We apply neutron diffraction, high-resolution synchrotron x-ray diffraction, magnetization measurements, electronic structure calculations, and quantum Monte-Carlo simulations to unravel the structure and magnetism of (CuCl)LaTa${}_2$O${}_7$. Despite the pseudo-tetragonal crystallographic unit cell, this compound features an orthorhombic superstructure, similar to the Nb-containing (Cu$X$)LaNb${}_2$O${}_7$ with $X$ $=$ Cl and Br. The spin lattice entails dimers formed by the antiferromagnetic fourth-neighbor coupling $J_4$, as well as a large number of nonequivalent interdimer couplings quantified by an effective exchange parameter $J_{\text{eff}}$. In (CuCl)LaTa${}_2$O${}_7$, the interdimer couplings are sufficiently strong to induce the long-range magnetic order with the Néel temperature $T_N\simeq 7$ K and the ordered magnetic moment of 0.53${\mu }_B$, as measured with neutron diffraction. This magnetic behavior can be accounted for by $J_{\text{eff}}/J_4\simeq 1.6$ and $J_4\simeq 16$ K. We further propose a general magnetic phase diagram for the (CuCl)LaNb${}_2$O${}_7$-type compounds, and explain the transition from the gapped spin-singlet (dimer) ground state in (CuCl)LaNb${}_2$O${}_7$ to the long-range antiferromagnetic order in (CuCl)LaTa${}_2$O${}_7$ and (CuBr)LaNb${}_2$O${}_7$ by an increase in the magnitude of the interdimer couplings $J_{\text{eff}}/J_4$, with the (CuCl)La$M$${}_2$O${}_7$ ($M$ $=$ Nb, Ta) compounds lying on different sides of the quantum critical point that separates the singlet and long-range-ordered magnetic ground states.

• Received 16 June 2012

DOI:https://doi.org/10.1103/PhysRevB.86.064440