Interplay of atomic displacements in the quantum magnet(CuCl)LaNb2O7

Abstract

We report on the crystal structure of the quantum magnet ͑CuCl͒LaNb 2 O 7 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 ͓␣-͑CuCl͒LaNb 2 O 7 ͔ and find two high-temperature polymorphs. The ␥-͑CuCl͒LaNb 2 O 7 phase, stable above 640 K, is tetragonal with a sub = 3.889 Å, c sub = 11.738 Å, and the space group P4 / mmm. In the ␥-͑CuCl͒LaNb 2 O 7 structure, the Cu and Cl atoms are randomly displaced from the special positions along the ͕100͖ directions. The ␤ phase ͑a sub ϫ 2a sub ϫ c sub , space group Pbmm͒ and the ␣ phase ͑2a sub ϫ 2a sub ϫ c sub , space group Pbam͒ 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 ␥ → ␤ transformation while a cooperative tilting of the NbO 6 octahedra in the ␣ phase further eliminates the disorder of the Cu atoms. The low-temperature ␣-͑CuCl͒LaNb 2 O 7 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 ͑CuCl͒LaNb 2 O 7 and related compounds with unusual magnetic properties.

Magnetic correlation in the square-lattice spin system (CuBr)Sr2Nb3O10: A neutron diffraction study

“…32,33 The values of other crystal structure parameters, such as fractional atomic coordinates, bond lengths, and bond angles, are also in a good agreement with the previously reported values from an x-ray diffraction study and are not reported here. No structural change has been observed down to the lowest measured temperature 2 K. It may be mentioned that Tassel et al 39 and independently Tsirlin et al 40 reported the existence of a very small orthorhombic distortion in the similar compound (CuCl)LaNb 2 O 7 , which belongs to the Dion-Jacobson series with n = 2. Due to the limited resolution available at D20, we cannot exclude the existence of such a distortion in (CuBr)Sr 2 Nb 3 O 10 .…”

Field-induced evolution of magnetic ordering in the quantum spin system (CuBr)Sr2Nb3O10with aRitter1, Yusuf2, Bera3 et al. 2013Phys. Rev. B6120View full textAdd to dashboardCite

“…In the case of the n = 2 compound (CuCl)LaNb 2 O 7 , an orthorhombic splitting was observed in high-resolution synchrotron XRD while no splitting was detectable for (CuCl)LaTa 2 O 7 by XRD. 5,11 Neutron-diffraction data for both compounds revealed, however, weak superlattice reflections, which are indexable in a four times larger unit cell with , the high intensity data of D20 were, therefore, used to verify and to confirm the absence of any superlattice reflections at RT and low temperatures (2 K). As a conclusion, one can state that the average crystal structure of (CuBr)Sr 2 Nb 3 O 10 stays tetragonal in space group P 4/mmm over the studied temperature range between RT and 2 K. However, there exists a strong disorder of the Br atoms (within the Cu-Br layers) and of one of the oxygen atoms (within the NbO 6 perovskite blocks), which lowers the symmetry locally.…”

Ground-state phase diagram of the spin-12square-latticeJ1-J2model with plaquette structur

“…This plaquette model, however, has been questioned recently by Rosner and coworkers. 55 In this paper we consider the J 1 -J 2 model on the square lattice with plaquette structure (see Fig. 1) merging this way the two mechanisms to destroy magnetic LRO, namely frustration and local singlet formation.…”