We have identified a potential quantum spin liquid by tuning the magnetic interactions on a square lattice through d10-d0 cation mixing. MuSR shows dynamic magnetism down to 0.25 K in Sr2Cu(Te0.5W0.5)O6, and a strong T-linear dependence to the low temperature specific heat. Frustrated magnetism on the square lattice is described by the J1-J2 spin-1/2 Heisenberg square lattice model, where the region at J2/J1~0.5 is currently under debate. However no experimental evidence of a compound with J2/J1~0.5 had been found previously. In Sr2Cu(Te0.5W0.5)O6 structural disorder and magnetic frustration are inherently linked with each Cu square containing either Te6+ or W6+ promoting either a dominant J1 or J2 interaction. On triangular lattices it has been proposed that a disorder-induced gapless QSL state is possible. We therefore wish to use PDF to study the local structure of Sr2Cu(Te0.5W0.5)O6.