New S=1/2 Alternating Chain Compound - High Pressure Form of (VO)$\text{}_{2}$P$\text{}_{2}$O$\text{}_{7^{-}}$

The high-pressure phase of (VO)$\text{}_{2}$P$\text{}_{2}$O$\text{}_{7}$ comprises a unique kind of spin-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.