Rational Design of (NH4)Cu[PO4] with a Spin Gapped, Distorted Honeycomb Layer

Abstract

Water (H2O) molecules are often used to depolymerize crystal structures. Conversely, polycondensation of basic building units of crystal structures can be realized by removing water molecules from hydrous compounds. Herein, we report our rational design and synthesis of a novel copper phosphate (NH4)Cu[PO4] from a hydrothermal route with minimal water used. The title compound keeps the layered structural feature of its hydrous analogue (NH4)Cu[PO4]·H2O, but the isolated centrosymmetric [Cu2O8] dimers in the latter further polymerize into a two‐dimensional (2D) corrugated honeycomb layer in the former. Interestingly, the layers of corrugated honeycomb arrangement of Cu atoms in the bc plane are composed of a relatively rare compressed [CuO5] trigonal bipyramid, rather than the axially elongated [CuO4(OH2)] square pyramid in (NH4)Cu[PO4]·H2O. Magnetic susceptibility χ(T) of the title compound increases steadily to a maximum at 58 K but falls off rapidly to almost zero with decrease in temperature, suggesting the existence of a spin gap and long range ordering for the 2D S = 1/2 corrugated honeycomb layer.