Interplay of atomic displacements in the quantum magnet (CuCl ) LaNb 2O7

Alexander A. Tsirlin, Artem M. Abakumov, Gustaaf Van Tendeloo, Helge Rosner

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16 Citations (Scopus)

Abstract

We report on the crystal structure of the quantum magnet (CuCl) LaNb 2 O7 that was controversially described with respect to its structural organization and magnetic behavior. Using high-resolution synchrotron powder x-ray diffraction, electron diffraction, transmission electron microscopy, and band-structure calculations, we solve the room-temperature structure of this compound [α- (CuCl) LaNb2 O7] and find two high-temperature polymorphs. The γ- (CuCl) LaNb2 O7 phase, stable above 640 K, is tetragonal with asub =3.889Å, csub =11.738Å, and the space group P4/mmm. In the γ- (CuCl) LaNb2O7 structure, the Cu and Cl atoms are randomly displaced from the special positions along the {100} directions. The β phase (asub ×2 asub × csub, space group Pbmm) and the α phase (2 a sub ×2 asub × csub, space group Pbam) are stable between 640 K and 500 K and below 500 K, respectively. The structural changes at 500 and 640 K are identified as order-disorder phase transitions. The displacement of the Cl atoms is frozen upon the γ→β transformation while a cooperative tilting of the NbO 6 octahedra in the α phase further eliminates the disorder of the Cu atoms. The low-temperature α- (CuCl) LaNb2O7 structure thus combines the two types of the atomic displacements that interfere due to the bonding between the Cu atoms and the apical oxygens of the NbO 6 octahedra. The precise structural information resolves the controversy between the previous computation-based models and provides the long-sought input for understanding (CuCl) LaNb2O7 and related compounds with unusual magnetic properties.

Original languageEnglish
Article number054107
JournalPhysical Review B - Condensed Matter and Materials Physics
Volume82
Issue number5
DOIs
Publication statusPublished - 12 Aug 2010
Externally publishedYes

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