The structural and magnetic properties of the one-dimensional spin-chain compound ${\mathrm{Cu}}_{2-y}{\mathrm{Zn}}_y{\mathrm{V}}_2{\mathrm{O}}_7$ were investigated by means of x-ray diffraction, static susceptibility, and electron spin resonance. The substitution of nonmagnetic ${\mathrm{Zn}}^{2+}$ for ${\mathrm{Cu}}^{2+}$ ions induces a polymorphic transition from the $\alpha$ phase, found for $y<\mathrm{}0.15,$ to the $\beta$ phase, which occurs for $y>~\mathrm{0.15.}$ This transition has a pronounced influence on the magnetic properties, which can be understood consistently by assuming that the intrachain Dzyaloshinsky-Moriya interaction is present in the $\alpha$ phase, but absent in the $\beta$ phase. This interpretation is confirmed by a symmetry analysis of the structural and magnetic properties. Our results indicate that the local coordination geometry of Cu in the two phases is decisive for the magnetic properties.

• Received 6 September 2002

DOI:https://doi.org/10.1103/PhysRevB.67.214410