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We apply neutron diffraction, high-resolution synchrotron x-ray diffraction,magnetization measurements, electronic structure calculations, and quantumMonte-Carlo simulations to unravel the structure and magnetism of(CuCl)LaTa2O7. Despite the pseudo-tetragonal crystallographic unit cell, thiscompound features an orthorhombic superstructure, similar to the Nb-containing(CuX)LaNb2O7 with X = Cl and Br. The spin lattice entails dimers formed by theantiferromagnetic fourth-neighbor coupling J4, as well as a large number ofnonequivalent interdimer couplings quantified by an effective exchangeparameter Jeff. In (CuCl)LaTa2O7, the interdimer couplings are sufficientlystrong to induce the long-range magnetic order with the Neel temperature TN~7 Kand the ordered magnetic moment of 0.53 mu_B, as measured with neutrondiffraction. This magnetic behavior can be accounted for by Jeff/J4~1.6 andJ4~16 K. We further propose a general magnetic phase diagram for the(CuCl)LaNb2O7-type compounds, and explain the transition from the gappedspin-singlet (dimer) ground state in (CuCl)LaNb2O7 to the long-rangeantiferromagnetic order in (CuCl)LaTa2O7 and (CuBr)LaNb2O7 by an increase inthe magnitude of the interdimer couplings Jeff/J_4, with the (CuCl)LaM2O7 (M =Nb, Ta) compounds lying on different sides of the quantum critical point thatseparates the singlet and long-range-ordered magnetic ground states.

(CuCl)LaTa2O7 and quantum phase transition in the (CuX)LaM2O7 family (X = Cl, Br; M = Nb, Ta)