Crystal structure of La4Mg3W3O18 layered oxide

authors	Khalyavin, DD; Senos, AMR; Mantas, PQ
nationality	International
journal	JOURNAL OF PHYSICS-CONDENSED MATTER
keywords	GROUP-THEORETICAL-ANALYSIS; X-RAY; POWDER DIFFRACTION; PEROVSKITES; SYSTEM
abstract	The crystal structure of A-site deficient La4M93W3O18 perovskite has been solved by x-ray powder diffraction in combination with group theoretical analysis. Above 700 K, the crystal structure is orthorhombic (space group Ibam; 2a(p) x 4a(p) x 2a(p) type superstructure) and presents a sequence of [LaO]-[Mg1/2W1/2O2]-[La1/3O]'-[Mg1/2W1/2O2]-[LaO]-[Mg1/2W1/2O2]-[La1/3O]"-[Mg1/2W1/2O2] layers stacked along the b axis. The lanthanum ions and the vacancies in the [La1/3O]' and [La1/3O]" layers are ordered and form rows along the c axis. A half-period shift along the a direction between these layers leads to a quadrupling of the primitive perovskite unit cell in the b direction. The ordering of the vacancies in the lanthanum poor layers is connected with the ionic ordering between Mg2+ and W6+ in the neighbouring [Mg1/2W1/2O2] blocks. Around 700 K, due to an anti-phase rotation of the octahedra, a continuous phase transition mediated by the Gamma(2)(+) irreducible representation from orthorhombic (Ibam, a(0)b(0)c(0)) to monoclinic (C2/m, a(0)b(0)c(-)) symmetry takes place.
publisher	IOP PUBLISHING LTD
issn	0953-8984
year published	2005
volume	17
issue	17
beginning page	2585
ending page	2595
digital object identifier (doi)	10.1088/0953-8984/17/17/006
web of science category	Physics, Condensed Matter
subject category	Physics
unique article identifier	WOS:000229222200011