New S=1/2 alternating chain compound - High pressure form of (VO)(2)P2O7-

The high-pressure phase of (VO)(2)P2O7 comprises a unique kind of spin-1/2 Heisenberg alternating antiferromagnetic chain, in contrast with the ambien t pressure phase containing two crystallographically different chains. Magn etic susceptibility and high-field magnetization data showed the presence o f a single spin gap of 23 (magnetization) similar to 27 (susceptibility) K, not double as observed for the ambient pressure phase. This result shows t hat the two kinds of chains of the ambient pressure phase have single, diff erent spin gaps.