Variation of cation distribution with temperature and its consequences on thermal expansion for Ca₃Eu₂(BO₃)₄

The structure of calcium europium orthoborate, Ca₃Eu₂(BO₃)₄, was determined using high-resolution powder X-ray diffraction data collected at the ID22 beamline (ESRF) under ambient conditions, as well as at high temperature. Rietveld refinement allowed determination of the lattice constants and structural details, including the Ca/Eu ratios at the three cationic sites and their evolution with temperature. Clear thermal expansion anisotropy was found, and slope changes of lattice-constant dependencies on temperature were observed at 923 K. Above this temperature the changes in occupation of the Ca/Eu sites occur, exhibiting a tendency towards a more uniform Eu distribution over the three Ca/Eu sites. Possible structural origins of the observed thermal expansion anisotropy are discussed.

Computing details

Dieuropium tricalcium tetraborate

Crystal data

Ca3·Eu2·(BO3)4	V = 968.73 (1) Å^3
Mr = 659.40	Z = 4
Orthorhombic, Pnma	Synchrotron radiation, λ = 0.400218 Å
Hall symbol: -P 2ac 2n	T = 295 K
a = 7.20378 (2) Å	white
b = 15.57492 (5) Å	cylinder, 1 × 0.7 mm
c = 8.63406 (3) Å

Data collection

ID22 diffractometer, transmission geometry	Data collection mode: transmission
Radiation source: synchrotron radiation, ESRF, beamline ID22	Scan method: continuous
Channel cut Si 111 monochromator	2θmin = 2.500°, 2θmax = 33.00°, 2θstep = 0.002°
Specimen mounting: packed into borosilicate capillary

Refinement

Rp = 6.04	19750 data points
Rwp = 9.11	Profile function: Voigt
Rexp = 1.89	Background function: manually determined, fitted at the last stage of refinement
RBragg = 6.735

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

	x	y	z	Biso*/Beq	Occ. (<1)
Ca1	0.51946 (14)	0.08348 (6)	0.82354 (12)	1.69 (2)	0.5309 (12)
Eu1	0.51946 (14)	0.08348 (6)	0.82354 (12)	1.69 (2)	0.4691 (12)
Ca2	0.69687 (19)	0.12768 (8)	0.34419 (16)	1.69 (2)	0.7403 (10)
Eu2	0.69687 (19)	0.12768 (8)	0.34419 (16)	1.69 (2)	0.2597 (10)
Ca3	0.32762 (18)	0.25	0.53358 (15)	1.69 (2)	0.4575 (16)
Eu3	0.32762 (18)	0.25	0.53358 (15)	1.69 (2)	0.5425 (16)
B1	0.333 (3)	0.4488 (11)	0.447 (2)	6.0 (4)
B2	0.710 (3)	0.25	0.655 (3)	6.0 (4)
B3	0.490 (4)	0.25	0.134 (3)	6.0 (4)
O1	0.5542 (10)	0.1752 (4)	0.0660 (9)	4.54 (10)
O2	0.7541 (11)	0.3240 (4)	0.7248 (8)	4.54 (10)
O3	0.4074 (12)	0.4068 (5)	0.5674 (10)	4.54 (10)
O4	0.2036 (11)	0.5085 (5)	0.4873 (9)	4.54 (10)
O5	0.3946 (11)	0.4619 (5)	0.3005 (10)	4.54 (10)
O6	0.3950 (15)	0.25	0.2692 (13)	4.54 (10)
O7	0.6619 (15)	0.25	0.5035 (12)	4.54 (10)

Geometric parameters (Å, º)

Ca1—Eu1	0.00000	Ca3—Eu3	0.00000
Ca1—O5^i	2.261 (8)	Ca3—O6	2.33 (1)
Ca1—O3^ii	2.359 (8)	Ca3—O7	2.42 (1)
Ca1—O2^ii	2.379 (8)	Ca3—O2^iii	2.442 (7)
Ca1—O2^iii	2.430 (8)	Ca3—O2^ix	2.442 (7)
Ca1—O4^iv	2.438 (8)	Ca3—O1^x	2.445 (7)
Ca1—O4^v	2.546 (8)	Ca3—O1^xi	2.445 (7)
Ca1—O1^vi	2.547 (7)	Ca3—O3^ii	2.525 (8)
Ca1—O3^v	2.953 (8)	Ca3—O3	2.525 (8)
Ca2—Eu2	0.00000	B1—O3	1.34 (2)
Ca2—O5^vii	2.353 (8)	B1—O5	1.35 (2)
Ca2—O7	2.363 (7)	B1—O4	1.37 (2)
Ca2—O4^i	2.465 (8)	B2—O2^ii	1.34 (1)
Ca2—O6^vii	2.574 (8)	B2—O2	1.34 (1)
Ca2—O5^ii	2.614 (8)	B2—O7	1.35 (3)
Ca2—O1	2.715 (7)	B3—O6	1.36 (3)
Ca2—O1^viii	2.788 (8)	B3—O1^ii	1.38 (2)
Ca2—O3^ii	2.890 (8)	B3—O1	1.38 (2)
Ca2—O6	2.963 (8)
O5—B1—O3	132.1 (16)	O7—B2—O2^ii	119.7 (11)
O4—B1—O5	111.1 (13)	O1^ii—B3—O6	122.2 (9)
O4—B1—O3	113.9 (14)	O1—B3—O1^ii	114.8 (18)
O2—B2—O2^ii	119 (2)	O1—B3—O6	122.2 (9)
O7—B2—O2	119.7 (11)

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