High-pressure form of $(\mathrm{VO}{)}_2{\mathrm{P}}_2{\mathrm{O}}_7$: A spin-$\frac{1}{2}$ antiferromagnetic alternating-chain compound with one kind of chain and a single spin gap

$(\mathrm{VO}{)}_2{\mathrm{P}}_2{\mathrm{O}}_7,$ known as a quantum spin chain compound, was found to undergo a structural transition when treated at 2 GPa and $700°\mathrm{C}.$ The high-pressure phase as studied by powder x-ray and neutron diffractions 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, high-field magnetization, and specific-heat 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 is consistent with recent magnetization and NMR studies which indicated that the two kinds of chains of the ambient pressure phase have single, different spin gaps.