High-pressure form of (VO)(2)P2O7: A spin-1/2 antiferromagnetic alternating-chain compound with one kind of chain and a single spin gap

(VO)(2)P2O7, known as a quantum spin chain compound, was found to undergo a structural transition when treated at 2 GPa and 700 degrees C. The high-pr essure phase as studied by powder x-ray and neutron diffractions comprises a unique kind of spin-1/2 Heisenberg alternating antiferromagnetic chain, i n contrast with the ambient pressure phase containing two crystallographica lly different chains. Magnetic susceptibility, high-field magnetization, an d specific-heat data showed the presence of a single spin gap of 23 (magnet ization)-27 (susceptibility) K, not double as observed for the ambient pres sure phase. This result is consistent with recent magnetization and NMR stu dies which indicated that the two kinds of chains of the ambient pressure p hase have single, different spin gaps.