| Abstract
| - The crystal structure of α-AgCuPO4 was determined, and the crystal structures and magnetic properties of α-AgCuPO4 and β-AgCuPO4 were compared. The magnetic susceptibilities of α-AgCuPO4 and β-AgCuPO4 show a maximum at ∼75 and ∼50 K, respectively, and are well simulated by an antiferromagnetic alternating-chain model leading to a spin gap of ∼94 and ∼62 K, respectively.
- The crystal structure of the low-temperature form of AgCuPO4 (i.e., α-AgCuPO4) was determined by powder X-raydiffraction and was compared with that of the high-temperature form of AgCuPO4 (i.e., β-AgCuPO4). The magneticproperties of the two forms were examined by measuring their magnetic susceptibilities and evaluating the relativestrengths of their spin-exchange interactions on the basis of spin-dimer analysis. Both forms of AgCuPO4 havelayers of Cu2P2O8 alternating with silver-atom double layers; β-AgCuPO4 has two Cu2P2O8 layers per unit cell,while α-AgCuPO4 has one. The coordinate environment of each Cu2+ ion is close to being a distorted squarepyramid in α-AgCuPO4, but it is close to being a distorted trigonal bipyramid in β-AgCuPO4. The magneticsusceptibilities of α- and β-AgCuPO4 are well simulated by an antiferromagnetic alternating-chain model, whichleads to J/kB = −146.1 K and αJ/kB = −75.8 K for α-AgCuPO4, and J/kB = −82.6 K and αJ/kB = −31.7 K forβ-AgCuPO4 (with the convention in which the spin-exchange parameter between two adjacent spin sites is writtenas 2J). The spin gaps, Δ/kB, obtained from these parameters are 93.7 K for α-AgCuPO4 and 62.3 K for β-AgCuPO4.The strongest spin exchange in both forms of AgCuPO4 comes from a super-superexchange path, and this interactionis stronger for α-AgCuPO4 than for β-AgCuPO4 by a factor of ∼2, in good agreement with the experiment. Ouranalysis supports the use of this model for β-AgCuPO4 and indicates that the spin lattice of α-AgCuPO4 would bebetter described by a two-dimensional net made up of weakly interacting alternating chains.
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