Year: 2010
Type: article
Abstract: We report on the crystal structure of the quantum magnet $(\text{CuCl}){\text{LaNb}}_{2}{\text{O}}_{7}$ 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 $[\ensuremath{\alpha}\text{\ensuremath{-}}(\text{CuCl}){\text{LaNb}}_{2}{\text{O}}_{7}]$ and find two high-temperature polymorphs. The $\ensuremath{\gamma}\text{\ensuremath{-}}(\text{CuCl}){\text{LaNb}}_{2}{\text{O}}_{7}$ phase, stable above 640 K, is tetragonal with ${a}_{\text{sub}}=3.889\text{ }\text{\AA{}}$, ${c}_{\text{sub}}=11.738\text{ }\text{\AA{}}$, and the space group $P4/mmm$. In the $\ensuremath{\gamma}\text{\ensuremath{-}}(\text{CuCl}){\text{LaNb}}_{2}{\text{O}}_{7}$ structure, the Cu and Cl atoms are randomly displaced from the special positions along the {100} directions. The $\ensuremath{\beta}$ phase (${a}_{\text{sub}}\ifmmode\times\else\texttimes\fi{}2{a}_{\text{sub}}\ifmmode\times\else\texttimes\fi{}{c}_{\text{sub}}$, space group $Pbmm$) and the $\ensuremath{\alpha}$ phase ($2{a}_{\text{sub}}\ifmmode\times\else\texttimes\fi{}2{a}_{\text{sub}}\ifmmode\times\else\texttimes\fi{}{c}_{\text{sub}}$, 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 $\ensuremath{\gamma}\ensuremath{\rightarrow}\ensuremath{\beta}$ transformation while a cooperative tilting of the ${\text{NbO}}_{6}$ octahedra in the $\ensuremath{\alpha}$ phase further eliminates the disorder of the Cu atoms. The low-temperature $\ensuremath{\alpha}\text{\ensuremath{-}}(\text{CuCl}){\text{LaNb}}_{2}{\text{O}}_{7}$ 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 ${\text{NbO}}_{6}$ octahedra. The precise structural information resolves the controversy between the previous computation-based models and provides the long-sought input for understanding $(\text{CuCl}){\text{LaNb}}_{2}{\text{O}}_{7}$ and related compounds with unusual magnetic properties.
Source: Physical Review B
Language: Latvian
Subfield: Condensed Matter Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2074589828