A new BaCa(CO₃)₂ polymorph

A new polymorph of the double carbonate BaCa(CO₃)₂, `a C2 phase', has been synthesized. Its structure has been obtained by density-functional-theory-based (DFT-based) model calculations and has been refined by Rietveld analysis of X-ray powder diffraction data. The structure of the new polymorph differs significantly from those of the established polymorphs barytocalcite, paralstonite and alstonite. The unit-cell parameters of the new monoclinic (space group C2) compound are a = 6.6775 (5), b = 5.0982 (4), c = 4.1924 (3) Å, β = 109.259 (1)°. The new compound has been further characterized using Raman spectroscopy. This work shows that earlier studies have misidentified the products of an established synthesis route and that findings based on the incorrect identification of the synthesis product concerning the suitability of barytocalcite as a matrix for the retention of radioactive isotopes will need to be reconsidered.

Computing details

Barium Calcium Carbonate

Crystal data

O6C2BaCa	β = 109.2586 (13)°
Mr = 297.42	V = 134.73 (3) Å^3
Monoclinic, C2	Z = 1
a = 6.6775 (5) Å	Dx = 3.666 Mg m^−3
b = 5.0982 (4) Å	Cu Kα radiation
c = 4.1924 (3) Å	T = 293 K

Data collection

PANalytical XPert PRO diffractometer

2θmin = 10°, 2θmax = 100°, 2θstep = 0.03°

Refinement

Rp = 0.060	R(F^2) = 0.13286
Rwp = 0.084	30464 data points
Rexp = 0.049

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
O1	−0.1043 (6)	0.5037 (2)	0.7367 (5)	0.0782 (15)*
C3	0.0	0.6259 (3)	1.0	0.058 (2)*
O	0.5	0.3722 (2)	1.0	0.0782 (15)*
Ca	0.0	0.1032 (14)	0.5	0.0456 (3)*	0.5
Ba	0.0	0.1032 (14)	0.5	0.0456 (3)*	0.5