Structure and phase transitions of CaTaOAlO₄

The structure of CaTaOAlO₄ (CTAO) has been determined using X-ray powder diffraction and density functional methods in combination with ²⁷Al MAS NMR spectroscopy. A structural phase transition occurs near room temperature in CTAO as indicated by heat-capacity measurements, lattice strain data and infrared spectroscopy. Rietveld analysis of the powder diffraction data does not indicate deviation from monoclinic symmetry C2/c. But the observed quadrupolar coupling of the Al atom is reproduced by the electronic structure calculations only in a structure with space-group symmetry P2₁/n, distinguished by two different Ta coordination environments. The atomic coordinates of this low-temperature structure of CTAO are obtained by computational force relaxation within the experimental unit cell determined at 170 K.

Computing details

Figures

(crystal)

Crystal data

CaTaAlO5	V = 399.14 (2) Å^3
Mr = 328.01	Z = 4
Monoclinic, P21/n	? radiation, λ = ? Å
a = 6.6298 (2) Å	θ = 11.5–59.1°
b = 8.9515 (2) Å	T = 170 K
c = 7.3502 (1) Å	× × mm
β = 113.792 (1)°

Data collection

h = ?→?	l = ?→?
k = ?→?

Refinement

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

	x	y	z	Biso*/Beq
Ca	0.00635	0.33695	0.70829
Al	0.01053	0.31462	0.2493
Ta1	0	0	0
Ta2	0	0	0.5
O1	0.98863	0.07911	0.7462
O2	0.19148	0.43423	0.43596
O3	0.81411	0.44241	0.09542
O4	0.12217	0.19782	0.12117
O5	0.89633	0.2026	0.38334