We report a synchrotron x-ray-scattering study of the diluted spin-Peierls (SP) material ${\mathrm{Cu}}_{1-x}{\mathrm{Mg}}_x{\mathrm{GeO}}_3.$ In a recent paper we have shown that the SP dimerization attains long-range order only for $x<\mathrm{}{x}_c\sim 0.022\pm \mathrm{0.001.}$ Here we report that the SP transition is reentrant in the vicinity of the critical concentration $x_c.$ This is manifested by broadening of the SP dimerization superlattice peaks below the reentrance temperature $T_r,$ which may mean either the complete loss of the long-range SP order or the development of a short-range ordered component within the long-range ordered SP state. Very long relaxation times and related marked hysteresis are found in samples with Mg concentrations in the vicinity of $x_c.$ The reentrant SP transition is likely related to the competing Néel transition which occurs at a temperature similar to $T_r.$ We argue that impurity-induced competing interchain interactions play an essential role in these phenomena.

• Received 26 August 1999

DOI:https://doi.org/10.1103/PhysRevB.61.9527