New S=1/2 Alternating Chain Compound - High Pressure Form of (VO)₂P₂O_7-

Abstract

The high-pressure phase of (VO) 2 P 2 O 7 comprises a unique kind of spain-1/2 Heisenberg alternating antiferromagnetic chain, in contrast with the ambient pressure phase containing two crystallographically different chains. Magnetic susceptibility and high-field magnetization data showed the presence of a single spin gap of 23 (magnetization) 27 (susceptibility) K, not double as observed for the ambient pressure phase. This result shows that the two kinds of chains of the ambient pressure phase have single, different spin gaps. . A recent neutron scattering study on single crystals [4] showed the validity of the alternating chain model with a major AF interaction (J1) between a pair of VO5 pyramids bridged by ΡO4 tetrahedra and a minor one (J2) between a pair of edge sharing VO5 pyramids. Interestingly, the data also revealed the existence of a second energy gap of about twice as large as the first one as predicted theoretically [5]. However, this compound comprises two slightly different chains [6,7], making it difficult to determine whether these chains have single but different spin gaps or have the same double gaps. High-field magnetization and NMR studies have shown the former is the case [8].We found that this compound undergoes a pressure-induced transition to a similar but simplified structure with a unique kind of chain [9]. The crystal structures of the high-pressure (HP) and the ambient-pressure (AP) phases are illustrated in Fig. 1. Both these are made of edge-sharing pairs of VO5 pyramids and PO4 tetrahedra, while the arrangement is simpler in the HP phase. In the (205) .