5)
O1—H1B···O10B^vii	0.83 (2)	1.93 (3)	2.709 (5)	156 (5)
O1—H1B···O10C^vii	0.83 (2)	2.30 (5)	3.11 (4)	164 (5)
O2—H2B···O3^viii	0.83 (2)	2.21 (3)	2.959 (2)	152 (5)
O2—H2B···O11D^ix	0.83 (2)	2.27 (5)	2.836 (18)	126 (5)

Symmetry codes: (iv) −x+1, y+1/2, −z+1/2; (v) x, −y+1/2, z+1/2; (vi) x+1, −y+3/2, z−1/2; (vii) −x+1, y−1/2, −z+3/2; (viii) x, −y+3/2, z+1/2; (ix) −x+1, −y+2, −z+1.