Single-crystal neutron-diffraction study of La2Cu0.95Li0.05O4.

Abstract

${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ possesses an orthorhombic superstructure based upon a tetragonal ${\mathrm{K}}_{2}$Ni${\mathrm{F}}_{4}$-type subcell for which a number of different space groups have been reported. A single-crystal neutron diffraction study, performed with large flux-grown crystals, confirms the nature of the substructure and provides $a=5.359(1)$, $b=5.363(1)$, and $c=13.127(2)$ \AA{}A. The orthorhombic strain, $\ensuremath{\epsilon}=\frac{(b\ensuremath{-}a)}{\frac{1}{2}(a+b)}$, is 7.64\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$, an order of magnitude smaller than for pure ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ as a result of incorporation of Li, derived from the flux, in the Cu site. The apparent space group is Pccn, but systematic examination of structural models in all possible centrosymmetric orthorhombic subgroups which may accommodate a ${\mathrm{K}}_{2}$Ni${\mathrm{F}}_{4}$-type substructure led to satisfactory refinement with positive-definite temperature factors only in space group Bmab-${D}_{2h}^{18}$. A substantial number of very weak reflections were detected which violate the required systematic absences of this space group. Through structure-factor calculation, after reindexing based on a probable twin law, and through dark-field transmission electron microscopy, the "forbidden" reflections are shown to arise from a 0.302(2) volume fraction of domains in twinned orientation which otherwise escaped detection because of the small value of $\ensuremath{\epsilon}$. Final agreement indices for 346 reflections with $I\ensuremath{\ge}\ensuremath{\sigma}(I)$ are $R(F)=3.22%$, $R({F}^{2})=2.57%$, ${R}_{w}({F}^{2})=3.07%$; the goodness of fit, $S$, is 1.13. The La and oxygen sites are found to be fully occupied within standard deviations of 0.4 at.%. The occupancy of the Cu site is ${\mathrm{Cu}}_{0.950}$${\mathrm{Li}}_{0.050(3)}$, in excellent accord with the results of a bulk chemical analysis.